Indian Journal of Chemistry

Vol. 51B, May 2012, pp. 739-745

Synthesis and antibacterial evaluation of benzazoles

tethered dihydro[1,3]oxazines
Davinder Prasada, Rajesh Kumar Rohillab, Nilanjan Royb, Mahendra Nath*a
a
Department of Chemistry, University of Delhi, Delhi 110 007, India
b
Department of Biotechnology, National Institute of Pharmaceutical Education and Research,
SAS Nagar, Punjab 160 062, India
E-mail: mnath@chemistry.du.ac.in
Received 29 August 2011; accepted (revised) 17 February 2012

Synthesis of various benzazoles tethered 1,3-oxazines such as 1H-benzo[d]azolophenyl-3,4-dihydro-2H-benzo-

[e][1,3]oxazines 2a-j, 2-(3-(1H-benzo[d]oxazol-2-yl)phenyl)-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine 3, 2-(3-(1H-
benzo-[d]imidazol-2-yl)phenyl)-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine 4 and 3-(3-(benzo[d]oxazol-2-yl)phenyl)-7-
(benzo[d]thiazol-2-yl)-3,4-dihydro-2H-benzo[e][1,3]oxazine 5 has been accomplished through three-component one-pot
Mannich type condensation-cyclization reaction of substituted phenols or β-naphthol with formaldehyde and benzazolo-
phenyl amines at 80-90°C. All the newly synthesized compounds have been characterized on the basis of elemental and
spectral analyses. Moreover, these compounds have been evaluated for their in vitro antibacterial efficacy against two Gram-
positive bacteria viz S. epidermidis and S. aureus and two Gram-negative bacteria viz E. coli and P. aeruginosa.
Interestingly, the compounds, 2f-i have displayed significant antibacterial efficacy with IC50 (0.208-1.106 µg/mL).

Keywords: One-pot methodology, antibacterial efficacy, benzazoles, dihydro[1,3]oxazines, polymerization

The 1,3-oxazine derivatives are an important class of Results and Discussion

heterocyclic compounds which occupy an unique Chemistry
place in material and medicinal chemistry due to
The synthetic routes of title compounds are
their diverse physical and biological properties such
outlined in Scheme I. The starting compounds 2-(3-
as monomer for polymer formation1, photochromic
aminophenyl)benzoxazole, 2-(3-aminophenyl)benz-
agents2, antibacterial3,4, non-steroidal progesterone
thiazole and 2-(3-aminophenyl)-1H-benzimidazole
receptor modulators5, antifungal4, antimalarial6,
were synthesized according to the literature
antituberculor7, antitumor8,9 and anti-HIV agents10,11.
procedure29 via the condensation reaction of m-
In addition, 1,3-oxazine nucleus is a part of many
aminobenzoic acid with corresponding o-substituted
biologically important natural products12,13 and other
aryl amines in polyphosphoric acid at 160°C, while 2-
synthetic bioactive molecules14-17 including
(3-hydroxy-phenyl)benzthiazole was prepared30 by
Efavirenz, a benzoxazinone derivative for the
the reaction of 2-aminothiophenol with 3-
treatment of HIV-1 infections18. Similarly,
hydroxybenzaldehyde in the presence of ceric
benzazoles such as benzoxazoles, benzthiazoles and
ammonium nitrate and 30% H2O2 in acetonitrile at
benzimidazoles are known to display a wide range of
pharmacological properties19-28 and some of them are 25°C. The physical and spectral data of these known
common to 1,3-oxazines. Thus, it was contemplated compounds are in agreement with those of reported
to synthesize benzazole tethered dihydro-1,3-oxazine data31-34. The benzazolophenyl amines 1 on reaction
derivatives in order to obtain more potent with corresponding phenols or β-naphthol and 37%
antibacterial agents. Hence, a series of novel aqueous formaldehyde solution at 80-90°C afforded
heteroarene tethered dihydro[1,3]oxazines has been 1H-benzo[d]azolophenyl-3,4-dihydro-2H-benzo[e]-
prepared by using one-pot methodology to evaluate [1,3]oxazines 2a-j, 2-(3-(1H-benzo[d]oxazol-2-yl)-
their antibacterial activity. phenyl)-2,3-dihydro-1H-naphtho-[1,2-e][1,3]oxazine
740 INDIAN J. CHEM., SEC B, MAY 2012

N
O

N
3

b X= O
H
N X

N X N
b a
N N
O X = NH N O
1 NH2
R3 R1

c X=O R2
4 2a-j
O
N
O
N S

N
5
Reagents and conditions: a) Substituted phenol, HCHO, 80-90ºC, 4 hr; b) β-Naphthol, HCHO, 80-90ºC, 4 hr; c) 2-(3-
Hydroxyphenyl)benzothiazole, HCHO, 80-90ºC, 5 hr.
Scheme I

13
3 and 2-(3-(1H-benzo-[d]imidazol-2-yl)phenyl)-2,3- C NMR of 2c were assigned to N-CH2-O and N-
dihydro-1H-naphtho[1,2-e][1,3]oxazine 4 in moderate CH2-Ar groups, respectively. The mass spectral
to good yields (Scheme 1). Under similar reaction analysis gave further evidence for the formation of 3-
conditions, the scope of the reaction was further (3-(benzo[d]thiazol-2-yl)phenyl)-6-chloro-3,4-dihyd-
explored by reacting 2-(3-hydroxyphenyl)benz- ro-2H-benzo[e][1,3]oxazine 2c by showing [M]+ ion
thiazole with 37% aqueous formaldehyde solution and peak at m/z 378.0157 for the molecular formula,
2-(3-aminophenyl)benzoxazole to obtain 3-(3- C21H15ClN2OS. In addition, the thermally-activated
(benzo[d]oxazol-2-yl)phenyl)-7-(benzo[d]-thiazol-2- cure behavior of synthesized dihydro-1,3-oxazines
yl)-3,4-dihydro-2H-benzo[e][1,3]-oxazine 5 in 25%
was studied by differential scanning calorimetry
yield. All the newly prepared compounds were
(DSC). The DSC thermogram of compound 2c has
characterized on the basis of spectral and elemental
analyses. The IR spectrum of compound 2c has shown shown an endothermic peak at 134.84°C which was
two characteristic absorption peaks at 1217 cm-1 due attributed to its melting point and the characteristic
to asymmetric stretching of C-O-C bond and at 1048 exothermic peak with onset at 265.92°C and maxi-
cm-1 which corresponds to the symmetric stretching of mum at 270.90°C was observed due to ring opening
C-O-C bond. In the proton NMR of 2c, two chara- polymerization of the oxazine ring system35. The
cteristic peaks of benzoxazine ring were observed at δ amount of heat of polymerization was found to be
5.44 corresponding to N-CH2-O protons and at δ 4.73 175.8 J/g. The typical synthetic methods, characteri-
corresponding to N-CH2-Ar protons. Similarly, two zation and DSC data of compounds 2a-j, 3, 4 and 5
characteristic carbon signals at δ 79.1 and 50.3 in the are presented in the experimental section.
 PRASAD et al.: SYNTHESIS OF BENZAZOLES TETHERED DIHYDRO[1,3]OXAZINES 741

Table I — In vitro antibacterial activity of compounds 2a-j, 3, 4 and 5

Compd X R1 R2 R3 Yield IC50 (µg/mL)

(%) Se Sa Ec Pa
2a O H Cl H 80 * * * *
2b O H Br H 72 * * * *
2c S H Cl H 84 * * * *
2d S H Br H 84 * * * *
2e NH H F H 58 * * * *
2f NH H Cl H 85 0.852 0.208 1.106 0.957
2g NH H Br H 82 0.402 0.208 1.106 0.947
2h NH H I H 77 * 0.419 * 0.957
2i NH Cl Cl H 86 * * * 0.957
2j NH H CH3 H 85 * * * *
3 - - - - 86 * * * *
4 - - - - 85 * * * *
5 - - - - 25 * * * *
Tetracycline - - - - - 0.080 0.120 0.090 0.060
Se, S. epidermidis; Sa, S. aureus; Ec, E. coli; Pa, P. aeruginosa
*Activity: >500 µg/mL.

In vitro antibacterial efficacy donating methyl groups, respectively at position 6

were found inactive against all the tested bacterial
The in-vitro antibacterial activity of compounds
strains. These results imply that instead of the nature,
2a-j, 3, 4 and 5 and standard drug tetracycline was
the size of substituent at position 6 of benzoxazine
carried out against two Gram-positive bacteria viz. S.
ring is crucial for the antibacterial activity. Further, an
epidermidis and S. aureus and two Gram-negative
introduction of O or S in place of NH in the
bacteria viz. E. coli and P. aeruginosa. The activity
benzimidazole nucleus as in compounds 2a-d led to
data are presented in Table I. Though, target
complete loss of antibacterial activity against tested
compounds were not as active as standard drug,
bacterial strains. In contrast, their benzazolophenyl-
tetracycline but compounds 2f-i were found signi-
2,3-dihydro-1H-naphtho[1,2-e][1,3]-oxazine counter-
ficantly potent with low IC50 values in the range of
parts 3 and 4 and 3-(3-(benzo[d]oxazol-2-yl)phenyl)-
0.208-1.106 µg/mL. The most active compound 2g
7-(benzo[d]thiazol-2-yl)-3,4-dihydro-2H-benzo[e]-
possessing bromo substituent at position 6 in the
[1,3]oxazine 5 were presenting antibacterial activity at
fused aryl ring of dihydro-1,3-benzoxazine moiety has
displayed better antibacterial effect against S. more than 500 µg/mL concentration.
epidermidis, S. aureus, E. coli and P. aeruginosa with
Conclusions
IC50 values 0.402, 0.208, 1.106 and 0.947 µg/mL,
respectively. The other compound 2f with chloro In summary, various benzazole tethered di-
substituent at position 6 was found equipotent to 2g hydro[1,3]oxazines have been successfully prepared
except it showed slightly higher IC50 values against S. by using simple one-pot methodology and evaluated
epidermidis (0.852 µg/mL) and P. aeruginosa (0.957 for their in vitro antibacterial efficacy. Among all,
µg/mL). An exchange of bromo substituent at position compounds 2f and 2g have displayed significant
6 in 2g by iodo group as in 2h led to exhibit activity with IC50 values in the range of 0.208-1.106
significant antibacterial effect only against S. aureus µg/mL against all the tested bacterial strains. Further,
(IC50 0.419 µg/mL) and P. aeruginosa (IC50 0.957 it was noticed that the size of the substituent present
µg/mL). Introduction of two chlorine atoms at at position 6 in the fused aryl ring of dihydro-1,3-
positions 6 and 8 in 2i did not improve the activity but benzoxazine moiety is crucial for the activity in
was selectively effective against P. aeruginosa (IC50 addition to 1H-benzimidazole nucleus. Within the
0.957 µg/mL). In addition, the compounds 2e and 2j series, the compound 2i possessing chloro substituent
containing electron withdrawing fluoro and electron in the 6,8 positions of dihydro-1,3-benzoxazine ring,
742 INDIAN J. CHEM., SEC B, MAY 2012

displayed significant and selective efficacy against P. ArH), 6.79 (d, J = 8.5 Hz, 1H, ArH), 5.45 (s, 2H, N-
aeruginosa with IC50 value 0.957 µg/mL. Henceforth, CH2-O), 4.72 (s, 2H, N-CH2-Ar); 13C NMR (100
these results are useful and could be guide for the MHz, CDCl3): δ 162.83, 152.79, 150.65, 148.43,
development of potent antibacterial agents in the 141.93, 129.92, 128.15, 128.01, 126.43, 125.65,
future. 125.14, 124.56, 121.89, 120.70, 120.67, 119.94,
118.39, 116.85, 110.57, 78.90, 50.20; HRMS (ESI):
Experimental Section m/z [M]+ calcd. for C21H15ClN2O2: 362.0822; found:
All the chemicals were purchased from Sigma- 362.1564. Anal. Calcd. for C21H15ClN2O2.0.6H2O: C,
Aldrich and used without any further purification. 67.51; H, 4.37; N, 7.50. Found: C, 67.42; H, 4.45; N,
Thin layer chromatography was performed on 7.45%.
precoated Merck silica gel 60 F254 plates and spots 3-(3-(Benzo[d]oxazol-2-yl)phenyl)-6-bromo-3,4-
were developed under UV light (254 nm) or in iodine dihydro-2H-benzo[e][1,3]oxazine, 2b: Rf: 0.70 (20%
chamber. All the compounds were purified by column EtOAc in hexane); m.p. 153.9°C; Tmax (DSC
chromatography using silica gel (60-120 mesh). The exotherm): 260.5°C; IR (KBr): 1605 (C=N), 1548,
1
H and 13C NMR spectra were recorded on Bruker 1463, 1377, 1246, 1001, 972, 931, 863, 818, 793, 760,
300 or 400 MHz spectrometer. The IR spectra were 744, 722, 688 cm-1; 1H NMR (300 MHz, CDCl3): δ
obtained on a Perkin Elmer IR spectrometer and 7.98 (s, 1H, ArH), 7.83 (d, J = 7.4 Hz, 1H, ArH),
peaks are given in reciprocal centimeter (cm-1). Mass 7.79-7.76 (m, 1H, ArH), 7.60-7.57 (m, 1H, ArH),
spectra were recorded on Waters Micromass LCT 7.44-7.34 (m, 3H, ArH), 7.25-7.18 (m, 3H, ArH), 6.73
mass spectrometer. Elemental analyses were (d, J = 8.6 Hz, 1H, ArH), 5.43 (s, 2H, N-CH2-O), 4.71
determined on Elementar Analysensysteme GmbH (s, 2H, N-CH2-Ar); 13C NMR (100 MHz, CDCl3): δ
VarioEL V3.00 and CHN values were found within 162.83, 153.32, 150.67, 148.43, 141.96, 130.92,
±0.4 of theoretical values for all the final compounds. 129.94, 129.38, 128.19, 125.14, 124.58, 122.47,
The melting points were obtained by Perkin-Elmer 120.73, 119.97, 118.86, 116.92, 112.95, 110.59,
Differential Scanning Calorimetry. 78.92, 50.16; HRMS (ESI): m/z [M]+ calcd. for
C21H15BrN2O2: 406.0317; found: 406.5157. Anal.
General procedure for the synthesis of 2a-j, 3 and 4 Calcd. for C21H15BrN2O2.0.4H2O: C, 60.86; H, 3.84;
A mixture of amine 1 (1 mmol), substituted phenol N, 6.76. Found: C, 60.90; H, 3.89; N, 6.56%.
or β-naphthol (1 mmol) and formalin (37%, w/v, 2 3-(3-(Benzo[d]thiazol-2-yl)phenyl)-6-chloro-3,4-
mmol) was heated at 80-90°C for 2 hr. The additional dihydro-2H-benzo[e][1,3]oxazine, 2c: Rf: 0.69 (20%
amount of formalin (2 mmol) was added to the EtOAc in hexane); m.p. 134.8°C; Tmax (DSC
reaction mixture and heating was continued for exotherm): 270.9°C; IR (Nujol): 1597, 1452, 1365,
another 2 hr. The reaction mixture was diluted with 1260, 1217, 1161, 1048, 986, 951, 880, 819, 759, 726,
water (10 mL) and product was extracted with ethyl 682 cm-1; 1H NMR (400 MHz, CDCl3): δ 8.08 (d, J =
acetate (20 mL × 3 times). The organic layers were 10 Hz, 1H, ArH), 7.91 (d, J = 8.8 Hz, 1H, ArH), 7.87
combined and washed with 10% NaOH solution (15 (s, 1H, ArH), 7.61 (d, J = 8.8 Hz, 1H, ArH), 7.52-7.47
mL × 3 times) followed by water (15 mL × 3 times). (m, 1H, ArH), 7.41-7.36 (m, 2H, ArH), 7.21 (d, J =
The organic layer was dried over anhydrous sodium 7.6 Hz, 1H, ArH), 7.09-6.99 (m, 2H, ArH), 6.78 (d, J
sulfate and evaporated under reduced pressure to = 10 Hz, 1H, ArH), 5.44 (s, 2H, N-CH2-O), 4.73 (s,
obtain the crude compound. The product was purified 2H, N-CH2-Ar); 13C NMR (75 MHz, CDCl3): δ 167.9,
by column chromatography on silica gel using ethyl 154.0, 152.9, 148.6, 135.1, 134.7, 130.0, 128.0, 126.5,
acetate/hexane as eluent. 126.3, 125.7, 125.2, 123.2, 122.0, 121.6, 121.0, 120.2,
3-(3-(Benzo[d]oxazol-2-yl)phenyl)-6-chloro-3,4- 118.4, 116.8, 79.1, 50.3; HRMS (ESI): m/z [M]+
dihydro-2H-benzo[e][1,3]oxazine, 2a: Rf: 0.85 (30% calcd. for C21H15ClN2OS: 378.0594; found: 378.0157.
EtOAc in hexane); m.p. 98.3°C; Tmax (DSC Anal. Calcd. for C21H15ClN2OS.0.4H2O: C, 65.33; H,
exotherm): 280.3°C; IR (Nujol): 1603 (C=N), 1552, 4.12; N, 7.26. Found: C, 65.59; H, 3.97; N, 7.20%.
1476, 1456, 1376, 1234, 1160, 998, 936, 874, 821,
743 cm-1; 1H NMR (300 MHz, CDCl3): δ 8.00 (s, 1H, 3-(3-(Benzo[d]thiazol-2-yl)phenyl)-6-bromo-3,4-
ArH), 7.84 (d, J = 7.6 Hz, 1H, ArH), 7.81-7.78 (m, dihydro-2H-benzo[e][1,3]oxazine, 2d: Rf: 0.75 (20%
1H, ArH), 7.62-7.59 (m, 1H, ArH), 7.46-7.36 (m, 3H, EtOAc in hexane); m.p. 159.2°C; Tmax (DSC
ArH), 7.28-7.26 (m, 1H, ArH), 7.11-7.06 (m, 2H, exotherm): 255.1°C; IR (Nujol): 1597, 1508, 1462,
 PRASAD et al.: SYNTHESIS OF BENZAZOLES TETHERED DIHYDRO[1,3]OXAZINES 743

1377, 1313, 1263, 1160, 1049, 951, 876, 819, 760, 3-(3-(1H-Benzo[d]imidazol-2-yl)phenyl)-6-bro-
726, 687 cm-1; 1H NMR (400 MHz, CDCl3): δ 8.08 (d, mo-3,4-dihydro-2H-benzo[e][1,3]oxazine, 2g: Rf:
J = 9.2 Hz, 1H, ArH), 7.90 (d, J = 9.2 Hz, 1H, ArH), 0.72 (50% EtOAc in hexane); m.p. 201.8°C; Tmax
7.87 (s, 1H, ArH), 7.61 (d, J = 10.8 Hz, 1H, ArH), (DSC exotherm): 210.3°C; IR (KBr): 3053(NH), 1608
7.52-7.48 (m, 1H, ArH), 7.41-7.36 (m, 2H, ArH), (C=N), 1476, 1448, 1409, 1365, 1275, 1228, 1157,
7.26-7.19 (m, 3H, ArH), 6.68 (d, J = 10.4 Hz, 1H, 1120, 1096, 1009, 973, 943, 855, 746, 609 cm-1; 1H
ArH), 5.46 (s, 2H, N-CH2-O), 4.71 (s, 2H, N-CH2- NMR (300 MHz, DMSO-d6): δ 12.83 (s, 1H, NH),
Ar); 13C NMR (100 MHz, CDCl3): δ 167.90, 154.02, 7.92 (s, 1H, ArH), 7.64 (m, 2H, ArH), 7.51 (m, 1H,
153.35, 148.60, 135.05, 134.71, 130.92, 130.00, ArH), 7.40-7.37 (m, 2H, ArH), 7.24-7.22 (m, 4H,
129.39, 126.32, 125.22, 123.23, 122.53, 121.60, ArH), 6.74-6.70 (m, 1H, ArH), 5.55 (s, 2H, N-CH2-
121.04, 120.28, 118.86, 116.84, 112.94, 79.10, 50.22; O), 4.77 (s, 2H, N-CH2-Ar); HRMS (ESI): m/z [M]+
HRMS (ESI): m/z [M]+ calcd. for C21H15BrN2OS: calcd. for C21H16BrN3O: 405.0477; found: 405.4429.
422.0088; found: 422.0244. Anal. Calcd. for Anal. Calcd. for C21H16BrN3O.H2O: C, 59.45; H,
C21H15BrN2OS.0.5H2O: C, 58.34; H, 3.73; N, 6.48. 4.28; N, 9.90. Found: C, 59.52; H, 4.22; N, 9.51%.
Found: C, 58.21; H, 3.63; N, 6.33%. 3-(3-(1H-Benzo[d]imidazol-2-yl)phenyl)-3,4-di-
3-(3-(1H-Benzo[d]imidazol-2-yl)phenyl)-6-fluo- hydro-6-iodo-2H-benzo[e][1,3]oxazine, 2h: Rf: 0.75
ro-3,4-dihydro-2H-benzo[e][1,3]oxazine, 2e: Rf: (50% EtOAc in hexane); m.p. 215.3°C; Tmax (DSC
0.70 (50% EtOAc in hexane); m.p. 211.5°C; Tmax exotherm): 218.7°C; IR (KBr): 3047(NH), 1607
(DSC exotherm): 216.1°C; IR (KBr): 3040 (NH), (C=N), 1585, 1534, 1474, 1447, 1403, 1364, 1319,
1604 (C=N), 1497, 1476, 1449, 1410, 1364, 1245, 1276, 1228, 1189, 1157, 1122, 1009, 968, 940, 853,
1225, 1198, 1167, 1132, 1011, 968, 929, 866, 815, 821, 766, 747, 683, 598, 476 cm-1; 1H NMR (300
795, 766, 745, 688 cm-1; 1H NMR (300 MHz, DMSO- MHz, DMSO-d6): δ 12.90 (s, 1H, NH), 7.99 (s, 1H,
d6): δ 12.96 (s, 1H, NH), 7.99 (s, 1H, ArH), 7.73-7.71 ArH), 7.73-7.70 (m, 2H, ArH), 7.57 (m, 2H, ArH),
(m, 2H, ArH), 7.59 (m, 1H, ArH), 7.50-7.44 (m, 1H, 7.48-7.43 (m, 2H, ArH), 7.30-7.23 (m, 3H, ArH), 6.65
ArH), 7.32-7.27 (m, 3H, ArH), 7.13-7.02 (m, 1H, (d, J = 8.7 Hz, 1H, ArH), 5.61 (s, 2H, N-CH2-O), 4.82
ArH), 7.00-6.96 (m, 1H, ArH), 6.86-6.82 (m, 1H, (s, 2H, N-CH2-Ar); HRMS (ESI): m/z [M]+ calcd. for
ArH), 5.58 (s, 2H, N-CH2-O), 4.83 (s, 2H, N-CH2- C21H16IN3O: 453.0338; found: 453.1175. Anal. Calcd.
Ar); 13C NMR (75 MHz, DMSO-d6): δ 157.61, for C21H16IN3O: C, 55.64; H, 3.56; N, 9.27. Found: C,
154.47, 151.28, 150.14, 150.11, 148.02, 131.04, 55.79; H, 3.87; N, 8.92%.
129.79, 122.55, 122.46, 118.66, 118.47, 117.56, 3-(3-(1H-Benzo[d]imidazol-2-yl)phenyl)-6,8-di-
117.46, 114.85, 114.52, 114.21, 113.54, 113.23, chloro-3,4-dihydro-2H-benzo[e][1,3]oxazine, 2i: Rf:
78.30, 48.75; HRMS (ESI): m/z [M]+ calcd. for 0.74 (50% EtOAc in hexane); m.p. 238.4°C; Tmax
C21H16FN3O: 345.1277; found: 345.3566. Anal. (DSC exotherm): 242.9°C; IR (KBr): 3081(NH), 1608
Calcd. for C21H16FN3O.0.5H2O: C, 71.17; H, 4.84; N, (C=N), 1590, 1541, 1482, 1465, 1433, 1399, 1363,
11.86. Found: C, 70.94; H, 4.59; N, 11.60%. 1316, 1240, 1187, 1147, 1118, 1010, 974, 931, 859,
3-(3-(1H-Benzo[d]imidazol-2-yl)phenyl)-6-chlo- 831, 794, 764, 747, 710, 683 cm-1; 1H NMR (300
ro-3,4-dihydro-2H-benzo[e][1,3]oxazine, 2f: Rf: MHz, DMSO-d6): δ 12.87 (s, 1H, NH), 7.93 (s, 1H,
0.75 (50% EtOAc in hexane); m.p. 208.9°C; Tmax ArH), 7.68 (d, J = 7.2 Hz, 1H, ArH), 7.58 (m, 2H,
(DSC exotherm): 212.7°C; IR (KBr): 3047 (NH), ArH), 7.44-7.39 (m, 2H, ArH), 7.29 (s, 1H, ArH),
1604 (C=N), 1585, 1476, 1448, 1409, 1364, 1249, 7.25-7.20 (m, 3H, ArH), 5.69 (s, 2H, N-CH2-O), 4.81
1232, 1194, 1159, 1121, 1096, 1011, 967, 950, 881, (s, 2H, N-CH2-Ar); HRMS (ESI): m/z [M]+ calcd. for
870, 817, 794, 765, 745, 688, 635 cm-1; 1H NMR C21H15Cl2N3O: 395.0592; found: 395.4139. Anal.
(300 MHz, DMSO-d6): δ 12.92 (s, 1H, NH), 8.00 (s, Calcd. for C21H15Cl2N3O.0.4H2O: C, 62.51; H, 3.95;
1H, ArH), 7.74-7.71 (m, 2H, ArH), 7.57 (m, 1H, N, 10.41. Found: C, 62.80; H, 4.20; N, 10.02%.
ArH), 7.48-7.46 (m, 1H, ArH), 7.33-7.17 (m, 5H, 3-(3-(1H-Benzo[d]imidazol-2-yl)phenyl)-3,4-di-
ArH), 6.84 (d, J = 8.7 Hz, 1H, ArH), 5.62 (s, 2H, N- hydro-6-methyl-2H-benzo[e][1,3]oxazine, 2j: Rf:
CH2-O), 4.84 (s, 2H, N-CH2-Ar); HRMS (ESI): m/z 0.75 (50% EtOAc in hexane); m.p. 212.9°C; Tmax
[M]+ calcd. for C21H16ClN3O: 361.0982; found: (DSC exotherm): 218.4°C; IR (KBr): 3045 (NH),
361.9693. Anal. Calcd. for C21H16ClN3O.0.5H2O: C, 1605 (C=N), 1582, 1501, 1493, 1459, 1444, 1409,
68.02; H, 4.62; N, 11.33. Found: C, 68.30; H, 4.40; 1367, 1313, 1292, 1276, 1259, 1227, 1174, 1138,
N, 11.02%. 1120, 1009, 971, 952, 917, 812, 792, 764, 742, 682,
744 INDIAN J. CHEM., SEC B, MAY 2012

599, 467, 442 cm-1; 1H NMR (300 MHz, DMSO-d6): the reaction mixture after 2.5 hr and the heating was
δ 12.84 (s, 1H, NH), 7.91 (s, 1H, ArH), 7.65-7.63 (m, continued for additional 2.5 hr. After completion of
2H, ArH), 7.51 (d, J = 7.2 Hz, 1H, ArH), 7.41-7.36 the reaction, the mixture was cooled to RT and diluted
(m, 1H, ArH), 7.23-7.14 (m, 3H, ArH), 6.94 (s, 1H, with water (10 mL). The product was extracted with
ArH), 6.88 (d, J = 8.1 Hz, 1H, ArH), 6.64 (d, J = 8.1 ethyl acetate (10 mL×3 times). The organic layers
Hz, 1H, ArH), 5.49 (s, 2H, N-CH2-O), 4.71 (s, 2H, N- were combined and washed with 10% aqueous NaOH
CH2-Ar) 2.18 (s, 3H, CH3); HRMS (ESI): m/z [M]+ solution (10 mL× 3 times) followed by water (10
calcd. for C22H19N3O: 341.1528; found: 341.3740. mL×3 times). The organic layer was dried over
2-(3-(Benzo[d]oxazol-2-yl)phenyl)-2,3-dihydro- anhydrous sodium sulfate and evaporated under
1H-naphtho[1,2-e][1,3]oxazine, 3: Rf: 0.80 (20% reduced pressure. The crude product was purified
EtOAc in hexane); m.p. 156.4°C; Tmax (DSC over silica gel column by using 15% ethyl acetate in
exotherm): 270.2°C; IR (KBr): 1598 (C=N), 1548, hexane as eluent.
1455, 1377, 1295, 1229, 1153, 1009, 942, 910, 821, Rf: 0.70 (30% EtOAc in hexane); m.p. 196.5°C;
745, 682 cm-1; 1H NMR (300 MHz, CDCl3): δ 7.98 (s, Tmax (DSC exotherm): 264.7°C; IR (CHCl3): 1602
1H, ArH), 7.72-7.62 (m, 4H, ArH), 7.56 (d, J = 8.7 (C=N), 1550, 1484, 1452, 1380, 1244, 1191, 1114,
Hz, 1H, ArH), 7.47-7.41 (m, 2H, ArH), 7.30-7.15 (m, 1000, 935, 860, 791, 758, 684 cm-1; 1H NMR (300
5H, ArH), 6.98 (d, J = 8.7 Hz, 1H, ArH), 5.41 (s, 2H, MHz, CDCl3): δ 8.06 (s, 1H, ArH), 8.03 (s, 1H, ArH),
N-CH2-O), 4.95 (s, 2H, N-CH2-Ar); 13C NMR (100 7.88 (d, J = 7.8 Hz, 1H, ArH), 7.83 (d, J = 7.5 Hz,
MHz, CDCl3): δ 162.93, 152.19, 150.65, 149.04, 1H, ArH), 7.79-7.76 (m, 1H, ArH), 7.67-7.57 (m, 3H,
141.97, 131.08, 129.85, 128.99, 128.64, 128.40, ArH), 7.50-7.25 (m, 6H, ArH), 7.18 (d, J = 7.8 Hz,
128.08, 126.73, 125.07, 124.52, 123.73, 120.90, 1H, ArH), 5.51 (s, 2H, N-CH2-O), 4.80 (s, 2H, N-
120.69, 120.42, 119.92, 118.74, 116.94, 112.32, CH2-Ar); HRMS (ESI): m/z [MH]+ calcd. for
110.57, 78.63, 48.33; HRMS (ESI): m/z [M]+ calcd. C28H20N3O2S: 462.1276; found: 462.0231.
for C25H18N2O2: 378.1368; found: 378.0103. Anal.
Calcd. for C25H18N2O2: C, 77.50; H, 4.94; N, 7.23. In vitro antibacterial assay
Found: C, 77.78; H, 5.25; N, 6.93%. The in-vitro antibacterial activity of all the prepared
2-(3-(1H-Benzo[d]imidazol-2-yl)phenyl)-2,3-di- compounds was evaluated against Gram-positive
hydro-1H-naphtho[1,2-e][1,3]oxazine, 4: Rf: 0.80 bacteria viz. S. epidermidis and S. aureus and Gram-
(50% EtOAc in hexane); m.p. 200°C; IR (KBr): 3047 negative bacteria viz. E. coli and P. aeruginosa and
(NH), 1605 (C=N), 1584, 1493, 1472, 1442, 1406, results are shown in Table I. The antimicrobial
1367, 1230, 1188, 1098, 1058, 1010, 952, 810, 742, susceptibility testing was carried out by using National
692 cm-1; 1H NMR (300 MHz, DMSO-d6): δ 12.84 (s, Committee for Clinical Laboratory Standards
1H, NH), 8.01 (s, 1H, ArH), 7.92 (d, J = 9 Hz, 1H, (NCCLS) micro-dilution assay. Briefly, the bacterial
ArH), 7.84 (d, J = 8.1 Hz, 1H, ArH), 7.74-7.63 (m, strains were grown in standard media until exponential
3H, ArH), 7.56-7.51 (m, 2H, ArH), 7.42-7.33 (m, 3H, growth was achieved. Tests were performed in a 96-
ArH), 7.20-7.16 (m, 2H, ArH), 7.06 (d, J = 9 Hz, 1H, well microtitre plate in a final volume of 100 µL. Test
ArH), 5.61 (s, 2H, N-CH2-O), 5.08 (s, 2H, N-CH2- compounds were dissolved in 5% DMSO at an initial
Ar); HRMS (ESI): m/z [M]+ calcd. for C25H19N3O: concentration of 500 µg and serially diluted in plate.
377.1528; found: 377.4210. Anal. Calcd. for Each well was then inoculated with ~2-5 × 105
C25H19N3O.H2O: C, 75.93; H, 5.35; N, 10.63. Found:
bacterial cells and incubated at 37°C for 24 hr with
C, 76.05; H, 5.65; N, 10.42%.
shaking at 200 rpm. One well containing cells and 5%
DMSO without any test compound (growth control),
Synthesis of 3-(3-(benzo[d]oxazol-2-yl)phenyl)-7-
and one well containing only growth medium (sterility
(benzo[d]thiazol-2-yl)-3,4-dihydro-2H-benzo[e]-
control) were used as controls. Growth of bacteria was
[1,3]oxazine, 5
determined using Power wave200 microplate scanning
A mixture of 2-(3-aminophenyl)benzoxazole (100 spectrophotometer (Bio-Tech Instruments, Winooski,
mg, 0.475 mmol), 3-(benzo[d]thiazol-2-yl)phenol VT, USA). Percent survival was calculated using
(108 mg, 0.475 mmol) and formalin (37% w/v, 82 µL, growth without any compound as 100% survival. The
0.950 mmol) was heated at 80-90°C. A second lot of IC50 values are calculated using Grafit 4.0 software
formalin (37% w/v, 82 µL, 0.950 mmol) was added to (Erithacus Software Ltd., Horley, Surrey, UK).
 PRASAD et al.: SYNTHESIS OF BENZAZOLES TETHERED DIHYDRO[1,3]OXAZINES 745

Acknowledgements 16 Tabuchi Y, Ando Y, Kanemura H, Kawasaki I, Ohishi T,

Koida M, Fukuyama R, Nakamuta H, Ohta S, Nishide K &
The authors thank University of Delhi, India for Ohishi Y, Bioorg Med Chem, 17, 2009, 3959.
providing a research grant under the scheme to 17 Wang S, Li Y, Liu Y, Lu A & You Q, Bioorg Med Chem Lett,
strengthen R & D Doctoral Research programme. One 18, 2008, 4095.
18 Vrouenraets S M E, Wit F W N M, van Tongeren J & Lange J
of the authors (DP) is grateful to Council of Scientific M A, Expert Opin Pharmacother, 8, 2007, 851.
and Industrial Research (CSIR), New Delhi, India, for 19 Aiello S, Wells G, Stone E L, Kadri H, Bazzi R, Bell D R,
Senior Research Fellowship. Stevens M F G, Matthews C S, Bradshaw T D & Westwell A
D, J Med Chem, 51, 2008, 5135.
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