Triton-B catalyzed, efficient one-pot synthesis of carbazates

Article abstract of DOI:10.1007/s00706-008-0955-8

A quick and efficient, one-pot synthesis of carbazates was accomplished in high yields by the reaction of various primary, secondary, and tertiary alkyl halides with a variety of substituted hydrazines using the benzyltrimethylammonium hydroxide (Triton-B)/CO₂ system. The reaction conditions are mild with simpler work-up procedures than the reported methods.

Full text of DOI:10.1007/s00706-008-0955-8

Monatsh Chem 139, 1463–1466 (2008)
DOI 10.1007/s00706-008-0955-8
Printed in The Netherlands
Triton-B catalyzed, efficient one-pot synthesis of carbazates
Devdutt Chaturvedi1, Amit K. Chaturvedi2, Nisha Mishra2, Virendra Mishra2
1
Bio-Organic Chemistry Division, Indian Institute of Integrative Medicine, Jammu-Tawi, J&K, India
2
Synthetic Research Laboratory, Department of Chemistry, B.S.A.P.G. College, Mathura, U.P., India
Received 11 April 2008; Accepted 4 May 2008; Published online 31 July 2008
# Springer-Verlag 2008
Abstract A quick and efficient, one-pot synthesis of
carbazates was accomplished in high yields by the
reaction of various primary, secondary, and tertiary
alkyl halides with a variety of substituted hydrazines
using the benzyltrimethylammonium hydroxide (Tri-
ton-B)=CO2 system. The reaction conditions are mild
with simpler work-up procedures than the reported
methods.
conditions, such as use of strong bases, high reaction
temperatures, and long reaction times [11]. Thus, we
were prompted to embark on the improved proce-
dures. Our group [12] has been engaged during the
past several years in the development of new meth-
odologies for the preparation of carbamates, dithio-
carbamates, and related compounds using cheap,
abundantly available, and safe reagents like CO2
and CS2. Recently [13], we have found that benzyl-
trimethylammonium hydroxide (Triton-B) is the best
catalyst for the synthesis of carbamates, dithiocarba-
mates, and dithiocarbonates (xanthates). We report
here an efficient, one-pot synthesis of carbazates
from a variety of primary, secondary, and tertiary
alkyl halides and substituted hydrazines using the
Triton-B=CO2 system.
Keywords Alkyl halides; Benzyltrimethylammonium hy-
droxide; Carbon dioxide; Substituted hydrazines; Carbazates.
Introduction
Organic carbazates have attracted much attention due
to their numerous remarkable medicinal, industrial,
and synthesis applications [1, 2]. They have exten-
sively been used as pharmaceuticals [3], agrochem-
icals [4], intermediates in organic synthesis [5], for
protection of amino groups in peptide synthesis [6],
as linkers in solid phase organic synthesis [7], and as
donor ligands in complexation reactions with transi-
tion metals [8]. To satisfy their demand, their syn-
thesis has been changed from the use of costly and
toxic chemicals like phosgene [9] and its derivatives
[10] directly or indirectly, to the abundantly avail-
able cheap and safe reagents like CO2. However,
their formation using CO2 employed harsh reaction
Results and discussion
Substituted hydrazine was taken in dry DMSO and
purified CO2 gas was bubbled in it at 60ꢀC for
30 min with constant stirring. Now, Triton-B was
slowly added with constant stirring. The reaction
was continued for another 30 min, and then the cor-
responding alkyl halide was added. The reaction was
further continued until completion as checked by
TLC (see Table 1). It is proposed that the Oꢁ of the
carbazate ion produced will attack the electrophilic
carbon of the respective alkyl halide to afford carba-
zates in high yields (80–98%) at room temperature
in 3–5 h, as mentioned in Table 1. The reaction
proved to be successful and the desired products
Correspondence: Devdutt Chaturvedi, Bio-Organic Chem-
istry Division, Indian Institute of Integrative Medicine,
Canal Road, Jammu-Tawi 18000, J&K, India. E-mail:
ddchaturvedi002@yahoo.co.in

1464
D. Chaturvedi et al.
Table 1 Conversion of alkyl halides into carbazates 1–16a
Product
R1
R2
R3
X
R
Time=h
Yields=%b
1
2
3
4
5
6
7
8
n-C3H7
PhCH2CH2
PhCH2
Ph
H
H
H
H
Me
H
H
H
H
C4H9
C4H9
H
H
H
H
H
H
H
H
H
H
H
H
H
C4H9
H
H
H
Br
Br
Cl
Cl
Br
Cl
Br
Br
Br
Br
Br
Cl
Cl
Cl
Br
Br
4-MeO-Ph
Ph
Ph
3
3
4
94
96
87
92
90
85
85
80
83
89
88
96
96
98
86
82
Bn
Bn
4.5
4.5
4.5
4.5
5
4.5
4.5
4.5
3
3
3
4.5
5
C2H5
Ph-4-MeO
C3H7
C3H7
C3H7
C4H9
C4H9
C5H11
C7H15
C9H19
C3H7
Ph-3-NO2
Ph-4-NO2
Ph-2,4-NO2
Naphthyl
Ph
Ph
n-C4H9
Ph
9
10
11
12
13
14
15
16
n-C4H9
Ph
Ph
C3H7
CH3
H
H
Ph
a
All the products were characterized by IR, NMR, and mass spectroscopic data
Isolated yields
b
Scheme 1
were isolated and their structures confirmed by vari-
experimental convenience. This synthesis protocol
developed is believed to offer a more general method
for the formation of carbon–oxygen bonds essential
to numerous organic syntheses.
ous spectroscopic and analytical techniques. Thus,
various substituted hydrazines were reacted with a
variety of primary, secondary, and tertiary alkyl
halides using the Triton-B=CO2 system to afford
the corresponding carbazates in good to excellent
yields (Table 1) with the reaction conditions as
shown in Scheme 1.
We tried several solvents like n-heptane, n-hexane,
acetonitrile, benzene, toluene, methanol, dichloro-
methane, chloroform, DMSO, dimethylformamide,
and hexamethylphosphoric triamide of which dry
DMSO proved to be the most suitable one at room
temperature.
In conclusion, we developed a convenient and
efficient protocol for the one-pot, three-component
coupling of various substituted hydrazines with a
variety of primary, secondary, and tertiary alkyl
halides via a CO2 bridge using Triton-B. This meth-
od generates the corresponding carbazates in good to
excellent yields. Furthermore, this method exhibits
substrate versatility, mild reaction conditions, and
Experimental
Chemicals were procured from Merck, Aldrich, and Fluka
chemical companies. Reactions were carried out under
Argon. IR spectra (4000–200 cmꢁ1) were recorded on a
Bomem MB-104-FTIR spectrophotometer using neat tech-
nique, whereas NMRs were scanned on an AC-300F NMR
(300MHz) instrument using CDCl3 and TMS as internal stan-
dard. Elemental analysis were conducted by means of a Carlo-
Erba EA 1110-CNNO-S analyzer and agreed favorably with
calculated values.
Typical experimental procedure
To a stirred solution (under Ar) of 3 mmol substituted hydra-
zine in 5 cm3 anhyd. DMSO carbon dioxide was continuously
bubbled and 2 cm3 Triton-B were added at 60ꢀC. Then the
mixture was stirred for 0.5 h at which point 3 cm3 of the
required alkyl halide were added over a period of 5 min.
The stirring was further continued until the completion of

Triton-B catalyzed, efficient one-pot synthesis of carbazates
1465
reaction (cf. Table 1). The reaction mixture was poured
into 20cm3 water and the organic layer was extracted with
3ꢂ10 cm3 EtOAc. The organic layer was washed with 20cm3
0.1 N HCl, 25cm3 saturated NaHCO3 solution, 30 cm3 brine,
and then dried (Na2SO4) and concentrated to get the desired
compound.
ꢁ ¼ 69.3, 107.6, 114.8, 118.8, 128.5, 129.9, 133.6, 143.6,
148.7, 160.6 (C¼O) ppm; MS: m=z ¼ 317.
N0-(4-Nitrophenyl)hydrazine carboxylic acid butyl ester
(7, C11H15N3O4)
1
Oil; IR (neat): ꢀꢀ¼ 1682 cmꢁ1; H NMR (CDCl3): ꢁ ¼ 0.99
(t, J ¼ 7.2 Hz, 3H, CH3), 1.36 (m, 2H, CH2CH3), 1.57 (m,
2H, OCH2CH2), 4.04 (br, NHArNO2), 6.92–8.15 (m, 4H,
Ar–H), 8.0 (br, NH) ppm; 13C NMR (CDCl3): ꢁ ¼ 13.8,
21.7, 32.3, 63.7, 113.5, 124.6, 138.8, 143.3, 159 (C¼O)
ppm; MS: m=z ¼ 253.
N0-(4-Methoxyphenyl)hydrazine carboxylic acid butyl ester
(1, C12H18N2O3)
Oil; IR (neat): ꢀꢀ¼ 1680 cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.96 (t,
3H, J ¼ 7.3 Hz), 1.34 (m, 2H), 1.86 (m, 2H), 3.73 (s, 3H), 4.12
(t, J ¼ 6.5 Hz, 2H), 4.85 (m, NH), 6.74–7.66 (m, 4H), 8.0 (br,
NH) ppm; 13C NMR (CDCl3): ꢁ ¼ 13.7, 19.5, 32.5, 63.5,
112.5, 114.9, 134.5, 152.4, 160.6 (C¼O) ppm; MS (EI):
m=z ¼ 238.
N0-(2,4-Dinitrophenyl)hydrazine carboxylic acid butyl ester
(8, C11H14N4O6)
Oil; IR (neat): ꢀꢀ¼ 1681cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.98 (t,
J ¼ 7.2Hz, 3H, CH3), 1.36 (m, 2H, CH2CH3), 1.59 (m, 2H,
SCH2CH2), 4.08 (br, NHArNO2), 7.19–9.50 (m, 3H, Ar–H),
8.10 (br, NH) ppm; 13C NMR (CDCl3): ꢁ ¼ 13.8, 19.3, 31.8,
63.8, 113.6, 119.2, 130.2, 132.8, 139.7, 143.3, 160 (C¼O)
ppm; MS: m=z ¼ 298.
N0-Phenylhydrazine carboxylic acid 3-phenyl propyl
ester (2, C16H18N2O2)
Oil; IR (neat): ꢀꢀ¼ 1685cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 1.92 (m,
2H), 2.56 (t, 2H, J ¼ 7.2 Hz, -PhCH2), 4.10 (t, J ¼ 6.5 Hz, 2H),
4.67 (br, NHPh), 6.66–7.12 (m, 10H, Ar–H), 8.0 (br, NH)
ppm; 13C NMR (CDCl3): ꢁ ¼ 32.4, 34.4, 63.5, 112.6, 119.4,
125.7, 128.8, 129.6, 138.7, 161 (C¼O) ppm; MS: m=z ¼ 270.
N0-Naphth-2-ylhydrazine carboxylic acid butyl ester
(9, C15H18N2O2)
Oil; IR (neat): ꢀꢀ¼ 1681cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.96 (t,
J ¼ 7.2Hz, 3H, CH3), 1.36 (m, 2H, CH2CH3), 1.57 (m, 2H,
OCH2CH2), 4.05 (br, H, Ar–NH), 4.12 (t, J ¼ 7.0 Hz, 2H),
6.76–7.55 (m, 7H, Ar–H), 8.02 (br, NH) ppm; 13C NMR
(CDCl3): ꢁ ¼ 13.9, 22.1, 32.5, 33.9, 107.4, 117.2, 121.3, 124.5,
126.6, 127.2, 133.5, 142.6, 161 (C¼O) ppm; MS: m=z ¼ 258.
N0-Phenylhydrazine carboxylic acid phenethyl ester
(3, C15H16N2O2)
1
Oil; IR (neat): ꢀꢀ¼ 1681 cmꢁ1; H NMR (CDCl3): ꢁ ¼ 2.83
(2H, t, J ¼ 6.7 Hz, PhCH2CH2O), 4.42 (t, 2H, J ¼ 7.2 Hz,
PhCH2O), 4.77 (br, H, PhNH), 6.69–7.15 (m, 10H, Ar–H),
8.05 (br, NH) ppm; 13C NMR (CDCl3): ꢁ ¼ 35.5, 65.9, 112.3,
118.6, 128.5, 129.5, 140.3, 142.5, 165.4 (C¼O) ppm; MS:
m=z ¼ 256.
N0-Phenylhydrazine carboxylic acid 1-butylpentyl ester
(10, C16H26N2O2)
Oil; IR (neat): ꢀꢀ¼ 1682cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.99 (t,
J ¼ 7.2Hz, 6H, CH3), 1.33 (m, 4H, CH2CH2CH), 1.38 (m, 4H,
CH2CH3), 1.54 (m, 4H, CHCH2), 3.95 (t, OCH), 4.15 (br,
NHAr), 6.66–7.18 (m, 5H, Ar–H), 8.0 (br, NH) ppm; 13C
NMR (CDCl3): ꢁ ¼ 14.3, 23.1, 28.5, 36.2, 72.7, 112.2,
119.3, 129.0, 142.4, 158 (C¼O) ppm; MS: m=z ¼ 278.
N0-Butylhydrazine carboxylic acid benzyl ester
(4, C12H18N2O2)
Oil; IR (neat): ꢀꢀ¼ 1680 cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.99 (t,
J ¼ 7.2 Hz, 3H, CH3), 1.34 (m, 2H, CH2CH3), 1.56 (m, 2H,
CH2CH2CH3), 2.15 (br, NH), 2.66 (m, 2H, NHCH2), 5.13 (s,
2H, PhCH2), 7.10–7.19 (m, 5H, Ar–H), 8.0 (br, NH) ppm; 13
C
N0-Phenylhydrazine carboxylic acid 1,1-dibutylpentyl ester
NMR (CDCl3): ꢁ ¼ 13.8, 20.3, 31.6, 51.2, 69.3, 126.8, 127.6,
128.5, 141.8, 158 (C¼O) ppm; MS: m=z ¼ 222.
(11, C20H34N2O2)
1
Oil; IR (neat): ꢀꢀ¼ 1684 cmꢁ1; H NMR (CDCl3): ꢁ ¼ 0.96
N0-Butylhydrazine carboxylic acid sec-butyl ester
(5, C9H20N2O2)
(t, J ¼ 7.2 Hz, 9H, CH3), 1.29 (m, 4H, CH2CH2C), 1.33 (m,
4H, CH2CH3), 1.50 (m, 4H, CHCH2), 4.0 (br, H, NH–Ar),
6.67–7.19 (m, 5H, Ar–H), 8.0 (br, NH) ppm; 13C NMR
(CDCl3): ꢁ ¼ 14.3, 23.5, 26.8, 39.8, 72.4, 112.5, 119.3,
129.6, 142.2, 162 (C¼O) ppm; MS: m=z ¼ 334.
Oil; IR (neat): ꢀꢀ¼ 1681 cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.98 (t,
J ¼ 7.2 Hz, 3H, CH3), 1.15 (t, J ¼ 7.0 Hz, 3H, CH3), 1.38 (m,
2H, CH2CH3), 1.42 (d, J ¼ 6.5 Hz, 3H, CHCH3), 1.56 (m,
2H, CH3CH2CH2), 2.0 (br, NH), 2.66 (m, 2H, NHCH2),
4.20 (m, CHCH3), 8.0 (br, NH) ppm; 13C NMR (CDCl3):
ꢁ ¼ 8.2, 13.8, 19.2, 20.5, 29.3, 71.4, 156.9 (C¼O) ppm; MS:
m=z ¼ 188.
N0-Butylhydrazine carboxylic acid hexyl ester
(12, C11H24N2O2)
Oil; IR (neat): ꢀꢀ¼ 1684cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.98 (t,
J ¼ 7.0 Hz, 6H, CH3), 1.30 (m, 4H, CH2CH2CH2CH3), 1.36
(t, J ¼ 7.0 Hz, 2H, CH2CH3), 1.58 (m, 2H, NHCH2CH2), 1.63
(t, J ¼ 6.5 Hz, 2H, CH2N), 2.0 (br, 2H, NH), 2.66 (t, 2H,
NHCH2), 4.10 (t, 2H, J ¼ 7.2 Hz, OCH2), 8.0 (br, NH) ppm;
13C NMR (CDCl3): ꢁ ¼ 13.7, 14.1, 20.2, 23.1, 28.6, 31.5, 32.6,
69.5, 164.5 (C¼O) ppm; MS: m=z ¼ 216.
N0-(3-Nitrophenyl)hydrazine carboxylic acid 4-methoxy-
benzyl ester (6, C15H15N3O5)
Oil; IR (neat): ꢀꢀ¼ 1682 cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 3.73 (s,
3H, OCH3), 4.05 (br, H, NHPhOMe), 5.34 (s, 2H), 6.66–7.69
(m, 8H, Ar–H), 8.1 (br, NH) ppm; 13C NMR (CDCl3):

1466
D. Chaturvedi et al.
N0-Phenylhydrazine carboxylic acid n-octyl ester
(13, C15H24N2O2)
(1999) Chin Chem Lett 10:365; c) Chaturvedi K, Jaiswal
AK, Mishra KN, Pandey OP, Sengupta SK (1998)
ACH-Models Chem 135:93; d) Briggs GC, Cornell
CL, Mansfield DJ, Turner RM (1999) PCT Int Appl
1999, WO9923066, CAN: 130:337920
Oil; IR (neat): ꢀꢀ¼ 1685 cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.96 (t,
J ¼ 7.2 Hz, 3H, CH3), 1.30 (m, 8H, CH2), 1.35 (m, 2H,
CH2CH3), 1.63 (m, 2H, OCH2CH2), 4.0 (br, PhNH), 4.12 (t,
2H, J ¼ 7.0 Hz, OCH2), 6.66–7.25 (m, 5H, Ar–H), 8.0 (br,
NH) ppm; 13C NMR (CDCl3): ꢁ ¼ 14.5, 23.10, 27.5, 30.5,
32.5, 63.6, 112.2, 129.6, 118.9, 142.2, 163 (C¼O) ppm;
MS: m=z ¼ 264.
5. a) Connolly TJ, Crittal AJ, Ebrahim AS, Ji G (2000) Org
Process Res Dev 4:526; b) Gordan MS, Krause JG,
Linneman-Mohr MA, Parchue RR (1980) Synthesis
3:244; c) Hauser MJ (1966) J Org Chem 31:968
6. a) Sakakibara S, Honda I, Naruse M, Kanaoka M (1969)
Experimentia 25:576; b) Sakakibara S, Honda I, Takada
K, Miyoshi M, Ohnishi T, Okumura K (1969) Bull Chem
Soc Jpn 42:809; c) Quibell M, Turnell WG, Johnson T
(1993) J Chem Soc Perkin Trans I 22:2843; d) Gray CJ,
Quibell M, Baggett N, Hammerle T (1992) Int J Pept
Protien Res 40:351
N0-Butylhydrazine carboxylic acid decyl ester
(14, C15H32N2O2)
Oil; IR (neat): ꢀꢀ¼ 1683 cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.97 (s,
3H, CH3), 0.99 (s, 3H, CH3), 1.29 (m, 12H, CH2), 1.34 (m,
4H, CH2CH3), 1.57 (m, 2H, CH2CH2CH3), 2.0 (br, NH), 2.65
(m, 2H, NHCH2), 4.12 (t, J ¼ 7.0 Hz, 2H, OCH2), 8.0 (br, NH)
ppm; 13C NMR (CDCl3): ꢁ ¼ 13.7, 14.5, 20.3, 23.1, 28.9,
30.6, 30.9, 31.5, 32.5, 63.5, 160 (C¼O) ppm; MS: m=z ¼ 272.
7. a) Huang JY, Choi HS, Lee DH, Yoo SE, Gong YD (2005)
J Comb Chem 7:136; b) Hwang JY, Choi HS, Lee DH,
Gong YD (2005) J Comb Chem 7:816
8. a) Bolzati C, Benini E, Cavazza-Ceccato M, Cozzala E,
Malago E, Agostini S, Tisato F, Rofosco F, Bandoli G
(2006) Bioconjugate Chem 17:419; b) Ali MA, Mirza
AH, Butcher RJ, Krause KA (2006) Transit Metal Chem
31:79; c) Singh R, Kaushik NK (2004) Main Group Met
Chem 27:327
9. Dyker H, Scherkenbeck J, Gondol D, Goehrt A, Harder A
(2001) J Org Chem 66:3760
10. a) Dutta AS, Morley JS (1975) J Chem Soc Perkin Trans I
1712; b) Nara S, Sakamoto T, Miyazawa E, Kikugawa Y
(2003) Synth Commun 33:87
N0-Phenylhydrazine carboxylic acid 1-propylbutyl ester
(15, C14H22N2O2)
Oil; IR (neat): ꢀꢀ¼ 1680 cmꢁ1; 1H NMR (CDCl3): ꢁ ¼ 0.97 (s,
3H, CH3), 1.33 (m, 4H, CH2CH3), 1.54 (m, 4H, CHCH2), 3.95
(m, H, CH–O), 4.1 (br, H, NH–Ar), 6.66–7.22 (m, 5H, Ar–
H), 8.0 (br, NH) ppm; 13C NMR (CDCl3): ꢁ ¼ 14.5, 20.1, 38.4,
72.8, 112.5, 118.3, 129.6, 143.3, 160 (C¼O) ppm; MS:
m=z ¼ 250.
N0-Phenylhydrazine carboxylic acid 1-phenyl ethyl ester
11. a) Saxena A, Tandon JP (1983) Polyhedron 2:443; b) Fox
DL, Ruxer JT, Liver RM, Alford KL, Salvatore RN
(2004) Tetrahedron Lett 45:401
(16, C15H16N2O2)
1
Oil; IR (neat): ꢀꢀ¼ 1682 cmꢁ1; H NMR (CDCl3): ꢁ ¼ 1.69
(d, J ¼ 7.2Hz, 3H, CH3), 4.2 (br, NH–Ph), 5.42 (m, CH–O),
6.66–7.22 (m, 10Ar–H), 8.0 (br, NH) ppm; 13C NMR
(CDCl3): ꢁ ¼ 23.4, 74.2, 112.5, 118.9, 126.5, 128.5, 129.7,
141.3, 142.5, 163.5 (C¼O) ppm; MS: m=z ¼ 256.
12. For Reviews see: a) Chaturvedi D, Ray S (2007) Curr
Org Chem 11:987; b) Chaturvedi D, Mishra N, Mishra V
(2007) Curr Org Synthesis 3:308; For research work
see: c) Chaturvedi D, Kumar A, Ray S (2002) Synth
Commun 32:2651; d) Chaturvedi D, Kumar A, Ray S
(2003) Tetrahedron Lett 44:7637; e) Chaturvedi D, Ray
S (2005) Lett Org Chem 2:742; f) Chaturvedi D, Ray S
(2005) J Sulfur Chem 26:365; g) Chaturvedi D, Ray S
(2006) J Sulfur Chem 27:265; h) Chaturvedi D,
Ray S (2006) Tetrahedron Lett 47:1307; i) Chaturvedi
D, Mishra N, Mishra V (2006) Chinese Chem Lett
17:1309; j) Chaturvedi D, Ray S (2007) Tetrahedron Lett
48:149; k) Chaturvedi D, Mishra N, Mishra V (2007) J
Sulfur Chem 28:39; l) Chaturvedi D, Mishra N, Mishra V
(2007) J Sulfur Chem 28:607; m) Chaturvedi D, Mishra
N, Mishra V (2007) Tetrahedron Lett 48:5043; n)
Chaturvedi D, Mishra N, Mishra V (2008) Monatsh Chem
139:267; o) Chaturvedi D, Mishra N, Mishra V (2008)
Synthesis:355
Acknowledgements
Authors wish to thank SIAF division of CDRI for providing
spectroscopic and analytical data.
References
1. Ragnarsson U (2001) Chem Soc Rev 30:205
2. Schmidt EU (2001) Hydrazine and its derivatives pre-
paration, properties, applications, 2nd edn. Wiley-inter-
science, New York
3. a) Pedras MSC, Jha M (2006) Bioorg Med Chem 14:4958;
b) Shailendra, Bharti N, Naqvi F, Azam A (2002) Helv
Chim Acta 85:2713; c) Bharti N, Maurya MR, Naqvi F,
Azam A (2000) Bioorg Med Chem Lett 10:2243; d) Bharti
N, Mannar MR, Fehmida N, Bhattacharya A, Bhattacharya
S, Azam A (2000) Eur J Med Chem 35:481
13. a) Chaturvedi D, Ray S (2006) Monatsh Chem 137:201;
b) Chaturvedi D, Ray S (2006) Monatsh Chem 137:311;
c) Chaturvedi D, Ray S (2006) Monatsh Chem 137:459;
d) Chaturvedi D, Ray S (2006) Monatsh Chem 137:465;
e) Chaturvedi D, Ray S (2006) Monatsh Chem 137:1219
4. a) Sengupta SK, Pandey OP, Rao GP (1999) Sugarcane
Pathology 1:279; b) Chen HS, Li ZH, Han YH, Wang ZW