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Protecting Groups

Protective groups are used in organic synthesis to selectively modify functional groups and prevent unwanted reactions. Protective groups must react selectively, be removable, and be non-toxic. Common protective groups include ethers for alcohols like methyl ethers and silyl ethers; ketals and acetals for ketones and aldehydes; and carbamates like tert-butyl carbamate and benzyl carbamate for amines. Protective groups are added and removed under mild conditions to selectively modify one functional group at a time.

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86 views16 pages

Protecting Groups

Protective groups are used in organic synthesis to selectively modify functional groups and prevent unwanted reactions. Protective groups must react selectively, be removable, and be non-toxic. Common protective groups include ethers for alcohols like methyl ethers and silyl ethers; ketals and acetals for ketones and aldehydes; and carbamates like tert-butyl carbamate and benzyl carbamate for amines. Protective groups are added and removed under mild conditions to selectively modify one functional group at a time.

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zatty kim
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Protecting groups in organic

synthesis
O A selective reaction at one reactive site in a multifunctional
compound can be achieved using protective groups so that
other active sites are blocked.
O Protecting groups must fulfill a number of requirements
O React selectively in good yield
O Must be selectively removed at a later stage
O Must be non-toxic
this will react as well so it must be protected first

1. Base
2. CH 3I
Hydroxyl (OH) protecting groups

A. OH as ethers
1. Methyl ethers
O ROH ROMe
O Formation : NaH, MeI, THF or CH2N2, silica
O Cleavage : AlBr3, EtSH, Me3SiI

NaH, CH3I AlBr3, EtSH

chemical transformations
2. Methoxymethyl ether, MOM

O R-OH R-OCH2OMe (stable


to base and mild acid)

O Formation: MeOCH2Cl, NaH, THF


O Cleavage: Me2BBr2
3. Methoxyethoxymethyl ethers, MEM

O R-OH R-OCH2OCH2CH2OMe

O Formation: MeOCH2CH2OCH2Cl, NaH, THF


O Cleavage: Lewis acids such as ZnBr2, TiCl4

TiCl4
4. Benzyl ethers, R-Obn

O R-OH R-OCH2Ph , stable to acid


and base

O Formation: KH, PhCH2Cl, THF


O Cleavage: H2/ PtO2 or Li/ NH3
B. OH as silyl ethers

O R-OH R-O-SiR3
O Formation: R3Si-Cl, pyridine, DMAP
R3Si-Cl, DCM, imidazole,
DMAP
O Cleavage: F- such as TBAF, HF

(CH3)3SiCl HF

pyridine
Ketones and aldehydes protecting groups
O Ketones and aldehydes are protected a cyclic and acyclic
ketals and acetals
O Stable to base; removed with acid

MeOH. H+

(CH2OH)2, H +

Benzene, -H2O

CH2(CH2OH)2

H+, Benzene
Amines protecting groups

O Carbamates
1. Tertiary butyl carbamate (BOC)

BOC anhydride

O Cleavage: Strong acid (3M HCl), CF3COOH


2. Allyl carbamate (Alloc)

Pd(0), Et3SiH
3. Benzyl carbamate , CBz

(CBz-Cl)

cleavage

O Cleavage: Hydrogenolysis
O PdCl2, Et3SiH
O BBr3

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