The Art and Science of

Total Synthesis

REVIEWS

The Art and Science of Total Synthesis at the Dawn

of the Twenty-First Century**
K. C. Nicolaou,* Dionisios Vourloumis, Nicolas Winssinger, and Phil S. Baran
Dedicated to Professor E. J. Corey for his outstanding contributions to organic synthesis

At the dawn of the twenty-first century, the state of the art and science of
total synthesis is as healthy and vigorous as ever. The birth of this exhilarating, multifaceted, and boundless science is marked by Whlers synthesis
of urea in 1828. This milestone event
as trivial as it may seem by todays
standardscontributed to a demystification of nature and illuminated the
entrance to a path which subsequently
led to great heights and countless rich
dividends for humankind. Being both a
precise science and a fine art, this
discipline has been driven by the constant flow of beautiful molecular architectures from nature and serves as the
engine that drives the more general
field of organic synthesis forward.
Organic synthesis is considered, to a
large extent, to be responsible for some

of the most exciting and important

discoveries of the twentieth century in
chemistry, biology, and medicine, and
continues to fuel the drug discovery
and development process with myriad
processes and compounds for new
biomedical breakthroughs and applications. In this review, we will chronicle the past, evaluate the present, and
project to the future of the art and
science of total synthesis. The gradual
sharpening of this tool is demonstrated
by considering its history along the
lines of pre-World War II, the Woodward and Corey eras, and the 1990s,
and by accounting major accomplishments along the way. Today, natural
product total synthesis is associated
with prudent and tasteful selection of
challenging and preferably biologically
important target molecules; the dis-

1. Prologue
Your Majesty, Your Royal Highnesses, Ladies and Gentlemen.
In our days, the chemistry of natural products attracts a very
lively interest. New substances, more or less complicated,
[*] K. C. Nicolaou, D. Vourloumis, N. Winssinger, P. S. Baran
Department of Chemistry
and The Skaggs Institute for Chemical Biology
The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
and
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
Fax: ( 1) 858-784-2469
E-mail: kcn@scripps.edu
[**] A list of abbreviations can be found at the end of the article.
Angew. Chem. Int. Ed. 2000, 39, 44 122

covery and invention of new synthetic

strategies and technologies; and explorations in chemical biology through
molecular design and mechanistic
studies. Future strides in the field are
likely to be aided by advances in the
isolation and characterization of novel
molecular targets from nature, the
availability of new reagents and synthetic methods, and information and
automation technologies. Such advances are destined to bring the power of
organic synthesis closer to, or even
beyond, the boundaries defined by
nature, which, at present, and despite
our many advantages, still look so far
away.
Keywords: drug research natural
products synthetic methods total
synthesis

more or less useful, are constantly discovered and investigated. For the determination of the structure, the architecture
of the molecule, we have today very powerful tools, often
borrowed from Physical Chemistry. The organic chemists of
the year 1900 would have been greatly amazed if they had
heard of the methods now at hand. However, one cannot say
that the work is easier; the steadily improving methods make
it possible to attack more and more difficult problems and the
ability of Nature to build up complicated substances has, as it
seems, no limits.
In the course of the investigation of a complicated
substance, the investigator is sooner or later confronted by
the problem of synthesis, of the preparation of the substance
by chemical methods. He can have various motives. Perhaps
he wants to check the correctness of the structure he has
found. Perhaps he wants to improve our knowledge of the
reactions and the chemical properties of the molecule. If the

 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000

1433-7851/00/3901-0045 $ 17.50+.50/0

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K. C. Nicolaou et al.

substance is of practical importance, he may hope that the

synthetic compound will be less expensive or more easily
accessible than the natural product. It can also be desirable to
modify some details in the molecular structure. An antibiotic
substance of medical importance is often first isolated from a
microorganism, perhaps a mould or a germ. There ought to
exist a number of related compounds with similar effects; they
may be more or less potent, some may perhaps have
undesirable secondary effects. It is by no means, or even
probable, that the compound produced by the microorganismmost likely as a weapon in the struggle for existenceis
the very best from the medicinal point of view. If it is possible
to synthesize the compound, it will also be possible to modify
the details of the structure and to find the most effective
remedies.

K. C. Nicolaou

D. Vourloumis

The synthesis of a complicated molecule is, however, a very

difficult task; every group, every atom must be placed
in its proper position and this should be taken in its most
literal sense. It is sometimes said that organic synthesis
is at the same time an exact science and a fine art. Here
nature is the uncontested master, but I dare say that
the prize-winner of this year, Professor Woodward, is a good
second.[1]
With these elegant words Professor A. Fredga, a member of
the Nobel Prize Committee for Chemistry of the Royal
Swedish Academy of Sciences, proceeded to introduce R. B.
Woodward at the Nobel ceremonies in 1965, the year in which
Woodward received the prize for the art of organic synthesis.
Twenty-five years later Professor S. Gronowitz, then a member of the Nobel Prize Committee for Chemistry, concluded

N. Winssinger

P. S. Baran

K.C. Nicolaou, born in Cyprus and educated in England and the US, is currently Chairman of the Department of Chemistry
at The Scripps Research Institute, La Jolla, California, where he holds the Darlene Shiley Chair in Chemistry and the
Aline W. and L. S. Skaggs Professorship in Chemical Biology as well as Professor of Chemistry at the University of
California, San Diego. His impact on chemistry, biology, and medicine flows from his works in organic synthesis described
in nearly 500 publications and 70 patents as well as his dedication to chemical education, as evidenced by his training of
over 250 graduate students and postdoctoral fellows. His recent book titled Classics in Total Synthesis,[3] which he coauthored with Erik J. Sorensen, is used around the world as a teaching tool and source of inspiration for students and
practitioners of organic synthesis.
Dionisios Vourloumis, born in 1966 in Athens, Greece, received his B.Sc. degree from the University of Athens and his
Ph.D. from West Virginia University under the direction of Professor P. A. Magriotis, in 1994, working on the synthesis of
novel enediyne antibiotics. He joined Professor K. C. Nicolaous group in 1996, and was involved in the total synthesis of
epothilones A and B, eleutherobin, sarcodictyins A and B, and analogues thereof. He joined Glaxo Wellcome in early 1999
and is currently working with the Combichem Technology Team in Research Triangle Park, North Carolina.
Nicolas Winssinger was born in Belgium in 1970. He received his B.Sc. degree in chemistry from Tufts University after
conducting research in the laboratory of Professor M. DAlarcao. Before joining The Scripps Research Institute as a
graduate student in chemistry in 1995, he worked for two years under the direction of Dr. M. P. Pavia at Sphinx
Pharmaceuticals in the area of molecular diversity focusing on combinatorial chemistry. At Scripps, he joined the
laboratory of Professor K. C. Nicolaou, where he has been working on methodologies for solid-phase chemistry and
combinatorial synthesis. His research interests include natural products synthesis, molecular diversity, molecular evolution,
and their application to chemical biology.
Phil S. Baran was born in Denville, New Jersey in 1977. He received his B.Sc. degree in chemistry from New York University
while conducting research under the guidance of Professors D. I. Schuster and S. R. Wilson, exploring new realms in
fullerene science. Upon entering The Scripps Research Institute in 1997 as a graduate student in chemistry, he joined the
laboratory of Professor K. C. Nicolaou where he embarked on the total synthesis of the CP molecules. His primary research
interest involves natural product synthesis as an enabling endeavor for the discovery of new fundamental processes and
concepts in chemistry and their application to chemical biology.

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his introduction of E. J. Corey, the 1990 Nobel prize winner,
with the following words:
...Corey has thus been awarded with the Prize for three
intimately connected contributions, which form a whole.
Through retrosynthetic analysis and introduction of new
synthetic reactions, he has succeeded in preparing biologically
important natural products, previously thought impossible to
achieve. Coreys contributions have turned the art of synthesis
into a science...[2]
This description and praise for total synthesis resonates
today with equal validity and appeal; most likely, it will be
valid for some time to come. Indeed, unlike many one-time
discoveries or inventions, the endeavor of total synthesis[36] is
in a constant state of effervescence and flux. It has been on the
move and center stage throughout the twentieth century and
continues to provide fertile ground for new discoveries and
inventions. Its central role and importance within chemistry
will undoubtedly ensure its present preeminence into the
future. The practice of total synthesis demands the following
virtues from, and cultivates the best in, those who practice it:
ingenuity, artistic taste, experimental skill, persistence, and
character. In turn, the practitioner is often rewarded with
discoveries and inventions that impact, in major ways, not
only other areas of chemistry, but most significantly material
science, biology, and medicine. The harvest of chemical
synthesis touches upon our everyday lives in myriad ways:
medicines, high-tech materials for computers, communication
and transportation equipment, nutritional products, vitamins,
cosmetics, plastics, clothing, and tools for biology and
physics.[7]
But why is it that total synthesis has such a lasting value as a
discipline within chemistry? There must be several reasons for
this phenomenon. To be sure, its dual nature as a precise
science and a fine art provides excitement and rewards of rare
heights. Most significantly, the discipline is continually being
challenged by new structural types isolated from natures
seemingly unlimited library of molecular architectures. Happily, the practice of total synthesis is being enriched constantly
by new tools such as new reagents and catalysts as well as
analytical instrumentation for the rapid purification and
characterization of compounds.
Thus, the original goal of total synthesis during the first part
of the twentieth century to confirm the structure of a natural
product has been replaced slowly but surely with objectives
related more to the exploration and discovery of new
chemistry along the pathway to the target molecule. More
recently, issues of biology have become extremely important
components of programs in total synthesis. It is now clear that
as we enter the twenty-first century both exploration and
discovery of new chemistry and chemical biology will be
facilitated by developments in total synthesis.
In this article, and following a short historical perspective of
total synthesis in the nineteenth century, we will attempt to
review the art and science of total synthesis during the
twentieth century. This period can be divided into the preWorld War II Era, the Woodward Era, the Corey Era, and the
1990s. There are clearly overlaps in the last three eras and
many more practitioners deserve credit for contributing to the
evolution of the science during these periods than are
Angew. Chem. Int. Ed. 2000, 39, 44 122

mentioned. The labeling of these eras is arbitrarynot

withstanding the tremendous impact Woodward and Corey
had in shaping the discipline of total synthesis during their
time. As in any review of this kind, omissions are inevitable
and we apologize profusely, and in advance, to those
whose brilliant works were omitted as a result of space
limitations.

2. Total Synthesis in the Nineteenth Century

The birth of total synthesis occurred in the nineteenth
century. The first conscious total synthesis of a natural product
was that of urea (Figure 1) in 1828 by Whler.[8] Significantly,
this event also marks the beginning of organic synthesis and
OH
O
NH2

O
NH2

Me

OH

HO
HO

O
OH
OH

urea

acetic acid

glucose

[Whler, 1828][8]

[Kolbe, 1845][9]

[Fischer, 1890][12]

Figure 1. Selected nineteenth century landmark total syntheses of natural

products.

the first instance in which an inorganic substance

(NH4CNO:ammonium cyanate) was converted into an organic substance. The synthesis of acetic acid from elemental
carbon by Kolbe in 1845[9] is the second major achievement in
the history of total synthesis. It is historically significant that,
in his 1845 publication, Kolbe used the word synthesis for
the first time to describe the process of assembling a chemical
compound from other substances. The total syntheses of
alizarin (1869) by Graebe and Liebermann[10] and indigo
(1878) by Baeyer[11] spurred the legendary German dye
industry and represent landmark accomplishments in the
field. But perhaps, after urea, the most spectacular total
synthesis of the nineteenth century was that of ()-glucose
(Figure 1) by E. Fischer.[12] This total synthesis is remarkable
not only for the complexity of the target, which included, for
the first time, stereochemical elements, but also for the
considerable stereochemical control that accompanied it.
With its oxygen-containing monocyclic structure (pyranose)
and five stereogenic centers (four controllable), glucose
represented the state-of-the-art in terms of target molecules
at the end of the nineteenth century. E. Fischer became the
second winner of the Nobel Prize for chemistry (1902), after
J. H. vant Hoff (1901).[13]

3. Total Synthesis in the Twentieth Century

The twentieth century has been an age of enormous
scientific advancement and technological progress. To be
sure, we now stand at the highest point of human accomplishment in science and technology, and the twenty-first century
promises to be even more revealing and rewarding. Advances
47

REVIEWS
in medicine, computer science, communication, and transportation have dramatically changed the way we live and the
way we interact with the world around us. An enormous
amount of wealth has been created and opportunities for new
enterprises abound. It is clear that at the heart of this
technological revolution has been science, and one cannot
deny that basic research has provided the foundation for this
to occur.
Chemistry has played a central and decisive role in shaping
the twentieth century. Oil, for example, has reached its
potential only after chemistry allowed its analysis, fractionation, and transformation into myriad of useful products such
as kerosene and other fuels. Synthetic organic chemistry is
perhaps the most expressive branch of the science of
chemistry in view of its creative power and unlimited scope.
To appreciate its impact on modern humanity one only has to
look around and recognize that this science is a pillar behind
pharmaceuticals, high-tech materials, polymers, fertilizers,
pesticides, cosmetics, and clothing.[7] The engine that drives
forward and sharpens our ability to create such molecules
through chemical synthesis (from which we can pick and
choose the most appropriate for each application) is total
synthesis. In its quest to construct the most complex and
challenging of natures products, this endeavorperhaps
more that any otherbecomes the prime driving force for
the advancement of the art and science of organic synthesis.
Thus, its value as a research discipline extends beyond
providing a test for the state-of-the-art. It offers the opportunity to discover and invent new science in chemistry and
related disciplines, as well as to train, in a most rigorous way,
young practitioners whose expertise may feed many peripheral areas of science and technology.[6]

3.1. The Pre-World War II Era

The syntheses of the nineteenth century were relatively
simple and, with a few exceptions, were directed towards
benzenoid compounds. The starting materials for these target
molecules were other benzenoid compounds, chosen for their
resemblance to the targeted substance and the ease by which
the synthetic chemist could connect them by simple functionalization chemistry. The twentieth century was destined to
bring dramatic advances in the field of total synthesis. The
pre-World War II Era began with impressive strides and with
increasing molecular complexity and sophistication in strategy design. Some of the most notable examples of total
synthesis of this era are a-terpineol (Perkin, 1904),[14]
camphor (Komppa, 1903; Perkin, 1904),[15] tropinone (Robinson, 1917; Willsttter, 1901),[1617] haemin (H. Fischer,
1929),[18] pyridoxine hydrochloride (Folkers, 1939),[1920] and
equilenin (Bachmann, 1939)[21] (Figure 2). Particularly impressive were Robinsons one-step synthesis of tropinone
(1917)[16] from succindialdehyde, methylamine, and acetone
dicarboxylic acid and H. Fischers synthesis of haemin[18]
(1929). These total syntheses are among those which will be
highlighted below. Both men went on to win a Nobel Prize for
Chemistry (Fischer, 1929; Robinson, 1947).[13]
48

K. C. Nicolaou et al.

Figure 2. Selected landmark total syntheses of natural products from 1901

to 1939.

3.2. The Woodward Era

In 1937 and at the age of 20 R. B. Woodward became an
assistant professor in the Department of Chemistry at
Harvard University where he remained for the rest of his
life. Since that time, total synthesis and organic chemistry
would never be the same. A quantum leap forward was about
to be taken, and total synthesis would be elevated to a
powerful science and a fine art. Woodwards climactic
contributions to total synthesis included the conquest of some
of the most fearsome molecular architectures of the time. One
after another, diverse structures of unprecedented complexity
succumbed to synthesis in the face of his ingenuity and
resourcefulness. The following structures (some are shown in
Figure 3) are amongst his most spectacular synthetic achievements: quinine (1944),[22] patulin (1950),[23] cholesterol and
cortisone (1951),[24] lanosterol (1954),[25] lysergic acid (1954),[26]
strychnine (1954),[27] reserpine (1958),[28] chlorophyll a (1960),[29]
colchicine (1965),[286] cephalosporin C (1966),[30] prostaglandin F2a (1973),[31] vitamin B12 (with A. Eschenmoser) (1973),[32]
and erythromycin A (1981).[33] Some of these accomplishments
will be briefly presented in Section 3.5.
Woodward brought his towering intellect to bear on these
daunting problems of the 1940s, 1950s, and 1960s with
distinctive style and unprecedented glamour. His spectacular
successes were often accompanied by appropriate media
coverage and his lectures and seminars remained legendary
for their intellectual content, precise delivery, and mesmerizing style, not to mention their colorful nature and length!
What distinguished him from his predecessors was not just his
powerful intellect, but the mechanistic rationale and stereochemical control he brought to the field. If Robinson
introduced the curved arrow to organic chemistry (on paper),
Woodward elevated it to the sharp tool that it became for
teaching and mechanistic understanding, and used it to
explain his science and predict the outcome of chemical
reactions. He was not only a General but, most importantly, a
generalist and could generalize observations into useful
theories. He was master not only of the art of total synthesis,
but also of structure determination, an endeavor he cherished
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Natural Products Synthesis

H
H
O
H

HO

OH

OH

patulin (1950)[23]

H
H
MeO2C H

reserpine (1958)[28]

O
H
OMe

Me
H

O Me
Me
H
H2N

OH

NH2

Me

Me

O H
O P
O
O
H

MeO

NH2

HO

H OH

HO

PGF2 (1973)[31]

H
O

marasmic acid (1976)[288]

O

cephalosporin C (1966)[30]

isolongistrobine (1973)[287]

OHC

OMe OMe

illudalic acid

CO2H

(1977)[289]

illudinine

(1977)[289]

HO

H
N

R
O

MeO

Me

Me

Me

OMe

OHC

OAc

OH

HO

NHAc

colchicine (1965)[286]
CO2H

illudacetalic acid

penems
(1977)[289]

HO
S

N
O

H
OH

MeO

Me

OH

CO2H

vitamin B12 (1973)[32]

[with A. Eschenmoser]

OH
O

Me

OMe

O
CO2

N
HO

H H H
N
S

H3N

Me O

Me

MeO
OH

HO

Me
HO

O H

NMe2

NH2

OH

chlorophyll a (1960)[29]

O
H
N

6-demethyl-6-deoxytetracycline (1962)[285]

OMe

Me

CN

lanosterol (1954)[25]

NH2
N

OMe

Co

NH

HN

lysergic acid (1954)[26]

strychnine (1954)[27]

Me MeH

MeO2C

H2N

Me
HO

Mg

MeO

Me

NMe

O
H

H2N

N
N
H

cortisone (1951)[24]

quinine (1944)

Me

[22]

H
N

CO2H

Me

O
Me

MeO

Me

OH

R'

OH
Me
HO
O

OH

Me

Me
O

O
CO2H

(1978)[290]

OMe

NMe2
Me

Me

Me
OH
O
Me
erythromycin A (1981)[33]

Figure 3. Selected syntheses by the Woodward Group (1944 1981).

throughout his career. He clearly influenced the careers of not

only his students, but also of his peers and colleagues, for
example, J. Wilkinson (sandwich structure of ferrocene), K.
Block (steroid biosynthesis), R. Hoffmann (Woodward and
Hoffmann rules), all of whom won the Nobel Prize for
chemistry.[13]
His brilliant use of rings to install and control stereochemical centers and to unravel functionality by rupturing
them is an unmistakable feature of his syntheses. This theme
appears in his first total synthesis, that of quinine,[22] and
appears over and over again as in the total synthesis of
reserpine,[28] vitamin B12 ,[3, 32] and, remarkably, in his last
synthesis, that of erythromycin.[33] Woodwards mark was that
of an artist, treating each target individually with total
mastery as he moved from one structural type to another.
He exercised an amazing intuition in devising strategies
toward his targets, magically connecting them to suitable
starting materials through elegant, almost balletlike, maneuvers.
However, the avalanche of new natural products appearing
on the scene as a consequence of the advent and development
of new analytical techniques demanded a new and more
systematic approach to strategy design. A new school of
thought was appearing on the horizon which promised to take
the field of total synthesis, and that of organic synthesis in
general, to its next level of sophistication.

3.3. The Corey Era

In 1959 and at the age of 31 E. J. Corey arrived at Harvard
as a full professor of chemistry from the University of Illinois,
Angew. Chem. Int. Ed. 2000, 39, 44 122

Urbana-Champaign. His dynamism and brilliance were to

make him the natural recipient of the total synthesis baton
from R. B. Woodward, even though the two men overlapped
for two decades at Harvard. Coreys pursuit of total synthesis
was marked by two distinctive elements, retrosynthetic
analysis and the development of new synthetic methods as
an integral part of the endeavor, even though Woodward
(consciously or unconsciously) must have been engaged in
such practices. It was Coreys 1961 synthesis of longifolene[34]
that marked the official introduction of the principles of
retrosynthetic analysis.[4] He practiced and spread this concept
throughout the world of total synthesis, which became a much
more rational and systematic endeavor. Students could now
be taught the logic of chemical synthesis[4] by learning how
to analyze complex target molecules and devise possible
synthetic strategies for their construction. New synthetic
methods are often incorporated into the synthetic schemes
towards the target and the exercise of the total synthesis
becomes an opportunity for the invention and discovery of
new chemistry. Combining his systematic and brilliant approaches to total synthesis with the new tools of organic
synthesis and analytical chemistry, Corey synthesized hundreds of natural and designed products within the thirty year
period stretching between 1960 and 1990 (Figure 4)the year
of his Nobel Prize.
Corey brought a highly organized and systematic approach
to the field of total synthesis by identifying unsolved and
important structural types and pursuing them until they fell.
The benefits and spin-offs from his endeavors were even more
impressive: the theory of retrosynthetic analysis, new synthetic methods, asymmetric synthesis, mechanistic proposals,
and important contributions to biology and medicine. Some of
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K. C. Nicolaou et al.

Figure 4. Selected syntheses by the Corey Group (1961 1999).

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his most notable accomplishments in the field are highlighted
in Section 3.5.
The period of 1950 1990 was an era during which total
synthesis underwent explosive growth as evidenced by
inspection of the primary chemical literature. In addition to
the Woodward and Corey schools, a number of other groups
contributed notably to this rich period for total synthesis[35]
and some continue to do so today. Indeed, throughout the
second half of the twentieth century a number of great
synthetic chemists made significant contributions to the field,
as natural products became opportunities to initiate and focus
major research programs and served as ports of entry for
adventures and rewarding voyages.
Among these great chemists are G. Stork, A. Eschenmoser,
and Sir D. H. R. Barton, whose sweeping contributions began
with the Woodward era and spanned over half a century. The
Stork Eschenmoser hypothesis[35] for the stereospecific
course of biomimetic cation cyclizations, such as the conversion of squalene into steroidal structures, stimulated much
synthetic work (for example, the total synthesis of progesterone by W. S. Johnson, 1971).[36] Storks elegant total syntheses
(for example, steroids, prostaglandins, tetracyclins)[3739] decorate beautifully the chemical literature and his useful
methodologies (for example, enamine chemistry, anionic ring
closures, radical chemistry, tethering devices)[4043] have found
important and widespread use in many laboratories and
industrial settings.
Similarly, Eschenmosers beautiful total syntheses (for
example, colchicine, corrins, vitamin B12 , designed nucleic
acids)[4447] are often accompanied by profound mechanistic
insights and synthetic designs of such admirable clarity and
deep thought. His exquisite total synthesis of vitamin B12
(with Woodward), in particular, is an extraordinary achievement and will always remain a classic[3] in the annals of
organic synthesis. The work of D. H. R. Barton,[48] starting
with his contributions to conformational analysis and biogenetic theory and continuing with brilliant contributions
both in total synthesis and synthetic methodology, was
instrumental in shaping the art and science of natural products
synthesis as we know it today. Among his most significant
contributions are the Barton reaction, which involves the
photocleavage of nitrite esters[49] and its application to the
synthesis of aldosterone-21-acetate,[50] and his deoxygenation
reactions and related radical chemistry,[51] which has found
numerous applications in organic and natural product synthesis.
It seemed for a moment, in 1990, that the efforts of the
synthetic chemists had conquerred most of the known
structural types of secondary metabolites: prostaglandins,
steroids, b-lactams, macrolides, polyene macrolides, polyethers, alkaloids, porphyrinoids, endiandric acids, palitoxin
carboxyclic acid, and gingkolide; all fell as a result of the
awesome power of total synthesis. Tempted by the lure of
other unexplored and promising fields, some researchers even
thought that total synthesis was dead, and declared it so. They
were wrong. To the astute eye, a number of challenging and
beautiful architectures remained standing, daring the synthetic chemists of the time and inviting them to a feast of
discovery and invention. Furthermore, several new structures
were soon to be discovered from nature that offered
Angew. Chem. Int. Ed. 2000, 39, 44 122

unprecedented challenges and opportunities. To be sure, the

final decade of the twentieth century proved to be a most
exciting and rewarding period in the history of total synthesis.

3.4. The 1990s Era

The climactic productivity of the 1980s in total synthesis
boded well for the future of the science, and the seeds were
already sown for continued breakthroughs and a new
explosion of the field. Entirely new types of structures were
on the minds of synthetic chemists, challenging and presenting
them with new opportunities. These luring architectures
included the enediynes such as calicheamicin and dynemicin,
the polyether neurotoxins exemplified by brevetoxins A and
B, the immunosuppressants cyclosporin, FK506, rapamycin,
and sanglifehrin A, taxol and other tubulin binding agents,
such as the epothilones eleutherobin and the sarcodictyins,
ecteinascidin, the manzamines, the glycopeptide antibiotics
such as vancomycin, the CP molecules, and everninomicin
13,384-1 (see Section 3.5).
Most significantly, total synthesis assumed a more serious
role in biology and medicine. The more aggressive incorporation of this new dimension to the enterprise was aided and
encouraged by combinatorial chemistry and the new challenges posed by discoveries in genomics. Thus, new fields of
investigation in chemical biology were established by synthetic chemists taking advantage of the novel molecular
architectures and biological action of certain natural products.
Besides culminating in the total synthesis of the targeted
natural products, some of these new programs expanded into
the development of new synthetic methods as in the past, but
also into the areas of chemical biology, solid phase chemistry,
and combinatorial synthesis. Synthetic chemists were moving
deeper into biology, particularly as they recognized the
timeliness of using their powerful tools to probe biological
phenomena and make contributions to chemical and functional genomics. Biologists, in turn, realized the tremendous
benefits that chemical synthesis could bring to their science
and adopted it, primarily through interdisciplinary collaborations with synthetic chemists. A new philosophy for total
synthesis as an important component of chemical biology
began to take hold, and natural products continued to be in
the center of it all. In the next section we briefly discuss a
number of selected total syntheses of the twentieth century.

3.5. Selected Examples of Total Syntheses

The chemical literature of the twentieth century is adorned
with beautiful total syntheses of natural products.[35] We have
chosen to highlight a few here as illustrative examples of
structural types and synthetic strategies.
Tropinone (1917)
Perhaps the first example of a strikingly beautiful total
synthesis is that of the alkaloid ()-tropinone (1 in Scheme 1)
reported as early as 1917 by Sir R. Robinson.[5, 16] In this
elegant synthesiscalled biomimetic because of its resem51

REVIEWS

K. C. Nicolaou et al.

a)
Me
N

Mannich reaction

CO2H

CHO
NMe
O

CO2H

H
O

H2NMe
CHO

CHO

Mannich reaction

1: tropinone

b)

H2NMe

Me

- H2 O

Equilenin (1939)

NMe
NMe

+ H2O

CHO
H

2: succin-dialdehyde

OH

7
O

[intermolecular Mannich reaction]

Me

Me

CO2H

CO2H

10

H
CO2

CO2H

O H

Me O

Dieckmann
cyclization

CO2H

CO2H

9
[intramolecular Mannich reaction]

The first sex hormone to be constructed in the laboratory by

total synthesis was equilenin (1 in Scheme 3). The total
synthesis of this first steroidal structure was accomplished in
a)

HCl
-2 CO2

O2 C

Me

Me
N

prior to elimination of the latter functionalities. In contrast to

the rather brutal reagents and conditions used in this
porphyrins synthesis, the tools of the trade when Woodward faced chlorophyll a, approximately thirty years later,
were much sharper and selective.

HO
O
H CO2H

H
MeO

HO

1: equilenin

4: Butenandt's ketone

Scheme 1. a) Strategic bond disconnecions and retrosynthetic analysis of

()-tropinone and b) total synthesis (Robinson, 1917).[16]
Me

blance to the way nature synthesizes tropinoneRobinson

utilized a tandem sequence in which one molecule of
succindialdehyde, methylamine, and either acetone dicarboxylic acid (or dicarboxylate) react together to afford the natural
substance in a simple one-pot procedure. Two consecutive
Mannich reactions are involved in this synthesis, the first one
in an inter- and the second one in an intramolecular fashion.
In a way, the total synthesis of ()-tropinone by Robinson was
quite ahead of its time both in terms of elegance and logic.
With this synthesis Robinson introduced aesthetics into total
synthesis, and art became part of the endeavor. It was left,
however, to R. B. Woodward to elevate it to the artistic status
that it achieved in the 1950s and to E. J. Corey to make it into
the precise science that it became in the following decades.

Me

CO2Me

HO

Haemin (1 in Scheme 2), the red pigment of blood and the

carrier of oxygen within the human body, belongs to the
porphyrin class of compounds. Both its structure and total
synthesis were established by H. Fischer.[5, 18] This combined
program of structural determination through chemical synthesis is exemplary of the early days of total synthesis. Such
practices were particularly useful for structural elucidation in
the absence of todays physical methods such as NMR
spectroscopy, mass spectrometry, and X-ray crystallography.
In the case of haemin, the molecule was degraded into smaller
fragments, which chemical synthesis confirmed to be substituted pyrroles. The assembly of the pieces by exploiting the
greater nucleophilicity of pyrroles 2-position, relative to that
of the 3-position, led to haemins framework into which the
iron cation was implanted in the final step. Among the most
remarkable features of Fischers total synthesis of haemin are
the fusion of the two dipyrrole components in succinic acid at
180 190 8C to form the cyclic porphyrin skeleton in a single
step by two CC bond-forming reactions, and the unusual way
in which the carbonyl groups were reduced to hydroxyl groups
52

CO2H

CO2Me

HO

Arndt-Eistert reaction

Reformatsky
reaction

a. (CO2Me)2, MeONa
b. 180 C, glass

b)

MeO

Me

CO2Me

(90%)

MeI, MeONa
(92%)

O
MeO

CO2Me
O

MeO

[Reformatsky reaction] a. BrZnCH2CO2Me

[dehydration] b. SOCl2, py
[saponification] c. KOH, MeOH
a. CH2N2
Me
Me
CO2Me
CO2H
b. NaOH
d. Na-Hg
c. SOCl

Me

CO2H

H
O

MeO

Cl

MeO

Haemin (1929)

CO2H

Me

Me

H
MeO

[Dieckmann cyclizationdecarboxylation sequence]

CO2Me

CO2Me

a. MeONa
b. HCl, AcOH
CO2Me

MeO

10

CO2H

MeO

3a
(84% overall)
a. CH2N2
b. Ag2O, MeOH [-N2]

[Arndt-Eistert
reaction]

(39% overall)
CO2H

Me O

HO

(92%)

1: equilenin

Scheme 3. a) Strategic bond disconnections and retrosynthetic analysis of

equilenin and b) total synthesis (Bachmann et al., 1939).[21]

1939 by Bachmann and his group at the University of

Michigan.[21, 52] This synthesis featured relatively simple
chemistry as characteristically pointed out by the authors:
The reactions which were used are fairly obvious ones...[21]
Specifically, the sequence involves enolate-type chemistry, a
Reformatsky reaction, a sodium amalgam reduction, an
Arndt Eistert homologation, and a Dieckmann cyclization decarboxylation process to fuse the required cyclopentanone ring onto the pre-existing tricyclic system of the
starting material. As the last pre-World War II synthesis of
note, this example was destined to mark the end of an era; A
new epoch was about to begin in the 1940s with R. B.
Woodward and his school of chemistry at the helm.
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

Me

Me

Me
Me
Me
Me

Me

Br

Me

Me

1: haemin

HO2C

b)

Me

HO

Me

Me

Me

CO2H

Me

5
CO2H

NH HN

Me

HO2C
H

HO

Me

N
H

4
Br

Me

OHC

N
H

Fe
N

Me

NH HN

CO2Et

N
H

CO2H

6
Me

Me

Me

Me

HBr
Me

N
H

Me

Me

N
H

Me

H
NH HN
Me

Me

Me

a. H2SO4

HCO2H

b.
Me

HCl

CO2Et

N
H

Me

OHC

Me

CO2Et

N
H

11

Me

12

CO2H

Me

piperidine
[Knoevenagel]

CO2Et

N
H

Me

N
H

Me

Me

Me

Me

CO2Et

N
H

Me

CO2Et

N
H

17

16
HO2C

HO2C

Me

HO2C

CO2Et

N
H

NH HN
Me

10

HO2C

HO2C

H2 O

Me

NH HN
Me

HBr,
Br2

Me

15
HO2C

Me
HO

9 Br

Na/Hg

CO2Et

N
H

13

HO2C

Me

HO2C

HO2C

14
Me

Me

NH HN
Me

Me

CO2H

EtO2C

Me

NH HN
Me

Me

H Me

Br

CO2Et

CO2Et

N
H

22

CO2Et

N
H

21

Br

CO2Et

N
H

19

20

CO2Et

N
H

Br

Br

18
O

HO2C

EtO2C
Me

Me

Me

CO2Et
NH HN

Me

HO2C

HO

N
H

CO2Et

N
H

22

CO2Et

H
CO2H

23

H
H
Me

Me

NH HN

- Br Br + Me

Me

Me

CO2H

HO2C

Me
Me

Me

Me

Me

Me

NH HN
Me

Br

Me

NH HN

CO2H

Br

Br
NH HN

Me

Me [fusion in succinic acid]

27
HO2C

CO2H

26
Me

NH HN
Me Br

CO2H

CO2H

NH HN

Me

CO2H

25

28

29
HO2C

Me

Br

[CO2]
HO2C

NH HN

NH HN

HO2C

24

Br

NH HN

NH HN

Me

Me
Me

HO2C

CO2H

CO2H

CO2H

HO2C

[oxidation]
O

Me
Me

a. Fe3
b. Ac2O, AlCl3

HN

Me

HN

c. H
Me

NH

Me

NH

HO2C

31

30
CO2H

Me
O

KOH,EtOH,

Me

Me

HN

OH

[reduction]

[Friedel-Crafts acylation]
Me

HO

Me

Me
CO2H

Me

NH

O2C

32

a. /H
[dehydration]

Me

Me

CO2

Cl
Me

1: haemin
HO2C

HO2C

N
Fe

b. Fe3
Me

CO2H

Scheme 2. a) Strategic bond disconnections and retrosynthetic analysis of haemin and b) total synthesis (Fisher, 1929).[18]

Before we close this era of total synthesis and enter into a

new one, the following considerations might be instructive in
atempting to understand the way of thinking of the pre-World
War II chemists as opposed to those who followed them. The
rather straightforward synthesis of equilenin is representative
of the total syntheses of pre-World War II erawith the
exception of Robinsons unique tropinone synthesis. In
contemplating a strategy towards equilenin, Bachmann must
have considered several possible starting materials before
recognizing the resemblance of his target molecule to
Butenands ketone (4 in Scheme 3). After all, three of
equilenins rings are present in 4 and all he needed to do
was fuse the extra ring and introduce a methyl group onto the
Angew. Chem. Int. Ed. 2000, 39, 44 122

cyclohexane system in order to accomplish his goal. The issue

of stereochemistry of the two stereocenters was probably left
open to chance in contrast to the rational approaches towards
such matters of the later periods. Connecting the chosen
starting material 4 with the target molecule 1 was apparently
obvious to Bachmann, who explicitly stated the known nature
of the reactions he used to accomplish the synthesis.
Since the motivations for total synthesis were strongly tied
to the proof of structure, one needed a high degree of
confidence that the proposed transformations did indeed lead
to the proposed structure. Furthermore, the limited arsenal of
chemical transformations did not entice much creative deviation from the most straightforward course. This high degree of
53

REVIEWS
confidence that synthetic chemists had in their designed
strategies was soon to decrease as the complexity of newly
discovered natural products increased, thus catalyzing the
development of novel strategies and new chemistry in
subsequent years. In addition, advances in theoretical and
mechanistic organic chemistry as well as new synthetic tools
were to allow much longer sequences to be planned with a
heightened measure of confidence and considerable flexibility
for redesign along the way.
Strychnine (1954)
As the most notorious poison[53] of the Strychnos plant
species, strychnine (1 in Scheme 4) occupied the minds of

K. C. Nicolaou et al.
structural chemists for a rather long time. Its gross structure
was revealed in 1946[54] and was subsequently confirmed by
X-ray crystallographic analysis.[55] In 1952, Sir Robert Robinson commented that strychnine: For its molecular size it is
the most complex substance known.[56] This estimation had
not, apparently, escaped R. B. Woodwards attention who had
already been fully engaged in strychnines total synthesis. In
1948 Woodward put forth the idea that oxidative cleavage of
electron-rich aromatic rings might be relevant in the biogenesis of the strychnos alkaloids.[57] This provocative idea was
implemented in his 1954[27] synthesis of strychnine, which
established Woodward as the undisputed master of the art at
the time. The total synthesis of ()-strychnine by Woodward
(Scheme 4) ushered in a golden era of total synthesis and

Scheme 4. a) Strategic bond disconnections and retrosynthetic analysis of ()-strychnine and b) total synthesis (Woodward et al., 1954).[27]

54

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

installed unprecedented confidence in, and respect for, the
science of organic synthesis. Although several of its steps were
beautifully designed and executed, perhaps the most striking
feature is its reliance on only the simplest of reagents to carry
out what seemed to be rather complex chemical transformations. With its challenging molecular structure, the molecule
of strychnine continued to occupy the minds of several
subsequent practitioners of the art and several other total
syntheses have since appeared in the literature.[58, 59]
Penicillin (1957)
Few discoveries of the twentieth century can claim higher
notoriety than that of penicillin (1 in Scheme 5). Discovered
in 1928 by Alexander Fleming[60] in the secretion of the mold
Penicillium notatum, penicillin was later shown to possess
remarkable antibacterial properties by Chain and Florey.[61]
Following a massive development effort known as the
Anglo American penicillin project[62, 63] the substance was

introduced as a drug during World War II and saved countless

lives. Its molecular structure containing the unique and
strained b-lactam ring was under the cloud of some controversy until Dorothy Crowfoot-Hodgkin confirmed it by X-ray
crystallographic analysis.[64]
Not surprisingly, penicillin immediately became a highly
priced synthetic target attracting the attention of major
players in total synthesis of the time. Finally, it was Sheehan
and Henery-Logan[65] at the Massachusetts Institute of
Technology who delivered synthetic penicillin V by total
synthesis of the enchanted molecule, as Sheehan later
called it.[66] Their synthesis, reported in 1957 and summarized
in Scheme 5, was accompanied by the development of the
phthalimide and tert-butyl ester protecting groups and the
introduction of an aliphatic carbodiimide as a condensing
agent to form amide bonds andin the eventpenicillins
fragile b-lactam ring. With this milestone, another class of
natural products was now open to chemical manipulation and
a new chapter in total synthesis had begun.
Reserpine (1958)

a)
Ring formation

Amide formation
H
N

PhO

H
S

Me

Me

Me

Me

OH

Me

HClH2N

CO2H

2
Me

NH2

HN

5: valine

4
O
O

Me

(75%)

Me

Ac2O, 60 C

CO2H

Me

(72-80%)

Me

HS

CO2H

ClCH2COCl

CO2H

Me

tBuO2C HN

CO2H
Lactamization
1:penicillin V

b)

PhtN

PhtN

Me

Cl

Me

O
Cl

OAc
SH

Me

Me

[isomerization]

Me

Me

Me

Me

11

Me

O
H

Me

Me
O

O
N

10

Cl

Cl

(75%) H2S, NaOMe [Michael addition]

O

Me Me

Me O OMe
OMe Me

HS

HS

Me

12

13

Me

Me

HS
O

CHO

Me
HClH2N

tBuO2C

NaOAc
a. N2H4

tBuO2C HN

Me

HClH2N

(82%)

Me

H
N

H
S

PhO

tBuO2C HN

CO2H

16

PhOCH2COCl,
Et3N
(70%) O

b. HCl, H2O

Me

Me

tBuO2C HN

Me
CO2H

H
S
N

O
O

Me
Me

a. KOH (1.0 equiv)

b. DCC, H2O, dioxane
(12%)

H
N

(100%)
H
S

PhO
HO2C HN

Me
Me

CO2H

CO2K

[potassium salt of 1]

Me

19
a. HCl
b. py, acetone, H2O

H
N

Me

CO2H

18

PhO

CO2H

O
H

3a

Me

S
N

CO2H

4: D-penicillamine
hydrochloride

tBuO2C

17

HCO2H
Ac2O

(74%)
a. brucine
b. resolution
Me
c. HCl, H2O
Me
d. HCl

Me
CO2H

15

Me

HS

Me

S
N
H

CO2Me (100%)

14

Me
Me

b. Me2CO

N
H

Me

a. tBuONa
b. tBuOCHO

a. HCl, H2O

Me

20

Scheme 5. a) Strategic bond disconnections and retrosynthetic analysis of

penicillin V and b) total synthesis (Sheehan et al., 1957).[65]

Angew. Chem. Int. Ed. 2000, 39, 44 122

Reserpine (1 in Scheme 6), a constituent of the Indian

snakeroot Rauwolfia serpentina Benth., is an alkaloid substance with curative properties[67] for the treatment of hypertension, as well as nervous and mental disorders.[68] Reserpine
was isolated in 1952 and yielded to structural elucidation in
1955 (Schlittler and co-workers)[69] and to total synthesis in
1958 (Woodward et al.).[28] The first total synthesis of reserpine (Scheme 6), considered by some as one of Woodwards
greatest contributions to synthesis, inspires admiration and
respect by the manner in which it exploits molecular
conformation to arrive at certain desired synthetic objectives.
During this synthesis, Woodward demonstrated brilliantly the
power of the venerable Diels Alder reaction to construct a
highly functionalized 6-membered ring, to control stereochemistry around the periphery of such a ring, and most
importantly, to induce a desired epimerization by constraining
the molecule into an unfavorable conformation by intramolecular tethering. All in all, Woodwards total synthesis of
reserpine remains as brilliant in strategy as admirable in
execution. It was to be followed by several others.[70]
The synthesis of reserpine appropriately represents Woodwards approach to total synthesis. Even though Woodward
did not talk about retrosynthetic analysis, he must have
practiced it subconsciously. In his mind, reserpine consisted of
three parts: the indole (the AB unit, see Scheme 6), the
trimethoxybenzene system, and the highly substituted E-ring
cyclohexane. Given the simplicity of the first two fragments
and their obvious attachment to fragment 3, Woodward
concerned himself primarily with the stereoselective construction of 3 and the stereochemical problem encountered in
completing the architecture of the CD ring system. He
brilliantly solved the first problem by employing the Diels
Alder reaction to generate a cyclic template onto which he
installed the required functionality by taking advantage of the
special effects of ring systems on the stereochemical outcomes
of reactions. He addressed the second issue, that of the last
stereocenter to be set at the junction of rings C and D, by
55

REVIEWS

K. C. Nicolaou et al.

a)
MeO

Lactamization

MeO

B
N
H

C-C bond
formation

A
B
N O
H

N
H

H
MeO2C

Imine
formation

N
H

OMe

MeO2C

OMe

OAc
OMe

OMe

1: reserpine

OMe

Esterification

O
H

O
H

Diels-Alder
reaction

O
H
MeO2C

CO2Me

MeO2C

E
OAc

MeO2C
H

OMe

[Meerwein-Pondorff-Verley
reduction]

b)
O

[Diels-Alder
reaction]
O

AcOH

OH
H

HO

13

B
C
N
N
H H
D

OAc

B
N
H

NaBH4

MeO2C

OAc

POCl3

H
OAc

MeO2C

OMe

N
H

OAc

15

OAc
R
HN

MeO

R = CO2Me

17

H
N H

H
O

MeO

HN
O

18

19

tBuCO2H,
[isomerization]

OMe

OMe

A
B
C
N
N
H H
D
H

H
N

NaOMe, MeOH,

H
O

E
OH

21

H
HN

MeO

MeO2C

OMe

20

py

Cl

OMe
OMe

MeO

OMe

B
C
N
N
H H
D
H

H
O

MeO2C

a. MeOH/CHCl3
(+)-CSA
b. resolution
c. 1 N NaOH
OMe

O
OMe

23

OMe

[esterification]

22

OMe
OMe

Chlorophyll a (1 in Scheme 7), the green pigment of plants

and the essential molecule of photosynthesis, is distinguished
from its cousin molecule haemin by the presence of two extra
hydrogen atoms (and, therefore, two chiral centers) in one of
its pyrrole rings, the presence of the phytyl side chain, and the
encapsulation of a magnesium cation rather than an iron
cation. Its total synthesis by R. B. Woodward et al. in 1960[29]
represents a beautiful example of bold planning and exquisite
execution. This synthesis includes improvements over Fischers routes to porphyrin building blocks and, most importantly, a number of clever maneuvers for the installment of the
three stereocenters and the extra five-membered ring residing
on the periphery of the chlorin system of chlorophyll a. The
chemical synthesis of chlorophyll a is a significant advance
over Fischers total synthesis of haemin,[18] and must have
given Woodward the confidence, and prepared the ground, for
his daring venture towards vitamin B12 in which he was to be
joined by A. Eschenmoser (see p. 61).

OMe

a. KOH, MeOH
b. DCC, py

N H

Chlorophyll a (1960)

16

MeO

OMe

MeO

OAc

B
N Cl
H

OMe
OAc

OMe

17

MeO2C

MeO

OMe

B
N
H

MeO

OMe

NH2

OMe

B
O
N
H

MeO2C

14
NaBH4, MeOH
[reductive amination-lactamization]

MeO

OH

OMe

MeO2C

10

OMe

MeO

H2O

MeO

OH

HO2C

H
H H2SO4 H

O
H
MeO2C

NaOMe
MeOH

11

a. CH2N2
b. Ac2O
c. OsO4
d. HIO4
e. CH2N2

Br

OMe

Br

O H

NBS

H2Cr2O7 H
AcOH

HO
H

OH

OMe

12

[elimination-conjugate addition]

Zn

Br
H

H
OH

Zn

Br2

Br

OH

OH
H

5+6 CO2Me

MeO2C

Zn

O Al(OiPr)3,
H
iPrOH

A
B
C
N
N
H H
D
H

H
O

OMe

MeO2C
OMe

1: ()-reserpine

OMe
OMe

Scheme 6. a) Strategic bond disconnections and retrosynthetic analysis of

reserpine and b) total synthesis (Woodward et al., 1958).[28]

56

cleverly coaxing his polycycle into an unfavorable conformation (through intramolecular tethering), which forced an
isomerization to give the desired stereochemistry.
These maneuvers clearly constituted unprecedented sophistication and rational thinking in chemical synthesis
design. While this rational thinking was to be further
advanced and formalized by Coreys concepts on retrosynthetic analysis, the stereocontrol strategies of this era were to
dominate synthetic planning for some time before being
complemented and, to a large degree, eclipsed by acyclic
stereoselection and asymmetric synthesis advances which
emerged towards the end of the century.

Longifolene (1961)
The publication of the total synthesis of longifolene (1 in
Scheme 8) in 1961 by Corey et al.[34] is of historical significance in that in it Corey laid out the foundation of his
systematic approach to retrosynthetic analysis. Our thinking
about synthetic design has been profoundly affected and
shaped by the principles of retrosynthetic analysis ever since,
and the theory is sure to survive for a long time to come.
Coreys longifolene synthesis[34] exemplifies the identification
and mental disconnection of strategic bonds for the purposes
of simplifying the target structure. The process of retrosynthetic analysis unravels a retrosynthetic tree with possible
pathways and intermediates from which the synthetic chemist
can choose the most likely to succeed and/or most elegant
strategies. The total synthesis of longifolene itself, shown in
Scheme 8, involves a Wittig reaction, an osmium tetroxidemediated dihydroxylation of a double bond, a ring expansion,
and an intramolecular Michael-type alkylation to construct
the longifolene skeleton. This synthesis remains a landmark in
the evolution of the art and science of total synthesis.
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

Hofmann elimination reaction

Me
H2N
Me

Me
OHC

Me
HN

Mg
Me

Me

NH

Me

H
H

Dieckmann cyclization
H

Me

Me

H2N
NC

Me

NH

NH

HN

HBr

NH

Me

NH

Me

HN

AcOH
b. H2S

Me

a. MeO2C
Me b. NaOH
c. CH2N2

HN

Me a. EtNH2,

NaBH4

HN

MeO2C

CO2Me

CO2Me

CN
Me

Me

Me

CO2Et

HN

HN

HCl

Me

CO2Et

formation]

NC

HN

HN

Me

CO2Et [thioaldehyde
MeO2C

Me
Cl

CN

OHC

SHC

MeO2C

NH

Me

Me
NH

Me

Me

Me

4: phytol

CO2Me

H3N

Me

Me

Me

Me

MeO2C

OHC

H2N

Me

CO2Et

1: chlorophyll a

b)

OH

+
Me

Me
Me

Ester
formation

Me

HN

NH

Me

O
CO2Me
Me

Cl

CO2Et

Me

HCl

AcHN

H3N
N
Me

Me

AcHN

Me

NH HN

Me

NH HN

Me

Me a. I2 [oxidation]

Me
Me

NH

Me

Me

NH HN

Me

Me

Me

AcOH,

(50% overall)

Me

Me

NH HN

Me

b. Ac2O, py
NH HN

AcHN

Me

Me

NH

Me

NH HN

Me

Me

CO2Me

CO2Me

CO2Me

10

CO2Me

CO2Me

CO2Me

CO2Me

11

NH

CO2Me

13

12

Me

NH

Me

NH

Me a. resolution

Me

Me

HN

Me

H
H

CHO CO2Me

CO2Me

b. CH2N2 Me

b. NaOH, H2O Me

HN

Me

H
HO2C

19

CO2Me

14

HO

NH

HN

Me

O2, hv

Me

H
O CHO

CO2Me

NH

HN

CO2Me

MeO2C
MeO2C

17

Me

[Hofmann
elimination] Me
Me a. HCl,
MeOH
b. Me2SO4, Me
Me
NaOH
H

[photooxygenation]

MeO2C
MeO2C

18

Me

Me

[highly specific
photochemical
Me
Me cleavage of the
cyclopentadiene ring]

AcOH/

[hydrolysis]

Me

NH

HN

Me
CO2Me

MeO2C
MeO2C

16

Me

15

[cyanohydrin lactone
formation]

HCN, Et3N

Me

NH

a. KOH, MeOH

with quinine
Me

Me

Me

Me
CO2Me

CO2Me

AcHN
Me

Me

CO2Me

CO2Me

CO2Me

NH

[oxidation] Me

Me

O
CO2Me

air

Me

HN

Me

H
H
NC
CO2Me

Me

Me

a. Zn, AcOH
b. CH2N2
c. MeOH, HCl

[reduction]
[methylation]
[methanolysis]

20

Me

Me

NH

HN

H
H
MeO2C
CO2Me

Me
Me

Me

[Dieckmann
cyclization]

Me

b. H ,

Me

HN

Me

H
H

CO2Me

21

NH

NaOH, py

Me

a. NaOH, H2O
4

Me

c. Mg(OEt)2

[ester exchangemagnesium
insertion sequence]

Me

Me

Me

MeO2C
CO2Me

N
Mg

H
O

22

O
CO2Me
Me

1: chlorophyll a
Me

Me

Me
Me

Scheme 7. a) Strategic bond disconnections and retrosynthetic analysis of chlorophyll a and b) total synthesis (Woodward et al., 1960).[29] The locking of 2
and 9 together through formation of a schiff base forces the cyclization to proceed with the desired regioselectivity.

Lycopodine (1968)
Lycopodine (1 in Scheme 9), first isolated in 1881, is the
most wildly distributed alkaloid from the genus lycopodium.[71] In addition to the great challenge of synthesizing this
novel polycyclic framework in a stereocontrolled manner, one
must effectively address the challenge posed by the C13
quaternary center, which is common to all four rings. Gilbert
Stork was one of the first to successfully complete the total
synthesis of lycopodine.[72] This masterfully executed synthesis
Angew. Chem. Int. Ed. 2000, 39, 44 122

features a unique aza-annulation strategy which utilizes the

Stork enamine methodology[73] (a generally useful strategy to
generate and trap enolates regiospecifically) to construct
quinolone systems, a stereospecific cationic cyclization to
establish the C13 quaternary center, and a series of functional
group manipulations to elaborate the resulting aromatic ring
into ring D. Several syntheses of lycopodine have since
appeared,[74] each featuring a unique strategy complementary
to Storks beautiful synthesis.
57

REVIEWS
a)

K. C. Nicolaou et al.
Alkylation

Me

Me

Michael Me
addition

a)

Me
O

Allylic

Me

H oxidation

Me

OMe

H
O
H

Olefination

Me

1: longifolene

N
N

Me

Me

Cl3C

Lactamization
O

Me

pinacol
rearrangement

Me

1: lycopodine

Me

5: Wieland-Miescher
ketone

OH
OTs

Ozonolysis
CO2Me

Me

Me

N
H

Cationic
cyclization
2

Stork enamine

Me
O

CHO
O

OMe
MeO

OMe

CO2Et

Conjugate
addition

b)

HO

a.

OH

pTsOH,
b. Ph3P=CHMe

Me

a. OsO4, py
b. pTsCl, py

Me

O Me

7
O

[pinacol
(31% overall) rearrangement]

4 H
OTs
LiClO4,
CaCO3

b)
MeO

HO

O
O

OH

O Me

Et3N ,

a.

Me
O
O

HS

Me Me
S

SH

BF3Et2O
b. LiAlH4,

Me

Me

H
Me
Me

a. Na, NH2NH2,
b. CrO3, AcOH

O
H

Me

Me

H
Me

N
H

[Stork
enamine]
Ar

H2N
N

Me

(55%) O

Me

c. KOtBu
d. O
OMe

12

11

Cl

Me

a. LiAlH4
b. Li-NH3
[Birch
reduction]

N
H

OMe

Scheme 8. a) Strategic bond disconnections and retrosynthetic analysis of

longifolene and b) total synthesis (Corey et al., 1961).[34]

H
H

(49% overall)

Me

10

H3PO4:HCO2H (1:1)
H

Me

a. MeLi,
b. SOCl2, py

N
O

CCl3

13

OMe

Cl3C

O3, MeOH
H

Me

Me

Cephalosporin C (1966)
Cephalosporin C (1 in Scheme 10) was isolated from
Cephalosporium acremonium in the mid-1950s[75] and was
structurally elucidated by X-ray crystallographic analysis in
1961.[76] Reminiscent of the penicillins, the cephalosporins
represent the second subclass of b-lactams, several of which
became legendary antibiotics in the latter part of the
twentieth century. Having missed the opportunity to deliver
penicillin, the Woodward group became at once interested in
the synthesis of cephalosporin C and, by 1965, they completed
the first total synthesis of the molecule.[30]
This total synthesis of cephalosporin C was the sole topic of
Woodwards 1965 Nobel lecture in Stockholm. Indeed, in a
move that broke tradition, R. B. Woodward described on that
occasion for the first time, and in a breathtaking fashion, the
elegant synthesis of cephalosporin C. Highlights of this synthesis, which is summarized in Scheme 10, include the
development of the azodicarboxylate-mediated functionalization of the methylene group adjacent to the sulfur atom of
l-cysteine, the aluminum-mediated closure of the aminoester
to the b-lactam functionality, the brilliant formation of
cephalosporins sulfur-containing ring, and the use of the
b,b,b-trichloroethyloxy moiety to protect the hydroxyl group.
This total synthesis stands as a milestone accomplishment in
the field of natural product synthesis.

Ar

(20-25%
of
desired
isomer)

Me

[cationic
cyclization]

10

H
N

Me

H2N

1: longifolene

58

OH

H
N

Me

OH
H

[conjugate
addition]
(90%)

OMe

Me

Ph3CNa; MeI (60%)

Me

a. LiAlH4
b. MeMgBr,
CuCl2

Me
Me 8

OMe

OMe

(36%)

(10-20%)

CH3

b. K2CO3, H2O
[decarboxylation]

O Me

2 N HCl,

[Isomerization;
Michael addition]
a. NaOEt, 7

CO2Et

Me

Me

EtO2C

Me

H
H

O
O

N
O
Cl3C

N
O

16

CO2Me

O
Cl3C

15

CO2Me

Cl3C

H
O
O

N
OH

Me

OH

14

SeO2
N
H2O2
(30%
O
overall)O

CO2Me

Cl3C

CHO

CO2Me

a. NaOMe [formate methanolysis]

b. Zn, MeOH [deprotection of amine]
Me

Me

N
H

Me

N
O

H
H

a. LiAlH4
b. CrO3-H2SO4

O
H

17
MeO

H
H

1: lycopodine

18

Scheme 9. a) Strategic bond disconnections and retrosynthetic analysis of

()-lycopodine and b) total synthesis (G. Stork et al., 1968).[72]

Prostaglandins F2a and E2 (1969)

The prostaglandins were discovered by von Euler in the
1930s[77] and their structures became known in the mid-1960s
primarily as a result of the pioneering work of Bergstrm and
his group.[78] With their potent and important biological
activities and their potential applications in medicine,[79] these
scarce substances elicited intense efforts directed at their
chemical synthesis. By 1969 Corey had devised and completed
his first total synthesis of prostaglandins F2a (1 in Scheme 11)
and E2 .[80] These syntheses amplified brilliantly Coreys
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

CO2H

Conjugate
addition

O
H

NH H

H2N
H

Cyclization
O

CH2OAc

MeO2C
H

H2N

Me
CO2Me

N N

MeO2C
H

Me

tBuOC(O)N

S
Me

N3
H

tBuOC(O)N

Me

MeO2C

Me

MeO2C
H

Me

a. acetone
MeO2C
H
b. tBuOCOCl
N
c.
CH
2
2
SH
tBuOC(O)N

HO2C
H

OAc
H

tBuOC(O)N

6: L-(+)-cysteine

b)

H
Me

HO2C
H
SH

Me

1: cephalosporin C

H2N

CHO

tBuOC(O)N

Amide bond formation

CO2H

CO2CH2CCl3

NH

N CO Me
2

S N

Me

Me CO2Me

OAc

CO2Me

MeO2C H
H
S

CO2Me

MeO2C H
H

tBuOC(O)N

Me

tBuOC(O)N

[oxidation]

Me

12

tBuOC(O)N
Me

MeO2C
H

Pd(OAc)4

N
S

Me

13

CO2Me

tBuOC(O)N

CO2Me

Me

tBuOC(O)N

CO2Me

Me

MeO2C
H

NH

Me

11

MeO2C
H

OAc

Me

CO2Me
N

10

Me CO2Me

MeO2C OAc
N
H
N

OAc

NH

S N

tBuOC(O)N

CO2Me

Me

Me

14

CO2Me

Me

15

OAc

[-N2]
MeO2C
H

OAc
H

tBuOC(O)N
Me

MeO2C
H

tBuOC(O)N

Me

Me

5: major product

MeO2C
H

OAc
H

OAc

tBuOC(O)N

S
Me

Me

17: minor product

Me

16

AcO, MeOH
O
MeO2C
OH a. MeSO2Cl
H
H b. NaN3
tBuOC(O)N
S

MeO2C
H

tBuOC(O)N
Me

Me

Me

18

Me

[lactamization]

CHO

H
NHCO2CH2CCl3
H

N H
H

O
CO2CH2CCl3

a.

27

CO2CH2CCl3
O
N

TFA

H
tBuOC(O)N

Me

N H
H

CO2CH2CCl3

py

[equilibration]

Me

CH2OAc

CO2H
O

CH2OAc

N
H

N H
H

O
CO2CH2CCl3

28

23

H
NHCO2CH2CCl3

CHO

CH2OAc

O
CO2CH2CCl3

25
24
b. CCl3CH2OH, DCC

26

NHCO2CH2CCl3 H2N
H
CO2H
CO2H

CO2CH2CCl3
H

22

DCC

Me

CO2CH2CCl3

CO2CH2CCl3

a. BH3 [neutral reducing agent]

b. Ac2O,
CO2CH2CCl3
py
O
H
NHCO2CH2CCl3

CHO
S

21

CO2CH2CCl3
N

Me

NaO

20

H
S

CCl3

CHO

HO2C H

HO

HO CO2H a. CCl3CH2OH, pTsOH

b. NaIO4
O
O

19: tartaric acid

NH

H
a. Al/Hg/MeOH
tBuOC(O)N
b. iBu3Al

[SN2 with inversion

of configuration]

OH

[reduction of azide]

N3

Zn, AcOH

NH2

N H
H

H
CO2H

[reductive removal of
the protecting groups]

1: cephalosporin C

Scheme 10. a) Strategic bond disconnections and retrosynthetic analysis of

cephalosporin C and b) total synthesis (Woodward et al., 1966).[30]

retrosynthetic analysis concepts and demonstrated the utilization of the bicycloheptane system derived from a Diels
Alder reaction as a versatile key intermediate for the synthesis of several of the prostaglandins. A large body of
Angew. Chem. Int. Ed. 2000, 39, 44 122

Scheme 11. a) Strategic bond disconnections and retrosynthetic analysis of

()-PGF2a and b) the total synthesis (Corey et al., 1969).[80]

synthetic work[8183] followed the initial Corey synthesis and

myriad prostaglandin analogues have since been synthesized,
aiding both biology and medicine tremendously.
Coreys original strategy evolved alongside the impressive
developments in the field of asymmetric catalysis, many of
which he instigated, which culminated by the 1990s, in a
refined, highly efficient and stereocontrolled synthesis of the
prostaglandins.[84] Thus, in its original version, the Corey
synthesis of prostaglandins F2a and E2 was nonstereoselective
and delivered the racemate and as a mixture of C15 epimers.
Then, in 1975, came a major advance in the use of a chiral
auxiliary to control the stereochemical outcome of the crucial
Diels Alder reaction to form the bicyclo[2.2.1]heptane
system in its optically active form.[85] The theme of chiral
auxiliaries to control stereochemistry played a major role in
the development of organic and natural products synthesis in
the latter part of the century. In addition to the contributions
59

REVIEWS

K. C. Nicolaou et al.

of Corey, those of A. I. Myers,[86] D. A. Evans,[87] W. Oppolzer,[88] and H. C. Brown[89] as well as many others helped shape
the field.
Finally came the era of catalyst design and here again the
prostaglandins played a major role in providing both a driving
force and a test. In a series of papers, Corey disclosed a set of
chiral aluminum- and boron-based[90, 91] catalysts for the
Diels Alder reaction (and several other reactions) that
facilitated the synthesis of an enantiomerically enriched
intermediate along the route to prostaglandins. And, finally,
the problem of stereoselectivity at C15 was solved by the
introduction of the oxazaborolidine catalyst (CBS) by Corey
in 1987.[92] These catalysts not only refined the industrial
process for the production of prostaglandins, but also found
uses in many other instances both in small scale laboratory
operations and manufacturing processes of drug candidates
and pharmaceuticals. For a more in-depth analysis of the
Corey syntheses of prostaglandins F2a and E2 and other
advances on asymmetric catalysis, the reader is referred to
ref. [4] and other appropriate literature sources.

Progesterone (1971)
Progesterone (1 in Scheme 12), a hormone that prepares
the lining of the uterus for implantation of an ovum, is a
member of the steroid class of compounds that is found
ubiquitously in nature. Its linearly fused polycyclic carbon
framework is characteristic of numerous natural products of
steroidal or triterpenoid structures. A daring approach to
progesterones skeleton by W. S. Johnson[93] was inspired by
the elucidated enzyme-catalyzed conversion[94] of squalene
oxide into lanosterol or to the closely related plant triterpenoid dammaradienol. This biomimetic strategy was also
encouraged by the Stork Eschenmoser hypothesis, which
was proposed in 1955[35] to rationalize the stereochemical
outcome of the biosynthetic transformation of squalene oxide
to steroid. According to this postulate it was predicted that
polyunsaturated molecules with trans CC bonds, such as
squalene oxide, should cyclize in a stereospecific manner, to
furnish polycyclic systems with trans,anti,trans stereochemistry at the ring fusion.
This brilliant proposition was confirmed by W. S. Johnson
and his group through the biomimetic total synthesis of
progesterone (Scheme 12). A tertiary alcohol serves as the
initiator of the polyolefinic ring-closing cascade, in this
instance, but other groups have also been successfully
employed in this regard (for example, acetal, epoxide). The
methylacetylenic group performed well as a terminator of the
cascade in the original work. A number of new terminating
systems have since been successfully employed (for example,
allyl or propargyl silanes, vinyl fluoride). The work of W. S.
Johnson was complemented by that of van Tamelen[95] and
others[3, 4] who also explored such biomimetic cascades.

Tetrodotoxin (1972)
Tetrodotoxin (1 in Scheme 13) is the poisonous compound
of the Japanese puffer fish and its structure was elucidated by
60

Scheme 12. a) Strategic bond disconnections and retrosynthetic analysis of

progesterone and b) total synthesis (Johnson et al., 1971).[93]

Woodward in 1965.[96] By 1972 Kishi and his group had

published the total synthesis[97] of this highly unusual and
challenging structure. This outstanding achievement from
Japan was received at the time with great enthusiasm and
remains to this day as a classic in total synthesis. The target
molecule was reached through a series of maneuvers which
included a Diels Alder reaction of a quinone with butadiene,
a Beckman rearrangement to install the first nitrogen atom,
stereoselective reductions, strategic oxidations, unusual functional group manipulations, and, finally, construction of the
guanidinium system. As a highly condensed and polyfuncAngew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

HO

C-N Bond
formation

N O
H
HO

NH2
AcHN

O
O

1: tetrodotoxin

H
CH2OH

HN
H2N

OH

OH
OAc
CH2OAc

AcO H

OH

OAc

trans-Esterification/
epoxide opening

Orthoester
formation
Epoxide opening/
cyclization

HO

AcO
OAc
AcNH

, SnCl4

Me

Me

[Lewis acid catalyzed N

Diels-Alder
HO
reaction]

O
Me

a. MsCl, Et3N
b. H2O,
(61%)

[Beckmann
rearrangement]

AcN O
H

Me

(83%)

HO

O
AcNH

Diels-Alder
reaction

CH2OAc

O
O

b)

Baeyer-Villiger
oxidation

Me

Me

[regio- and stereoselective reduction] a. NaBH4, MeOH

(72%)
[epoxide-mediated etherification] b. mCPBA, CSA

a. SeO2
b. NaBH4
(100%) O

H
OH
O

O AcN
H OAc

9
a. mCPBA
b. Ac2O, py
c. TFA, H2O;
Ac2O, py

O
O
AcO AcN OAc
H

10

O
AcHN

OH

14

OH

HO
H2N
AcHN

O
H

OAc

OAc
CH2OAc
H

O
S
H2N

OH

AcN O
H

O
OAc

a. KOAc, AcOH
b. Ac2O, CSA,

CH2OAc

OAc

N
H
AcO

H O
H

13

a. H5IO6
b. NH4OH
(9% overall)

OAc

15

OH

OAc

OAc

AcO

Me
O

O
O
AcHN
O
(100%)
c. vacuum,
H O
[Baeyer-Villiger O
300 C
AcO
oxidation]
AcO AcN OAc [acetate elimination]
H
H
O
(80%)
11
3
a. OsO4, py
(65%)
b. (MeO)2CMe2, CSA
c. Et3O BF4 , Na2CO3; AcOH

BF4 ;
Ac2O, py
b. acetamide
(50%)

a. BF3, TFA
b. TFA, H2O
HO

c. Al(OiPr)3, iPrOH,
d. Ac2O, py
(86% overall)

mCPBA

H O a. Et O
3

AcO

H
HO

OAc

AcO-

CH2OAc
H

OAc
N

AcHN

OH

HO

8
d. (EtO)3CH, CSA; Ac2O, py [diethylketal formation]
e.
[ethyl enol ether formation]
f. mCPBA, K2CO3
[epoxidation]
g. AcOH
[epoxide opening and acetylation]
(53% overall)
OAc

Me

O AcN
H OAc

a. CrO3, py
b. BF3Et2O,

CH2OAc
H

OAc
CH2OAc
H

OAc
H2N

H O
a. BrCN, NaHCO3 AcO
H
b. H2S
O
(100%)
12

HO

H
H

OH
CH2OH

HN
H2N

N O
H
HO H

H
O
O

OH

1: tetrodotoxin

Scheme 13. a) Strategic bond disconnections and retrosynthetic analysis of

tetrodotoxin and b) total synthesis (Kishi et al., 1972).[97]

tional molecule, tetrodotoxin was certainly a great conquest

and elevated the status of both the art and the practitioner,
and at the same time was quite prophetic of things to come.
Vitamin B12 (1973)
The total synthesis of vitamin B12 (1 in Scheme 14),
accomplished in 1973 by a collaboration between the groups
of Woodward and Eschenmoser,[3, 32] stands as a monumental
achievement in the annals of synthetic organic chemistry.
Angew. Chem. Int. Ed. 2000, 39, 44 122

Rarely before has a synthetic project yielded so much

knowledge, including: novel bond-forming reactions and
strategies, ingenious solutions to formidable synthetic problems, biogenetic considerations and hypotheses, and the seeds
of the principles of orbital symmetry conservation known as
the Woodward and Hoffmann rules.[98] The structure of
vitamin B12 was revealed in 1956 through the elegant X-ray
crystallographic work of Dorothy Crowfoot-Hodgkin.[99] The
escalation of molecular complexity from haemin to chlorophyll a to vitamin B12 is interesting not only from a structural
point of view, but also in that the total synthesis of each
molecule reflects the limits of the power of the art and science
of organic synthesis at the time of the accomplishment.
One of the most notable of the many elegant maneuvers of
the Woodward Eschenmoser synthesis of vitamin B12 is the
photoinduced ring closure of the corrin ring from a preorganized linear system wrapped around a metal template,
which was an exclusive achievement of the Eschenmoser
group. The convergent approach defined cobyric acid (2 in
Scheme 14) as a landmark key intermediate, which had
previously been converted into vitamin B12 by Bernhauser
et al.[100] The synthesis of vitamin B12 defined the frontier of
research in organic natural product synthesis at that time. For
an in depth discussion of this mammoth accomplishment, the
reader is referred to ref. [4].
Erythronolide B (1978)
The macrolide antibiotics, of which erythromycin is perhaps
the most celebrated, stood for a long time as seemingly
unapproachable by chemical synthesis. The origin of the
initial barriers and difficulties was encapsulated in the
following statement made by Woodward in 1956, Erythromycin, with all our advantages, looks at present hopelessly
complex, particularly in view of its plethora of asymmetric
centers.[101] In addition to the daunting stereochemical
problems of erythromycin and its relatives, also pending was
the issue of forming the macrocyclic ring. These challenges
gave impetus to the development of new synthetic technologies and strategies to address the stereocontrol and macrocyclization problems.
The brilliant total synthesis of erythronolide B[102] (1 in
Scheme 15), the aglycon of erythromycin B, by Corey et al.
published in 1979, symbolizes the fall of this class of natural
products in the face of the newly acquired power of organic
synthesis. Additionally, it provides further illustration of the
classical strategy for the setting of stereocenters on cyclic
templates. The synthesis began with a symmetrical aromatic
system that was molded into a fully substituted cyclohexane
ring through a short sequence of reactions in which two
bromolactonizations played important roles. A crucial Baeyer Villiger reaction then completed the oxygenated stereocenter at C6 and rendered the cyclic system cleavable to an
open chain for further elaboration.
As was the case in many of Coreys syntheses, the total
synthesis of erythronolide B was preceded by the invention of
a new method, namely the double activation procedure for the
formation of macrocyclic lactones employing 2-pyridinethiol
esters.[103] This landmark invention allowed the synthesis of
61

REVIEWS

K. C. Nicolaou et al.

Scheme 14. a) Strategic bond disconnections and retrosynthetic analysis of ()-vitamin B12 , b) key synthetic methodologies developed in the course of the
total synthesis, c) and final synthetic steps in the Woodward-Eschenmoser total synthesis of vitamin B12 (Woodward Eschenmoser, 1973).[32]

62

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

Me

O
Me

Me
Me

Me
O

Me
OBz
O

HO

OH

Me

OH

OTBS
O

OTBS

OBz
Me

Me

Me

Me

1: erythronolide B

Bromolactonization

Me

Br

Me

Bromolactonization

Me

Me

Me

NaOMe

(72%)

Me

10

Me

Me
O

13
a. H2, Raney-Ni
b. BzCl

12

OBz

OBz

a. LDA
Me
b. MeI
(75%
O overall) BzO
Me

Br2, KBr

Me

Me

(91%)

Me

Me

O
Me

11

[bromolactonization]

CO2H

OBz
Me

Me

BzO
Me

Me

O
Br

Me

a. KOH, H2O (98%)

b. resolution

(93%)

Me
O

Me

nBu3SnH
Me
AIBN

Me

CO2H

(76%)

a. LiOH
Me
b. CrO3, H2SO4
(80%)

OBz
Me

Me

Me

CH3CO3H

BzO
Me

(70%) BzO

Me

Me

CO2H

[Baeyer-Villiger
oxidation]

Me

16

14

5
a. H2O2, Na2WO4
b. resolution
c. ClCO2Et
Me d. NaBH4 OMe
e. POCl3,
(76% overall)

O
Me

18

15
a.
Li
b. Amberlyst IRC-50
c. ArSO2Cl, py
Me d. Me2CuLi
Me
I
O e. TBSCl, imid.
Me
f. LDA; MeI
OMe g. [Cp2ZrHCl]
OTBS
Me h. I2, CCl4
Me

19

Ph3P

17
N

S S

(65%)

OBz
Me

Me

BzO
Me

O O O
S

tBuLi, MgBr2

Me

(90%)

OH
Me

Me

Me

Me

a. AcOH
HO
b. LiOH
OBz
H2O2 Me

Me

BzO
Me

O O O

Me

Me

OH

iPr
Me

Me

OH

tBu
N

N
S S

23
Ph3P;

Me

iPr

PhMe,
(50% )

a. MnO2
(98% )
b. H2O2, NaOH

10
OH

Me
OH
OH
OH

Me

24

a. H2, Pd/C [epoxide reduction]

b. K2CO3, MeOH
[epimerization at C-10]
c. HCl

Me

1: erythronolide B

Me

Me
O

Me

Me
OH

Me
O

Me

O
Me

O
Me

Me

25

Scheme 15. a) Strategic bond disconnections and retrosynthetic analysis of

erythronolide B and b) total synthesis (Corey et al., 1978).[102]

Angew. Chem. Int. Ed. 2000, 39, 44 122

Monensin[105] (1 in Scheme 16), isolated from a strain of

Streptomyces cinamonensis, is perhaps the most prominent
member of the polyether class of antibiotics. Also known as
ionophores, these naturally occurring substances have the
ability to complex and transport metals across membranes,
thus exerting potent antibacterial action.[106, 107] These structures are characterized by varying numbers of tetrahydropyran, tetrahydrofuran, and/or spiroketals. Kishis total synthesis
of monensin,[108] which followed his synthesis of the simpler
ionophore lasalocid,[109] represents a milestone achievement
in organic synthesis (Scheme 16). This accomplishment demonstrates the importance of convergency in the total synthesis
of complex molecules and is one of the first examples of
stereoselective total synthesis through acyclic stereocontrol,
and elegantly marked the application of the Cram rules within
the context of natural-product synthesis. By unraveling the
spiroketal moiety of the molecule Kishi was able to adopt an
aldol-based strategy to couple monensins two segments. A
series of daring reactions (for example, hydroborations,
epoxidations) on acyclic systems with pre-existing stereocenters allowed the construction of the two heavily substituted fragments of the molecule which were then successfully
coupled and allowed to fold into the desired spiroketal upon
deprotection. Kishis beautiful synthesis of monensin also
provided a demonstration of the importance of 1,3-allylic
strain in acyclic conformational preferences, which in turn can
be exploited for the purposes of stereocontrolled reactions
(for example, epoxidation).
A second total synthesis of monensin was accomplished in
1980 by W. C. Still and his group (Scheme 17).[110] Just as
elegant as Kishis synthesis, the Still total synthesis of
monensin demonstrates a masterful application of chelationcontrolled additions to the carbonyl function. A judicious
choice of optically active starting materials as well as a highly
convergent strategy that utilized the same aldol spiroketalization sequence as in Kishis synthesis allowed rapid access to
monensins rather complex structure.

O
O

Me

Me

Me
O

OH
Me

Me

Me
Me
O

Me

22

Me

Me

Me

Me

Me Me

20

tBu

Me
OH
Me

OTBS
Me

Me

Me

HO

OTBS

d. Amberlyst IRC-50
e. KOH
(61% overall)

Me

Me

Me

Me

OH
Me

Me

Me

OMe

Me

Zn(BH4)2

OBz

21 HO
a. KOH
b. CH2N2
c. HBr,

Me
OBz

OH

Me
OH

OBz

OBz

Me

Me

Me

CO2H

HO2C

5
O

(96%)
Me

HO

O [bromolactonization]
Br
Me
Br2, KBr Me

Me

Br

Me Al/Hg

BzO
Me

a. BH3THF;
Me
H2O2, NaOH
b. CrO3, H2SO4

Me

Me

Me

Me

Me

OH

Monensin (1979, 1980)

OBz
Br

Me

Me 7

S
Me

Me

O O O

Baeyer-Villiger
oxidation

8 Me

b)

Me

Alkylation

Me

Me

Me
BzO
Me

C-C bond formation

Lactonization

OH

Me
Me

OH Functionalization

several macrolides including erythronolide B and, most

significantly, catalyzed the development of several improvements and other new methods for addressing the macrocyclization problem.[104] Soon to follow Coreys synthesis of
erythronolide B was Woodwards total synthesis of erythromycin A.[33]

Me

Me

Me

OH

Me

OBz

Endiandric Acids (1982)

The endiandric acids (Scheme 18) are a fascinating group of
natural products discovered in the early 1980s in the
Australian plant Endiandra introsa (Lauraceae) by Black
et al.[111] Their intriguing structures and racemic nature gave
rise to the so called Black hypothesis for their plant origin,
which involved a series of non-enzymatic electrocyclizations
from acyclic polyunsaturated precursors (see Scheme 18).
Intrigued by these novel structures and Blacks hypothesis for
their biogenetic origin, we directed our attention towards
their total synthesis. Two approaches were followed, a
63

REVIEWS

K. C. Nicolaou et al.

a) Aldol condensation

Spiroketalization
O

HO

Me

Me

Me

O
Me

Me H

Me
CO2H

a.

OMe
Me

Me

Ph3P

Me

H Me

CO2Me

HO O

MeO

Me
H

BH3;
KOH, H2O2
(85%)

Me

OH

Me

BH3

O
Me

Me

[8:1 mixture]

MeO

a. O
(MeO)2P

a. KH, MeI
b. H2, 10% Pd/C
c. resolution
d. PCC
(77% overall)

OBn

H Me

HO O

OBn

b. LiAlH4
c. BnBr, KH
(66% overall)

CO2Et
Me

Me

Et

MeO

Me

H Me

1: monensin HO

b)
a. nBuLi, MeI
CN
b. KOH
c. LiAlH4
5
d. PCC

Me

Et

Me
O OH

OBn

H
HO O

OMe

Me

Et

OMe O

Me

CO2Me

OMe

Me

Me
O
Me

b. LiAlH4
(73%)

Me

O
Me

Me

(80%)
Me a. mCPBA

Me

Me

Me

b. KOH aq.
c. resolution

CO2H

OH

(35% overall)

PPh3

14

13

15

a. PhCHO, CSA
b. LiAlH4-AlCl3 (1:4)
OH c. resolution
HO

HO

CH3C(OEt)3
CH3CH2CO2H,

Et
H

(93%)

OBn

a. PCC
b.

Et

HO

[Johnson
orthoester Claisen
rearrangement]

16

Me

Me

Me

Ar
O

Et H

[bromoetherification]

26

KO2,
[18]crown-6
DMSO

Me

Ar
H

Et H

H H

OH

27
Me
Me
MeO OH
OH

Ar

OH
H

Et H

Me

OH

Me

11
[12:1 mixture]

Et

21 OH

O
Et

Et

Ar

mCPBA

OH

(36%)

[7:2 mixture]
24

(78%)

OH

[hydroxyl-directed
epoxidation]
22

23

a. NaOMe, MeOH
b. (CH3O)3CH
MeOH, CSA

Me
Me
Me
a. Cl3CCOCl, py
Me
b. OsO4, py
Ar
OBz
O
c. BzCl, py
O
O
H
Et H
H HO
d. CrO3, H2SO4
CCl3
28
O
Me

H
Me

PPh3

[Wittig reaction]
25

20
a. pTsCl
O
b. LiAlH4
c. CSA
Ar
d. OsO4, NaIO4

Me

a. LiAlH4
Ar
b. PCC
OBn c. MeOC H MgBr
6 4
H
d. CrO3, H2SO4
e. BCl3
(31% overall)

EtO2C

OBn

OMe OH

(47%)

Me

15

Me

Me

NBS Ar
O
OH
(57%)
H
Et H

Br

Me

19

Me

Me

Me

18

17

EtO

BH3; H2O2

Et

Et

OBn

MgBr

a. MOMBr,
OMe OBn O
OMe OBn OMOM
a. CH2N2
PhNMe2
MeO2C
HO2C
H
b. BnBr, KH O
b. HCl
c. O3 , MeOH
Me Me Me
Me Me Me
c. PCC
12
2
(33% from 11)

Me

Me

12 steps

OH

10

Me

(53% overall)

Me

Me

Me

MeMgBr
O

Et H

H H

OMe

(22% from 4)

OHC

MeO O

Me

Et H

H H

Li, EtOH
NH3 (l)

OH

OMe

[Birch reduction]

Me
OH

Me

MeO

H O Et H O H H O OMe

31

OH

30

a. (CH3O)3CH
MeOH, CSA
b. O3 , MeOH
c. MgBr2

Me

Me

Me

MeO
H

Et H

H H

OH
OMe

29

a. O3 , MeOH
b. HCl, MeOH
c. MeLi
OH
H
Me

Me
O OH

Et
O

HO

Me

iPrNMgBr, 2

H Me

(21%, 92% based on recovered SM)

OBn
Me
OMe
Me

HO O

Et

Me

Me

Me

OH

O H
O

a. H2 , 10% Pd/C
b. CSA, H2O

Me

c. NaOH-MeOH (1:5)

Me

H Me

Me

[8:1 mixture]
MeO
32

Et
O

Me H

H
OMe

Me

HO O
CO2Me

MeO

HO

Me

H Me

H
HO

CO2Na
HO

Me

Me

1-Na: (+)-monensin sodium salt

Scheme 16. a) Strategic bond disconnections and retrosynthetic analysis of monensin and b) total synthesis (Kishi et al., 1979).[108]

stepwise (Scheme 19 b) and a direct one-step strategy

(Scheme 19 c). Both strategies involve an 8-p-electron electrocyclization, a 6-p-electron electrocyclization, and a Diels
Alder-type [42] cycloaddition reaction to assemble the
polycyclic skeletons of endiandric acids. The total synthesis[112]
of these architecturally interesting structures demonstrated a
number of important principles of organic chemistry and
verified Blacks hypothesis for their natural origin. In
particular, the one-pot construction of these target mole64

cules from acyclic precursors from the endiandric acid cascade

is remarkable, particularly if one considers the stereospecific
formation of no less than four rings and eight stereogenic
centers in each final product.
Efrotomycin (1985)
Efrotomycin (1 in Scheme 20; see p. 67), the most complex
member of the elfamycin class of antibiotics[113] that includes
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

Scheme 17. a) Strategic bond disconnections and retrosynthetic analysis of monensin and b) total synthesis (Still et al., 1980).[110]

aurodox, was isolated from Nocardia lactamdurans. [114] Its

molecular structure, which contains nineteen stereocenters
and seven geometrical elements of stereochemistry, presented
considerable challenge to the synthetic chemists of the 1980s,
particularly in regard to the oligosaccharide domain and the
all-cis-tetrasubstituted tetrahydrofuran system. The total synAngew. Chem. Int. Ed. 2000, 39, 44 122

thesis of efrotomycin, accomplished in 1985 in our laboratories,[115] addressed both problems by devising new methodologies for the stereoselective construction of glycosides and
tetrahydrofurans. Scheme 20 summarizes this total synthesis
in which the two-stage activation procedure for the synthesis
of oligosaccharides utilizing thioglycosides and glycosyl
65

REVIEWS

K. C. Nicolaou et al.

CO2R

CO2R
Ph

Ph

a CO2R

CO2R

Ph

Ph

CO2R

RO2C

CO2R
Ph Ph

H
H
HO2C

endiandric acid D

Ph

Ph

endiandric acid E
Ph

H
H

CO2H
HO2C

endiandric acid F

Diels-Alder
Ph

endiandric acid G
Diels-Alder

H
HO2C

H
H

endiandric acid A

H
H

CO2H

Ph

Diels-Alder

Ph

H
H

H
H CO H
2

Ph
Ph

RO2C

H
H

endiandric acid B

CO2H

H
H

Ph

endiandric acid C

Scheme 18. The endiandric acid cascade (Black et al., R Me, H). a) Conrotatory 8-p-electron cyclization; b) disrotatory 6-p-electron cyclization.[111]

Scheme 19. a) Strategic bond disconnections and retrosynthetic analysis of endiandric acids A C, b, c) total synthesis, and d) biomimetic synthesis of
endiandric acid methyl esters A C (Nicolaou et al., 1982).[112]

66

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

OMe

Glycosidation
H

O
Me

O
Me

Wittig
olefination

OH

Me

OMe H

H
N

Me
O
Me

OH

OMe
OTBS

Me

OMe H

H
N

Me

Me
O

Me
OMe

OH

Me

Me

Me

Me

Me

Ph3P

OH

1: efrotomycin

Me

OH

TBSO

OH

Me

Me

O
O

Me

N
PhS

Me
HO OH

Me

Amide
formation

CCl3

OMe Me

OMe
OH

OMe

HO

Glycosidation

Me

Br

OBn

b)

a. LiCuMe2; TMSCl
b. O3; Me2S
c.
OMe

a. KCH2S(O)CH3
b. TBSCl, imid.

MeO2C

OMe H

OTBS

CuLi

MeO2C
O

7
Me

Me

OH Me
OMe
OH

a. nBu2SnO,
b. BnBr
c. KH, MeI

O
Me

OMe
OBn

Me

(55%)

OMe Me

d. NBS, DAST

12

MeO

Me

a. H2, 5% Pd/C
b. TBSCl, imid.
c. PhSTMS, ZnI2

OMe Me

OTBS

Me

Me

5, 6

Me
TMSO

10

Me

OTMS

OMe
OMe
OTBS

a. AgClO4, SnCl2
b. NBS, DAST

O
O

O
HO

NBS, AIBN

Ph

(100%)

c. PhSTMS, ZnI2;
K2CO3, MeOH
(70%)

14

a. Swern [O]
b. tBuOK,

Me
O

a. [Rh], H2
b. LiAlH4
c. CSA, acetone

Me
O

Me

15

OH
Me Me

(70%)

17

18

OMe
O

(80%)

OH

AcO

16

Me

a. (-)-DET, Ti(iPrO)4
tBuOOH
b. BnOC(O)Cl, py
c. AlCl3; H2O
(65%)

HO

O
O

c. DIBAL-H
(85%)

Me

a. AcOH, H2O
b. NaOH/EtOH

Me Me

19

Me

O
Me

Me

(85%)

O
H

OMe

Me

(85%)

Me

OH

O
O

a. (MeO)2CMe2,
CSA
b. K2CO3, MeOH;
CSA
c. RuO2, NaIO4

O
Me Me O

20

a. AcOH, H2O
b. PCC

c.

Me

22

Me

Me

O
Me

a. AlMe3, 10
b. HFpy
c. DDQ, MeOH

OH

24

Me
OMe H

H
N

O
Me

OH

OMe
OH

Me

Me

(26% overall)

21

Me

HO
O

Me Me

Me

OMe
O

Me

P(O)Ph2

(59%)
OMe

Me
Me
CO2H

O
O

Me

OMe
OH

(86%)
Me
Me

, nBuLi

Me

Me

Me
OH

Me

Me

Me

O
O

HO
O

CH3CH2CH2CO2Et,
LDA

Me

O
Me

OEt
O

23
c. 16, AgClO4, SnCl2
d. K2CO3, MeOH
OMe

(86%)

Me
Me

OH

OH

OMe
OAc

Me

Me
Me

Me

OMe

OH

Me
CO2Me

F
Me

TBSO

Me

MeO3P(O)

SPh

OPMB

TBSO

(63%)

a. nBu3SnH, AIBN
b. TBSCl, imid.

Br
OBz

HO

13

Me

OMe

OMe
OTBS

OMe
OMe

OMe

Me

a. TBAF
b. Ac2O, 4-DMAP
OMe

Me

OMe

(66%)

Me
O

H2N

d. KH, MeI
e. AcOH, H2O

(90%)

11

MeO

Me

Me
OMe H

1: efrotomycin

Me

Me
HO OH

Me

OH

Scheme 20. a) Strategic bond disconnections and retrosynthetic analysis of efrotomycin and b) total synthesis (Nicolaou et al., 1985).[115]

fluorides[116] as well as the base-induced zip-type diepoxide

opening were highlighted as powerful methods for organic
synthesis. Numerous applications and extensions of these
synthetic technologies have since followed.[117]
Okadaic acid (1986)
Okadaic acid[118] (1 in Scheme 21) is a marine toxin isolated
from Halichondria Okadai. Besides its shellfish toxicity,
Angew. Chem. Int. Ed. 2000, 39, 44 122

okadaic acid exhibits potent inhibition of certain phosphatases and is a strong tumor promotor. With its three spiroketal
moieties and seventeen stereogenic centers, the molecules
polycyclic structure presented a serious challenge to synthetic
chemistry. The first total synthesis of okadaic acid was
achieved in 1984 by the Isobe group in Japan[119] and was
followed by those of Forsyth[120] and Ley.[121] The Isobe
synthesis of okadaic acid, summarized in Scheme 21, highlights the use of sulfonyl-stabilized carbanions in synthesis, the
67

REVIEWS

K. C. Nicolaou et al.

Scheme 21. a) Strategic bond disconnections and retrosynthetic analysis of okadaic acid and b) total synthesis (Isobe et al., 1986).[119]

control of stereochemistry through chelation, and the power

of the anomeric effect to exert stereocontrol in spiroketal
formation.
Amphotericin B (1987)
The polyene macrolide family of natural products is a
subgroup of the macrolide class, which poses formidable
challenges to synthetic organic chemistry. Among them,
68

amphotericin B[122] (1 in Scheme 22), isolated from Streptomyces nodosus, occupies a high position as a consequence of
its complexity and medical importance as a widely used
antifungal agent. Its total synthesis[123] in 1987 by our group
represented the first breakthrough within this class of complex molecules. This total synthesis featured the recognition
of subtle symmetry elements within the target molecule that
allowed the utilization of the same starting material to
construct two, seemingly unrelated, intermediates and the
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

Horner-Wadsworth-Emmons;
a)
hydrogenation
Ester bond formation
OH

HO

Me

OH

OH

OH

OH

Me

OH

OTHP

TBSO

O
H
O

Me
OH

OTBS

Ph

TBSO
CO2Et

(EtO)2P
O

NH

TBSO

Cl3C

Ketophosphonate-aldehyde
macrocyclization

1: amphotericin B

4
O

NH2

OH

O
O

Me

Glycosidation
Phosphonate-aldehyde
condensation

Me
O

CO2H

Me

OBn O

(MeO)2P

MeO

Me

Phosphonate-aldehyde
condensation;
ring closure

OAc

Me
OTBS
N3

b)
HO

TPSO
O
OH
OH

(+)-8: (+)-xylose
OH
O
HO

HO

OH

7 steps

a. acetonide
formation
b. TPSCl, imid.
c. PhOC(S)Cl, py
d. nBu3SnH, AIBN

OBn O

(38% overall)

9a

BnO

(32% overall)

O
HO

(MeO)2P

OTBS

9b

()-9: ()-xylose

OH

OR1

a. PPTS [cyclization]
OMe
a. H2, Pd/C
O
Ph b. NBS, MeOH
(63%)
b. imid.
BnO
O
O
AcO
O
O
OR2 OR3 O
O
c. TBSOTf
c. 5 steps (53%)
d. LiOH
MeO
12
13
e. CH2N2
O
f. PDC
g. CH2N2
mixture of R1, R2 = acetonide, R3 = TBS and R1 = TBS, R2, R3 = acetonide
h. K2CO3,
OR1
MeOH
OMe
a. Et2AlC CH2OTPS
OTBS
HO
i. PDC
b. NaH, BnBr
2 OR3
j. (MeO)2P(O)CH2Li
O
O
O
OR
O
c. TBAF
CO2Me

(16% overall)

14

HO

OTPS

PCl5

BnO

(86%)

BnO

Cl

11

(67% overall)
OH

BnO

a. KSAE
b. Red-Al
c. TBSCl, imid.
d. H2, Pd(OH)2/C

Ph

O
O

16

17

LiCuMe2

BnO

e. PhCH(OMe)2, CSA
f. SO3py
TBSO
(47% overall)
a. H2, 10% Pd/C
b. Me2C(OMe)2, CSA
OBn c. CSA, MeOH
d. PCC

OH

BnO

OBn

Me

18

20

B(nBu)2
O
O

O
O

OTHP

Me

Me
O 21

Me

a. LDA, 6
b. DIBAL-H
c. MnO2

OTHP

TBSO

Me

Me

(86% overall)

Me

TBSO

26

Me

Me

(69%)

(60%)

22

TBSO

Me

OH

TBSO

Me

14

O
Me

O
HO

Me

OR2

Me

OR3

O
N

Me

23 Ph

OTBS

a. K2CO3, [18]crown-6
b. NaBH4

CO2Me

(MeO)2P

28

(70%)

Me

OMe

DCC, 4-DMAP
27

24

25

Ph

(72%)

Me

OCO2tBu

OR1
Me

a. LiBH4
b. tBuCOCl, py
c. TBSOTf, lut.
d. AcOH, THF, H2O
e. PhSSPh, nBu3P

OH

Me

(48% overall)

TBSO

SPh

a. Raney Ni
b. DHP, CSA
c. DIBAL-H
d. PCC, NaOAc

Me

[Evans' aldol]

a. LDA, 6
b. MeOH, PPTS
c. DIBAL-H
d. MnO2

Ph

TBSO

19

(88%)

P(OMe)2

HO

K2CO3 ,

OCHO MeOH

OEt
O

OTBS

H
O

(65% overall)

a. LDA; MeSSMe
Ph b. LDA; 5
c. TBAF

SMe

d. Red-Al

(MeO)2P

a. (EtO)3CH, AcOH
EtO2C
OH
b. LiAlH4
BnO
c. BnBr, NaH
BnO
OH
EtO2C
(87%)
15: (+)-diethyl-L-tartrate

a. H2, Pd/C
b. L-Selectride
(75% overall)
c. TPSCl, imid.
d. TBAF
BnO
O
e. MsCl, Et3N
f. NaI, acetone
g. (MeO)2P(O)H, NaH

10 steps

(68% overall)

10

OTPS
O

NaH, DME

(67% overall)

O
OH
Me
HO

Me

OH

OH

OH

MeO

OH

OH

O
H

CO2H

Me

1: amphotericin B

O
OH

Me
OH
NH2

Me

a. HFpy
b. HS(CH2)3SHEt3N
[azide reduction]
c. MeOH, CSA
d. LiOH, THF, H2O
(54% overall)

7
PPTS (cat.)
[glycosidation]
(40% overall)

TBSO

OMe

O
Me

O TBSO

OTBS

O
H

CO2Me

Me

29

OH

Scheme 22. a) Strategic bond disconnections and retrosynthetic analysis of amphotericin B and b) total synthesis (Nicolaou et al., 1987).[123]

Angew. Chem. Int. Ed. 2000, 39, 44 122

69

REVIEWS
employment of the then newly discovered Sharpless asymmetric epoxidation reaction[124] to stereoselectively construct
the 1,3-diol systems.
The Horner-Wadsworth-Emmons process[125] emerged as
the most valuable reaction of the synthesis, having been
utilized five times to construct carbon carbon double bonds.
Particularly striking was the application of an intramolecular
ketophosphonate aldehyde condensation to construct the
38-membered ring of amphotericin B. A further, notable
feature within this total synthesis is the strategy through which
the carbohydrate moiety was installed stereoselectively on a
derivative of amphoteronolide B to construct the challenging
b-1,2-cis-glycoside bond of the target molecule. Important in
this field is also Masamunes elegant synthesis of 19-dehydroamphoteronolide B.[126]

K. C. Nicolaou et al.
a)

Epoxidation

Epoxide opening
and lactonization

O
HO
HO

HO

Aldol
reaction

C-C bond
formation

Ring closure

MeO

Baeyer-Villiger
oxidation

O
OMe

O
OMe

tBu
H

Tandem vicinal
di-functionalization

tBu

OH

4 O

Intramolecular ketene-olefin
[2+2] cycloaddition

a.

b)

OMe

Ginkgolide B (1988)

OMe

OMe

OMe

a. [tBu2Cu(CN)Li2]
b. TMSCl, Et3N

OMe

b. 6N HCl
(75%)

OMe

a. Cy2BH
b. AcOH; H2O2
c. 1N HCl
d. pH 11; pH 3

MeO

tBu
O

12

(65%)

b. LDA; PhNTf2

(86%)

tBu

a. TiCl4, O

MeO

tBu

TMSO

Ginkgolide B (1 in Scheme 23) is a highly functionalized

natural substance isolated from the Ginkgo biloba tree, widely
known for its medicinal properties.[127] The structural elucidation of ginkgolide B in 1967 was a major accomplishment of
the Nakanishi group.[128] Its total synthesis by the Corey group
in 1988[129] stands as a landmark achievement in organic
synthesis. Despite its relatively small size, ginkgolide B
proved to be stubborn in its defiance to chemical synthesis,
primarily because of its highly unusual bond connectivity.
Among its most striking structural features are the tert-butyl
group which occurs rather rarely in nature, the eleven
stereogenic centers of which two are quaternary, and its six
five-membered rings. The Corey synthesis of ginkgolide B
abounds with brilliant strategies and tactics, but most
impressive is, perhaps, the intramolecular [22] ketene cycloaddition reaction, which contributed substantially to the
construction of the required carbon framework by delivering
two of the most challenging rings.

11

MeO

10

[Pd(PPh3)], CuI, nPrNH2

[Sonogashira coupling]
(76-84%)

tBu

TfO

CO2H

a. (COCl)2
(80%) b. nBu N,
3
MeO

[ketene-olefin
[2 + 2]
cycloaddition]

[Baeyer-Villiger
oxidation]

(86%)

Ph3COOH, NaOH

tBu

tBu

13

O 4

tBu

OH

14

a. HS(CH2)3SH, TiCl4
(75%)
b. PDC, AcOH
H OMe

a. LiNEt2
b.
N

PhO2S O

H
tBu

MeO

then (MeO)3P
(72%)

OH

18

(68%)

H
tBu

LDA, HMPA
O

OH

tBuO

tBuO

15

O O

O
O

Me
HO

Me

tBu

OH

Ph3COOH,

(80%)

OH

H
tBu BnMe3N-OiPr; O

tBu

a. HIO4, MeOH, H2O

b. CSA, MeOH

OMe

16

17
a. NBS, hv
(40%) b. AgNO3
c. PPTS, py

Palitoxin (1989, 1994)

Ph

c. CSA
(75%)

OH

70

OMe

Isolated from soft corals of the Palythoa genus, palitoxin (1

in Scheme 24) is endowed with toxic properties exceeded only
by a few other substances known to man.[130] Both its
structural elucidation and total synthesis posed formidable
challenges to chemists. While the gross structural elucidation
of palitoxin was reported independently by the groups of
Hirata[131] and Moore[132] in 1981, its total synthesis had to
await several more years of intense efforts. Finally, after
heroic efforts from Kishi and his group the synthesis of
palitoxin carboxylic acid was published in 1989[133] and that of
palitoxin itself in 1994[134] (see Scheme 24). The synthesis of
palitoxin holds a special place in the history of total synthesis
in that palitoxin is the largest secondary metabolite to be
synthesized in the laboratory, both in terms of molecular
weight and number of stereocenters. Most importantly, this
mammoth endeavor led to the discovery and development of
a number of useful synthetic reactions. Amongst them are the
improvement of the NiCl2/CrCl2-mediated coupling reaction

OH

Hydroxylation

1: ginkgolide B

H
tBu

O
O

OH

Oxidation

H
tBu

Me

H
tBu

OH

19

20
CSA (92%)

[lactol oxidation]

O
O

HO
HO

Me H
HO

O
O

H
tBu

OH

1: ()-ginkgolide B

a. I2, CaCO3
O
b. BF3Et2O
(89%)

HO
TBSO
H
O

Me H
HO

O
O

22

OH

OH
O
H
tBu

a.TBSOTf
O
b. OsO4, py
(65%)

HO

Me H
HO

O
O

H
tBu

OH

21

Scheme 23. a) Strategic bond disconnections and retrosynthetic analysis of

ginkgolide B and b) total synthesis (Corey et al., 1988).[129]

between iodo-olefins and aldehydes, a modified, refined

method for the Suzuki palladium-catalyzed coupling reaction
leading to conjugated dienes, and a new synthesis of N-acyl
ureas.
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

84

a)

O
O
OH
O

HO
85

98

OH

HO

d
e

N
H

N
H

OH
OH

HO

HO
OH
OH
OH

OH
OH

23

Me

THPO

Me
37

Me

OPMB

Me O

TBS
Me

PPh3

11
AcO
a. CrCl2, NiCl2, 11
b. Ac2O
c. PPTS, MeOH
d. [RuCl2(PPh3)3]

OMe

OPMB

OPMB

PMBO
OPMB

22

Me

OPMB
OMe
O

38

115

OBz

HO

N
H

NH2
SePh

12
j. camphor sulfonyl
oxaziridine
[cis-trans isomerization] k. hv

[Suzuki coupling]
OTBS
Me
OTBS

OTBS TBSO

TBSO

77

HO B

(7.5% overall)

a. [Pd(PPh3)4], 2
b. LiCH2P(O)(OMe)2
84

OH

OTBS

(72% overall)

51

O
OBz

BzO
BzO

OTBS

TEOCNH

Me

37

OTBS

85

a. Ketophosponate 10,
NaH; then 3
b. LiBH4
c. Ac2O
d. DDQ, Ac2O
e. HClO4
f. LiOH
g.TBAF
h. AcOH
O
i. py,

23

51

OBz

Me

OH
OBz

BzO
BzO

Me

a. 7, nBuLi, THF; then 8

Me

37

OPMB

OPMB
OPMB
OMe
O

Me

(64-46% overall)

51

O
OBz

38

PMBO
PMBO
PMBO

Me

99
98

TBSO
TBSO

OPMB

22

84

O
O

PMBO

b. H2, 10% Pd/C

c. PPTS, MeOH
d. Swern [O]
(ca. 55% overall
from 14)

37

Me

Me

OPMB

O
OAc
23

BzO
BzO

OPMB

OPMB
OPMB

OPMB

OPMB

PMBO
TBS
Me PMBO
PMBO

MeO

OBz

22

OTBS
H
OTBS

MeO
53

Me

PMBO

Me

OTBS
O
TBSO

OPMB

38

23

TBSO
51

OBz

AcO

OPMB

Me

Me

OH
OBz

BzO
BzO

Wittig reactions;
hydrogenation

a. 5, nBuLi, THF, -78 C; then 6

b. H2, 10% Pd/C
c. TBAF, THF, 25 C
d. MsCl, Et3N
e. NaI, 2-butanone
f. Ph3P, DMF

OPMB
OMe
O

Me

OPMB

OTBS
OTBS

37

MeO

OPMB

22

OPMB

22

THPO

OTBS

OPMB

PMBO

PMBO
O

OTBS

PMBO
PMBO
PMBO

OPMB

PPh3
O

OAc
OTBS

PMBO
PMBO
PMBO
28

Me

Horner-WadsworthEmmons reaction

1: palytoxin

75

OPMB

23

OH

OH

Me

Me

H
OH

53

51

b)

O
OAc

OH

52

37

HO
HO

NiCl2 /CrCl2
coupling

HO

Me

OH

38

PMBO
TBS
Me PMBO
PMBO

MeO

AcO

HO

Me

OPMB

OH

23

Me

OH

OH

22

Me

HO

Me

OH

OH

NiCl2 /CrCl2 coupling

OH

77

HO B

OH

HO
Me HO

Me HO

OTBS

Suzuki coupling

HO

Me
OTBS

TEOCNH

76

75

OH

H2N

Amide bond
formation
f

HO

OH
Me
OH

HO

TBSO

OTBS

97 93

OTBS TBSO

77

OTBS

Wittig 98
reaction

O
115

O
115

OH

OTBS

85

99

OH

HO

OH

99

84

TBSO
TBSO

99
98

a. CrCl2, NiCl2
b. PDC
c. Ph3P=CH2
d. PPTS
e. Swern [O]
f. LiCH[B(OCH2CH2CH2O)]2;
then EtOAc, brine/1N HCl

O
115

TEOCNH

TBSO
TBSO

OTBS

85

TBSO

OTBS

97 93

1: palytoxin
OTBS

O
77

Me
OTBS

OTBS TBSO

OAc
OTBS

OTBS

OTBS
O

OTBS
OTBS

O
O

99
98

TBSO
TBSO

OTBS
Me
OTBS
OTBS

13

TBSO

OTBS

OTBS
O

OTBS TBSO

TEOCNH

85

OTBS

O
115

84

TBSO

TBSO
77

OTBS

OCPh2(C6H4-p-OMe)

MeO

14

MeO

H
OTBS

P
O

10

Scheme 24. a) Strategic bond disconnections and retrosynthetic analysis of palytoxin and b) highlights of the total synthesis (Kishi et al. 1989, 1994).[133, 134]

Angew. Chem. Int. Ed. 2000, 39, 44 122

71

REVIEWS
Cytovaricin (1990)
Cytovaricin (1 in Scheme 25) is a 22-membered macrolide,
isolated from Streptomyces diastatochromogenes in 1981,[135]

K. C. Nicolaou et al.
which is endowed with impressive antineoplastic activity and
complex molecular architecture. Possessing seventeen stereogenic centers on its main framework, a spiroketal, and a
glycoside moiety with four additional stereocenters, cytovar-

Scheme 25. a) Strategic bond disconnections and retrosynthetic analysis of cytovaricin, b) key asymmetric alkylation and aldol reactions, and c) outline of
the total synthesis (Evans et al., 1990).[137]

72

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

icin presented a considerable challenge to synthetic chemistry
in the 1980s. Its structural elucidation by X-ray crystallographic analysis in 1983[136] opened an opportunity for Evans
et al. to apply their elegant alkylation and aldol methodology
for acyclic stereoselection to the solution of the cytovaricin
problem. Indeed, by 1990 the group reported a beautiful total
synthesis[137] that clearly demonstrated the new concepts of
stereochemical control by acyclic stereoselection as opposed
to the classical methods applied previously to solve such
problems. It is instructive to compare this synthesis to the
cyclic-template strategy used by Corey,[102] Woodward,[33] and
Stork[138] to achieve stereochemical control in their syntheses
of the erythromycin macrolide framework. This impressive
use of acyclic stereocontrol through the use of the Evans
chiral oxazolidone certainly propelled the area of polyketide
synthesis, a class of compounds that are rather readily
accessible synthetically by todays standards.

Calicheamicin g I1 (1992)
The arrival of calicheamicin g I1 [139] (1 in Scheme 26) and its
relatives, collectively known as the enediyne anticancer
antibiotics,[140] on the scene in the 1980s presented an entirely
new challenge to synthetic organic chemistry. Isolated from
Micromonespora echinospora ssp calichensis, this fascinating
natural product provided a unique opportunity for discovery
and invention in the areas of chemistry, biology, and medicine.
Its novel molecular structure is responsible for its powerful
biological properties, which include strong binding to duplex
DNA, double-strand cleavage of the genetic material by
formation of a benzenoid diradical, andas a consequence
potent antitumor and antibiotic activity.
The structure of calicheamicin g I1 is comprised of a carbohydrate domain and an enediyne core carrying a trisulfide
moiety that acts as a triggering device for the cascade of
events which leads, via a Bergman cycloaromatization,[141] to
the diradical species and DNA rupture. The oligosaccharide
domain of calicheamicin g I1 is endowed with high affinity for
certain DNA sequences, and acts as the delivery system of the
molecule to its biological target. The highly strained 10membered enediyne system, the novel oligosaccharide fragment, and the trisulfide unit are but some of the unusual and
challenging features of calicheamicin g I1 . Even more challenging, of course, was the chartering of the proper sequence for
assembling all these functionalities into the final structure.
Two groups rose to the challenge, ours (1992)[142] and that of
S. J. Danishefsky (1994).[143]
Notable features of our total synthesis of calicheamicin g I1
(Scheme 26) are the installment of the sulfur atom in the
carbohydrate domain through a stereospecific [3,3]-sigmatropic rearrangement and the [32] olefin nitrile oxide
cycloaddition reaction employed in the construction of the
enediyne core. That a molecule of such complexity could be
assembled in the laboratory in less than five years after its
structural elucidation in 1987 is an accurate reflection of the
high level of the state-of-the-art in the early 1990s. Just as
impressive is Danishefskys synthesis of calicheamicin, which
can be found in the original literature.[143]
Angew. Chem. Int. Ed. 2000, 39, 44 122

Strychnine (1993)
Although ()-strychnine had succumbed to the ingenuity
of Woodward in 1954 (see Scheme 4) it can still be considered
a target of choice to demonstrate the application of new
reactions and novel strategies by virtue of its abundant
stereochemical features densely packed in a heptacyclic
framework. Almost 40 years after Woodwards seminal
synthesis, Overmans synthesis of strychnine[58] (Scheme 27;
see p. 76) stands as a testimony to the evolution of organic
synthesis. Indeed, powerful palladium-mediated reactions
were used to expedite the assembly of the crucial intermediate
13 (Scheme 27) in a stereospecific fashion, thereby setting the
stage for the key tandem aza-Cope rearrangement and
Mannich reaction. This tandem reaction proved to be
particularly efficient and well-suited to afford an advanced
tricyclic system with concomitant formation of the quaternary
center stereospecifically, under mild conditions, and in nearly
quantitative yield. The sophisticated sequence of reactions
which ultimately led to Overmans ()-strychnine synthesis
deserves special mention for its elegance.
Rapamycin (1993)
Rapamycin (1 in Scheme 28; see p. 77) is an important
molecule within the field of immunosuppression that was first
isolated in 1975[144] from Streptomyces hygroscopicus, a
bacterial strain found in soil collected in Rapa Nui (Easter
Island), and structurally elucidated in 1978.[145] Its potent
immunosuppressive properties are reminiscent of those of
cyclosporin and FK506, whose biological and medical importance, particularly in the field of organ transplants, became
evident in the 1980s.[146] Although the structures of rapamycin
and FK506 possess striking similarities, the former is considerably more complex and attracted serious attention from the
synthetic chemists in the late 1980s and early 1990s. By 1995
there were four total syntheses of rapamycin,[147150] the first
being reported from this group in 1993 (Scheme 28).[147] This
asymmetric synthesis of rapamycin is an example of high
convergency and acyclic stereoselection, and is perhaps
known best for the way in which the macrocyclic ring was
formed. A palladium-catalyzed reaction based on Stilles
chemistry allowed a stitching cyclization process to proceed, to furnish the required conjugated triene system
concurrently as it formed the 29-membered ring of the target
molecule.[151]
Taxol (1994)
Taxol (1 in Scheme 29; see p. 78), one of the most
celebrated natural products, was isolated from the Pacific
yew tree and its structure was reported in 1971.[152] Its arduous
journey to the clinic took more than 20 years, being approved
by the Food and Drug Administration (FDA) in 1992 for the
treatment of ovarian cancer.[153] Synthetic chemists were
challenged for more than two decades as taxols complex
molecular architecture resisted multiple strategies toward its
construction in the laboratory. Finally, in 1994, two essentially
simultaneous reports[154, 155] described two distinctly different
73

REVIEWS

K. C. Nicolaou et al.

total syntheses of taxol. These first two syntheses, by our

group[154] and that of Holton,[155] were followed by those of
Danishefsky,[156] Wender,[157] Mukaiyama,[158] and Kuwajima.[159] All these syntheses, which are characterized by novel
strategies and brave tactics, contributed enormously to the
advancement of total synthesis and enabled investigations in
biology and medicine.
Amongst the most notable features of our total synthesis of
taxol (Scheme 29) are the boron-mediated Diels Alder
reaction to construct the highly functionalized C ring, the
application of the Shapiro and McMurry coupling reactions,
and the selective manner in which the oxygen functionalities
were installed onto the 8-membered ring of the molecule.
Because of the great drama associated with cancer, this and
the other syntheses of taxol received headliner publicity. The
art and science of total synthesis was once again brought to
the attention of the general public.

Zaragozic Acid (1994)

A new natural product with unprecedented molecular
architecture often gives impetus to synthetic endeavors
directed at its total synthesis. Such was the case with zaragozic
acid A (1 in Scheme 30; see p. 79) whose structure was
released essentially simultaneously in 1992 by groups from
Merck[160] and Glaxo[161] (the latter naming the compound
squalestatin S 1).[162] Isolated from a species of fungi, zaragozic
acid A exhibits impressive in vitro and in vivo inhibition of
cholesterol biosynthesis by binding to squalene synthase.[163]
Zaragozic acid A, like its many relatives, possesses an unusual
tricarboxylic acid core, whose highly oxygenated nature
added to its novelty and complexity as a synthetic target.
The distinguishing features of our synthesis[164] of zaragozic
acid A (Scheme 30) include the utilization of the Sharpless
asymmetric dihydroxylation reaction[165] to install the first two
oxygen-bearing stereocenters onto a complex prochiral diene
system and a multi-step, acid-catalyzed rearrangement to
secure the zaragozic acid skeleton.
The synthesis of zaragozic acid was also accomplished and
reported at approximately the same time as ours by the groups
of Carreira (zaragozic acid C)[166] and Evans (zaragozic
acid C).[167] In addition, Heathcock et al.[168] reported another
total synthesis of zaragozic acid A in 1996.

Swinholide A (1994)
Swinholide A (1 in Scheme 31; see p. 80), a marine natural
product with antifungal and antineoplastic activity, was
originally isolated from the Red Sea sponge Theonella
swinhoei.[169a] Its structure was fully established in the late
1980s by X-ray cystallographic analysis.[169b] The structure of
swinholide A has C2 symmetry and is distinguished by two
conjugated diene systems, two trisubstituted tetrahydropyran
systems and two disubstituted dihydropyran systems, a 44membered diolide ring, and thirty stereogenic centers. Its
challenging molecular architecture coupled with its scarcity
74

Scheme 26. a) Strategic bond disconnections and retrosynthetic analysis of

calicheamicin g I1 and b) total synthesis (Nicolaou et al., 1992).[142]

and biological action prompted several groups to undertake

synthetic studies towards its total synthesis. Two laboratories,
that of I. Paterson at Cambridge[170] and ours[171] have
succeeded in the task.
Angew. Chem. Int. Ed. 2000, 39, 44 122

Natural Products Synthesis

REVIEWS

Scheme 26. (Continued)

Angew. Chem. Int. Ed. 2000, 39, 44 122

75

REVIEWS

K. C. Nicolaou et al.
elucidated (1981).[177] The beauty of brevetoxins molecular
architecture, which accommodates eleven rings and twentythree stereogenic centers, attracted immediate attention from
the synthetic community. This neurotoxin, whose mechanism
of action involves the opening of sodium channels, shows
remarkable regularity in its structure. Thus, all rings are transfused and each contains an oxygen atom. All ring oxygens are
separated by a CC bond and each is flanked by two synarranged hydrogen or methyl substituentsexcept for the
first which carries a carbonyl to its left and the last which is
flanked by two anti-oriented hydrogens. With its imposing
structure, brevetoxin B presented a formidable and daunting
problem to synthetic organic chemistry. Not only did new
methods need to be developed for the construction of the
various cyclic ether moieties residing within its structure, but,
most importantly, the right strategy had to be devised for
the global assembly of the molecule.
After several abortive attempts, brevetoxin B was finally
conquered, and the total synthesis was reported in 1995 from
these laboratories (Scheme 33).[178] Along with the accomplishment of the total synthesis, this twelve-year odyssey[179]
yielded a plethora of new synthetic technologies for the
construction of cyclic ethers of various sizes. Prominent
among them are (see Scheme 33 b): a) the regio- and stereoselective routes to tetrahydrofuran, tetrahydropyran, and

Patersons total synthesis,[170] shown in Scheme 31 (see

p. 80), came first and was accompanied by the development
and application of a number of various types of asymmetric
boron-mediated aldol reactions to form key CC bonds.
Indeed, this new aldol methodology[172] was utilized to install
three contiguous chiral centers in two steps with high
diastereoselectivity (9 !12 in Scheme 31), and represents a
most welcomed progress in acyclic stereocontrol. Our total
synthesis of swinholide A[171] (Scheme 32; see p. 81) featured
two relatively new, at the time, methods for CC bond
construction in complex-molecule synthesis, namely the
Ghosez cyclization[173] to form a,b-unsaturated b-lactones
from orthoester sulfones and epoxides, and the dithianestabilized anion opening of cyclic sulfates.[174] The macrolactonization was performed by the Yamaguchi reagent[175] in
both strategies. Both total syntheses are highly convergent
and demonstrated the power of the art in acyclic stereoselection and large-ring construction and stand as important
achievements in the field of macrolide synthesis.
Brevetoxin B (1995)
Brevetoxin B (1 in Scheme 33; see p. 82), an active
principle of the poisonous waters associated with the red
tide phenomena,[176] was the first structure of its kind to be
a)
N

H
N

1: ()-strychnine

b)
OH

OtBu

EtO2C

N
H H

HO

N
H

O
OtBu

(89%)

tBuO

HO

15

R2N

[Mannich
reaction]
(98%)

R 2N

O 4

a. LDA, NCCO2Me
b. 5% HCl-MeOH,
[carboxymethylation;
imine formation;
tautomerization]
OtBu
(70%)

N
H

H
HO

1: ()-strychnine

[epoxide opening]
a. NaH,
b. KOH, H2O
(62%)

CO2Me

OtBu

TIPSO

[Pd2(dba)3], Ph3As, CO
O
MeN

7
OtBu

NMe

(80%)

11

R 2N

OtBu

a. tBuOOH,
Triton-B
HO b. Ph P=CH
3
2
c. TBAF
(84%)

a. MsCl, iPr2NEt
b. LiCl, DMF
R2N
OtBu c. NH COCF , NaH
2
3
(83%)

N
H

Zn:

N
H H

OtBu

12

N
H

N
H

N
H H
O

OH

OMe

16

ZnO

OMe

OH

17 OH
H

CH2(CO2H)2
Ac2O,
(65%)

HO

20

TIPSO

R2N

Zn
10% H2SO4,
MeOH,

N
H

H
H

[isomerizations]

HO2C

OtBu

13

H
N
H H

[lactamization]
H

Me3Sn

OtBu

F3COCHN

R2N

R2N

MeN

NMe
MeN

14

10

(CH2O)n,
Na2SO4,
HO

HO

TIPSO
I

a. CrO3,
H2SO4
b. L-Selectride;
then PhNTf2 Me3Sn
c. Me6Sn2, [Pd(Ph3P)4]
OtBu
(78%)

tBuO

NMe

tBuO

[3,3]
[aza-Cope
rearrangement]

MeN
N

NMe

3 OH

a. NaCNBH3,
TiCl4
HO
b. DCC, CuCl
H2O
c. DIBAL-H
d. TIPSCl
(57%)

CO2Me

H CO2Et

AcO

O
F3COCHN
O

[chemo-and stereo-selective TIPSO

reduction]

a. MeOCOCl, py
b. NaH, [Pd2(dba)3],
O

AcO

lactone
formation; 2: Wieland- Carboxymethylation;
imine formation;
reduction Gumlich
tautomerization
aldehyde

HO

Tandem Mannichaza-Cope
N rearrangement

Epoxide
opening

19 CO2H

OH

a. NaOMe, MeOH
b. DIBAL-H

H
N
H H

HO

(65%)

H
N
H H
CO2Me

18

OH

Scheme 27. a) Strategic bond disconnections and retrosynthetic analysis of ()-strychnine and b) total synthesis (Overman et al., 1993).[58]

76

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

Epoxide
opening

Me

Me

OMe O

Me

N
H

SnnBu3

H
Me

O
Me

nBu3Sn

OMe

OH

MeMeO

OH

OMe O

OH

I
I

Me

b)

Me

a. nBu3SnH, cat.
Me

TMS

b. I2

OH
Me

(81%)

Me

Me

Me

TMS
O

(80%)

OMe O

Me

,LDA

c. K2CO3, MeOH
(65%)

Li

10

OMe

Me

Me

Me

a. DDQ
OMe OTIPS

(68%)

11
PmBO

b. LiOH

Me

TMS

(88%)

[2-thienylCu(CN)Li]

OTBDPS

NiII/CrII coupling

OMe

a. HO

OMe

Me

(70%)

OH

OH

Esterification
H

a. CSA, MeOH
b. CF3SO2Cl, Et3N

Me

a. LiI, LiAlH4
O b. NaH, MeI

OH
OMe OTIPS

7
O

cat. = [Mo(allyl)Br(CO)2(CH3CN)2]

Me
TMS

MeO

OMe Me

tBuLi;

O
O

Me
OTES

Me

HN
3
Takai reaction

OTES OTIPS

1: rapamycin

TMS

Aldol reaction

HO
OTIPS

Amide bond formation

OH

Stille couplings
"stitching macrocyclization"

Me

Me

b. Swern [O]
(92%)

14

Me

Me

a. TIPSOTf TMS
b. NIS

I
OMe OTIPS PMBO

OMe OH

(95%)

13

Me

PMBO

12

Me

OH

nBuLi, tBuOK;

Me

(+)-Ipc2B

(+)-Ipc2BOMe,
BF3Et2O

15

OTBS

(75%)

16

b. O3; Me2S

17

Me

HO

PmBO

OTBS

CrCl2, NiCl2,

Me

Me

(93%)

Me

OMe Me

Me

PMBO

OMe

OPMB
CHO

Me

Me

OMe Me

Me

Ph

26

Me

Me

20

PMBO

OTIPS

PMB
OH

a. LiBH4
b. pTsCl, Et3N, 4-DMAP

OMe
OMe Me

OTBDPS

O
O

Ph

Me

Me

Me

Me

Me

OTBDPS

O
O

OMe Me

OMe

OH
Me

Me

31

OMe

PMBO

O
Me

Me

(60%)

OMe

OTIPS

I
OMe Me

Me

Me

OTBDPS

PMB

29

Me
Me

Me

O
OH

OMe O
SnnBu3

Me

O
Me

OMe Me

Me

O
OH
N
H
O

OH

OH

OMe O

O
O

Me

(27%)
H

CO2H

DIC, iPr2NEt
b. OsO4, NMO
c. Pb(OAc)4
d. CHI3, CrCl2

N
O

(87%)

2
[PdCl2(CH3CN)2], iPr2NEt

OTBDPS

nBu3Sn

a.

SnnBu3

OMe Me

OMe Me

a. DDQ
b. TESOTf
c. 3, DIC, HOBt

Me

O
Me

Me

30

PMB
OH

Boc

OMe
Me

OH

OMe O

24

Boc

OTES OTIPS

Me

OTIPS

(81%)

Me
OTES

OTBS

iPr2NEt, LiCl
(96%)

OTBS

OH

HO
OTIPS
OMe O

Me

OMe

Me

Me

Me

28

(70%)

Ph

25

27

Me

d. Swern [O]
e. HFpy
f. Swern [O]
g. HF, CH3CN

Me

O
N
P(O)(OEt)2

OMe

Ph

PMBO

c. LiEt3BH

Me

nBu2BOTf, Et3N,
(86%)

[Evans asymmetric aldol reaction]

Me

c. TBDPSCI, imid.
(75%)

OTBDPS

Me

23

Me

a. [RhCl(Ph3P)3], Et3SiH
b. aq. HF

OMe

Cp2ZrHCl; I2
(85%)

OTBS

OTIPS

PMBO

OTBS

Me 19

[Nozaki-Takai-Hiyama-Kishi reaction]
(83%)

22

PMBO

b. nBuLi; MeI
(98%)

21

a. TIPSOTf
b. HFpy
c. Swern [O]

Me

a. CBr4, Ph3P, Zn

18

Me

Me

OTBS

(72%)
PMBO

PMBO

PMBO

a. NaHMDS, PMBBr

OTBS

Me

Me

32

[inter- followed by intramolecular

Stille couplings]

O
OMe

Me

OH

O
Me

OH

OMe Me

Me

OMe
OH

Me

1: rapamycin

Scheme 28. a) Strategic bond disconnections and retrosynthetic analysis of rapamycin and b) highlights of the total synthesis (Nicolaou et al., 1993).[147]
Angew. Chem. Int. Ed. 2000, 39, 44 122

77

REVIEWS

K. C. Nicolaou et al.

a)
AcO
Ph

O
Ph

N
H

silicon-induced hydroxy ketone cyclization to oxepanes;

f) nucleophilic additions to thiolactones as an entry to
medium and large ring ethers; g) thermal cycloadditions of
dimethyl acetylene dicarboxylate with cyclic enol ethers as an
entry to medium size oxocyclic systems; and h) the novel and
unprecedented chemistry of dithiatopazine. For a more
detailed analysis of this total synthesis, the reader should
consult ref. [3].

McMurry coupling

Esterification
O

OH
Me

Me
O

Oxetane formation

OH

O
H
OBz OAc

HO

Oxygenation

Shapiro coupling
1: Taxol
TPSO
TESO

Me

Ph

Diels-Alder reaction

b)

OAc

Me

5
Cl

(85%)

CN

[Diels-Alder
reaction]

OAc

Me

(65%)
O

NNHSO2Ar

Me

OH

12
HO

10

PhB(OH)2

Me

OH

EtO2C

OB

[Diels-Alder
reaction]

OTBS

EtO2C

OEt

Me

a. TBSCl,
imid.
b. H2NNHSO2Ar
(68%)

CN
Cl

OH

OH

KOH, tBuOH

O
Me

Diels-Alder reaction
4

Me

Dynemicin A (1995)

NNHSO2Ar

OBn

Ph

Me

OTBS

[boronate
cleavage]

Ph

HO

OH

11

13

[lactone migration]

Me
TBDPSO

a. LiAlH4
b. CSA, MeO

OBn

OMe

Me

c. TPAP, NMO

TBDPSO

(63% overall)
O
O

15

Me

OTBS

nBuLi

(82%)
16

[Shapiro reaction]

Li

a. [V(O)(acac)2],
TBSO
OTBDPS
tBuOOH
OBn
Me
Me
b. LiAlH4
c. KH, COCl2
(50%)

TBSO
Me

a. TBSOTf
O
OH
b. LiAlH4
Me
c. CSA, MeOH
EtO
H
d. TBDPSCl, imid.
OH
OTBS e. KH, nBu4NI, BnBr
O
(46% overall)
O
O
14

OBn

H
OH

Me

O
O

17

OTBDPS

a. TBAF
HO
b. TPAP, NMO
Me
c. TiCl3(DME)15
Zn/Cu
(17%)
O
[McMurry
O
coupling]
O

18

OH

OBn
Me

OBn

H
O

19

a. Ac2O, 4-DMAP
b. TPAP, NMO
c. BH3THF; H2O2
AcO

OTES
Me

Me

O
O
O 21

OMs
OH
OAc

AcO

a. HCl, MeOH
b. Ac2O, 4-DMAP
c. H2, Pd(OH)2/C
d. TESCl, py
e. MsCl, 4-DMAP
(46% overall)

(48%)

OBn
Me

Me

OH
O

O
O

20

a. K2CO3, MeOH d. PhLi [selective carbonate opening]

e. PCC [allylic oxidation] (34% overall)
b. nBu4NOAc
c. Ac2O, 4-DMAP f. NaBH4
AcO
Me

AcO

OTES
Me

Ph

HO
HO

22

O
H
OBz OAc

Ph

a. NaHMDS, 2
b. HFpy

N
H

Me

OH
Me

O
OH
HO

1: Taxol

O
H
OBz OAc

Scheme 29. a) Strategic bond disconnections and retrosynthetic analysis of

taxol and b) total synthesis (Nicolaou et al., 1994).[154]

oxepane systems employing specifically designed hydroxy

epoxides; b) the silver-promoted hydroxy dithioketal cyclization to didehydrooxocanes; c) the remarkable radical-mediated bridging of bis(thionolactones) to bicyclic systems; d) the
photoinduced coupling of dithionoesters to oxepanes; e) the
78

Dynemicin A[180] (1 in Scheme 35; see page 84), a dark blue

substance with strong antitumor properties and a member of
the enediyne class of antitumor antibiotics that includes
calicheamicin g I1 (Scheme 26), possesses a striking molecular
architecture.[140, 181] Isolated from Micromonospora chersina,
dynemicin includes in its structure a highly strained 10membered enediyne ring, and a juxtaposition of epoxide,
imine, and anthraquinone functionalities. The lure provided
by this fascinating DNA-cleaving molecule resulted in intense
synthetic studies directed towards its total synthesis. In 1993
Schreiber et al. first reported the total synthesis of di- and
trimethoxy derivatives of dynemicin methyl ester (1 in
Scheme 34; see p. 84).[182] This synthesis relies on the powerful
intramolecular Diels Alder reaction to construct the complex enediyne region of the molecule and a series of selective
follow-up reactions to reach the methylated dynemicin
targets.
Myers et al. reported the first total synthesis of dynemicin
itself in 1995.[183] Their synthesis, summarized in Scheme 35,
highlights a stereoselective introduction of the ene diyne
bridge, the use of a quinone imine as the dienophile in a regioand stereoselective Diels Alder reaction, and a number of
other novel steps to complete the total synthesis. The second
total synthesis of dynemicin was reported from the Danishefsky laboratory[184] (Scheme 36; see p. 85) and features a
double Stille-type coupling in its assembly of the enediyne
grouping. All three syntheses project admirable elegance and
sophistication.
Ecteinascidin 743 (1996)
A marine-derived natural substance, ecteinascidin (1 in
Scheme 37) possesses an unusual molecular architecture and
extremely potent antitumor properties. Isolated from the
tunicate Ecteinascidia turbinata, ecteinascidin 743 is comprised of eight rings, including a 10-membered heterocycle,
and seven stereogenic centers.[185] Prompted by its attractive
molecular architecture, impressive biological action, and low
natural abundance, Corey et al. embarked on its total synthesis, and in 1996 they published the first total synthesis[186] of
ecteinascidin 743 based on a brilliant strategy (Scheme 37; see
p. 86).
The plan was inspired, at least in part, by the proposed
biosynthesis of the natural product. Of the many powerful
transformations in Coreys total synthesis of ecteinascidin 743, at least three stand out as defining attributes; an
intramolecular Mannich bisannulation sequence was instrumental in establishing the bridging aromatic core to the
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

Scheme 30. a) Strategic bond disconnections and retrosynthetic analysis of zaragozic acid A and b) total synthesis (Nicolaou et al., 1994).[164]

piperazine ring, which allowed the formation of the desired

aminal functionality, while two asymmetric Pictet Spengler
reactions played key roles in forming the isoquinoline rings.
The centerpiece of the synthesis is, however, the generation
and biomimetic quinone methide capture by the sulfur atom
to construct the 10-membered lactone bridge. The masterful
use of substrate topology to predict reactivity, inflict asymmetry, and achieve selectivity is amply demonstrated throughout Coreys synthesis.
Finally, the success in recognizing subtle retrosynthetic
clues left by nature and applying them in the context of a
chemical synthesis elevates this total synthesis to a unique
Angew. Chem. Int. Ed. 2000, 39, 44 122

level of brilliance. This impressive accomplishment also

speaks for the efficiency that total synthesis has reached and
the complex natural product analogues which can be synthesized in large quantities.[187]
Epothilone A (1997)
Appearing in the mid-1990s, epothilones A (1 in
Scheme 38; see p. 87) and B[188] stimulated intense research
activities in several laboratories.[189] The impetus for their total
synthesis came not so much from their modestly complex
macrolide structures but more so from their potent tubulin79

REVIEWS

K. C. Nicolaou et al.

a)

OMe
OMe
Me

OH

Me

Me
OH

Me

Me

Yamaguchi
esterification

OH
O

Ar

OH

Me

Me

Me

Dimerization

Me

HO

OTBS

tBu
Me

Me

tBu
CO2Me

OH

Me

Vinylogous Mukaiyama
aldol reaction

Me

Brown's
syn-crotylboration

Mukaiyama coupling

Me

O
HO

Me
Yamaguchi
macrolactonization

TBSO

Me

MeO

O
OH

Me
HO2C

OH
OMe

Me

OH

Me

O
OMe

O
Me
O

Si

MeO

Me
O

Me

Me

O
Ar

Me

Paterson antialdol reaction

3
OMe

OMe

1: swinholide A

b)

[catalytic asymmetric
epoxidation]
a. (+)-DIPT, Ti(OiPr)4,

Me

Me

OMe

Me

[hydroxy-directed
reductive opening of epoxide]

OH

OMe

a. O3; Me2S; HCl

Me
b. Ph3P=C(Me)CHO

H2C=CHCH2TMS
TMSOTf
(96%)

(79%)

b. Red-Al
(34%)

OH

Me

a. O3; Me2S; HCl

Me
b. NaH, MeI

OH

tBuOOH

OMe

OMe

7
Me

Me

Me

tBu

OMe

[hydroboration-oxidation]
a. thexylborane;
OBn
H2O2, NaOH Me
b. (imid)2C=S
c. nBu3SnH
[deoxygenation]
(70%)

Me

Si

tBu

13
(93%)
Me
Me

Me

Me

Me

tBu

OMe

Si

TBSO

25
MeO
Me

[stereocontrolled Mukaiyama coupling/

chelation-mediated 1,3-syn reduction/
1,3-diol protection sequence]

11

Me

]2B

TBSO

HO

Me

TBSO

; H2O2
[Brown's syn
-crotylboration]

24

23
O

Me

[Horner-Emmons reaction] a. (MeO)2P(O)CH2CO2Me, nBuLi

b. TBSOTf
c. K2CO3, MeOH
(87%)
d. Dess-Martin [O]
Me
OAc

(80%)

TMSO

TiCl2(OiPr)2

20

OTBS

BzO
TBSO

a. CH2N2
b. HFpy, py Me

10
B(cC6H11)2

Me

[Paterson anti-aldol coupling]

(84%)

OMe

(97%)

MeO

Me

BzO

(83%)

20

HO2C

Si

OBn

MeO2C

tBu

Me

OBn
OH

12

Me

tBu

tBu

Me

b. tBu2Si(OTf)2
(78%)

Me

a. 25, LiHMDS, TMSCl, Et3N

b. 14, BF3OEt2
c. nBu2BOMe; LiBH4; H2O2
d. p-MeOC6H4CH(OMe)2, CSA
e. NaOH, H2O, MeOH

Me
OBn a. Me4NBH(OAc)3 Me

Me

a. MeOTf
b. PdCl2, MeO2C
CuCl, O2
(74%)
[Wacker oxidation]

MeO2C

14 tBu Si tBu

Me
O

Me

OMe

(cC6H11)2BCl, Me
Et3N

Me
OBn

a. H2, 10% Pd/C

b. Swern [O]

CHO

(82%)

Me

21

[Luche reduction]
a. NaBH4, CeCl37H2O
b. Ac2O, iPr2NEt

Me

OH

BF3OEt2
[vinylogous Mukaiyama
aldol reaction]
(85%)

BzO

22

]2BCl

Me

Me

O
Ar

Me

TMSOTf,
HO
iPr2NEt
(61%) BzO

O
BzO

(94%)

Cl

19

a.
b.

18

OMe

, iPr2NEt

16

Cl

O
Me

CHO

BzO

17
(56%)
[asymmetric aldol reaction]

15

OMe

(65%)
OMe

OMe
Me
Ar

O
OH

Me

OH
OMe
O

Me
Me

Me
O

Ar

2,4,6-Cl3C6H2COCl, Et3N, 3;
4-DMAP, 2

Me

OTBS

Me

OH
O

Me

[Yamaguchi
esterification]
(54%)

Me

OTBS

Me

Me

CO2Me
O

Me

tBu
O
tBu Si
O

Me

2
OTBS
Me

Me

Me

OH

OH
O

OH

Ar

OH

O
Me

Me
Me

O
MeO

Me
O

Me

O
HO

OH

Me
Me

Me

Me

d. 2,4,6-Cl3C6H2COCl, Me
Et3N; 4-DMAP
O
OMe
e. HF/H2O/MeCN
MeO
(27%)
O
[Yamaguchi
Me macrolactonization]

O
OMe

a. TBSCl, Et3N
b. HFpy, py
c. Ba(OH)28H2O,
MeOH

Me

OH

OH

Me
Me

26
O

OMe

OMe

OMe

1: swinholide A

Scheme 31. a) Strategic bond disconnections and retrosynthetic analysis of swinholide A and b) total synthesis (Paterson et al., 1994).[170]

80

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

OMe
OMe

Enders alkylation

Me

OH

Me

Me

Yamaguchi
esterification

OH
O

OH

Me

Ar

Me
Me

Me

Me
Yamaguchi
macrolactonization

TBSO

OTBS

HO

CO2Me

TMSO

Me

OH
Me

Me

MeO

Me

Me

O
HO

OH

HO2C

Me

OMe

MeO

Ghosez
lactonization

Me

OH

Dimerization

Mukaiyama aldol

Me

OTBS

Me

Me

O
Me
O

O
OMe

Me

Dithiane-cyclic
sulfate coupling

OH

TBSO

Me

2
Me

Me

O
Ar

Me

3
OMe

OMe

1: swinholide A

b)

a. Ac2O, Et3N,

Me

Me

4-DMAP

OH

Me

Me

HO

OH

BF3OEt2, TMSOTf HO

c. NaOMe, MeOH
(74%)
5: L-rhamnose
OH

OH

Me
Me

Me

Me

O
TBSO

a. nBu2SnO; CsF, MeI

b. NaH, CS2; MeI
OH
c. nBu3SnH, AIBN
[selective methylation/
Barton-McCombie deoxygenation
sequence]
(35%)

b. CH2=CHCH2TMS,

OBz O

a. H2, 10% Pd/C

b. SOCl2, Et3N Me
c. RuCl3, NaIO4
O
(94%)

12

TBSO

a. TiCl2(OiPr)2,
20 OTMS
b. TBSOTf
c. K2CO3, MeOH
Me
d. Swern [O]
BzO

OMe

Me

Me

Me
H

OMe

(68%)

OMe

d. O3 8
[Enders alkylation]
(67%)
Me

Me

Me

OBz OBn

11

OMe

Me

Me
O
OMe

OMe

OTBS e. TiCl , SH SH
4

f. DIBAL-H
g. TBSOTf
(50%)

21
a. tBuLi, HMPA;
b. H2SO4

a. DIBAL-H
b. BF3OEt2,
c. BzCl, Et3N
d. OsO4, NMO;
Pb(OAc)4
(60%)

Me
a. PhCHO, SmI2
b. TBSOTf
[1,3 anti-reduction;
Evans-Hoveyda modification
of the Tishchenko reduction]
(77%)

Me

Me

Me

Me
O

S
OBz OH

TBSO

OMe

(68% overall)

[selective desilylation]
Me

Me

Me

OH

OMe

Me

Me

OMe

Me
Me

Me

OMe

OTBS

a. NaOH, MeOH/THF/H2O
b. TMSOTf, iPr2NEt

Me

Me

Me
O

TBSO

OTMS O

O
Ar

(84%)
[Ghosez lactonization]

OMe

18

HO

a. PMBCO(NH)CCl3,
CSA
O
b. DIBAL-H
OMe c. (+)-Ipc B(allyl);
PMBO
2
Me
NaOH, H2O2
13
(74%)

OMe

Me

16

O
OH

nBuLi;
CO
2

Me

Me

;
I

14

PMBO
I

15

OMe

[Yamaguchi esterification]
a. 2,4,6-Cl3C6H2COCl, Et3N, 2,
4-DMAP
b. PPTS, MeOH
c. Ba(OH)28H2O
d. 2,4,6-Cl3C6H2COCl, Et3N;
4-DMAP
e. HF/H2O/MeCN
[Yamaguchi
macrolactonization]

Me

OMe

CO2Me

Ar

(82%)

PMBO

OH

Me

OH

OTBS

O
OH

10

OMe

Me

O
TBSO

OBn

a. MnO2
b. (MeO)2P(O)CH2CO2Me, nBuLi

(92% overall)
Me

23

Ar

OH

Me
O

OMe

Me

OTBS

Me

O
TBSO

OBn
O

a. NBS, AgClO4
b. nBu3B; NaBH4; H2O2, NaOH
c. p-MeO-C6H4CH(OMe)2, CSA
d. DIBAL-H
e. HFpy, py

[cleavage of dithiane functionality]

[syn 1,3-selective reduction]

Me

OTBS

22

OMe

Me

K2CO3, MeOH
(52% overall)

Me

Me
PMBO

Me

a. NaH, MeI OMe

OMe
17
b. PhSO2
OMe
DMPU, nBuLi; H2SO4;
pTsOH; Et3N, DBU

TMS

19

(72%)

Me

OBn

Me
OBn

TBSO

OBn
O

4
[aldol condensation]

OMe

TiCl4, Et3N

Me
O

OMe

Me

a. O3; NaBH4
b. I2, Ph3P, imid.
c. LDA,

(7% overall)
CO2H

OH

Me

Me

OH
O

OH

Me

OH

O
Me

OMe
O

Me
Me

O
Me

MeO

O
O
HO

OH
Me

Me
OH

OH

Me
Me

OTBS
OMe 1: swinholide A

Scheme 32. a) Strategic bond disconnections and retrosynthetic analysis of swinholide A and b) total synthesis (Nicolaou et al., 1995).[171]
Angew. Chem. Int. Ed. 2000, 39, 44 122

81

REVIEWS

K. C. Nicolaou et al.

binding properties and their potential to overshadow taxol as

superior anticancer agents. The first total synthesis of
epothilone A came from the Danishefsky laboratories in
1996[190] and was followed shortly thereafter by syntheses from
our laboratories[191] and from those of Schinzer.[192] Danishefskys first total synthesis of epothilone A (Scheme 38) featured a Suzuki coupling reaction to form a crucial CC bond
and an intramolecular enolate aldehyde condensation to
form the 16-membered macrocyclic lactone. This method as
well as others allowed the Danishefsky group to synthesize
several additional natural and designed members of the
epothilone family, including epothilone B,[193] for extensive
biological investigations.
Chemical biology was also on our minds in devising a
solution and a solid-phase total synthesis[194] of epothilone A
(1). As shown in Scheme 39 (see p. 87) this new solid-phase
paradigm of complex molecule total synthesis relied on a
novel olefin metathesis strategy.[195] Of special note is the
cyclorelease mechanism of this approach by which the 16membered epothilone ring was constructed with simultaneous
cleavage from the resin. Most importantly, this solid-phase
strategy allowed the application of Radiofrequency Encoded
Chemistry (REC; IRORI technology)[196] to the construction
of combinatorial epothilone libraries[197] for chemical biology
studies. The power of chemical synthesis of the 1990s in
delivering large numbers of complex structures for biological
screening was clearly demonstrated by this example of total
synthesis, marking, perhaps, a new turn for the science.

Eleutherobin (1997)
A marine natural product of some note, eleutherobin (1 in
Scheme 40; see p. 88) includes in its structure a number of
unique features. Isolated from an Eleutherobia species of soft
corals and reported in 1995,[198] this scarce natural product
elicited immediate attention from the synthetic community as
a result of its novel molecular architecture and tubulin binding
properties. Among the challenges posed by the molecule of
eleutherobin are its oxygen-bridged 10-membered ring and its
glycoside bond. Solutions to these problems were found in our
1997 total synthesis[199] as well as in Danishefskys total
synthesis,[200] which followed shortly thereafter. Scheme 40
summarizes our strategy to eleutherobin from ()-carvone.
Highlights include the intramolecular acetylide aldehyde
condensation to give the desired 10-membered ring and the
spontaneous intramolecular collapse of an in situ generated
hydroxycyclodecenone to form eleutherobins bicyclic framework. This total synthesis exemplified the power of chemical
synthesis in delivering scarce natural substances for biological
investigations.

Sarcodictyin A (1997)
Sarcodictyins A and B (1 and 2 in Scheme 41; see p. 88) are
two marine natural products discovered in 1987 in the
Mediterranean stoloniferan coral Sarcodictyon roseum.[201]
82

Scheme 33. a) Strategic bond disconnections and retrosynthetic analysis of

brevetoxin B, b) key synthetic methodologies developed for the formation
of polycyclic ethers and fundamental discoveries, and c) total synthesis of
brevetoxin B (Nicolaou et al., 1995).[178]
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

c)

Me
Me
a. Ac2O, py
O
OH b. BF Et O, TMSOTf,
3
2
O

OH
HO

[Barton-McCombie
deoxygenation reaction]
a. Im2C=S
OH
b. nBu3SnH, AIBN HO

TMS

HO

c. NaOMe, MeOH
OH d. TBDPSCl, imid.;
CSA, OMe
4: D-mannose

c. Amberlyst-15
d. nBu2SnO; BnBr
(35%)

O
H
TBDPSO

a. Swern [O]
OBn
b. AlMe3,
Me
MgBr2Et2O HO
K
(61%)
O

OBn

O
H
TBDPSO

H
TBDPSO

a. O3; Me2S, Ph3P

CO2Me
OBn
Me
b. CH2=CHMgBr
HO
c. TPSCl, imid.
K
d. O3; Me2S, Ph3P TBDPSO
O
H
H
e. Ph3P=CHCO2Me
TBDPSO
(34%)
8

a. NaH
[Intramolecular
conjugate addition]
b. DIBAL-H;
Ph3P=CHCO2Me

(62%)

SEt H
EtS

OTBS
Me

a. EtSH, Zn(OTf)2
b. CSA, MeOH

K
H

(69%)

c. SO3py, Et3N,
DMSO
(68%)

TBDPSO

Me

HO
HO

OH 10 steps

(49%)
H

O
H
H
TBDPSO
12

PPTS

TfO

Ph

b. LiHMDS, HMPA;
PhNTf2
(58%)

OBn

Me

20

J
HO

Me

a. DIBAL-H
b. mCPBA
c. Swern [O]

OBn

Me

Me
O

OH

Me

(58%)

Me

TBSO

OBn

OBn

Me

18

(83%)

(85%)

7 steps

e. NaClO2, NaH2PO4
f. TBAF

19

17

a. TBSO
PPh3
b. H2, Pd/C
c. CSA, MeOH
d. Swern [O]

OBn

Me

Ph

e. TBAF
f. PPTS
(65%)

G
H

Me
O

CO2Et d. CH2PPh3

16

Me

O
H
H
TBDPS TBDPSO

a. DIBAL-H
b. mCPBA
c. SO3py

O
H Me
TBS

(58%)

Me

OBn
Me

MeO2C
O
H
d. Ph3P=CHCO2Me
TBDPSO

F
O

e. TBAF

HO
OH H

OBn
Me

10

Me

a. 2,4,6-Cl3C6H2COCl,
Et3N; 4-DMAP

OBn

MeO2C

c. Swern [O]
Ph
d. Ph3P=C(Me)CO2Et
(77%)

OH

a. TBSCl, imid.
b. 9-BBN;
H2O2, NaOH

Me

15

11 TBDPSO

[6-endo ring closure]

Me Me
O

[(2-thienyl)(CN)CuLi]
[Murai coupling]

H
MeO2C

Me

OH

Me

14

Li

21

OTBS
Me

(89%)

13: 2-deoxy-D-ribose

Me

TBSO

Me

Ph

[6-endo hydroxy
epoxide
OBn
H
H
Me
cyclization]
HO
O
O
12 steps
CSA
K
I
J
(71%)
(55%)

a. LiHMDS, HMPA; PhNTf2

Me

E
R

R=

O
O

TBS
O

C
H

H
O

Me

O
H

Me
CHO

23

PivO

OBn

Me

HO
Me

TBSO

H
O

O
H H
Me

PivO

(64%)

Me

[Ni II/Cr II coupling]

Me

22

Me

b. CrCl2, NiCl2, TBSO

OBn

OBn

G
H

Me Me
O

H
O

O
H H
Me

a. DIBAL-H
b. Ph3P=CHCO2Et
c. DIBAL-H
d. (+)-DET, Ti(OiPr)4
Me H H
O
TBSO
tBuOOH
D
C
e. SO py

OBn

G
O

Me

a. TBAF
b. PPTS
c. TBSOTf
d. O3; Ph3P
e. MeMgCl
f. DMP

Me
O

C
H

Me

TBSO

(72%)

Me
H

O
H
H
CO2Me

OBn f. CH2=PPh3

27

H
O

O
H H
Me

Me

(69%)

OBn

Me

O
H H
Me

OBn

G
O

Me
OBn

26

A
O

PivO TESO

Me

H
O

Me
H O
H
EtS
OH

G
O
H

AgClO4,
NaHCO3,
SiO2, 4 MS

I
O

Me

Me

H
O

Me

G
H

A
O

H O

O
H

Me

H
O

Me
OBn

O
H H
Me

Me

OBn

G
O

Me

a. DIBAL-H
b. BF3Et2O, Et3SiH
c. Li, (l) NH3
d. pTsCl, py
e. NaI
f. TMS-imid.
g. Ph3P

OBn

29

Me

a. nBuLi, HMPA, 3
b. PPTS, MeOH

Me
H

(75%)

Me

H
O

D
H

Me

OTMS

O
H H
Me

Me

PPh3I

2
30
O

E
O
H
H
Me

H
HO

Me

H
O
H

H O

[methylenation]
Me

a. CH2=NMe2 I
b. HFpy

Me

38

(76%)

O
H

HO
Me

Me

H
O

OBn

Me

TBSO

a. Ph3SnH, AIBN [reductive desulfurization]

b. PCC [oxygenation]
c. TBAF
d. DMP
Me
(42% overall)
Me H
H

OH
EtS

O
H H
Me

O
H
H
Me

O
H

[hydroxy dithioketal
cyclization]

31

Me

Me

EtS

Me

OTBDPS

TBSO

25

Me
H

Me
H
O

d. CSA, MeOH
e. KH
(74%)

Me

[intramolecular
Horner-Wadsworth-Emmons
olefination]

OBn

28

Me

a. TBAF
b. BrCH2COCl, py
c. (MeO)3P
d. iPr2NEt, LiCl

OBn

G
H

Me

a. DIBAL-H
b. DMP
c. (MeO)2P(O)CH2CO2Me,
NaHMDS, [18]crown-6 TBSO Me

(65%)

Me

a. KH, CS2; MeI

b. nBu3SnH, AIBN
c. BH3THF
d. TESOTf
(47%)

Me

24

Me

Me

d. 2,4,6-Cl3C6H2COCl,
Et3N; 4-DMAP
OBn
(62%)

Me

21

Me

a. PPTS, H2O
b. BH3THF; H2O2, NaOH
c. LiOH, MeOH, H2O
O

OBn

G
H

Me

Me

Me
O

O
H

O
H

Me

O
H
H
Me

Me

HO

K
O

I
O

O
H

Me

1: brevetoxin B

Scheme 33. (Continued)

Angew. Chem. Int. Ed. 2000, 39, 44 122

83

REVIEWS

K. C. Nicolaou et al.

a)
CO2Me
O
H
O
N

Me
H
HN

OMe O

OMe

CO2Me

O
OCH3

Diastereoselective
OMe
epoxidation
Friedel-Crafts
1: tri-O-methyl dynemicin A
HO2C
methyl ester
OMe O

OMe
H

Br

OMe

CO2Me

Me

Allylic
3 OMe diazene
rearrangement

OBz

Pd-catalyzed
coupling

Me

O
N

MeO

MeO

O
H

Br

H
N

Stereoselective
imine attack

OH

MeO

Me

Pd-catalyzed
coupling

OMe

Tandem
macrolactonizationDiels-Alder

b)

a. ClCO2Me,
N

[Pd(PPh3)4]

MeO

TBSO

BrMg

MeO

Me

SiR3

OTBS b. TBAF

SnnBu3

Br

[Stille coupling]
(85%)

c. BrCH=CHCO2Me, (12%)
[Pd(PPh3)4]
[Sonogashira coupling]

Me

MeO2C
OBz
Me
H

O
H
N

9
H

a. LiOH
b. 4-DMAP,

H
N

Me

a. KOH
O b. py, MeO
Cl

OBz

(82%)

OMe 4

OMe

Cl

O
S NHNH2
O

H N

NH N

Me

O
O

OMe
O

H
N

[-N2]
H

HH
O

11

(57%)
OMe
a. AgOTf
b. K2CO3,
Me2SO4
[Friedel-Crafts]

H
OMe

OMe

O
OMe

a. MeAlCl2,
Et3SiH
b. SOCl2

CO2Me
O
H
N
O

CO2Me
OH

Br

Me

O
H
N

O
OMe O

Me
CO2Me
OMe

CO2Me

a. TMSOTf
b. DDQ

[Friedel-Crafts]
(51%)

OMe
H
OMe OH

OMe

13

OMe

H
HN

Me

MeO2C

Me
CO2Me
O
OMe
H
OMe O

OMe

OMe O

O O

11 O
a. DMP
b. NaClO2
(ca. 20%) c. LiOH
d. CH2N2
e. NaIO4

CO2Me
O
H
OMe O
N
COCl

OMe 12

OH

OMe

OMe

(82%)

Me
H

(ca. 50%)

10
(ca. 80%)

CO2Me

O
H

Me

Me

O
MeO

O
a. KHMDS,
H
MoOPh O
N
b. NaBH4
O
c. NaOMe
O d. (Cl3CO)2CO

OMe

9
[allylic diazene rearrangement]

CO2Me
O
H
O
N

OH

HO

BzO
O
Me
H

MeO

(33%)
[tandem Yamaguchi
macrolactonizationDiels-Alder]

a. CAN [oxidation of C-9]

b. MeAlCl2; then

O(CH2)3OBz

O
N

COCl
Cl

MeO

Cl

[oxidation at a and
deprotection at b]
a. CAN
b. Cs2CO3, MeI
[reprotection of b]

OMe

1: tri-O-methyl dynemicin A methyl ester

(50%)

a. mCPBA
b. DBU, MeOH

Me
OMe O

H
HN

CO2Me
O
OMe

a
3
OMe OH

OMe

14

Scheme 34. a) Strategic bond disconnections and retrosynthetic analysis of

tri-O-methyl dynemicin A methyl ester and b) total synthesis (Schreiber
et al., 1993).[182]

84

Scheme 35. a) Strategic bond disconnections and retrosynthetic analysis of

dynemicin A and b) total synthesis (Myers et al., 1995).[183]

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

Ring stitching
Carbonylation

Me
H
HN

OH

Me

MOMO
CO2H

O
OMe

OH

MOMO

1: dynemicin

CO2MOM
O

H
OH

H
N

OMe
H

Oxidation

Diels-Alder

Stereoselective I Me3Sn
imine attack
+

Diels-Alder
Imine formation

OH

Me

TMS
O

Me

O O

SnMe3

Me I
OH

[O]

OH
H

6
O

OTBS

4
TBSO Diastereoselective

OTBS

epoxidation

b)

a. K2CO3,

Br

OH

b. NaH,

CHO

EtO

ZnCl2
CHO

O
CO2Et
P
OEt

c. DIBAL-H
d. Swern [O]
(82%)

OMe

OH

(60%)

Me

Me

Me

Ph a. OsO4, NMO

Ph

BrMg

OTBS
O

Ph
Ph

10

Me

OMe

a. NH4OAc, HOAc, (87%)

b. TBSCl, imid.
TIPS

(90%)

[Diels-Alder]

OMe

O O

CAN

CHO

TIPS

Me

Cl

b. Ph2C(OMe)2;
TBSCl, imid.
(80%)

OTBS
OTBS

12

TBSO
OTBS 5

11

Resiniferatoxin (1997)

(80%)
TIPS

TIPS
Me

H
R

a. HCl
O

Me

H
R

Ph b. Swern [O]

c. Ph3P, CBr4
O Ph d. nBuLi
[Corey-Fuchs
OTBS
homologation]
13
(54%)

OTBS

Ph b. HCl

OAc

Ph c. NaH, TBSCl
d. Ac2O, Et3N
(60%)

OAc

14

OTBS

[selective triflation at C-5]

a. Tf2O
b. DMP
c. CrCl2
Me
[triflate
R H
CO2MOM
R H
N
removal]
N
O
O
d. MgBr2,
OMe
Et3N, CO2
H
e. MOMCl
17
f. CH2N2
OTBS
OTBS
(29%)
a.
TBAF
(60%)
2
b. PhI(OAc)2

TMS

TMS
O

R=

Me
OH
5
OH

O
R

a. AgNO3, NIS

16

CO2MOM
O

+
O

OMe
H

MOMO

18

OH
O

b. [Pd(PPh3)4],

OH
H

SnMe3

(74%)

OTBS

Me

[Tamura
homophthalic
O anhydride
protocol]

MOMO

Me

Me3Sn

(78%)

Cl

c. NH3, MeOH
d. mCPBA

LiHMDS

H
N

15

OTBS

a. [Pd(PPh3)4],
morpholine
O
b. NaH,

Me

Me

a. TBAF

R=

H
HN

MOMO

CO2H
O
OMe
H

-[CO2]

MOMO

OH

19

a. PhI(OCOCF3)2, THF
b. air, daylight

O
Me
OH

H
HN

O
H

A structural relative of phorbol ester,[206] resiniferatoxin (1

in Scheme 43; see p. 92)[207] was isolated from the E. resinifere
cactus species and exhibitsunlike phorbol but like capsaicin[208]binding affinity to the vanilloid receptor present in
sensory neurons. Besides its potential in biology and medicine, resiniferatoxin offers opportunities to the synthetic
chemist, among which is the application of new methods of
synthesis to the construction of the molecule. The structure of
resiniferatoxin contains an ABC ring skeleton with two trans
fusions. The C-ring carries five contiguous stereocenters, three
of which bear hydroxyl groups which are engaged in a benzyl
orthoester system. Following their success with phorbol
ester[209] the Wender group at Stanford reported the total
synthesis of resiniferatoxin in 1997.[210] This synthesis
(Scheme 43) brilliantly blends classical synthetic methods
with modern methodological advances in a strategy that
stands as a hallmark to the progression of natural product
synthesis. Highlights include an intramolecular [32] dipolar
cycloaddition reaction between an oxidopyryllium ion and a
terminal olefin to construct the BC framework, and a
transition metal-induced ring closure of an eneyne to form
the cyclopentane system (ring A).

Me
CO2H
OMe

OH

Their potent antitumor properties are due, at least in part, to

their tubulin-binding properties, resembling both eleutherobin and taxol in this regard.[202, 203] While the structural
similarities of the sarcodictyins to eleutherobin are apparent,
the correspondence of these molecules to taxol is not so
obvious.[203] Nevertheless, the excitement generated from
their taxol-like properties, coupled with their scarcity, led to
the launching of programs directed toward their total synthesis.
It is noteworthy that the impetus for the chemical synthesis
of these molecules in the 1990s was provided not only from
their structural novelty, but also from the desire to apply
organic synthesis as an enabling technology for chemical
biology. Thus, the total synthesis of sarcodictyins A and B,
accomplished in these laboratories in 1997,[204] went further
than delivering the natural substances. It was applied,
particularly in its solid-phase version (Scheme 41; see p. 88),
to the construction of combinatorial libraries for the purposes
of biological screening.[203, 205] That complex natural products
such as the sarcodictyins could be synthesized, at least
partially, on a solid phase is testament to the power and
potential of the recent advances in solid-phase chemistry.
Even more telling is the ability of synthetic chemistry at the
turn of the century to deliver combinatorial libraries of
complex natural or designed products such as those synthesized in this program and in the one described above for the
epothilones.

MOMO

MgBr2, Et2O

H
HN

CO2H
O
OMe

(15%)

OH 1: dynemicin

H
MOMO

OH

20

Scheme 36. a) Strategic bond disconnections and retrosynthetic analysis of

dynemicin A and b) total synthesis (Danishefsky et al., 1996).[184]

Angew. Chem. Int. Ed. 2000, 39, 44 122

Brevetoxin A (1998)
Within the polluted red tide waters often resides a more
powerful neurotoxin, and that is brevetoxin A (1 in
Scheme 42; see p. 90). Isolated from the dinoflagellate species
Ptychodiscus brevis Davis (Gymnodium breve Davis), breve85

REVIEWS

K. C. Nicolaou et al.

a)

HO
NH

MeO

Esterification

Spiro tetrahydroisoquinoline
formation
OMe

O
AcO
Me

HO
S H
H

Mannich bisannulation

Me

OH

Curtius rearrangement
1: ecteinascidin 743

Fl =

TBSO
O
OAllyl
O H
HN

Me

N
H

TBSO

Fl

H
NH

NHCO2All

CN

19

MOMO

OH

(87%)

OMOM

a. nBuLi
b. DMF Me

a. CH3SO3H
CHO b. NaH, Me
BnBr

OMOM

(64%)

(86%)

BOPCl;
OH
CO2All

OMe

HO

10

O
CO2All

OMe

11

12

a. piperidine
Me
AcOH, 9
OMe
b. [Pd(PPh3)4]
OMe
Et3N/HCO2H O
(93%)

Me

O
N

15

OMe
OMe

[Curtius
rearrangement]
[-N2]

O
HN

O
O

OBn

16

[Rh(cod)-(R,R)-dipamp}] BF4
Me
H2

[asymmetric
hydrogenation]
(97%, 96% ee)

OH

Me

23

Me

CN

Fl

NHCO2All

OBn

O
AcO

OMe

O
HN

OMe

O
O

Me

S H
O H
Me
H
H

a. 5
b. CF3CO2H
c. AgNO3 - H2O
(high yield)

Me

Me

N
H
O

OH

O
AcO
Me

24

CN

MOM OMe
O

Me

S H
O H
N

Me

N
O

H
O

1: ecteinascidin 743

17

O
S H
H

N
O

OBn

O
AcO

Me

NH MOM OMe

OMe

HO

CN

21

a. nBu3SnH, [PdCl2(Ph3P)2]
(70%)
O , DBU
b.
25

NH

CO2All
H

CO2All

N
MeO

N
N

HO

OH

Me N

OMe

(79%)
[tandem quinodimethane formation,O
deprotection and cyclization]

HO

Tf2O, DMSO; iPr2NEt; tBuOH;

(Me2N)2C=NtBu
[deprotects thiol]; Ac2O Me

OMe

OMe
OMe

20

Me

Me

(PhO)2P(O)N3,
Et3N

OBn

14

Me

O
OMe

OH

MOMO

OMe

13

(93%) BnOH
OBn

CN

HO

a. Tf2NPh, Et3N, 4-DMAP

b. TBDPSCl, 4-DMAP
AllylO
c. MOMBr, iPr2NEt
Me
d. [PdCl2(Ph3P)2], nBu3SnH
e. formalin, NaBH3CN
O
f. Me4Sn, [PdCl2(Ph3P)2],LiCl
O
(55% overall)
HO

OMe
O

CN

Me
H

22
[position-selective
angular hydroxylation]
a. (PhSeO)2O
(68%)
b. TBAF
c. EDCHCl, 4-DMAP, 2

CHO

O
OBn

O
TBDPSO

OBn

[-N2]
OBn

OMe
Me

a. nBuLi,
TMEDA Me
b. MeI

CO2All
H

OMe

NH2

MeO

N
OMOM

N
N

HO

Me

b)

a. DIBAL-H
b. KF2H2O, MeOH
c. CH3SO3H
[Mannich bisannulation]
(55%)

OH

Me

CO2All

OTBS

H
N

AllO

AllylO

OH
Me

HO

OTBS

OMe

OH

OMe

OMe

a. 4 , AcOH, KCN
b. Cs2CO3, allylbromide

(53%)

Me

CN

26

BF3Et2O, H2O
O

OH
O

Me

H
NH

O
O

OBn

a. BF3Et2O, 4 MS
b. H2, Pd/C
(73%)

Me

H
NCO2Bn

O
O

18

Scheme 37. a) Strategic bond disconnections and retrosynthetic analysis of ecteinascidin 743 and b) total synthesis (Corey et al., 1996).[186]

toxin A was structurally elucidated in 1986.[211] With its ten

fused ring structure and its twenty-two stereocenters, brevetoxin A rivals brevetoxin B in complexity, but as a synthetic
target it arguably exceeds the latter in difficulty and challenge
because of the presence of the 9-membered ring. Indeed, with
rings ranging in size from 5- to 9-membered, all sizes in
between included, brevetoxin A can be considered as the
ultimate challenge to the synthetic chemist as far as mediumsized ring construction is concerned. After a ten-year
campaign, our group reported the total synthesis of brevetoxin A (Scheme 42) in 1998.[212] As in the case of brevetoxin B,
this program was rich in new synthetic technologies and
strategies, which emerged as broadly useful spin-offs
(Scheme 42 c). Amongst the most important synthetic technologies developed during this program was the palladiumcatalyzed coupling of cyclic ketene acetal phosphates gen86

erated from lactones with appropriate appendages to afford

cyclic enol ether diene systems[213] suitable for a cycloaddition
reaction with singlet oxygen (24 !26 in Scheme 42). This
method provided the crucial turning point in solving the
problems associated with the 7-, 8-, and 9-membered rings of
the target and opened the gates for the final and victorious
drive to brevetoxin A.
Manzamine A (1998)
Manzamine A (1 in Scheme 44; see p. 93) is a spongederived substance (genera Haliclona and Pellina) with potent
antitumor properties. Disclosed in 1986,[214] the structure of
manzamine A, and those of its subsequently reported relatives,[215] attracted a great deal of attention from synthetic
chemists. The interest in the manzamines as synthetic targets
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

Suzuki coupling

a)
S
HO
O

Me
O

OH

HO

Me

BnO
O

TBSO

Me

OH
O

a. DHP. PPTS
b. TMS Li
BF3Et2O
c. MOMCl, iPr2NEt
d. PPTS, MeOH
e. Swern [O]
f. MeMgBr
g. TPAP, NMO
(36%)

TMS

a. nBuLi,
Ph2(O)P

Me
O
OMOM

a. TiCl4, MeO
Me

b. CSA

Me

OH

6 O

Br
Me

Me
TPSO

(78%)

H
O

a. LiAlH4
b. Et2Zn, CH2I2

Me

Me

(85%)

Me

a. Ph3P=CHOMe
(59%) b. pTsOH
c. Ph3P=CH2
d. PhI(OCOCF3)2, MeOH

(88%)

EtO2C

HO
O
O

OH

CO2H
O

HO

Me

(61% overall)

(+)-Ipc2B(All)

(96%)

NaHMDS; 10

16

17
Me
OTBS

Me

OH

13

O
I PPh3 (>70%)

Me

Me

OH

15

Me

OTBS

14

BnO

CO2H
O

12

a. HFpy
(ca. 95%)
b. Swern [O]

OTBS

LDA

ZnCl2
HO
S
Me
TBSO

N
O

Me

Me

Me

(84%)

OTBS

a. 1,4-butanediol, NaH
b. Ph3P, I2, imid.
c. Ph3P
(>90%)

Cl

a. nBu3SnH, AIBN
b. TPSCl, imid.
MeO
c. HS(CH2)3SH, TiCl4

[Suzuki coupling]
TPSO
O
O
15
(64%)
[Aldol macrocylization] a. KHMDS
(47%)
b. HFpy, py
c. TBSOTf
a. Dess-Martin [O]
S
S
b. HFpy
c. O O
TBSO

13

11

BnO

OMe
OMe
O
3 TPS

a. O3; Ph3P
b. NaClO2

Me

10

TBSO

OH

a. 9-BBN, 4; [PdCl2(dppf)],
Cs2CO3, Ph3As
b. pTsOH

Me

b. O3 ; Me2S
(85%)

Me

a. (+)-Ipc2B(All)
b. TBSOTf
(73%)

Me
Me

a. DIBAL-H
b. Ph3P=C(Me)CHO

Me

MgBr

TBSO

12

BnO

TPSO

14

TBSO

a. TBSCl, imid.
b. NaI, acetone
(98%)

11

Esterification

OTBSO

Me
O

Me
OTMS

OTBS O

NIS, MeOH

Me

Aldol reaction

a. Li2CuCl4 ,

HO

Me

N
O

b)

(34%)

BnO

a. TBSOTf
b. DDQ
c. Swern [O]

TBSO

OH

10

(87%)
[hetero Diels-Alder
reaction]

S
HO

BnO

Me

8
b. NIS, AgNO3
c. Cy2BH, AcOH
I
d. PhSH, BF3Et2O
e. Ac2O, py

3
S

OH

Emmons-type
homologation

OMe
O
TPS

b)

Me
OMe

Me

Me

Me

OH

1: epothilone A

Aldol reaction

Hetero Diels-Alder reaction

Me
O

1: epothilone A

HO

Me
O TBSO

Me

Epoxidation

S
TBSO

Olefin
metathesis/
cyclorelease

a)

Epoxidation

(41%)

1: epothilone A

CO2H

18 O

OH

O
O
TBS

HO

N
O

Me

Cl

PCy3
Ru

PCy3 Ph

HO

[olefin metathesis
reaction]
(52%)

O
O
TBS

Me
O

O
O
TBS

19
TFA
CH2Cl2

4 products [2 from the aldol reaction and

2 from the olefin metathesis reaction, ratio 3:3:1:3]

Scheme 38. a) Strategic bond disconnections and retrosynthetic analysis of

epothilone A and b) total synthesis (Danishefsky et al., 1996).[190]

(91%)

O
S
HO

Angew. Chem. Int. Ed. 2000, 39, 44 122

Cl

Me
O

OTBS

was heightened by a hypothesis put forward by Baldwin et al.

in 1992 for their biosynthesis.[216] By early 1999 two total
syntheses[217, 218] of manzamine A and evidence[219] supporting
the biosynthetic hypothesis had been reported.
Baldwins intriguing hypothesis for the biosynthesis of the
manzamine alkaloids postulates four simple starting materials
and an intramolecular Diels Alder reaction as the key
process to assemble the polycyclic framework (see
Scheme 45). The first total synthesis of manzamine A appeared from the Winkler group in 1998,[217] proceeded through
ircinal (2 in Scheme 44), itself a natural product, and involved

(ca. 80%) DCC, 4-DMAP, 5 [esterification]

Me

(ca. 90%)
(two diastereoisomers)
[Aldol reaction]

N
O

Me
O

OH

O O
Me

CF3

(85%)

HO

N
O

Me

[5:1 mixture of
diastereoisomers]

1: epothilone A

OH

chemistry employed to synthesize

combinatorial libraries

Scheme 39. a) Strategic bond disconnections and retrosynthetic analysis of

epothilone A and b) total synthesis (Nicolaou et al., 1997).[194, 197]

a photoinduced [22] cycloaddition reaction, a Mannich

closure, and an intramolecular N-alkylation to assemble the
polycyclic skeleton. In early 1999 the Baldwin group provid87

REVIEWS

K. C. Nicolaou et al.
Martin et al. in 1999 (Scheme 46; see p. 94)[218] on the other
hand involved an intramolecular Diels Alder reaction and
two olefin metathesis ring closures to construct the pentacyclic framework of the target molecule. All three approaches

a)

R'
O

Me

R'
Me

Me trans-Ketalization

O
Me

Cleavage
O

O
H
Me

OH

R''

Me

OH
H

OH

Me

OTIPS

Me

Resin

Me

Me

OTIPS

7
a. TIPSOTf
b. LiHMDS
c. Dess-Martin [O]
d. Et3N3 HF

OTES
Me
OTES
CHO

H
Me

OH
Me

Me

OH

(65%)
H

OH

Me

Me Loading

b)
H

R'
O

6
+

Me

R'''

Ester or carbonate
formation

Linker

Me

H
Me

Me

Functional group
3
manipulations

Me

R'''

Me

Me

Me

9
(>80%)

H2, [Rh(nbd)(dppb)]BF4
OAc
Me

a. Ac2O, py
b. PPTS, HO
11

Me

H
O

H
Me

Me

I PPh3

14

Me

Me

OAc
Me

Me

OTIPS

15

OH
OTIPS

O
O

Me

H
Me

10

O
H

Me

H
O

a. 1,4-butanediol,
NaH
Cl
13
b. Ph3P, I2,
imid.
Me
c. Ph3P
H
(>90%)

OAc
H

OH

Me
O

NaHMDS
(>95%)

OH

Me

c. Dess-Martin [O]
(85%)

OTIPS

12

Me

OTIPS

OTIPS

15

a. NaOMe
b. LG
c. TBAF

Me

(>81%)

R1
16

O
Me

R1

R1

Me

a. LG

R3

HO

Me

20

R3

19

Me
O

Me

21

R4

R1

d. CSA,

HO
Me

OH

(49-73%)
c. LG
25

or
DCC, 4-DMAP,
H2N

Me

17
a. (PhO)2P(O)N3,
DEAD, Ph3P
b. Ph3P, H2O

a. Dess-Martin [O]
b. NaClO2
c. DEAD, Ph3P,
HO

(42-86%)

R2

Me

18

b. PPTS,
O

H
Me

Me

R2

R5

d. PPTS, HO

R4
R1

22

R3

R3
26

O
Me

23

Me

(46-70%)
O

Scheme 40. a) Strategic bond disconnections and retrosynthetic analysis of

eleutherobin and b) total synthesis (Nicolaou et al., 1997).[199]

Me O

24: X = O, N

ed
evidence for their hypothesis through synthetic studies
culminating in the synthesis, albeit in low yield, of keramaphidin B (Scheme 45; see p. 93). The synthesis reported by
[219]

88

H
Me

R4

R3

H
Me

chemistry employed to synthesize

combinatorial libraries

Me

27

R3

HN

R5

Scheme 41. a) Strategic bond disconnections and retrosynthetic analysis of

a solid-phase sarcodictyin library and b) total synthesis (Nicolaou et al.,
1998).[205]
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

are elegant in their conception and brilliant in their execution,
demonstrating once again the ability of organic synthesis to
swiftly respond to new challenges posed by novel structures
from nature.
Vancomycin (1999)
Vancomycin (1 in Scheme 47 and 48; see p. 95), a representative of the glycopeptide class of antibiotics,[220] was
isolated in the 1950s and used for over four decades as a
weapon of last resort to combat bacterial disease. Isolated[221]
from the actinomycete Amycolatopsis orientalis, vancomycin
finally yielded to structural elucidation in 1982.[222] Within a
few years, it became the subject of synthetic investigations,
primarily as a consequence of its novel molecular architecture, important biological action and medical applications,
and intriguing mechanism of action. As a synthetic target,
vancomycin offered a unique opportunity to synthetic chemists to develop new synthetic technologies and strategies.
Among the most intriguing structural features of the molecule
were its two 16-membered bisaryl ether macrocycles and its
12-membered bisaryl ring system, each of which is associated
with an atropisomerism problem. The attachment of the two
carbohydrate moieties onto the heptapeptide aglycon system
added to the challenge presented by this target molecule.
By 1998 two groups, that of D. A. Evans[223] and ours,[224] had
reported independent total syntheses of the vancomycin
aglycon and by early 1999 the first total synthesis[225, 226] of
vancomycin itself appeared in the literature followed by
another report of the aglycon synthesis by the Boger group.[227]
Emanating from these laboratories, the total synthesis of
vancomycin is summarized in Scheme 48 (see p. 96). During
the vancomycin campaign, a number of new methods and
strategies were designed and developed, among which,
perhaps, the triazene-driven biaryl ether synthesis[228] is the
most prominent. The strategy employed modern asymmetric
reactions for the construction of the required amino acid
building blocks, which were then assembled into appropriate
peptides and cyclized to form the desired framework. While
the two biaryl ether macrocycles were formed by the triazenedriven cyclization, the bisaryl ring framework was assembled
by a sequential Suzuki coupling and a macrolactamization
reaction. Finally, the sugar units were sequentially attached
onto an appropriately protected aglycon derivative, which
afforded a protected vancomycin system in a stereoselective
manner from which free vancomycin was obtained.
The Evans synthesis of vancomycins aglycon, shown in
Scheme 47,[223] featured the stereocontrolled construction of
the amino acid building blocks and assembly to the heptapeptide backbone through a vanadium-mediated CC bond
forming reaction to construct the 12-membered biaryl ring
system and two nucleophilic aromatic substitutions activated
by o-nitro groups to form the two bisaryl ether macrocycles.
The synthesis of vancomycins aglycon[227] by Boger et al.
(Scheme 50; see p. 100) is distinguished by extensive studies
to determine the activation energy required to atropisomerize
each macrocycle, thereby allowing selective atropisomerization of the AB ring system in the presence of the COD
framework. These total syntheses added yet another distinAngew. Chem. Int. Ed. 2000, 39, 44 122

guished chapter to the annals of total synthesis and placed the

glycopeptide antibiotics on the list of conquests of synthetic
organic chemistry.
CP Molecules (1999)
CP-263,114 and CP-225,917 (1 and 2, respectively, in
Scheme 49; see p. 98), isolated from an unidentified fungus
by Pfizer scientists in 1997,[229] inhibit squalene synthase and
ras farnesyl transferase, and as such, represent important new
leads for cholesterol-lowering and anticancer drugs. Nature
molded within these structures an exotic display of delicate
and rare functionalities that beckoned to synthetic chemists
worldwide. The total synthesis of these compounds was finally
accomplished in 1999 in our laboratories after a relentless
campaign through a daunting synthetic labyrinth plagued with
manifold and unexpected obstacles.[230] This total synthesis is
retrosynthetically blueprinted in Scheme 49 a. A critical
disassembly maneuver called upon an intramolecular
Diels Alder reaction to simplify the bicyclic structure 6 of
the CP molecules to the open-chain precursor 7. Although this
retrosynthetic analysis serves as a conceptual overview of the
synthesis, it should be noted that it is actually the culmination
of several unsuccessful retrosyntheses by which the conversion of ketone 6 into the CP molecules was planned.
Commencing with dimethyl malonate (8), the synthesis of
the CP molecules proceeded smoothly through several
intermediates and finally yielded the desired acyclic precursur
7 stereoselectively (Scheme 49 b). When compound 7 was
treated with Me2AlCl in dichloromethane at 20 8C,
complete conversion to 6 through a Lewis acid catalyzed
intramolecular Diels Alder reaction was observed within
two minutes. The formidable task of stereoselectively installing the remote stereocenter at C7 was addressed by utilizing
dithiane chemistry (6 !15). The reason for such a high level
of diastereoselectivity (ca. 11:1) could possibly be a consequence of a shielding effect of the CP skeleton. Indeed, the
surprisingly close proximity of this side chain to the rest of the
molecule was quite apparent throughout the synthesis. The
stage was now set for the installation of the fused maleic
anhydride moiety. The synthesis of this delicate moiety was in
itself a great challenge due to unique environment surrounding ketone 15. The development of novel chemistry to
construct the anhydride was a result of persistence in the
face of several failed strategies.[231] Thus, ketone 15 was
smoothly converted to the enol triflate followed by palladiumcatalyzed carboxymethylation and exchange of the dithiane
for a dimethoxy ketal leading to the unsaturated ester 16.
After reduction with DIBAL-H, a Sharpless hydroxyldirected epoxidation of the allylic alcohol led to epoxide 17
selectively (ca. 10:1). Introduction of this electrophilic species
allowed for the placement of an additional carbon atom with
the correct oxidation state for the ensuing cascade sequence.
This carbon atom, in the form of a cyanide, was added to
epoxide 17 using Et2AlCN and proceeded with complete
regio- and stereospecificity (see 17 b). It was after considerable experimentation that we discovered that it was possible
to convert diol 18 in one synthetic operation. Thus, selective
mesylation of diol 18, followed by treatment with base and
89

REVIEWS
a)

K. C. Nicolaou et al.

Me
H
O

Me
O H

Lactonization

Me
H
OH

Lactonization H O
H

H
O Me

F
HO
H

H
TPSO

OH

HO

OH

TBDPSO

4: D-mannose

Ph

EtS

OBn

Ph

PPh3I

11

SEt

12

TBDPSO

H
H

(74%)

Ph

H
O Me

[acting as
Lewis acid and
nucleophile]

O
O

AlMe3

TBDPSO

Me

OH

HO

12 steps

OH

BnO

(18%)

Me

C
OH

HO

D
O

e. Ac2O, 4-DMAP
f. TFA
(91%)

O
H

22

Me

e. (+)-DET, Ti(iPrO)4
tBuOOH
f. SO3py
g. CH2=PPh3

OBn

I
TBSO

a. TBAF
b. PPTS
c. O3; NaBH4
d. PPTS,
Me2C(OMe)2
e. TBAF
(76%)

TBDPSO

a. SO3py, DMSO
b. MeO2C PPh3

Me

f. K2CO3, MeOH
g. TPAP, NMO
h. EtSH, Zn(OTf)2;

d. H2, Pd(OH)2/C
e. TBSOTf

PPTS
i. SO3py, DMSO

Me

c. H2, Raney Ni (W2)

d. LiAlH4
e. TBDPSCl, imid.
(81%)

a. Zn(OTf)2, EtSH
b. TBSCl
c. Ac2O

MeO2C

BnO

17

EtS

Me
H

TBDPSO

Me
Me

B
(65%)

Me

Me

B
OH

PivO

Me
H
OAc

D
H

23

Me
H H
O

OH
H

a. TPAP, NMO
b. Ph3P=(CH2)3CO2Me

O
H

Me
H

c. [{Ph3PCuH}6] TBDPSO
(65% overall)

H
TBDPSO

B
OH
H

TrO

Me
H H
O

D
H

OH

OH
H

Me

a. CH2=C(OMe)CH3, POCl3
b. Al2O3
H
TBDPSO
c. MsCl
d. Ph2PLi; H2O2
TrO
(78%)

Me

Me
Me

OH

D
H

Me
H
O

Me
O H

O
H

H O
H

OH
H

a. (PhO)2P(O)Cl,
KHMDS
b. nBu3SnCH=CH2
c. O2, hv

Me
Me
H

[reductive TBDPSO
cleavage of
endoperoxide]

OH
H

OH
H

PivO

a. nBuLi, 3

PPh2

Me
H
O

O
O
H

O
H

25

OMe

(49%)

b. KH, DMF TBDPSO

c. AcOH-THF

Me
O H

B
O
H
H

TrO

H O
H

Me
H
OH

F
HO
H

1: brevetoxin A

H
O Me

G
H

I
H

H
TBDPSO
O
MeO

O
H

H O
H

Me
H
OH

D
E

F
HO
H

30

OH
EtS

29

Me
O H

OH

J
H

EtS

Me

c. CH2=N(Me)2I
[Eschenmoser's salt]
(52%)

Me
H
OH

24

D
O

19

Me

H
TBDPSO

OH

(69%)

SnMe3

26

Me
H H
O

a. HFpy
b. Dess-Martin [O]

(94%)
Me

a. KHMDS, (PhO)2P(O)Cl
b. Me3SnSnMe3, [Pd(Ph3P)4]

a. H2, [RhCl(Ph3P)3]
b. H2, 10% Pd/C

18

Al(Hg)

O
Me

28

OH

Me
H
O

PivO

Me
O

OTBS

Me

O
H

OH
H

TBSO
H

HO

Me

D
H

20

Me

a. TBSCl, imid.
b. TPAP, NMO

27

PivO

G
H

Me

c. LiOH
OH d. 2,4,6-Cl C H COCl
3 6 2
PivO
[Yamaguchi lactonization]

Me

Me3Sn

H
TBDPSO

OH

(70%)
Me

(70%)

OBn

TBSO

Me

Me

(70%)

a. DIBAL-H
b. TrCl4-DMAP
c. TPAP, NMO;
DIBAL-H

OHC
H
EtS
O Me

c. 2,4,6-Cl3C6H2COCl
d. HFpy
e. [PhC(CF3)2O]2SPh2

OBn CO2Me

nBuLi; Cu-CCnPr
TfOCH2CH2OBn

OBn

a. LiOH
b. Li, NH3 (l)

OTBS

D
H

TBDPSO

21

a. PivCl, 4-DMAP
c. CSA, MeOH
d. TrCl4-DMAP

Me
H
O

(48%)

Me

Me

C
OH

BnO

Me
Me

OTBDPS

TBDPSO

c. IZn(CH2)2CO2Me,
[Pd(PPh3)4]
(68%)

Me

14

OBn CO2Me b. NaHMDS, Tf2NPh

(65%)

OTBS

OBn

a. Hg(OAc)2;
Li2[PdCl4], CuCl2,O2

OTBS

16

a. hexylborane;
H2O2, NaOH
b. TBDPSCl, imid.
c. H2, Pd/C
OMe
d. PPTS,
OMe

Me

15: D-glucose

H
TBDPSO

(94%)

OH

Epoxide opening

TBDPSO

13

TBDPSO

O 2
OBn
H
O SEt H O H

a. PPTS, MeOH
OBn
b. TBSCl, imid.
Me H
H
H
O
O
c. TPAP, NMO
Me
I
H
J
d. EtSH, BF3Et2O
O
O
O
e. SO3py, DMSO
H
H
H
(73%)
9

OBn

10

a. TBAF
Ph
b. AgClO4, NaHCO3
c. mCPBA
[hydroxy dithioketal
cyclization]

EtS
EtS

(55%)

TBDPSO

HO

EtS

nBuLi
(88%)

OMe

H
Me
O

a. 9-BBN; H2O2
b. SO3py, DMSO
c. MeO C PPh
2
3
d. DIBAL-H

OTBS

TBS
SEt

TBSO

(86%)

H
O

OHC

OBn

[6-endo hydroxy
epoxide cyclization]

a. TBAF; TPSCl, imid.

b. TBSOTf

(17%)

OH

Epoxide opening

PPh2

bis-Lactonization

OBn
TESO

15 steps

OH
OH

Me
H
O

1: brevetoxin A

b)

Me

TrO

Me

Horner-Wittig coupling

Dithioketal cyclization

HO

Dithioketal cyclization

Conjugate addition

H
O Me

G
H

I
H

OTBS

H
O Me

a. AgClO4, NaHCO3
b. mCPBA
c. BF3Et2O, Et3SiH
d. Dess-Martin [O]
e. NaClO2
f. CH2N2

OTBS

H
H

OTBDPS

TBDPSO

(48%)

Scheme 42. a) Strategic bond disconnections and retrosynthetic analysis of brevetoxin A, b) total synthesis (Nicolaou et al., 1998),[212] and c) key synthetic
methodologies developed in the course of the total synthesis (Nicolaou et al., 1998).[212]

90

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

c) Cyclic ketene acetal phosphates for the construction of medium and large cyclic ethers
O

Ph

H
O

O
H

Ph

H
O

O
H

(75%)
H
O

O
H

P(OPh)2
O

SnMe3

H
O

tri-n-butyl(2-ethoxyvinyl)tin
[Pd(PPh3)4], LiCl
(81%)

tri-n-butyl(1-ethoxyvinyl)tin
[Pd(PPh3)4], LiCl
(84%)
Ph O
H OEt
O
O
H

Ph

nBu3Sn
Ph O
[Pd(PPh3)4], LiCl
O
H
(96%)

Ph

hexamethylditin
[Pd(PPh3)4], LiCl

O
OTBS

(90%)

Ph

Ph

KHMDS
(PhO)2POCl

H
O

O
H

Me3Sn

[Pd(PPh3)4], LiCl (89%)

O
OTBS

Ph

H
O

O
H

OEt

Enol-phosphates and thioketal-mediated etherification for the

construction of the EFGH ring skeleton of brevetoxin A
H
O

O
Ph

O a. KHMDS,
(PhO)2P(O)Cl

O
H

Ph

H
O

O
Ph

b. nBu3SnCH=CH2,
[Pd(PPh3)4]

H
OH

O
H

b. H2,
Lindlar's
cat.

O
O
H

H
OH

a.

H
OH

F
HO
H

G
H

OH

H
O

O
a. O2, TPP Ph

O
H

H
HO

AgClO4
Ph

H
O

H H
O

b. Ph3SnH, AIBN

O
H

H EtS
OH

EtS

Novel stereocontrolled synthesis of the nonacene ring system

of brevetoxin A. Conformational-reactivity effects in nine-membered rings

S
O

1,4-diiodobutane
THF,

S O
O

O
O
Li

O
H

H
O

O
H

Synthesis of N-heterocycles via lactam-derived ketene aminal phosphates.

Asymmetric synthesis of cyclic amino acids

N
Boc
N
Boc

a. Me3SiCH2MgCl
Ni0
h. PhZnBr
0
Pd
g. Et3Al, Pd0

N H
Boc
f. nBu3Sn
Pd0

N
Boc

N
R

SiMe3
N CO Me
2
CO2Ph
Pd0

b. CO,
MeOH

O
O P(OPh)2

Pd0

N
Boc

d. Me3SnSnMe3
Pd0

e. Me3Si
CuI, Pd0

N SnMe
3
Boc

N
Boc
SiMe3

Scheme 42. (Continued)

c. nBu3Sn

O
H

finally acidic workup afforded the maleic anhydride 5. The

course of this dizzying domino sequence undoubtedly
involves formation of the unprecedented carbocyclic imino
butenolide 21 (a proven intermediate and new chemical
entity isolated for the first time) whereupon facile tautomerization to the electron rich and easily oxidizable 2-aminofuran 22 occurs.
Stepwise oxidation of the furan 22 followed by nitrogen
extrusion leads to anhydride 5. The remarkable efficiency
with which this reaction takes place (56 % overall yield,
seven transformations in one operation) is a testament to
the utility of tandem reactions in organic synthesis. After
some brief protecting group manipulations, the stage was set
for the aplication of another cascade reaction. It was found
that treatment of 23 with Dess-Martin-periodinane in
refluxing benzene led to the desired g-hydroxy lactonol in
63 % yield. This tandem reaction was based on the simple
ring chain tautomerization of hydroxy ketones and permitted the crucial oxidation to take place.[232]
The next key step involved the one-carbon elongation of
intermediate 4 by the classic Arndt Eistert reaction.
Because of the extreme steric hindrance of the carboxylic
acid derived from 4, a new method specifically tailored for
the preparation of hindered a-diazoketones was developed.[233] This new synthetic technology was based on the
expected extreme reactivity of the acyl mesylate species. In
the event, acyl mesylate 25 successfully activated the
hindered carboxylic acid for attack by diazomethane thus
leading to the requisite a-diazoketone for the ensuing Wolff
rearrangement. The final stage in the synthesis required
conversion of indole 27 into the CP molecules. Although the
conversion of 2 into 1 was known, the counterintuitive
conversion of the seemingly robust 1 into its hydrated
counterpart 2 appeared unlikely. We reasoned that LiOH
might be useful for effecting this conversion by virtue of its
unique solubility and reactivity. Not only did this LiOHmediated cascade reaction succeed in hydrolyzing the indole
amide of 27 to the corresponding carboxylic acid, it was also
able to induce ring opening of the stable pyran motif to
provide 2 directly. The conversion of 2 into 1 using acid
catalysis proceeded smoothly, thus completing the total
synthesis of the CP molecules.
In summary, the first total syntheses of these (racemic)
compounds was accompanied by a plethora of fundamental
discoveries, cascade reactions, and new synthetic technologies among which the following are, perhaps, most notable
(see Scheme 49 c, d): 1) the design and execution of a
cascade reaction involving no less than seven steps traversing through previously unknown chemical entities to construct the fused maleic anhydride moiety;[231] 2) the enlistment of another tandem sequence predicated on the ring
chain tautomerization of hydroxy ketones to sculpt the ghydroxy lactone moiety onto the bicyclic skeleton;[232]
3) development of a mild and effective method for the
construction of extremely hindered diazoketones using acyl
mesylates (Scheme 49 d, top);[233] 4) a new paradigm for the
two-step construction of strikingly complex natural-productlike heterocycles from commercially available chemicals (Scheme 49 d, middle);[234] 5) a new method for the
Angew. Chem. Int. Ed. 2000, 39, 44 122

91

REVIEWS

K. C. Nicolaou et al.

Scheme 43. a) Strategic bond disconnections and retrosynthetic analysis of resiniferatoxin and b) total synthesis (Wender et al., 1997).[210]

one-carbon
homologation
of
hindered
aldehydes
(Scheme 49 d, bottom);[235] and 6) the daring and counterintuitive conversion of the structurally robust CP molecule 1
into its hydrated CP derivative 2 passing through a multiplycharged intermediate in yet another cascade sequence.[230] The
total synthesis of the CP molecules stands as an instructive
example of how total synthesis can act as a driving force for
the discovery and development of new concepts and methods
in chemistry.
Aspidophytine (1999)
For over 25 years aspidophytine (1 in Scheme 51; see p. 101)
has remained an unanswered challenge for organic synthesis.
Best known for its use as an anticockroach/insecticidal
92

powderat least since the Aztec era in parts of Mexico and

Central America[236]its complex structure was not elucidated until 1973 by the groups of M. P. Cava, P. Yates, and D. E.
Zacharias.[237] The first total (enantioselective) synthesis of
this molecule was finally completed in 1999 by E. J. Corey and
co-workers and featured a rapid assembly of the aspidophytine core via a novel cascade sequence.[238] The hallmark of the
synthesis is the tandem sequence uniting dialdehyde 3 and
indole 2 in a one flask tandem operation. Also notable is the
conversion of acid 9 into lactone 11 by attack of the iminium
species 10. It is impressive that all four stereocenters (three
quatenary) of 1 are derived from one chiral center secured
early in the synthesis using the CBS reduction (see p. 58).
Aside from developing a breathtaking new domino sequence
to assemble the aspidophytine skeleton, this work raises the
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

a)
Pictet-Spengler cyclization

NH
N

Photoaddition

SN2 Displacement

CHO

N
H
H

OH

Mannich closure
H

1: manzamine A

N
H
H

redox exchange

CHO

H
N
N

HN

HN
N

OH

H H
N
N
Boc

N
H

b)
H H
N

1: manzamin B

NH2

(99%)
BocN

[Michael addition]

N
Boc

N
HO

OH

NH3

HO

HO

BocN

HO

b)

a. pTsOH,
I

Ph3P

b. KHMDS,

CO2Me

BocN

(20% overall)

a. TBSCl
b. LHMDS,
MeOCOCN
c. NaBH4
d. MsCl
e. DBU, C6H6
(62%)

H
HO

10

CO2Me

CO2Me

OH

14

a. TBAF
b. pTsCl
H
BocN
c. TFA
d. iPr2NEt
H
e. Lindlar's cat.
(76%)

OH

BocN

CO2Me

BocN

N
H

N
H

OH

TFA, 4
N

OH

DDQ
(50%)

[Pictet-Spengler]
(58%)
H

15

OH

16
1: manzamine A

Scheme 44. a) Strategic bond disconnections and retrosynthetic analysis of

manzamine A and b) total synthesis (Winkler et al., 1998).[217]

standards for the concise synthesis of extremely complex

alkaloids from simple starting materials.
Sanglifehrin A (1999)
Sanglifehrin A (Scheme 53; see p. 104) was originally isolated by a team of Novartis scientists from an actinomycete
Angew. Chem. Int. Ed. 2000, 39, 44 122

H
(0.2-0.3%)
N

N
H
H

a. DIBAL-H
b. Dess-Martin [O] (75%)

[Diels-Alder]

12
TBSO

HN

12

13

CHO

MeOH/
pH 7.3

DBU,
C 6H 6

b. Tf2O
(98%)

6
11

a. mCPBA
b. NaOMe
(69%)

(50%)

a. mCPBA
N

TBSO

TBSO

a. HCl
b. pTsCl
c. NaI
d. NaBH4

10
(77%)

BocN
N

11

CHO
N

OTHP

OH

py, AcOH

HO

O NH3

BocN

BocN

4: keramaphidin B

4: keramaphidin B

Scheme 45. a) Strategic bond disconnections and retrosynthetic analysis of

manzamine B and b) biomimetic total synthesis of keramaphidin B (Baldwin et al., 1998).[219]

strain found in a soil sample collected in Malawi.[239] This

molecule was found to display a very strong affinity for
cyclophillin A (20-fold higher than cyclosporin A) and significant immunosuppressive activity (10-fold lower than
cyclosporin A). Its mode of action seems to differ from other
cyclophillin binders such as cyclosporin A and thus it raises a
high interest for the understanding of immunosuppression
mechanisms.
Sanglifehrins chimeric structure is formed by a unique
[5.5]-spirolactam fragment, linked to a 22-membered macrolactone ring that contains two unusual amino acid residues
(piperazic acid and meta-tyrosine) as well as l-valine. Its
unprecedented molecular features as well as its novel biological properties made sanglifehrin A a prime target for total
synthesis. The first total synthesis of sanglifehrin A was
93

REVIEWS

K. C. Nicolaou et al.
Everninomicin 13,384-1 (1999)

a)
PictetSpengler
cyclization

N
H

N
N

Ring-closing
olefin metathesis

CO2Me

Tandem Stille/
Diels-Alder
reaction

OH
NBoc

Organolithium
addition

NH2

Wittig
2

1: manzamine A

N
H

Br
CO2Me

CO2Me

Br
O
NBoc

NBoc

NH

OTBDPS

OTPS

OTBDPS
OTBDPS

b)
NBoc
OTBDPS

Br

CO2 Na

a. LHMDS, CO2

(COCl)2;

O b. NaBH4

c. Na2CO3
(95%) TBDPSO

OH
N
Boc

CO2Me
N

CO2Me

NBoc

Br

5
+ Et3N
TBDPSO

OTBDPS

(79%)

SnnBu3

(68%) [Pd(Ph3P)4],
toluene,
MeO

OMe

H
H

O
N

a. DIBAL-H
b. Dess-Martin [O]
c. HC(OMe)3,H+ N
d.
Li
[stereocontrolled
alkyl lithium addition]
(26%)

[tandem StilleDiels-Alder]

CO2Me [allylic oxidation]

a. CrO3
b. HCl
H
c. Swern [O]
O
d. PPh3=CH2
NBoc
[double Wittig]
(30%)
2

Cl

Ru
PCy3 Ph

MeO

[olefin metathesis
reaction]

NBoc
OTBDPS
OTBDPS

8
(67%)

MeO

OMe

OMe

[olefin metathesis
reaction]
a.
Cl

a. KOH, MeOH
b. Et3N,

10

OH

O
Cl

CO2Me
H

PCy3

Cl

OTBDPS

NH

(75%)
11

Cl

N
H

PCy3
Ru
PCy3 Ph

b. HCl
c. DIBAL-H
d. Dess-Martin [O] H
e. TFA,H

OH

NH2

1: manzamine A

f. DDQ
(ca. 5% overall)

Scheme 46. a) Strategic bond disconnections and retrosynthetic analysis of

manzamine A and b) total synthesis (Martin et al., 1999).[218]

achieved in our laboratories in 1999.[240] As indicated in

Scheme 53, the two main domains of the molecule were
assembled by an intermolecular Stille coupling. The construction of the sensitive iodomacrocycle fragment was
performed by an esterification, two peptide couplings, and
eventually a regioselective intramolecular Stille coupling. The
synthesis of the spirolactam moiety involved the use of
Paterson aldol reactions to establish the first five stereocenters, while the spirolactam fragment was formed by
intramolecular cyclization of a suitable 9-hydroxy-5-ketoamide precursor. The developed chemistry demonstrated
once again the power of the Stille coupling reaction in the
synthesis of complex molecules and opened the way for the
construction of possible libraries for biological screening
purposes.
94

Everninomicin 13,384-1 (Ziracin; 1 in Scheme 52; see

p. 102),[241] a member of the orthosomicin class of antibiotics[242] and currently in clinical trials, is a promising new
weapon against drug-resistant bacteria including methicillinresistant Staphylococci and vancomycin-resistant Streptococci and Enterococci.[243] Isolated from Micromonospora carbonacea var africana (found in a soil sample collected from the
banks of the Nyiro River in Kenya), everninomicin 13,384-1
possesses a novel oligosaccharide structure containing two
sensitive orthoester moieties, a 1,1'-disaccharide bridge, a
nitrosugar, several b-mannoside bonds, and terminates with
two highly substituted aromatic esters.[244]
As a consequence of its unusual connectivity and polyfunctional and sensitive nature, everninomicin 13,384-1 constituted a formidable challenge to organic synthesis. After several
generations of glycosylation and protecting group strategies
had been explored and new synthetic methodologies were
developed, the total synthesis of everninomicin 13,384-1 was
finally completed in our laboratories in 1999.[245] Highlights of
this synthesis include: 1) the tin acetal-based stereocontrolled
formation of the 1,1'-disaccharide linkage;[246] 2) the 1,2migration of selenophenyl groups leading to selective orthoester formation based upon a modification of Sinay's
method;[247] and 3) the stereocontrolled formation of eight
unique glycoside bonds using a variety of techniques including
sulfur-, selenium-, and ester-based neighboring group participation.
Additional examples of natural products synthesized in the
twentieth century are given in Figures 5 8 (see pp. 105
108),[350458] but even this listing does not do justice to the
science of those whose brilliant contributions are not mentioned here as a result of the limited space available and
unintentional oversight. For a more complete picture, the
reader should consult the primary literature.

4. What Have We Learned from a Century of

Organic and Natural Product Synthesis?
During the twentieth century, we have come, through the
electronic theory and understanding of the nature of the
chemical bond and mechanistic insights, to adopt the arrow to
designate bond making and bond breaking. During this
revolutionary period for organic synthesis, we have also come
to understand and use conformational analysis, and to use
pericyclic reactions, anions and cations, as well as carbenes
and radicals in controlled ways to form and break chemical
bonds. The Woodward and Hoffmann rules brought understanding and generalization to pericyclic reactions such as the
Diels Alder reaction, the photoinduced [22] cycloadditions, and the various 1,3-dipolar cycloaddition reactions.
We have discovered new continents of chemistry and an
amazing number of synthetic reactions based on heteroatoms
and organometallic reagents and catalysts. Among the former
are the chemistries of nitrogen, phosphorous, boron, sulfur,
and silicon. Organometallic chemistry came a long way from
the sodium and Grignard reagents to cuprates and titanium
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

a)

SNAr-driven ring closure

OH

HO

OH

H
N
O
O

H
N

H
NH

NHMe

N
H

HO

MeO

OAll
OH

HO
O

Cl

H
N

H
N

NH2

H
N

HO

Addition of oxygen

MeO

Atropisomerization
NO2
F
HO

NH

OMs

Cl
H
N

O
O

NO2

OAll
HO

NHBoc
O

N
H

21
OPiv

a. AlBr3; then EtSH

OMs b. MeOH, 55 C

[atropisomerization and
transoid to cisoid
isomerization]
[>95:5 ratio of
NHTfa
atropisomers]

Cl
H
N

O
N
H

O
NH

(P)-23
NO2

N
Br
Bn

NO2

a. nBu2BOTf, Et3N

b. TMGA [N3 source]

c. LiOOH
(50%)

HO
O
HO

KHMDS;
then TsN3

N3

(78%, 80% de)

Bn

Bn

MeO

OMe

MeO

13

N
NCS

NO2

Sn(OTf)2

MeO
F

NO2

Bn

14

25

OAll

HO

HOAt, EDC
(86%)

NH2

TMSEO

O
O

HO

O
NHDdm

16

a. EDC,
HOBt
b. Ph3P-H2O

Boc

15 O

(53%)

NO2

N
H

OBn
OBn

Boc
N
Me

O
O

O
H

H
NH

N
H

NHDdm
OH

OBn

11

TFA

Cl

a. EDC,
HOBt
b. Li2CO3
c. TFA
15

a. N2O4
HO
b. LiOOH
H
c. [Pd(PPh3)4], morpholine
N
d. Pd/C, 1,4-cyclohexadiene
e. TFA
O
H

OH
O

OMe

18
d. EDC,HOBt
e. TFA; then
TFAA
(54%, 6 steps)

MeHN

O
NH

N
H

NHTfa
OBn

NH

OMe

Cl
OH

Cl
O
H

H
N
O

N
H

H
N
O

N
H

NHMe

(24%)
HO
HO

MeO

27
NO2

NMe

N
H

OBn
OBn

BnO

(62%)
Boc

H
N

NHDdm

NHBoc

26

OH
O

H
N

MeHN

3
HO2C

Boc
NMe

a. CsF [ca. 5:1 ratio of atropisomers]

b. Zn, AcOH
c. HBF4, tBuONO, CuCl, CuCl2

Cl

Cl
H
N

O
N
H

NHDdm

OAll

HO
OH
O

H
N

HO

a. HOBt, EDC
b. TBAF
(72%)

N
H

H
N

BnO

17

H
NH

O
N

NMe

OH
O

H
N

MeHN

(46%)

Boc

HO

NO2
F

Cl
O

H
N

c)

OH

HN

12

OBn
OBn

NO2

Cl

a. Boc2O; then
HCO2H, H2O2
b. LiOOH

Bn

OMe

F
Cl

c. AllBr, Cs2CO3
d. LDA
e. LiOH

11

a. BnBr, Cs2CO3 (M)-24

b. LiSEt

NH2

(65%)

BnO

10
Cl

H
N

MeHN
NHBoc

MeHN

(91%)

OMe

a. H2, Pd/C;
then Boc2O
b. LiOH
c. MeNH2,
EDC, HOBt

OH
OH

HO

O HN

MeHN

Cl

N3

H
N

OH

NHTfa
O

O HN

OH

OMs

H
N

OAll
O

OPiv

Cl

H
N

OMe
OMe

MeO

(54%)

O
HO

Oxidative biaryl coupling

OMe
OMe

MeO

OMe

b)

O
OH

5
OMe

NHBoc

N
H

MeHN

a. Zn, HOAc
b. NaNO2, H3PO4,Cu2O
c. [PdCl2(dppf)]CH2Cl2,
Et3N, HCO2H
HO
d. PivCl
e. TFA; then TFAA O
(68%)

OAllyl
OMs

HO

Cl
H
N

O
NH

OTf

OBn

HO

[ca. 5:1 ratio of

atropisomers] O
NHBoc
(79%)
O

OMe
OMe

22

Na2CO3;
then PhNTf2

MeHN

MeO

OMe
OMe

NHTfa

N
H

NH

Peptide bond formation

4
MeO

MeHN

OH

MeHN

OH
O

Cl

H N
H

Cl

MeHN

NO2
OMs

H
N

(55%)

OAll

NO2
O

NH

OBn functionality for oxidative

biaryl coupling
OBn

BnO

NHBoc

O
O

OMe
OMe

19

a. NaHCO3
b. 20, HOAt, HATU, collidine
c. HFpy

HO

O
NHDdm

20

Boc
N
Me

N
H

O HN
MeHN

OH
O

OH

MeHN

MeO

NHTfa

N
H

NH

OMs

HO2C

O
O

OMe

OMe

NO2

OH
Cl

[95:5 ratio of
atropisomers]
(65%)

OBn

OAllyl
TBSO

SNAr-driven
ring closure

NHTfa

N
H

NH

MeHN

NH2 Peptide bond formation

OH
1:
vancomycin
aglycon
OH

HO

VOF3, BF3Et2O,
AgBF4, TFA;
then NaHB(OAc)3

OH
O

Cl

H
N

N
H

Cl
O

NO2

NO2

Cl

NH2
OH
1: vancomycin aglycon
OH

OMe

Scheme 47. a) Strategic bond disconnections and retrosynthetic analysis of vancomycin aglycon, b) key methodology for unnatural amino acid synthesis, and
c) total synthesis (Evans et al., 1998).[223]

Angew. Chem. Int. Ed. 2000, 39, 44 122

95

REVIEWS

K. C. Nicolaou et al.

a)

HO
HO

HO
H2N

MeO

Glycosidation

Triazene

H
NH

N
H

Cl

TBSO
O

EtO2C

NHBoc

OC(NH)CCl3

BnO

BnO

Triazene-driven ring closure

Cl

TBSO

Macrolactamization

NH

H
N

OMe

N
H

HO

OMe
OMe

HO2C

Peptide bond formation

OH

nBuLi,
B(OMe)3;
then HCl

7
NHBoc

a. TMSCl, MeOH

a. MeI, K2CO3

b. Boc2O, K2CO3

b. I2, CF3CO2Ag
(84%)

NHCbz
EtO2C

NH2

Br

Br

N
Br

Br

NHDdm

Br

18

NHDdm
OMe
OMe

(53%)
19

OH

21
Boc
NMe

a. EDC,
HOBt
b. LiOH
(92%)

NH2

EtO

Cl
HO

HO

O
O

c. EDC, HOAt
HO
O

DdmHN

H
N

N
O

Boc

OH
O

Cl
H
N

O
O

TBS

23

d. TBSOTf
e. H2, Pd(OH)2/C
f. SO2Cl2
g. LiOH
(46%)

NH2
O 21

NHDdm

Cl
O

ONos

O
MeO
O

OH

OH

O 20

c.

[atropisomerization]

(M,M,P)-4: R1=H, R2=Cl

c. H2, Pd/C
a. NaI, I2, TMSCl
d. MeI, Cs2CO3
b. MeMgBr, iPrMgBr; then
e. Dess-Martin [O]; KMnO4
B(OMe)3; then H2O2
f. AlBr3
(34%)
(21-35%)

BnO

a. NaN3
b. SnCl2
(81%)

EtO

CO2Et

(P,M,P)-4: R1=Cl, R2=H

a. AD (95% ee)
b. TBSCl, imid.
c. DPPA, DEAD,
Ph3P
(66%)

Boc
NMe

N
H

BnO
MeO

N3

N
H

OTBS
O

H
N

16

Br

17

BnO

a. AD- (92% ee)

b. NosCl, Et3N

BnO

H
N

TBSO

NHBoc

HO2C

R1
O

Cl
O

H
NH

R2
O

TBSO

N
Br

NMe

N
H

OMe
30
OMe

MeO

N
Br

(52%)

22

H
N

a. Ph3P, H2O
b. Boc2O, Et3N
c. TBAF
d. TEMPO, NaOCl

H
N

24
EDC, HOAt
(86%)

Boc

15
N

HO

HO

14

OTBS
O

N
H

CuBrSMe2, py, K2CO3

[3:1 mixture of
(P,M,P):(M,M,P)]
(74%)

EtO2C 13

(61%)
OMe

OMe
OMe

29

Cl

NH2

a. NaNO2, HCl;
pyrrolidine, KOH
b. PCC
c. PPh3=CH2

(93%)
O

OH

TBSO

12

NH2

a. Br2, AcOH
b. LiAlH4

c. SO2Cl2
(76%)

HO

NH2
O

Br HO

H
N

10
Cl

a. TBSOTf
b. H2, Pd(OH)2/C

11

H
NH

BnO

OMe

, KOH

H
NH

OBn

c. AA
(41%, 87% ee)

H
N
O

9
a. BnBr, K2CO3
O
O
b.

Cl

NHBoc

OH

OH

OEt

TBSO

MeO

H
N

MeO

OMe
O

(93%)

OMe
N

MeO

OMe

MeO

BnO
MeO

B(OH)

(EtO)2

H
N

OMe

BnO

OH

a. FDPP
b. TBSOTf
c. TMSOTf
(70%)

NHBoc

Cl

(M,P)-28

NH2

(55%)

MeO

NH2

TBSO

N
Br

Br

BnO

HO

Suzuki biaryl coupling

H
N

N
H

26

N
N

Cl
O

NHDdm

(75%)

(68%)
O

a. PPh3=CH2
b. AD- (96% ee) BnO
c. nBu2SnO;
then BnBr

MeO

Boc
NMe

MeO

b. TBAF
c. Et3P-H2O
d. LiOH

[1:1 mixture of
atropisomers]
(67%)

MeO

H
N

N
H

BnO

OTBS
O

Cl
H
N

b)

a. CuBrSMe2,
py, K2CO3

27

MeO

OMe
OMe

OMe
OMe

Triazene-driven ring closure

NH2

N
H
OH

N3

AllocO

EtO2C

(85%)

AcO
AcO

NHCbz

18 , EDC, HOAt

H
N

N
H

AcO

OH
Br

TBSO

OTBS

(78%)
Cl

OH
Br

13

a. EDC, HOAt
b. TMSOTf

10

NH2
OH
1: vancomycin
OH

HO

NH2

(P)-25

OMe

HO

EtO2C
OMe
OMe

I
NHMe

N
H

N3

b. (PhO)2P(O)N3,
DEAD, Ph3P (95%)
MeO
c. LiOH (99%)

OH
O

H
N

OH
TBSO

BnO

Cl
H
N

MeO

Cl

O
HO

NHBoc

Cl
NHBoc

HO

a. [Pd(Ph3P)4],
Na2CO3
(56% plus 28%
(M)-atropisomer)

OMe

O 7

O
O

O
B(OH)

OH
O

H
N

BnO

H
NH

O
H

HO

H
N
O

N
H

NHMe

HO

NMe

H
N

OH
OH

NH2

26: vancomycin aglycon

24

Scheme 48. a) Strategic bond disconnections and retrosynthetic analysis of vancomycin aglycon, b) key methodology for unnatural amino acid synthesis, and
c) total synthesis of vancomycin (1) (Nicolaou et al., 1999).[224226]

96

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REVIEWS

Natural Products Synthesis

c)

OH

HO
O

H
N
O
O

Cl
H
N

H
NH

Cl

N
H

NH2
OH
26: vancomycin aglycon
OH

HO

c. CbzCl
(36%)
d. Al2O3KF

a. TBSOTf
b. CH2N2

OH

TBSO

Cl

OTBS

Cl
H
N

O
H

H
NH

H
N

N
H

MeO

Cbz
NMe

N
H

NH2
OTBS 27
OTBS
a. BF3Et2O

TBSO

b. nBu3SnH, [Pd(PPh3)4]

AcO
AcO

(70%)

AllocO

H
N

TBSO
O

NHMe

N
H

HO

O
O

OH
O

H
N

OC(NH)CCl3

AcO
AcO
OTBS
HO

O
O
O

O
TBSO

OTBS

Cl
H
N

O
O

O
H

H
NH

H
N

H
N

N
H

Cbz
NMe

N
H

MeO
F

Cl

NH2

OTBS 28
OTBS

TBSO

(84%)

BF3Et2O

O
NHCbz
AcO

AcO
AcO

AcO
CbzHN

OTBS
O
O

O
O

O
TBSO

OTBS

Cl
H
N

O
O

Cl

O
H

H
NH

H
N

N
H

H
N
O

N
H

Cbz
NMe

MeO
OTBS
OTBS

TBSO

NH2

29

a. HFpy c. Raney N:
b. K2CO3 d. LiOH
(65%)

HO
HO

HO
H2N

OH
O
O

O
O

HO
H
N
O
O

H
NH

HO
HO

Cl
H
N

N
H

Cl

O
NH2
OH
1: vancomycin
OH

Angew. Chem. Int. Ed. 2000, 39, 44 122

OH
O

H
N
O

N
H

NHMe

reagents. Of particular importance are the recent advances

made in catalysis using transition metals both for the
formation of carbon carbon bonds and for asymmetric
synthesis. The retrosynthetic analysis principles introduced
by Corey revolutionized strategy design in total synthesis,
while the many metal-catalyzed processes discovered during
the second half of the century facilitated the construction of
complex molecules in impressive ways. Most prominent
among these catalytic processes are the various palladiumcatalyzed reactions for carbon carbon bond formation[248]
and the olefin metathesis reaction[249] made synthetically
useful by the Grubbs[250] (ruthenium) and Schrock[251] (molybdenum) catalysts. During this period, we have also
witnessed the introduction of enzymes[252] as important tools
for organic synthesis and of catalytic antibodies[253] as
promising and useful reagents for synthetic and mechanistic
studies, and the application of genetic engineering to total
synthesis.[254]
In terms of stereochemical control, a journey through the
twentieth century reveals the dramatic progress from stereochemically random reactions to stereocontrolled processes,
first carried out in a reliable manner on cyclic templates and
later on acyclic systems. Acyclic stereoselection, both via
internal chiral auxiliaries and external catalysts, brought us
not only to diastereoselective processes, but also to asymmetric synthesis. Particularly impressive have been the
advances in asymmetric catalysis by which many building
blocks and final targets can nowadays be synthesized at will.
Classical optical resolution methods that characterized the
early total syntheses are being replaced by stereoselective
processes delivering only the desired enantiomer in high
enantiomeric excesses. Such processes include the Hajos
Wiechert cycloaldol/dehydration reaction catalyzed by optically active amino acids,[255] the Knowles asymmetric hydrogenation process for the industrial production of l-DOPA
employing soluble rhodium catalysts carrying chiral phosphane ligands,[256] the Takasago process for the production of
l-menthol via an asymmetric catalytic amino enamine
isomerization employing a rhodium BINAP catalyst,[257] the
Noyori asymmetric catalytic hydrogenation of ketones ((S)or (R)-[RuBr2(binap)] catalyst),[258] the Katsuki Sharpless
asymmetric catalytic epoxidation ((l)-()- or (d)-()-diethyl
tartrate/Ti(OiPr)4 catalyst),[259] the Sharpless dihydroxylation
reaction (OsO4 catalyst),[165] the Corey oxazaborolidinecatalyzed reduction of ketones,[92] and the Shibasaki carbon carbon bond forming reactions employing bimetallic
catalysts.[260]
Cascade reactions in which several transformations are
carried out in one reaction vessel in tandem have assumed
increasing importance in total synthesis.[261] Such cascades can
be defined as one-pot sequences involving fleeting intermediates, each of which leads to the formation of the next until a
stable product is formed, or pathways marked with distinct
intermediates that can be isolated if so desired, but which are
allowed to proceed to the next stage until the desired product
is obtained. By expanding the definition of cascade reactions
one can include the various one-pot reactions in which a
number of reagents and/or components are added sequentially to form a final product without isolation of intermediate
97

REVIEWS

K. C. Nicolaou et al.

Scheme 49. a) Strategic bond disconnections and retrosynthetic analysis of the CP molecules (1 and 2) and b) total synthesis (Nicolaou et al., 1999).[230]
c) New cascade reactions: the anhydride cascade, which features the first use of a highly unstable 2-aminofuran in the synthesis. The g-hydroxylactonal
cascade was developed as a mild method to construct the precursor to the fused g-hydroxylactone moiety, the g-hydroxylactonal. The pyran rupture cascade
traverses through a sequence of intermediates, including the tetraanion C. d) New synthetic technologies. (For further information see the text.)

98

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REVIEWS

Natural Products Synthesis

5. The Impact of Total Synthesis

In tracing the evolution of organic synthesis and total
synthesis to its present state, it is instructive to also consider
its impact on other scientific disciplines and on society.
Simply stated, this impact has been enormous and it boils
down to profoundly shaping our world by providing the
myriad synthetic materials we have around us today. To be
sure, total synthesis has been aided by its own appeal and
advances, but also by developments in analytical and
purification techniques and spectroscopic methods.

5.1. Driving and Testing the State-of-the-Art in Organic

Synthesis

Scheme 49. (Continued)

compounds or work-ups. Examples of cascade reactions

include the Robinson synthesis of tropinone,[16] the biomimetic synthesis of steroids,[262] the endriandric acid synthesis,[112] the Ugi three-component reaction,[263] the Vollhardt
synthesis of estrone,[358h] the synthesis of the CP molecules and
the many radical-based[264] and palladium-catalyzed[265] tandem sequences for the formation of polycyclic carbon frameworks.
Angew. Chem. Int. Ed. 2000, 39, 44 122

As the flagship of organic synthesis, total synthesis often

guides and demands new synthetic methods and strategies. It
also becomes the testing ground where new technologies and
strategic concepts are tested and judged for their applicability, efficiency, and practicality. In a way, total synthesis
provides the tough and real challenges to new synthetic
methods, often before they are passed over to those who use
them extensively in their daily research and/or for their
manufacturing needs. Indeed, the total synthesis of complex
natural products is frequently given as the reason for the need
to develop a new synthetic method to achieve a goal
unattainable by existing methods. Furthermore, newly appearing synthetic methods become convincingly useful once
they have been successfully applied to total synthesis.
Examples here include the Diels Alder reaction,[266] the
Wittig reaction,[267] the hydroboration reaction,[268] the Corey
dithiane reaction,[269] the Sharpless asymmetric epoxidation
reaction,[259] the various palladium-catalyzed coupling reactions,[270] the olefin metathesis reaction,[271] and last but not
least, the multitude of protecting groups available to the
synthetic chemists.[272]
It is important to note that total synthesis not only drives
organic synthesis forward in terms of synthetic technologies
and strategies, but also frequently leads to fundamental
theories and concepts. Thus, it was within the realm of the
total synthesis of natural products that the theories of
conformational analysis[273] (Barton and Hassel), the Woodward and Hoffmann rules,[98] and the Corey principles of
retrosynthetic analysis[3, 4, 34] crystallized.
Today our desire and ability to make contributions to
biology and medicine is driven to a large extent by total
synthesis, which can deliver not only scarce natural products
but also combinatorial libraries of related substances for
biological evaluation purposes.
Total synthesis not only dictates and demands the invention and development of new synthetic strategies, but it also
provides opportunities for the discovery of such methods and
techniques. Such discoveries are made either through rational
pursuit or simply by serendipity. Indeed, some of the most
dramatic and influential discoveries of our century are fruits
of serendipity. A remark made by Pasteur: Serendipity,
however, appears to be most generous to those positioned to
99

REVIEWS

K. C. Nicolaou et al.

a)

SNAr driven ring closure

OH

HO
O

O
O

H
N

H
NH

TBSO

OH
O

Cl
O

H
N

N
H

NHMe

N
H

MeO2C

HO
O

H
N
O

H
NH

Macrolactamization

16

OMe

Ea = 30.4 kcal mol1

OMe

MEMO

TBSO

NO2

O
NHBoc

H
N

N
H

NMe

MeO

MeO

MeO

BnO

OMe

a. Dess-Martin [O]
b. NaClO2

(64%, >99% ee)

(76%)

HO

NHCbz

HO2C

H
NH

MeO
OMe

NO2
N

MeO

HO

OH
O

H
N

NHBoc
HO

Boc
NMe

N
H

NC
MEMO

OMe

a.

NHCbz
OMe
OMe

ZrClCp2

NO2

NO2

OH

Cl
H
N

H
N

OBn

(M,M)-16

(50%)

HO

OMe
OBn

AD, BocCl

NHCbz
H
OMEM

OMe

(M,P)-16

a. nBu4NF-HOAc
b. LiOH
c. H2, Pd/C
d. EDC, HOBt

NHBoc
O

OMe
OMe

OMe
BnO

H
N

N
H

Ea = 25.1 kcal mol1

OBn

MeO2C

NHBoc

NHCbz

b)
OMe

Cl
O

MEMO

Suzuki biaryl coupling

BnO

TBSO

3:1
O

OMe
OMe

MeO

H
N

OH

120 C

Boc

N
H

HO

NC

MeO2C

OH
O

OMe

OH

Cl
O

OMe

Br

OH

Cl
H
N

(88%)
MeO

(M)-15

OMe
O

[Pd2(dba)3], (o-tolyl)3P
Na2CO3, toluene

NHBoc

SNAr driven ring

closure

B(OH)2

+
O

OH
1: vancomycin aglycon
OH

HO

NHCbz

MEMO

H
N

N
H

NH2 Peptide bond formation

HO

OH

Cl
H
N

OMe

Cl

17
a. HCO2H
b. EDC, HOBt

(58%)

OH

F
TBSO

9
b. TBSOTf
O

MeO2C

(45%)

8
O

10

HO2C

NH2

NH2

HO

OH F

NHBoc

HO

Cl
H
N

a. Boc2O
b. Br2-pyHBr
c. NaH, MeI

NO2

OMe
O

NHBoc

HO

H
N

H
NH

N
H

MEMO

Br
OH

11

MeO

12

(65-75%, 6:1 ratio of atropisomers)

CsF, DMSO

c)
NO2

NO2

OH

HO

TBSO

OMe

OMe

F
TBSO

10

a. EDC,
HOBt

HO
O

b. TBSOTf
(79%)

NHBoc

HO

EDC,
HOBt
(91%)

MeO2C

H
N

N
H

H
N

NHBoc
O

O
Br

O
MeO2C

N
H

OMe

NO2

OH

OH

140 C

TBSO

B Br

1:1.1
NHBoc

MeO2C

N
H

Boc2O

NHBoc

Br

(M)-14 R = NO2
(M)-15 R = Cl

a. H2, Raney Ni
b. tBuONO
(87%)
c. CuCl-CuCl2

N
H

Boc
NMe

d. Dess-Martin [O]
e. NaClO2
f. TMSCHN2
(9-11%)
g. H2O2, K2CO3
h. nBu4NF-HOAc
i. AlCl3, EtSH
OH

Ea = 26.6 kcal mol1

OMe

H
N

O
Br

N
H

OH
O

H
N

OMe
19
OMe

a. H2. Pd/C; tBuONO,

HBF4, CuCl2/CuCl
b. CF3CONMeTBS
c.

(50-60%)
O

NC

MeO

OMe

R
H
N

Cl
H
N

NO2

MEMO

K2CO3, CaCO3

TBSO

OMe
OMe

H
NH

13

Br

12

Boc
NMe

18

OMe
OMe

OMe

NH2

N
H

NC

(43%)

MeO2C

OH
O

H
N

HO
OMe

H
N

(P)-14
O
O

H
NH

HO
HO

O
H

Cl
H
N

N
H

Cl
OH
O

H
N
O

N
H

NHMe

O
NH2
OH
OH 1: vancomycin aglycon

Scheme 50. a) Strategic bond disconnections and retrosynthetic analysis of vancomycin aglycon, b) key methodology for unnatural amino acid synthesis, and
c) total synthesis (Boger et al., 1999).[227]

100

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REVIEWS

Natural Products Synthesis

a)

Oxidation

Cationic
cascade
reaction

NH2

CHO

OHC
O

N H
Me

MeO
OMe

+
N

MeO

Me

OMe

1: aspidophytine

b)
O

CeCl3,
THF

Br

TMS

OAc

a. R-CBS reduction
b. Na(Hg), MeOH

Br
TMS

MgBr

(82%)

OHC

b. NaIO4

Me 2

OMe

OiPr
OiPr

MeO

TMS

Me

7 OiPr
N

OiPr
OMe

TMS

(56%)

N:

a. OsO4, NMO

+
N

MeO

a. LDA, TBSCl,
-78oC, then
b. iPrOH, EDC
(57%)

CHO

NH2

CH3CN, then TFAA, 0C;

NaBH3CN

MeO

TMS

c. Ac2O, Et3N, 4-DMAP

(75%)

MeO

Br

N H
Me

MeO
TMS

Me

OMe

OMe

H+
N

OiPr

NaOH
OH

BH3CN

OiPr

(66%)
N

MeO
OMe

Me

N H
OMe Me

MeO

MeO
OMe

N H
Me

wishing to pursue the science of drug discovery and development process.

The pharmaceutical industry applies the knowledge gained
to discovering and manufacturing new drugs for the benefit
of society. That medicinal and combinatorial chemists have
so many tools at their disposal today in their quests for huge
numbers of novel and diverse small molecules is primarily
the result of the contributions of total synthesis and of
organic synthesis as a whole. A reminder of the importance
of chemical synthesis as one of the two main arms of the drug
discovery processthe other being identification of appropriate biological targetswill help appreciate total synthesis
within a larger perspective. Just as advances in molecular
biology facilitate drug discovery today by allowing the
elucidation of the human genome and proteome, so does
progress in total synthesis, which enables the construction of
the molecules needed to bind and modulate the function of
disease-associated biological targets.
The challenging and rewarding features of total synthesis
invite competition, which, like in any other human endeavor,
is both inevitable and healthy. Fortunately, and unlike many
other sciences, the creative nature of organic and natural
product synthesis allows equal opportunity for brilliant
contributions to all practitioners, whether they are the first
to finish or the last. And there are many things to discover
and invent in this science; only our imagination is the limit.

K3[Fe(CN)6], NaHCO3
OH
O

(81%)

6. Future Perspectives

a. OsO4
b. Pb(OAc)4

O
O

(71%)
MeO

N H
OMe Me

MeO

10

N H
OMe Me

11

N
O

MeO

OMe

H
Me

12

a. KHMDS,
PhNTf2
b. [Pd(PPh3)4],
nBu3SnH
(47%)

MeO

N H
OMe Me

1: aspidophytine

Scheme 51. a) Strategic bond disconnections and retrosynthetic analysis of

aspidophytine and b) total synthesis (Corey et al., 1999).[238]

detect and exploit the accidental, is worthy of remembering.

Serendipity will, no doubt, continue to be part of our science.

5.2. Drug Discovery and Development

While it is wonderful that total synthesis has made such
great leaps from the beginning of the twentieth century, its
greatest impact has been on the drug discovery and development process.[274] Indeed, the evolution of the drug discovery
and development process parallels closely that of total
synthesis. The two disciplines must be considered in unison,
for they are very synergistic and complementary. Academic
research focuses on organic and natural product synthesis,
which provides highly relevant basic knowledge and offers
superb education and training to young men and women
Angew. Chem. Int. Ed. 2000, 39, 44 122

The science and art of organic synthesis attracts many who

practice invention, discovery, and development of new
synthetic reactions and reagents for wider use. Such new
processes and engineered reactions are of paramount
importance to research chemists and manufacturers of
chemical products including pharmaceuticals. Other synthetic chemists adopt total synthesis as their main endeavor, with
the aim of designing and executing elegant strategies toward
complex targets. In judging such accomplishments, one has to
give credit not only to the beauty and efficiency of the
strategies and tactics, but also to the value of the exercise in
providing access to the target molecule and its analogues for,
usually but not always, biological studies. Yet, there are
others who attempt to combine target-oriented synthesis with
the discovery and development of new synthetic technologies.
And finally, there are those who aim to incorporate biology
into their total synthesis programs as well as methodology
development, thus elevating natural products to opportunities
for creative science in total synthesis, synthetic methodology,
and chemical biology.
All sub-disciplines of organic natural product synthesis are
to be equally respected as important to the advancement of
knowledge and the benefit of humankind. Furthermore, one
can choose which of these dimensions he or she will adopt in
their research programs, for all three have their place in
science. Indeed, the beauty of total synthesis lies in the
challenge and opportunities that it provides to make creative
and useful contributions to many other disciplines. It is,
therefore, up to the practitioner to imagine new directions and
101

REVIEWS

K. C. Nicolaou et al.

Scheme 52. a) Strategic bond disconnections and retrosynthetic analysis of everninomicin 13,384-1 and b, c)important synthetic methods; b) orthoester
formation by 1,2-migration of the phenylselanyl group followed by glycosylation (I !II !III !IV), and ring closure after syn-elimination
(V !VI !VII !VIII); c) stereoselective construction of 1,1'-disaccharides; d) synthesis of the building blocks and completion of the total synthesis of
everninomicin 13,384-1 (Nicolaou et al., 1999).[245]

102

Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

NH

a. TMSOTf
b. MeI, NaH
c. NaOH, MeOH
d. BnBr, NaH
(55%)
OMe
O

OBn

OAll

F
OBn

PhSe
OH

O
OMe

TIPSO

OCA

Me

Me

OMe

OH

MeO
O

PMBO
HO

OPMB

41

OMe

OR

O
OMe

HO

HO

MeO
O
Me

O
F

O
OMe

TBSO

OBn

OPMB

a. CH2Br2, NaOH
(66%) b. DDQ
c. NaH, ArC(O)F
d. nBu4NF, THF
OBn
O
F

MeO
O

CAO

O
OMe

TBSO

H
O

HO

OPMB

Me

Me

SPh

D
O

Me

OPMB

MeO
O

O
D
HO

O
OTBS

Me
Me

Me

Me

Me

PMBO
D
HO
Me

MeO
O

O
OTBS

OTIPS

Me

OTIPS

BnO

OTBS

D
Me

A2
OTBS

Me

O
OMe

A2

O O
TBSO

OTBS

OTBS

Me

O A
Me

O
O

Me
A2

O O
TBSO

OTBS

OTBS
O
F

OTBS

OTBS

OTBS

O
O
OMe

Me

O
O

A2

O O
TBSO

OTBS

OTBS

a. H2, 10% Pd/C, NaHCO3

b. nBu4NF, THF
Me

Me
MeO
O

O
D

C
HO

55

O
OMe

OMe
O

O
Me

(75%)

Me
O

O
O

a. NaIO4, MeOH
b.

OMe

Me

NO2

O
OAc
HN

Me

Me

OMe O

MeO
O

Cl

BnO

Me

Me
O O

OTBS
O

OTBS

(65%)

OMe

54

Me

O A

HO

O
OAc

OTBS
O

Me

Me

Me

A1

HO

Me

TBSO

OTBS

OTBS

OMe

Cl

OTBS

Me
MeO
O

O
O

H
O O

NO2

Me

Cl

PMBO

D
HO
SePh
Me

O
A1

46
a. LiI, DMF
b. nBu3SnH (53%)
c. nBu2SnO;
PMBCl

Me

O
OMe

MeO
O

BnO

OMe O
Cl

OTBS

47
a. nBu4NF
b. Ac2O
(69%)
c. nBuNH2
d. CCl3CN

OMe

Me

Me
O

Cl

O
OTBS

BnO

Me
D

OH

OTBS

MeO
O

F HO

OTBS

HO

OTBS

45

MeO
O

HO

OTBS

Me

Me

HO

A2

O O

OMe

Me

OMe O

OTIPS

pTs O

NO2

O A

a. TPAP, NMO
(74%) b. MeLi, Et2O
c. H2, 10% Pd/C
d. pTsCl, py

Me

a. TBSOTf
b. K2CO3, MeOH

Cl

a.Tf2O,
2,6-tBuPy
b. DDQ

(70%) SnCl , Et O
2
2

OH

(67%)

OTBS

SePh HO

BnO

Me

OTBS

26

Me

O
OMe

OMe

Me

O
C

Me

BnO

A1
O

O
O

OH

Me
O

OTIPS

MeO

23
Ph

OH

Me

OTBS

OBn

A1

44

Ph

OH

HO

Cl

A2

DDQ
(CA)2O, Et3N

OH

MeO
O

HO

O A

Me

BnO

D
H

OBn

OH

OMe O

OBn

O
O

H
O O

OMe

(55%)

A2

BnO

O
OMe

(78%)

Me

O
O

53

OBn

OBn

K2CO3, MeOH

OBn

Cl

O
OMe

HO

Me

Me
O

BnO

OBn

OBn

Me

OBn

A2

O O

OBn

OPMB

42

OMe

Me

(79%) H2, 10% Pd/C, EtOAc

H

43

OMe

Me

O
OMe

a. Dess-Martin [O]
b. Li(tBuO)3AlH
c. NaOH, MeOH

O
O

50: R = PMB
51: R = H
52: R = CA

OBz

OMe

(73%)

OBn

OBn

(75%) BnBr, NaH

Me

HO

A2

BnO

OBn

(90%)

HO
OBn

OMe

OR

OBn
O

a. TBSCl, NaH
(54%) b. OsO , NMO
4
c. nBu2SnO; BzCl

OBn
O

Me

a. nBu4NF, THF
b. Martin
sulfurane
c. K2CO3, MeOH

Me

48: R = Ac
49: R = H

40

OTBS

OBn

39

a. 34, SnCl2,
Et2O
b. K2CO3,
MeOH

O
O

(55%) BF3Et2O

Me

O
OMe

NH

RO

OBn

OBn

OPMB

O
OMe

O
OMe

HO

OAc
Cl3C

BzO

OTBS

OAc

O
Me

OBn
O

OBn

OMe

OBn

OBn
O

OMe
O

31

(69%)

a. NaIO4, MeOH
b.
TIPSO
c. nBu4NF
d. BzCl, E3N
(75%)

OBz

Me
MeO
O

PMBO

OAll

a. DDQ
b. TIPSOTf
c.[RhCl(Ph3P)3]; OsO4
d. nBu2SnO; CACl

Me
CCl3

HO

AllO

28

OMe

nBu

OBn

38

OMe

Sn

PMBO

OAll

O
OMe

PMBO

nBu

OBn
O

MeO

Me
OH

OMe

OH
O

F
O

O
OH

O
O
OMe

Me

O
H

O O
HO

A2
OH

OH

NO2
Me
OMe

1: everninomicin 13,384-1

CCl3

Scheme 52. (Continued)

Angew. Chem. Int. Ed. 2000, 39, 44 122

103

REVIEWS

K. C. Nicolaou et al.

Scheme 53. a) Strategic bond disconnections and retrosynthetic analysis of sanglifehrin A and b) total synthesis (Nicolaou et al., 1999).[240]

to constantly raise the bar to higher and higher expectations for

the art and science of organic and natural products synthesis.
Throughout its history the art and science of total synthesis
has demonstrated its nature as an aesthetically appealing
endeavor and as a scientifically important discipline. As a
science and an art, it has attracted some of the most creative
104

minds of the twentieth century and its impact on society is

paramount, if not fully appreciated by the general public. As
we close the chapter of the twentieth century and move into
the twenty-first century many may be wondering of the fate of
total synthesis. The best guides we have are history and the
present state-of-the-art. They both speak volumes of the vigor
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

H

O
Me

O
H

Me

HO

quinine

[Woodward, 1944]
[Gates, 1970]
[Uskokovic, 1970]
[Taylor, 1972]
[Hanaoka, 1982]

HN

Me

NH

[Gates, 1952]
[Ginsburg, 1954]
[Morrison, 1967]
[Kametani, 1969]
[Schwartz, 1975]
[Rice, 1980]
[Evans, 1982]
[Fuchs, 1987]
[Parker, 1992]
[Overman, 1993]
[Mulzer, 1996]

MeO2C

[Woodward, 1954]
[Julia, 1969]
[Ramage, 1976]
[Oppolzer, 1981]
[Rebek, 1984]
[Ninomyia, 1985]
OMe
HO

MeO
OH

estrone[358]

colchicine

H
Me

H Me
H

lupeol[360]

O
O

[Stork, 1971]

N
N
H
MeO2C

H 2N

O
HN

OH
N

MeO

R = Me: vinblastine[375]
R = CHO: vincristine[375]

H H
N

OAc

NH

HN

CO2Me

NH
OH
OH

Me

HO

HO
O

NH2
H

OMe
N

Me

cytochalasin B
[Stork, 1978]

Cl

MeO

Me

Me

Me

Me

MeO

N
HO H

Me
H

Me
O

N-methylmaysenine[373]
[Corey, 1978]
[Meyers, 1979]
[Isobe, 1984]

HO
Me

Me

Me

Me
OH

fumagillol

illudol[368]
[Matsumoto, 1971]
[Semmelhack, 1980]
[Vollhardt, 1991]
[Malacria, 1997]

OH

OMe O

OH

O
OH

OH

[361]

MeO

N
Me

OAc
CO2Me

vindoline[363]

Me

OH

daunomycinone[362]

[Bchi, 1975]
[Kutney, 1978]
[Ban, 1978]
[Danieli, 1984]
[Langlois, 1985]
[Rapoport, 1987]
[Kuehne, 1987]

[Wong, 1973]
[Hauser, 1981]
[Kende, 1976]
[Kimball, 1982]
[Swenton, 1978]
[Reddy, 1983]
[Kelly, 1980]
[Vogel, 1984]
[Kallmerten, 1980] [Rodrigo, 1984]
[Braun, 1980]
[Garland, 1988]

NH2

H Me

Me

thienamycin[371]

Me

HO
H

quassin[380]
[Grieco, 1980]
[Watt, 1990]
[Valenta, 1991]
[Shing, 1998]

H
MeO

aphidicolin[377]

[Trost, 1979]
[McMurry, 1979]
[Corey, 1980]
[Ireland, 1981]
[van Tamelen, 1983]
[Bettolo & Lupi, 1983]
[Tanis, 1985]
[Holton, 1987]
[Fukumoto, 1994]
[Iwata, 1995]

hirsutene[374]

OMe

MeO

Me

CO2H

ryanodol[376]

[372]

[Kishi, 1977]
[Fukuyama, 1987]

OH
H

OH

X = OMe: mitomycin A[369]

X = NH2: mitomycin C[369]

OH

OH

[Christensen, 1978] [Shibasaki, 1985]

[Kametani, 1980] [Hart, 1985]
[Hiraoka, 1986]
[Shiozaki, 1980]
[Hanessian, 1982] [Buynak, 1986]
[Deslongchamps, 1979]
[Evans, 1986]
[Ikegami, 1982]
[Fleming, 1986]
[Shinkai, 1982]
HO
[Yoshikoshi, 1982] [Georg, 1987]
CH2OH [Koga, 1982]
[Hatanaka, 1987]
[Ohno, 1988]
Me
[Grieco, 1984]
[Jacobi, 1996]
[Ley, 1985]

NH
Me

NH

Me

[Barton, 1962]
[Kametani, 1969]
[Koga, 1977]

[Corey, 1970]
[Mander, 1980]
[Yamada, 1989]

[Corey, 1972]
[Kim, 1997]
[Sorensen, 1999]
[Taber, 1999]

OH

galanthamine[357]

N
OMe O

CO2H

OH OH
HO HO

MeO

gibberellic acid[366]

OH
H H

[Kishi, 1977]
[Jocobi, 1984]

OH

Me

Me
OH

saxitoxin[370]

[Potier, 1976]

[Yamada, 1972]
[Inubushi, 1974]
[Kende, 1974]
[Roush, 1978]
[Martin, 1991]
[Williams, 1992]
[Uesaka, 1994]
[Sha, 1997]

OH
O

Me

Me

Me

dendrobine[367]

[Woodward, 1962]
[Muxfeldt, 1965]

Me

HO

[Harley-Mason, 1968]
[Saxton, 1969]
[Schlessinger, 1976]
[Winterfeldt, 1979]
[Takano, 1985]
[Fuji, 1987]

Me

H
Me Me

OC

eburnamine[382]

HO

OH

OH
O

HO

[Inubushi, 1969]
[Kametani, 1972]
[Wallace, 1979]
[Schultz, 1998]

Me

cepharamine[359]
H

Me

MeN

biotin
[Review[364]]

[Quinkert, 1980]
[Torgov, 1963]
[Bryson, 1980]
[Smith, 1963]
[Saegusa, 1981]
[Johnson, 1973]
[Ziegler, 1982]
[Cohen, 1975]
[Jung, 1984]
[Danishefsky, 1976]
[Money, 1985]
[Kametani, 1977]
[Posner,
1986]
[Oppolzer, 1980]
[Rao, 1991]
[Vollhardt, 1980]
[Oyasawara, 1992]
[Grieco, 1980]

[355]

[Echenmoser, 1961]
[van Tamelen, 1961]
[Nakamura, 1962]
[Scott, 1965]
[Woodward, 1965]
[Martel, 1965]
[Kaneko, 1968]
[Tobinaga, 1974]
[Kato, 1974]
[Evans, 1981]
[Boger, 1986]
[Magnus, 1987]
[Banwell, 1992]
[Cha, 1998]

[van Tamelen, 1958]

[Szntay, 1965]
[Stork, 1972]
[Kametani, 1975]
[Brown, 1980]
[Wenkert, 1982]
[Ninomiya, 1983]
[Martin, 1987]
[Tanol, 1994]

MeO

CO2H

OH

6-demethyl-6
-deoxytetracycline[356]

NHAc

yohimbine[354]

H
S

OH
NH2

OH

lysergic acid[353]

NMe2

MeO

HO

H
H

morphine[352]

NH

OMe

MeO

HO

Me O
H

Me

N
H H

[Woodward, 1951]
[Sarett, 1952]
[Kuwajima, 1986]
[Fukumoto, 1990]

[350]

HO2C

cortisone[351]

HO

H
H

MeO

OH
OH

OH

HO

[Hua, 1985]
[Tatsuta, 1979]
[Cossy, 1987]
[Hudlicky, 1980]
[Lacroix, 1989]
[D. R. Little, 1981]
[Sternbach, 1990]
[Mehta, 1981]
[Greene, 1990]
[Magnus, 1981]
[Rao, 1990]
[Wender, 1982]
[Paquette, 1990]
[Ley, 1982]
[Cohen,
1992]
[R.D. Little, 1983]
[Fukumoto, 1993]
[Curran, 1983]
[Oppolzer, 1994]
Me

CHO

OMe
HO

N Me

Me
O

MeO H
O
OMe

delphinine[378]
[Wiesner, 1979]

HO
O
OAc

NMe2
O
Me

OH
O

carbomycin B[383]

Me
O
Me

[Nicolaou, 1979]
[Tatsuta, 1980]

Figure 5. Selected natural product syntheses from the twentieth century.

of this art and science, and of its potential for further advances
and contributions. For one, nature has not yet finished
revealing its secrets to us, and many more novel, and presently
unimaginable, structures are destined to dazzle our eyes,
boggle our minds, and challenge our creativity. Furthermore,
the state of the art is comparatively only in its early stages of
development in light of natures seemingly magical and
powerful biosynthetic schemes. To be sure, the competitive
nature of the pharmaceutical and biotechnology industries
and their drive to discover and produce new cures for disease
Angew. Chem. Int. Ed. 2000, 39, 44 122

will demand new and sharper tools for organic synthesis.

Fueled by these and other industries, the discipline of total
synthesis will be there to attract talented individuals as
practitioners and to deliver the new tools needed for yet
higher efficiencies and selectivities.
Targeting more complex structures will demand more
effective reactions in terms of accomplishing bond constructions and functional-group transformations. The overall
efficiency has to be pushed higher and so does selectivity.
Cascade reactions and other novel strategies will have to be
105

REVIEWS

K. C. Nicolaou et al.
Me

OH

Me

Me

Me
Me

Me H

OH

coriolin[379]

MeO

Me

Me

Me

O
O

O
Me HO

N
H
N

NH

rifamycin S[387]

N
H
O

OH

O
[385]

Me
NH2

OH
HO

HO

[Kishi, 1985]
[Stork, 1990]
[Holmes, 1999]

N
H

cyanocycline

Me

daphnilactone A[399]
[Heathcock, 1989]

Me
O

HO

[Kishi, 1989]

Me
H
H
Me

Me
O
O

Me

OH

[Smith, 1989]

N
O

NH
Me
N

FK506[401]
[Merck, 1989]
[Schreiber,
1990]
[Danishefsky,
1990]
[Sih, 1990]
[Smith, 1994]
[Ireland, 1996]

CC-1065[396]
[Kelly, 1987]
[Boger, 1988]

H
N

H H

NH2

Me
O

H
H

Me

H H

huperzine[400]

NH

ikarugamycin[397] O
[Kozikowski, 1989]

Me

OMe

OMe

Me

Me

OH
O
OMe

OH
N
H

OH

O
H
O

NH
Me

echinosporin[388]

O
O
OMe

Me

CONH2
O

Me

OH

OH

MeO

Me

[Inoue, 1986]

ophiobolin C[392]

HO

H2N

Et

MeO

OH
O

Me

neosurugatoxin[390]

O
HO

Me

[Corey, 1982]
[White, 1986]
[Nakata & Oishi, 1986]
[Matsumoto, 1987]

NH

fredericamycin A[393]

Me

H
Me

H
N

OH

OH
O

O
H
N

OMe

[Kelly, 1986] [Bach, 1994]

[Clive, 1992] [Reddy, 1994]
[Julia, 1993] [Boger, 1995]
[Kita, 1999]

H
OHC
H
O

aplasmomycin[384]

OH

A[386]

Me

[Hanessian, 1986]
[Ley, 1990]
Me
[White, 1995]

HN

[Evans, 1986]
[Fukuyama, 1987]

OH

Br

HO
Me

O
Me

Me

O
Me

Me

O
B

MeO
HO
Me

OH HO
O

HO

histrionicotoxin[402]

[Ohno,1982]
[Hecht, 1982]
[Boger, 1994]

[Danishefsky, 1987]

HN

bleomycin A2

OH

R = OH: avermectin B1a[394]

R = OMe: avermectin A1a[395]

CN
H

Me

Me
O

[Gibbons, 1982]
[Boeckman, 1989]

[Fukuyama, 1990]
[Myers,1998]
[Corey, 1999]

ClSMe2+

HO
O
MeO

pleuromutilin[391]

N
H
OH
OH

OH
Me

Me O

[381]

Me

[Sih, 1981]
[Hirama, 1982]
[Girotra, 1983]
[Grieco, 1983]
[Heathcock, 1985]
[Keck, 1986]
[Kozikowski, 1987]
[Clive, 1988]
[Danishefsky, 1989]
[Burke, 1991]
[Hagiwara, 1995]

[Kishi, 1980]
[Hanessian, 1982]

NH

Me

Me

compactin[389]

Me

H H
N

NH

N
H

HO
O

Me

OMe

Me HN

H
Me

Me

R = CN: saframycin A

OH

Me

NH2

OH

R = H: saframycin B[381]

NH2

H2 N

HO

[Fukuyama, 1982]
[Kubo,1987]

H
N

Me

OH

Me

Me

[Isoe, 1984]
[Iwata, 1985]
[Wender, 1985]
[Piers, 1985]
[Funk, 1986]
[Magnus, 1987]
[Liu, 1988]
[Little, 1990]

NHCOCOMe
R

Me

H
N

HO

OH

OH

[Danishefsky, 1980]
[Helquist, 1981]
[Burke, 1982]
[Kende, 1982]
[Schlessinger, 1983]
[Vandewalle, 1983]
[Yoshii, 1983]
[Smith, 1984]

OH

quadrone[365]

[Danishefsky, 1980]
[Ikegami, 1980]
[Tatsuta, 1980]
[Trost, 1981]
[Mehta, 1982]
[Matsumoto, 1982]
[Magnus, 1983]
[Koreeda, 1983]
[Wender, 1983]
[Schuda, 1984]
[Funk, 1985]
[Little, 1985]
[Demuth, 1986]
[Curran, 1988]
[Weinges, 1993]
[Kuwajima, 1997]

NH2

Me

H
O

Me

MeO

Me

AcO

[Boeckman, 1989]
[Paquette, 1989]

O
O
H

N
H

koumine[398]
[Magnus, 1989]

Figure 6. Selected natural product syntheses from the twentieth century.

devised for delivering complex and diverse structures in single

operations in order to achieve such goals. New catalysts have
to be invented to bring about otherwise difficult or impossible
operations. There is little doubt that we can count on mother
nature to provide us with the targets and the opportunities to
invent and develop such methods in the future.
Solid-phase chemistry[275] is now gathering momentum in
natural product synthesis. Traditionally used for the synthesis
of peptides[276] and oligonucleotides,[277] this approach is now
being applied to construct small organic molecules in large
numbers, particularly for the purposes of drug discovery,[278]
catalyst development,[279] and material science.[280] Again, new
techniques, strategies, and tactics are needed to elevate this
endeavor to a higher level of sophistication, applicability, and
scope. As we have mentioned above, total synthesis is taking a
leading role in spearheading developments in new technologies for solid-phase and combinatorial chemistry. With the
strong foundation provided by such advances, automation of
synthetic and combinatorial chemistry should also be possible
106

and forthcoming. Indeed, automation technologies are already entering the realm of chemical synthesis laboratories.[281] Finally, a goal of paramount importance for the
ensuing century will undoubtedly involve the development of
synthetic strategies for the rapid construction of complex
natural products that rival or even surpass the efficiency of
nature.
In addition to natural products and their analogues,
synthetic chemists are also beginning to target complex and
exotic natural productlike structures for the purpose of
biological screening.[282] Such endeavors are clearly related
to total synthesis and are both inspired and facilitated by
advances in the latter field. Particularly enabling are those
contributions that include both new synthetic technologies
and novel molecular architectures. Examples of such endeavors include those from the Schreiber group,[283a] Sharpless
group,[283b] and our own[283c] (Figure 9; see p. 108).
In closing, in surveying the art and science of total synthesis
of the twentieth century, one is left with awe at its accomplishAngew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

Me
Me
Me

N
H

OH

OH
O

OH

HO

O
paspalinine[403]

OH

H2N

Me

H2N

O
OMe OH

[Smith, 1990]

isorobustin[405]

N
O
O H OH

O
H

OH

OH

OH

Me

Me

N
H H

Me

HO

H
OH

ambruticin[408]

O Me

Et

[Kende, 1990]

isorauniticine

halichondrin

Me

O
H
H
B[413] O

Me

Me

Br

OH

N
H

Me

MeO

Me

OH

N
Me

thebainone A[424]

HO

OH

Me

OCONH2

[Tius, 1992]

O
H
N

Me

NMe2

Me
OH
Me

Me
O

discodermolide[416]

lepicidin A[417]
[Evans, 1993]

OMe

Me

HO

OMe

OMe

H
OH

chlorothricolide

magellanine[431]

Cl

O
HN

HN
O

N
H

H
O

Me

H 2N

epoxydictimene[437]

H2N

[Schreiber, 1994]

(CH2)14

ptilomycalin[445]
[Overman, 1995]

[Overman, 1993]
[Ziegler, 1995]

Me

NHMe

[Danishefsky, 1995]
[Wood, 1997]

nPent
OH

Me
H
H

H
H

Me

staurosporine[435]

OH

O
O

Me

[Barrett, 1994]
[Weinreb, 1994]

MeO

OH HO

[Heathcock, 1994]

Me
Me

H
Me

HH

HO
H

Me
H

scopadulcic acid B[430]

petrosin[421]

papuamine[420]

H
N

[Terashima, 1994]
[Boger, 1996]

stenine[418]
[Hart, 1993]
[Wipf, 1995]

H
OBz
O

duocarmycin A[427]
O

Me
HO

H H
N

[Roush, 1994]

O
N
H

N
H

HO2C

H H
N

[423]

NH

H
H

Me

OMe

O
MeO2C

OH

[Coleman, 1994]

calphostin A[429]
H

Me

Me

HH

OMe

OH

[Overman, 1993]

Me

OCOPh

Me
H
H

Me

[Williams, 1993]

Me

OCOPh

MeO

Me

[Armstrong, 1998]

MeO

[Nicolaou, 1992]
[Yamamoto, 1995]
[Nakata, 1996]
[Mori, 1997]

OMe

R = Me: calyculin C (ent)[415]

O
OMe

member of the
clavularanes[434]

OH

Me

HO

H
O

OH

O
H
H

hemibrevetoxin B[414]

OH

[Evans, 1992]
[Shioiri, 1996]
[Masamune, 1994] [Smith, 1998]

Me

Me
OMe
OMe

R = H: calyculin A (ent)[415]

OH

CO2H

HO

Me

OH
CO2Me

[Wasserman and Boger, 1993]

Me
H

O MeO

isochrysohermidin[426]

H
H

HO P
O
MeHO

Me

OH

Me

O
Me

Me

Me

MeO2C
Me

Me

[Schreiber, 1993]
[Smith, 1995]
[Myles, 1997]
[Marshall, 1998]

OMe O

HO

Et
OH

CN

Me

[Kita, 1992]

Me
O

myrocin C[412]
[Danishefsky, 1992]

Me

O
OH

HO

discorhabdin C[425]

H
O

Me
H

N
H

NMe2 OH

N
H

Me

[Smith, 1991]

Me
OH

breynolide[422]

Br

OO

[Kishi, 1992]

Me

HO
Me

OAc

[Dauben, 1991]

rocaglamide[409]

O
HH

HO

kempene-2[411]

O
O

Me

HO
HO

H
O

CON(Me)2

OMe

O
HO

OH

Ph

HO

Me

Me

[Oppolzer, 1991]
Me

HO

Me

[410]

OH

[Trost, 1990]

[404]

[Danishefsky, 1990]

MeO2C

OMe

MeO
O

[Grieco, 1990]

indolizomycin

Me

Me

H
O

Me

[Schreiber, 1990]
H

Me

chaparrinone[407]

Me

hikizimycin[406]

CO2Me

OMe

OH

OH
N

H
OH
NH2

HO

[Barton, 1990]

HO
OH
Me

HO

O
AcO

7-deacetoxyalcyonin
acetate[438]
[Overman, 1995]

O
O

N
H

HN

OH

balanol[419]
[Nicolaou, 1994]
[Hu, 1994]
[Adams, 1994]
[Stadlwieser, 1996]
[Tanner, 1997]
[Naito, 1997]

OCONH2

OH

OH

syringolide 1[439]
[Wood, 1995]
[Kuwahara, 1995]
[Murai, 1996]
[Sims, 1997]
[Yoda, 1997]
[Wong, 1998]

OHC

NH

FR-900482[441]
[Fukuyama,1992]
[Danishefsky, 1995]

Figure 7. Selected natural product syntheses from the twentieth century.

ments and power. As a practitioner of the art, one is filled with

overwhelming pride to be part of such attractive and vigorous
endeavors and apprehensive in being responsible for conveying its true meaning and value to society.[284] But, most
importantly, the surveyor must be overly excited and optimistic about the future of the discipline and in transferring
this enthusiasm to the next generation of chemists. It would,
indeed, be of considerable interest to compare the
present state-of-the-art with that at the end of the twentyfirst century.
Angew. Chem. Int. Ed. 2000, 39, 44 122

Abbreviations
AA
Ac
acac
AD
AIBN
All
Alloc
9-BBN

asymmetric aminohydroxylation
acetyl
acetylacetonyl
asymmetric dihydroxylation
2,2'-azobisisobutyronitrile
allyl
allyloxycarbonyl
9-borabicyclo[3.3.1]nonane
107

REVIEWS

K. C. Nicolaou et al.
Me

Me

O
O

OH

OH

FR-90848

[Barrett, 1996]
[Falck, 1996]

O
H

N
H

Me

Me

Me

Me
HO

HO H

rubifolide

[456]

[Corey, 1997]
[Robichaud, 1998]
[Danishefsky, 1998]
[Yamada, 1999]

AcO
H

Me

Me

OH

Me

AcO
Me

Me

OH
HO
Me
O

OH
Me

O HO
O

X = Cl: spongistatin 1

[453]

X = H: spongistatin 2

[454]

OH

H H
N

O O

MeO

[Boger, 1999]

preussomerin I[446]

HO

[Heathcock, 1999]

[Roush, 1999]

OH

Me

N
Me

N
H

O
O

Me AcO
N

H
O
N

OH

[Taylor, 1999]
Me

Me
N

OH

N
O

luzopeptin B[452]
O

manumycin B[448]

OAc

H
N

OH

olivomycin A[450]

AcO
Me

MeO

Me

[Evans, 1998]

NH
O

H
O

OH
OMe

Me

HO

OH
O

[Kishi, 1998]

OH

H
O

OMe OH

Me

O
OH H
Me

phorboxazole A[449]

Me
iPrO2C
HO

OH

HO
H

Me

[Forsyth, 1998]

Me

OH

nBu
O

Me

[Romo, 1998]

Me
H
OH

Me

HO

OH

pateamine[444]

Me

H
N

Me

OH

Br

[Kishi, 1998]

Me

7[455]

OMe

Me
H2N

OMe

batrachotoxinin A[439]

Me

[Fuchs,1999]

Me

NMe2

cephalostatin

OAc

[Myers, 1998]

O
O

HO

OH

Me

[Kishi,1998]
[432]

OH

neocarzinostatin
chromophore[447]

Me

HO

[458]

[Paquette, 1997]

O
O

HO

MeO
Me
HO

pinnatoxin A (ent)

H
Me

HO
O

Me

Me
HO
Me

Me
N

OH
Me

salsolene oxide

Me

Me

Me
CO2

Me

Me

OH
OH

OH

Me

[Marshall, 1997]

dysidiolide[451]

MeO

Me

O
O

Me
O

HO

HN

Me

Me

[Danishefsky, 1997]
[Overman, 1998]

Me

HO
NMe2

hispidospermidin[443]

Me

Me
N

HN
Me

O
Me

Me

OH

lubiminol[442]
[Crimmins, 1996]

Me

OH

[Baldwin, 1996]

H
N

[Martin, 1996]

Me

CH2OH

O
N

trichoviridin[428]

croomine[433]

Me

NC

H O

O
HN

[436]

OH

N
H H
HO

O
OMe

OH

diepoxin [457]
[Wipf, 1999]

Figure 8. Selected natural product syntheses from the twentieth century.

X1
R1

NH

R2

N
O H
N

R4

R1
R

R2

R5
R2

R3

R1

R4
R

X1 = O, S; X2 = CR2, O, NR

2X

R5

BOP

X1

X2

7-8 steps

2 steps

Schreiber (1998)

Nicolaou (1999)
R2

O
R3

R3

O
R2

R1

R2

R5 X

R4

R2

Ar

O
R1

N N

NH

Ar1

X = SO2, CO, CH2

4-6 steps

R3

R1

4 steps

Ar1

O
R1

R1

4 steps

Sharpless (1999)

Figure 9. Novel, natural productlike, molecular architectures recently

synthesized for biological screening purposes (number of steps from
commercially available materials).[283]

BINAP
Bn
Boc
108

2,2'-bis(diphenylphosphanyl)-1,1'-binaphthyl
benzyl
tert-butyloxycarbonyl

Bz
CA
CAN
Cbz
Cod
Cp
CSA
Cy
DABCO
DAST
dba
DBN
DBU
DCB
DCC
Ddm
DDQ
DEAD
DEIPS
DET

benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluoride
benzoyl
chloroacetyl
cerium ammonium nitrate
benzyloxycarbonyl
cyclooctadiene
cyclopentadienyl
10-camphorsulfonic acid
cyclohexyl
1,4-diazabicyclo[2.2.2]octane
(diethylamino)sulfur trifluoride
trans,trans-dibenzylideneacetone
1,5-diazabicyclo[5.4.0]non-5-ene
1,8-diazabicyclo[5.4.0]undec-7-ene
3,4-dichlorobenzyl
N,N'-dicyclohexylcarbodiimide
4,4'-dimethoxydiphenylmethyl
2,3-dichloro-5,6-dicyano-1,4-benzoquinone
diethyl azodicarboxylate
diethylisopropylsilyl
diethyl trtrate
Angew. Chem. Int. Ed. 2000, 39, 44 122

REVIEWS

Natural Products Synthesis

DHP
DIAD
DIBAL-H
DIC

3,4-dihydro-2H-pyran
diisopropylazodicarboxylate
diisobutylaluminum hydride
5-(3,3-dimethyl-1-triazenyl)-1H-imidazole-4carboximide
DIPT
diisopropyl tartrate
DMA
N,N-dimethylacetamide
4-DMAP 4-dimethylaminopyridine
DMF
N,N-dimethylformamide
DMP
Dess-Martin-periodinane
DMPU
N,N-dimethylpropyleneurea
DMSO
dimethylsulfoxide
Dopa
3-(3,4-dihydroxyphenyl)alanine
DPPA
diphenyl phosphoryl azide
dppb
1,4-bis(diphenylphosphinyl)butane
dppf
1,1'-bis(diphenylphosphanyl)ferrocene
DTBMS di-tert-butylmethylsilyl
EDC
1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
FDPP
pentafluorophenyl diphenylphosphinate
Fmoc
9-fluorenylmethoxycarbonyl
HATU
O-(7-azabenzotriazol-1-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate
HBTU
O-benzotriazol-1-yl-N,N,N',N'-tetramethyluronium hexafluorophosphate
HMDS
bis(trimethylsilyl)amide
HMPA
hexamethylphosphoramide
HOAt
1-hydroxy-7-azabenzotriazole
HOBt
1-hydroxybenzotriazole
IBX
o-iodoxybenzoic acid
imid.
imidazole
Ipc
isopinocamphenyl
KSAE
Katsuki Sharpless asymmetric epoxidation
LDA
lithium diisopropylamide
lut.
2,6-lutidine
mCPBA 3-chloroperoxybenzoic acid
MOM
methoxymethyl
Ms
methanesulfonyl
MSTFA N-methyl-N-(trimethylsilyl) trifluoroacetamide
nbd
norbaranadine (bicyclo[2.2.1]hepta-2,5-diene)
NBS
N-bromosuccinimide
NIS
N-iodosuccinimide
NMO
4-methylmorpholine-N-oxide
Nos
4-nitrobenzolsulfonyl
OTf
trifluoromethanesulfonate
PCC
pyridinium chlorochromate
PDC
pyridinium dichromate
PG
protecting group
Pht
phthalimidyl
Piv
pivaloyl
PMB
p-methoxybenzyl
PPTS
pyridinium 4-toluenesulfonate
pTs
4-toluenesulfonyl
py
pyridine
Red-Al
sodium bis(2-methoxyethoxy)aluminum hydride
SEM
2-(trimethylsilyl)ethoxymethyl
TBAF
tetra-n-butylammonium fluoride
TBAI
tetra-n-butylammonium iodide
TBDPS
tert-butyldiphenylsilyl
TBS
tert-butyldimethylsilyl
Angew. Chem. Int. Ed. 2000, 39, 44 122

TEMPO
TEOC
TES
Tfa
TFA
TFAA
THF
THP
TIPS
TMGA
TMS
TPAP
TPS
Tr

2,2,6,6-tetramethyl-1-piperidinyloxy
trimethylsilylethylcarbonyl
triethylsilyl
trifluoroacetyl
trifluoroacetic acid
trifluoroacetic anhydride
tetrahydrofuran
tetrahydropyranyl
triisopropylsilyl
tetramethylguanidinium azide
trimethylsilyl
tetra-n-propylammonium perruthenate
triphenylsilyl
trityl

It is with enormous pride and pleasure that we wish to thank

our collaborators whose names appear in the references and
whose contributions made the described work possible and
enjoyable. We gratefully acknowledge the National Institutes of
Health (USA), Merck & Co., DuPont, Schering Plough,
Pfizer, Hoffmann-La Roche, Glaxo Wellcome, Rhone-Poulenc
Rorer, Amgen, Novartis, Abbott Laboratories, Bristol Myers
Squibb, Boehringer Ingelheim, Astra-Zeneca, CaPCURE, the
George E. Hewitt Foundation, and the Skaggs Institute for
Chemical Biology for supporting our research programs.
Received: June 10, 1999 [A 349]
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