Synthesis of new (Pyrazol-3-yl)-1,3,4-oxadiazole derivatives by unexpected aromatization during oxidative cyclization of 4,5-dihydro-1h-pyrazole-3- carbohydrazones and their biological activities

Article abstract of DOI:10.1002/jhet.1765

A series of novel 2-(4-(4-chlorophenyl)-1H-pyrazol-3-yl)-5-(Aryl)-1,3,4- oxadiazoles were synthesized by unexpected aromatization during oxidative cyclization of 4-(4-chlorophenyl)-4,5-dihydro-1H-pyrazole-3-carbohydrazones using chloramine-T as an oxidant. The hydrazones were derived from 4-(4-chlorophenyl)-4,5-dihydro-1H-pyrazole-3-carbohydrazide and various substituted aldehydes. The structure of the synthesized compounds was confirmed by FTIR, ¹H NMR, ¹³C NMR, and mass spectral data. The synthesized compounds were evaluated for their antitubercular and antioxidant activities. All the compounds 4a-h and 5-h showed good antitubercular activity against Mycobacterium tuberculosis (minimum inhibitory concentration = 25 μg/mL for 4f and 4g, 50-100 μg/mL for the rest). However, all the compounds exhibited poor antioxidant activity against 1,1-diphenyl-2-picryl- hydrazil free radical.

Full text of DOI:10.1002/jhet.1765

726
Synthesis of New (Pyrazol-3-yl)-1,3,4-oxadiazole Derivatives by Unexpected
Aromatization During Oxidative Cyclization of 4,5-Dihydro-1H-pyrazole-3-
carbohydrazones and Their Biological Activities
Vol 51
K. Jagadeesh Prathap, M. Himaja, Sunil V. Mali, and D. Munirajasekhara
a
a
a,b
*
aSchool of Advanced Sciences, VIT University, Vellore 632014, Tamil Nadu, India
bMedicinal Chemistry Division, Piramal Life Sciences, Nirlon Complex, Goregaon East, Mumbai 400 063, India
*
E-mail: jagadeeshprathap@yahoo.co.in
Received March 3, 2012
DOI 10.1002/jhet.1765
Published online 3 December 2013 in Wiley Online Library (wileyonlinelibrary.com).
A series of novel 2-(4-(4-chlorophenyl)-1H-pyrazol-3-yl)-5-(Aryl)-1,3,4-oxadiazoles were synthesized by
unexpected aromatization during oxidative cyclization of 4-(4-chlorophenyl)-4,5-dihydro-1H-pyrazole-3-
carbohydrazones using chloramine-T as an oxidant. The hydrazones were derived from 4-(4-chlorophenyl)-
4,5-dihydro-1H-pyrazole-3-carbohydrazide and various substituted aldehydes. The structure of the synthesized
compounds was confirmed by FTIR, 1H NMR, 13C NMR, and mass spectral data. The synthesized compounds
were evaluated for their antitubercular and antioxidant activities. All the compounds 4a–h and 5a–h showed
good antitubercular activity against Mycobacterium tuberculosis (minimum inhibitory concentration = 25 mg/mL
for 4f and 4g, 50–100 mg/mL for the rest). However, all the compounds exhibited poor antioxidant activity
against 1,1-diphenyl-2-picryl-hydrazil free radical.
J. Hetercycl Chem., 51, 726 (2014).
INTRODUCTION
antimicrobial [4], anticancer [5], insecticidal [6], anti-inflam-
matory [7], and antimycobacterial [8] activities. Biologically
relevant entities containing the 1,3,4-oxadiazole motif include
the human immunodeficiency virus integrase inhibitor I and
the angiogenesis inhibitor II (Fig. 1) [9,10]. The widespread
use of 1,3,4-oxadiazoles as a scaffold in medicinal chemistry
establishes this moiety as an important bioactive class of
heterocycles. These molecules are also utilized as pharmaco-
phores because of their favorable metabolic profile and ability
to engage in hydrogen bonding with the receptors.
Many pyrazole derivatives are acknowledged to possess a
wide range of bioactivities. The pyrazole motif makes up the
core structure of numerous biologically active compounds.
1H-Pyrazole and 4,5-dihydro-1H-pyrazole derivatives have
been found to possess diverse applications in medicine,
viz. antimicrobial [11–13], antiviral [14], antidiabetic [15],
anti-inflammatory [16], anticancer [17], insecticidal [18],
and antitubercular [19] activities.
The development of drug resistance to clinically used
agents has increased the demand for discovery of new chem-
ical scaffolds with antimicrobial activity. Tuberculosis is a
common and often deadly infectious disease caused by Myco-
bacterium tuberculosis in humans, and it is estimated that
one-third of the world’s population is currently infected with
TB bacillus and, annually, 2.0 million deaths were found [1].
The incidence of TB infection has steadily risen in the last de-
cade. The reemergence of TB infection has been linked to
coinfection with the human immunodeficiency virus [2],
and the treatment of tuberculosis has become a major problem
because of the emergence of multidrug-resistant strains and
extensively drug-resistant strains of M. tuberculosis [3]. In
this context, there is an urgent need for new chemical entities
to fight against this disease with a novel mechanism of action.
Five-membered heterocycles are privileged structures with
utility in medicinal chemistry. Among them, the 1,3,4-oxa-
diazole motif has been extensively used for many years as a
bioisosteric replacement of acid, ester, and amide functional-
ities in medicinal chemistry, and its derivatives have a wide
range of pharmaceutical and biological activities including
In view of this, we made an effort to synthesize some new
4,5-dihydro-1H-pyrazol-3-yl)-1,3,4-oxadiazoles 6 by treating
4,5-dihydro-1H-pyrazole-3-carbohydrazones 4 with chlora-
mine-T. Interestingly, we ended up with (pyrazol-3-yl)-
1,3,4-oxadiazoles 5 by unexpected aromatization during
© 2013 HeteroCorporation

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Synthesis of New (Pyrazol-3-yl)-1,3,4-oxadiazole Derivatives by Unexpected Aromatization
727
Figure 1. Biologically active compounds with the 1,3,5-oxadiazole motif.
oxidative cyclization of 4,5-dihydro-1H-pyrazole-3-carbohy-
drazones 4. The newly synthesized compounds were tested
for their antitubercular and antioxidant activities.
pyrazoline NH, respectively. Its LC-MS showed M + 1 peak
(239.1), and the FTIR also showed carbonyl and NH
stretching frequencies.
Pyrazoline-3-carbohydrazide 2 was condensed with differ-
ent aldehydes 3a–h in methanol to obtain corresponding
hydrazones 4a–h in high yields (Scheme 1). All these hydra-
zones have not been reported hitherto. The structure of the
hydrazones was confirmed by 1H NMR, MS, and FTIR spec-
RESULTS AND DISCUSSION
Chemistry.
The synthetic route for the preparation of
1
pyrazoline-3-carbohydrazone derivatives 4a–h and pyrazolo-
oxadiazole derivatives 5a–h is summarized in Scheme 1.
The starting compound methyl 4-(4-chlorophenyl)-4,
5-dihydro-1H-pyrazole-3-carboxylate 1 was treated with
hydrazine hydrate in methanol to obtain corresponding
carbohydrazide 2. The structure of 2 was confirmed by
tral data. In each case, their H NMR spectra in DMSO-d6
revealed two characteristic signals in the region d 8.11–8.15
and 8.26–8.44 that represents -N═CH-proton and -CONH
proton, respectively. Their FTIR spectra showed the N-H
stretch of the hydrazone as a broad band in the region
3449–3221 cmꢀ1. Further, their MS spectra showed [M + 1]
peak in ES positive mode.
1
spectral analysis. The H NMR spectrum of 2 in DMSO-d6
revealed three multiplets at d 3.31, 3.68, and 4.35 that
represent three nonequivalent protons of pyrazoline ring,
broad singlet at 4.21 represents -NH2 protons, two doublets
at d 7.18 and 7.36 with J= 8.4 correspond to 4-chlorophenyl
ring, and two singlets at 7.73 and 9.18 belong to CONH and
The oxidation of hydrazones 4a–h with chloramine-T [20]
to generate nitrilimines followed by intramolecular cyclization
with adjacent carbonyl group yielded 2-(4-(4-chlorophenyl)-
1H-pyrazol-3-yl)-5-(aryl)-1,3,4-oxadiazoles 5a–h instead of
expected 2-(4-(4-chlorophenyl)-4,5-dihydro-1H-pyrazol-3-yl)-
Scheme 1. Reaction sequence for the synthesis of 4,5-dihydro-1H-pyrazole-3-carbohydrazones 4a–h and (pyrazol-3-yl)-1,3,4-oxadiazole derivatives 5a–h.
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K. Jagadeesh Prathap, M. Himaja, S. V. Mali, and D. Munirajasekhar
Vol 51
Antioxidant activity.
All the newly synthesized
5-(aryl)-1,3,4-oxadiazoles 6. This is due to the unexpected
aromatization of 4,5-dihydropyrazoline ring to pyrazole
by second mole of chloramine-T under reflux conditions.
Reaction mechanism for the formation of 5a–h is given
in Scheme 2.
compounds 4a–h and 5a–h were screened for their free
radical scavenging activity. The free radical scavenging
activity of the synthesized compounds was determined by
the 1,1-diphenyl-2-picryl-hydrazil (DPPH) method [24]. The
samples were prepared at different concentrations in
methanol (10, 20, 50, 75, and 100 mg/mL). Butylated
hydroxy toluene (BHT), which is a good antioxidant, is
taken as a standard in this study. However, all the
compounds showed poor antioxidant activity (<30%) when
compared with standard BHT (97%). This low activity is
most probably due to the inability of all the compounds to
catch free radicals. The percentage of free radical scavenging
activity is shown in Table 3.
Two moles of chloramine-T reagent were needed for the
completion of the reaction with one mole of the substrate.
One mole reagent was utilized for oxidation of hydrazone
and another mole was consumed for the oxidation of 4,5-
dihydropyrazoline ring. All the synthesized 2-(4-(4-chloro-
phenyl)-1H-pyrazol-3-yl)-5-(aryl)-1,3,4-oxadiazoles 5a–hwere
characterized by FTIR, 1H NMR, and mass spectral analysis.
IR spectrum of oxadiazoles 5a–h showed the absence of
amide carbonyl frequency in the region 1730–1660 cmꢀ1
and showed a new peak at 1626–1590 cmꢀ1 because of
1
C═N frequency. H NMR showed peaks due to aromatic
CONCLUSION
protons and other substituents in the expected region. Further
13C NMR, DEPT-135, and mass spectra confirm the struc-
tures of 5a–h. The physical data of the compounds 2, 4a–h,
and 5a–h were given in Table 1.
In conclusion, some new pyrazolo-oxadiazole deriva-
tives were obtained by cyclization and unexpected aroma-
tization of 4,5-dihydropyrazoline-3-carbohydrazones by
chloramine-T trihydrate. Antitubercular activity study
revealed that compounds 4f, 4g showed potent activity
(25 mg/mL); 4a, 4b, 4c, 4d, 4e, 4h, 5a, 5c, 5d, 5e, 5f, 5g
(50 mg/mL) displayed good activity than pyrazinamide by
Alamar Blue assay method. The structure and biological
activity relationship of title compounds showed that
the presence of pyrazoline nucleus with carbohydrazone
moiety (-CONHN═C-) and 1,3,4-oxadiazole nucleus with
4-chlorophenyl pyrazole moiety as well as biologically
active fluoro, cyano, and methoxy groups attached to the
phenyl ring are responsible for good antitubercular activity.
All the compounds 4a–h and 5a–h displayed poor antiox-
idant activity when compared with standard BHT. This is
most probably due to the inability of all the compounds
to catch free radicals.
Biological activity
Antitubercular activity. The antimycobacterial activity of
the compounds 4a–h and 5a–h was assessed against M.
tuberculosis H37Rv (ATCC 27294) using microplate Alamar
Blue assay [21]. This methodology is nontoxic, uses a
thermally stable reagent, and shows good correlation with
proportional and BACTEC radiometric methods [22,23],
and the activity is expressed as the minimum inhibitory
concentration (MIC) in mg/mL. The MIC is defined as the
lowest drug concentration required to complete inhibition of
bacterial growth. Pyrazinamide was used as standard. All the
tested compounds 4a–h and 5a–h showed better in vitro
activity (MIC 25–50 mg/mL) than pyrazinamide (MIC of
100 mg/mL). The MICs of the compounds were depicted in
Table 2.
Scheme 2. Reaction mechanism for the formation of 2-(4-(4-chlorophenyl)-1H-pyrazol-3-yl)-5-(aryl)-1,3,4-oxadiazoles 5a–h from 4a–h.
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Synthesis of New (Pyrazol-3-yl)-1,3,4-oxadiazole Derivatives by Unexpected Aromatization
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Table 1
Physical data of the compounds 2, 4a–h, and 5a–h.
Compound
R
Yield (%)
mp (ꢁC)
m/z
2
84.2
89.6
84.7
88.2
85.1
88.6
83.4
90.2
91.7
61.4
62.6
64.7
58.7
62.7
60.2
58.3
59.6
173–175
187–188
196–197
191–193
185–187
191–192
214–215
187–189
163–165
219–220
242–244
214–216
146–148
246–247
255–257
215–216
224–226
238
326
416
386
386
344
351
380
386
333
412
382
382
340
347
376
382
4a
4b
4c
4d
4e
4f
4g
4h
5a
5b
5c
5d
5e
5f
H
3,4,5-Trimethoxy
3,5-Dimethoxy
2,4-Dimethoxy
4-Fluoro
4-Cyano
2,4,6-Trifluoro
2,4,6-Trimethyl
H
3,4,5-Trimethoxy
3,5-Dimethoxy
2,4-Dimethoxy
4-Fluoro
4-Cyano
2,4,6-Trifluoro
2,4,6-Trimethyl
5g
5h
given in Hz. IR spectra in KBr disk were recorded from 4000 to
400 cmꢀ1 on Avatar 330 FTIR spectrometer equipped with DTGS
detector (Thermo Scientific, India). Mass spectra were recorded
using Agilent 1100 MSD spectrometer (Agilent Technologies) in
electro spray mode. Compounds were checked for their purity by
TLC on silica gel G plates, and spots were located by exposing to
UV light and iodine vapors.
EXPERIMENTAL
General.
Analytical grade solvents and commercially
available reagents were used without further purification. The
column chromatography was carried out over silica gel (60–120
mesh), purchased from Sisco Research Laboratories Pvt Ltd.
Melting points were determined in open capillaries in electrical
melting point apparatus and are uncorrected. 1H NMR spectra
were recorded on 400-MHz and 500-MHz Bruker spectrometer in
DMSO-d6 or CDCl3 using TMS as an internal standard. Chemical
shifts are given in d relative to TMS; the coupling constants are
Procedure for the synthesis of 4-(4-chlorophenyl)-4,5-
dihydro-1H-pyrazole-3-carbohydrazide (2).
To a turbid
solution of methyl 4-(4-chlorophenyl)-4,5-dihydro-1H-pyrazole-
3-carboxylate 1 (5.0 g, 21 mmol) in methanol (25 mL) in an
Round Bottom flask was added hydrazine hydrate (98%, 63 mmol,
Table 2
Antitubercular activity of 4a–h and 5a–h against Mycobacterium tuber-
culosis H37Rv strain (ATCC 27294) by MABA method.a
Table 3
Antioxidant activity of 4a–h and 5a–h against DPPH free radical.
MIC (mg/mL)b
% inhibition at different concentrations (mg/mL)
Compound
M. tuberculosis H37Rv
Compound
10
20
50
75
100
4a
4b
50
50
4a
4b
4c
4d
4e
7
8
6
7
8
8
9
8
3
3
2
3
4
2
4
4
43
9
11
9
10
11
10
13
12
6
5
5
6
7
15
16
14
14
16
14
19
17
8
8
9
8
9
18
18
17
18
17
16
25
19
9
10
9
10
9
19
20
18
19
18
17
26
20
9
11
10
11
10
9
4c
4d
4e
4f
4g
4h
5a
5b
5c
5d
5e
5f
5g
5h
50
50
50
25
25
50
50
100
50
50
50
50
50
100
100
4f
4g
4h
5a
5b
5c
5d
5e
5f
6
7
6
59
8
9
8
82
8
5g
5h
BHT
10
10
95
12
11
97
Pyrazinamide
MIC, minimum inhibitory concentration.
aMicroplate Alamar Blue assay.
bSample concentrations tested from 0.2 to 100 mg/mL.
DPPH, 1,1-diphenyl-2-picryl-hydrazil; BHT, butylated hydroxy toluene.
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K. Jagadeesh Prathap, M. Himaja, S. V. Mali, and D. Munirajasekhar
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3.05 mL), and the resulting clear reaction mixture (slight
exothermic) was stirred for 16h at room temperature. After the
reaction was completed by TLC, excess methanol was evaporated
from the reaction mixture under reduced pressure, and the resultant
white precipitate was washed twice with little amount of water and
was air-dried to obtain the pure compound 2 as a white solid.
Yield 4.21 g (84.2%). mp 173–175ꢁC; IR υ (KBr) 3549, 3353,
3045, 2941, 1668 cmꢀ1; 1H NMR (400MHz) (DMSO-d6): d 3.31
(m, 1H, C5Ha of pyrazoline), 3.68 (m, 1H, C5Hb of pyrazoline),
4.21 (brs, 2H, NH2), 4.35 (m, 1H, C4H of pyrazoline), 7.18
(d, J = 8.4Hz, 2H, C2 and C6H of 4-Cl-Phenyl), 7.36 (d, J=8.4Hz,
2H, C3 and C5H of 4-Cl-Phenyl), 7.73 (s, 1H, -CONH-), 9.18 (s, 1H,
pyrazoline-NH); LC-MS: 239.1 (M + 1).
C5Ha of pyrazoline), 3.81 (s, 3H, 4-Methoxy), 3.84 (s, 3H, 2-
Methoxy), 3.90 (m, 1H, C5Hb of pyrazoline), 4.43 (m, 1H, C4H
of pyrazoline), 6.69 (d, J = 8.4 Hz, 1H, C5H of 2,4-
Dimethoxyphenyl), 6.74 (s, 1H, C3H of 2,4-Dimethoxyphenyl),
7.44 (d, J = 8.4 Hz, 2H, C2 and C6H of 4-Cl-Phenyl), 7.61
(d, J = 8.4 Hz, 2H, C3,C5H of 4-Cl-Phenyl), 7.73 (d, J = 8 Hz, 1H,
C6H of 2,4-Dimethoxyphenyl), 8.13 (s, 1H, -CH═N-), 8.26
(s, 1H, -CONH-), 11.58 (s, 1H, pyrazoline-NH); MS: 387.0 (M + 1).
N0-(4-Fluorobenzylidene)-4-(4-chlorophenyl)-4,5-dihydro-1H-
pyrazole-3-carbohydrazide (4e).
White solid (yield 0.64 g,
88.6%). mp 191–192ꢁC; IR υ (KBr) 3431, 3327, 3117, 2925,
2854, 1695, 1636 cmꢀ1; 1H NMR (300 MHz) (DMSO-d6): d 3.47
(m, 1H, C5Ha of pyrazoline), 3.88 (m, 1H, C5Hb of pyrazoline),
4.43 (m, 1H, C4H of pyrazoline), 7.21 (d, J = 8.4 Hz, 2H, C2 and
C6H of 4-Cl-Phenyl), 7.27 (d, J =8.7 Hz, 2H, C2 and C6H of 4-F-
Phenyl), 7.37 (d, J = 8.4Hz, 2H, C3,C5H of 4-Cl-Phenyl), 7.65
(t, J = 14.1Hz, 2H, C3,C5H of 4-F-Phenyl), 8.15 (s, 1H, -CH═N-
), 8.37 (s, 1H, -CONH-), 11.60 (s, 1H, pyrazoline-NH); MS:
345.1 (M+ 1), 367.1 (M+ Na).
General procedure for the synthesis of N0-(substituted
benzylidene)-4-(4-chlorophenyl)-4,5-dihydro-1H-pyrazole-3-
carbohydrazide derivatives (4a–h). To a stirred suspension
of 2 (2.1 mmol, 0.5 g) in methanol (8 mL) was added the
appropriate aldehyde
3 (2.1 mmol) and stirred at room
temperature for 30 min. The completion of the reaction was
confirmed by TLC, and the precipitate formed was diluted with
chilled water, filtered off, washed with water, and air-dried to
give the corresponding hydrazide-hydrazone derivatives 4. The
physical and spectral data of 4a–h are as follows.
N0-(4-Cyanobenzylidene)-4-(4-chlorophenyl)-4,5-dihydro-1H-
pyrazole-3-carbohydrazide (4f).
Off white solid (yield 0.64 g,
86.8%). mp 214–215ꢁC; IR υ (KBr) 3471, 3325, 3096, 2923, 2227,
1670 cmꢀ1; 1H NMR (300 MHz) (DMSO-d6): d 3.49 (m, 1H, C5Ha
of pyrazoline), 3.92 (m, 1H, C5Hb of pyrazoline), 4.45 (m, 1H, C4H
of pyrazoline), 7.21 (d, J=8.4Hz, 2H, C2 and C6H of 4-Cl-Phenyl),
7.37 (d, J= 8.4 Hz, 2H, C3,C5H of 4-Cl-Phenyl), 7.77 (d, J=8.4Hz,
2H, C2,C6H of 4-CN-Phenyl), 7.87 (d, J= 8.4 Hz, 2H, C3,C5H of 4-
CN-Phenyl), 8.27 (s, 1H, -CH═N-), 8.43 (s, 1H, -CONH-), 11.85
(s, 1H, pyrazoline-NH); MS: 352.1 (M + 1), 350.1 (M ꢀ 1).
N0-Benzylidene-4-(4-chlorophenyl)-4,5-dihydro-1H-pyrazole-
3-carbohydrazide (4a). Off white solid (yield 0.59 g, 86.2%). mp
187–188ꢁC; IR υ (KBr) 3441, 3247, 3129, 2943, 1674 cmꢀ1 1H
;
NMR (300 MHz) (DMSO-d6): d 3.48 (m, 1H, C5Ha of pyrazoline),
3.90 (m, 1H, C5Hb of pyrazoline), 4.44 (m, 1H, C4H of pyrazoline),
7.21 (d, J= 8.1 Hz, 2H, C2 and C6H of 4-Cl-Phenyl), 7.36–7.42 (m,
5H, C3,C5H of 4-Cl-Phenyl and C3,C4,C5H of Phenyl), 7.60 (m, 2H,
C2,C6H Phenyl), 8.15 (s, 1H, -CH═N-), 8.38 (s, 1H, -CONH-), 11.60
(s, 1H, pyrazoline-NH); LC-MS: 327.0 (M + 1).
N0-(2,4,6-Trifluorobenzylidene)-4-(4-chlorophenyl)-4,5-dihydro-
1H-pyrazole-3-carbohydrazide (4g). Off white solid (yield 0.7ꢀ21g
90.2%). mp 187–189ꢁC; IR υ (KBr) 3448, 3134, 2931, 1673 cm
;
1H NMR (400 MHz) (DMSO-d6): d 3.48 (m, 1H, C5Ha of pyrazoline),
3.91 (m, 1H, C5Hb of pyrazoline), 4.44 (m, 1H, C4H of pyrazoline),
7.20 (d, J= 8 Hz, 2H, C2 and C6H of 4-Cl-Phenyl), 7.27
(t, J=16.8Hz, 2H, C3,C5H of 2,4,6-trifluorophenyl), 7.37 (d,
J= 8.4 Hz, 2H, C3,C5H of 4-Cl-Phenyl), 8.21 (s, 1H, -CH═N-), 8.44
(s, 1H, -CONH-), 11.79 (s, 1H, pyrazoline-NH); LC-MS: 381.1 (M + 1).
N0-(2,4,6-Trimethylbenzylidene)-4-(4-chlorophenyl)-4,5-dihydro-
1H-pyrazole-3-carbohydrazide (4h). White solid (yield 0.68 g,
87.9%). mp 163–165ꢁC; IR υ (KBr) 3448, 3297, 3109, 2934,
N0-(3,4,5-Trimethoxybenzylidene)-4-(4-chlorophenyl)-4,5-dihydro-
1H-pyrazole-3-carbohydrazide (4b). Pale yellow solid (yield
0.74 g, 84.7%). mp 196–197ꢁC; IR υ (KBr) 3435, 3221, 3108,
2928, 1656 cmꢀ1 1H NMR (300 MHz) (DMSO-d6): d 3.46
;
(m, 1H, C5Ha of pyrazoline), 3.68 (s, 3H, 4-Methoxy), 3.79
(s, 6H, 3,5-Dimethoxy), 3.89 (m, 1H, C5Hb of pyrazoline), 4.43
(m, 1H, C4H of pyrazoline), 6.89 (s, 2H, C2,C6H of 3,4,5-
Trimethoxyphenyl), 7.21 (d, J = 8.4 Hz, 2H, C2 and C6H of
4-Cl-Phenyl), 7.37 (d, J = 8.4 Hz, 2H, C3,C5H of 4-Cl-Phenyl),
8.14 (s, 1H, -CH═N-), 8.28 (s, 1H, -CONH-), 11.60 (s, 1H,
pyrazoline-NH); 13C NMR (75 MHz) (DMSO-d6): d 48.25,
56.68, 57.27, 60.98, 104.80, 129.29, 129.82, 130.90, 132.15,
139.78, 141.45, 146.50, 147.47, 153.94, 158.80; LC-MS:
417.0 (M + 1).
1
1697 cmꢀ1; H NMR (300 MHz) (DMSO-d6): d 2.11 (s, 3H, 4-
Me), 2.22 (s, 6H, 2,6-Dimethyl), 3.46 (m, 1H, C5Ha of
pyrazoline), 3.88 (m, 1H, C5Hb of pyrazoline), 4.41 (m, 1H,
C4H of pyrazoline), 6.87 (s, 2H, C3,C5H of 2,4,6-
trimethylphenyl), 7.22 (d, J = 8.4 Hz, 2H, C2 and C6H of 4-Cl-
Phenyl), 7.37 (d, J = 8.4 Hz, 2H, C3,C5H of 4-Cl-Phenyl), 8.11
(s, 1H, -CH═N-), 8.71 (s, 1H, -CONH-), 11.45 (s, 1H,
pyrazoline-NH); MS: 369.1 (M + 1), 391.1 (M + Na).
N0-(3,5-Dimethoxybenzylidene)-4-(4-chlorophenyl)-4,5-dihydro-
1H-pyrazole-3-carbohydrazide (4c).
0.71g, 88.2%). mp 191–193ꢁC; IR υ (KBr) 3432, 3124, 2927,
1642cmꢀ1 1H NMR (400 MHz) (DMSO-d6): d 3.47 (m, 1H,
Pale yellow solid (yield
;
General procedure for the synthesis of 2-(4-(4-chlorophenyl)-
C5Ha of pyrazoline), 3.75 (s, 6H, 3,5-Dimethoxy), 3.89 (m, 1H,
C5Hb of pyrazoline), 4.43 (m, 1H, C4H of pyrazoline), 6.52
(s, 1H, C4H of 3.5-Dimethoxyphenyl), 6.74 (s, 2H, C2,C6H
of 3,5-Drimethoxyphenyl), 7.21 (d, J = 8 Hz, 2H, C2 and C6H of
4-Cl-Phenyl), 7.37 (d, J = 7.6 Hz, 2H, C3,C5H of 4-Cl-Phenyl),
8.15 (s, 1H, -CH═N-), 8.29 (s, 1H, -CONH-), 11.59 (s, 1H,
pyrazoline-NH); LC-MS: 387.1 (M+ 1).
1H-pyrazol-3-yl)-5-(aryl)-1,3,4-oxadiazole derivatives (5a–h).
A
mixture of 4 (1.55 mmol) and chloramine-T trihydrate (3.72 mmol) in
ethanol (10 mL) was refluxed under stirring for 4 h. The reaction
mixture was then concentrated under reduced pressure, and the
residue was extracted with ethyl acetate (25 mL). The extract was
washed with 10% NaHCO3 (10 mL), water (10 mL), and then dried
over Na2SO4. The solvent was evaporated, and the resulted crude
product was subjected to the column chromatography using ethyl
acetate–pet ether (12:88) as eluent to obtain 2-(4-(4-chlorophenyl)-
1H-pyrazol-3-yl)-5-(aryl)-1,3,4-oxadiazole derivatives 5. The physical
and spectral data of 5a–h are as follows.
N0-(2,4-Dimethoxybenzylidene)-4-(4-chlorophenyl)-4,5-dihydro-
1H-pyrazole-3-carbohydrazide (4d).
0.69g, 85.1%). mp 185–187ꢁC; IR υ (KBr) 3439, 3143, 2936,
1668cmꢀ1 1H NMR (400 MHz) (DMSO-d6): d 3.45 (m, 1H,
Pale yellow solid (yield
;
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

May 2014
Synthesis of New (Pyrazol-3-yl)-1,3,4-oxadiazole Derivatives by Unexpected Aromatization
731
2-(4-(4-Chlorophenyl)-1H-pyrazol-3-yl)-5-phenyl-1,3,4-oxadiazole
(5a). White solid (yield 0.32 g, 64.9%). mp 219–220ꢁC; IR υ
2-(4-(4-Chlorophenyl)-1H-pyrazol-3-yl)-5-(2,4,6-trifluorophenyl)-
1,3,4-oxadiazole (5g). Off white solid (yield 0.29 g, 58.3%).
1
mp 215-216ꢁC; IR υ (KBr) 3436, 3146, 1603 cmꢀ1; H NMR
(KBr) 3438, 3135, 1597 cmꢀ1 1H NMR (500 MHz) (DMSO-
;
(400 MHz) (CDCl3): 6.88 (t, J = 16.4 Hz, 2H, C3,C5H of
2,4,6-trifluorophenyl), 7.41 (d, J = 8.4 Hz, 2H, C2 and C6H of
4-Cl-Phenyl), 7.57 (d, J = 8.4Hz, 2H, C3,C5H of 4-Cl-Phenyl),
7.95 (s, 1H, pyrazole-H), 11.96 (br, 1H, pyrazole-NH);
LC-MS: 377.1 (M + 1).
d6): d 7.49 (d, J = 8.5 Hz, 2H, C2 and C6H of 4-Cl-Phenyl),
7.61–7.71 (m, 5H, C3,C5H of 4-Cl-Phenyl and C3,C4,C5H of
Phenyl), 7.99 (d, 2H, J = 8 Hz, C2,C6H Phenyl), 8.30 (s, 1H,
Pyrazole-H), 13.91 (brs, 1H, Pyrazole-NH); MS: APCI+ 323.0
(M + 1). Anal. Calcd For C17H11ClN4O: C, 63.26; H, 3.44; N,
17.36; Found: C, 63.39; H, 3.37; N, 17.29.
2-(4-(4-Chlorophenyl)-1H-pyrazol-3-yl)-5-(3,4,5-trimethoxyphenyl)-
1,3,4-oxadiazole (5b). Off white solid (yield 0.31 g, 62.6%). mp 242–
244ꢁC; IR υ (KBr) 3435, 3149, 2924, 1590 cmꢀ1; 1H NMR (400 MHz)
(DMSO-d6): d 3.74 (s, 3H, 4-Methoxy), 3.86 (s, 6H, 3,5-Dimethoxy),
7.16 (s, 2H, C2,C6H of 3,4,5-Trimethoxyphenyl), 7.51 (d, J= 8.4 Hz, 2H,
C2 and C6H of 4-Cl-Phenyl), 7.65 (d, J= 8.4 Hz, 2H, C3,C5H of 4-Cl-
Phenyl), 8.29 (s, 1H, Pyrazole-H), 13.89 (s, 1H, pyrazole-NH); 13C
NMR (75 MHz) (DMSO-d6): d 56.52, 60.70, 104.22, 118.81, 121.86,
128.65, 130.68, 130.93, 131.16, 132.43, 133.60, 141.04, 153.91, 160.49,
163.79; MS: ES+ 413.22 (M + 1). Anal. Calcd For C20H17ClN4O4: C,
58.19; H, 4.15; N, 13.57; Found: C, 58.12; H, 4.12; N, 13.39.
2-(4-(4-Chlorophenyl)-1H-pyrazol-3-yl)-5-(2,4,6-trifluorophenyl)-
1,3,4-oxadiazole (5h). White solid (yield 0.33 g, 66.8%). mp 224-
226ꢁC; IR υ (KBr) 3431, 3149, 2929, 1626 cmꢀ1 1H NMR
;
(400 MHz) (CDCl3): d 2.23 (s, 6H, 2,6-Dimethyl), 2.32 (s, 3H, 4-Me),
6.95 (s, 2H, C3,C5H of 2,4,6-trimethylphenyl), 7.38 (d, J= 8.4 Hz, 2H,
C2 and C6H of 4-Cl-Phenyl), 7.51 (d, J=8.4Hz, 2H, C3,C5H of 4-Cl-
Phenyl), 7.83 (s, 1H, pyrazole-H), 12.78 (br, 1H, pyrazole-NH); LC-
MS: 365.1 (M + 1). Anal. Calcd For C20H17ClN4O: C, 65.84; H, 4.70;
N, 15.36; Found: C, 65.69; H, 4.65; N, 15.17.
Antitubercular activity. Briefly, 200mL of sterile deionized
water was added to all outer perimeter wells of sterile 96-well
plate to minimized evaporation of medium in the test wells during
incubation. The 96-well plate received 100mL of the Middlebrook
7H9 broth, and a serial dilution of compounds was made directly
on plate. The final drug concentrations tested were 0.2 to 100mg/
mL. Plates were covered and sealed with parafilm and incubated
at 37ꢁC for 5 days. After this time, 25mL of freshly prepared
1:1 mixture of Almar Blue reagent and 10% tween 80 was added
to the plate and incubated for 24h. A blue color in the well was
interpreted as no bacterial growth, and a pink color was scored as
growth. The MIC was defined as lowest drug concentration that
prevented the color change from blue to pink.
2-(4-(4-Chlorophenyl)-1H-pyrazol-3-yl)-5-(3,5-dimethoxyphenyl)-
1,3,4-oxadiazole (5c). Light brown solid (yield 0.32 g, 64.7%). mp
214–216ꢁC; IR υ (KBr) 3447, 3148, 2921, 1602 cmꢀ1 1H NMR
;
(400 MHz) (DMSO-d6): d 3.83 (s, 6H, 3,5-Dimethoxy), 6.76 (s, 1H,
C4H of 3.5-Dimethoxyphenyl), 7.04 (s, 2H, C2,C6H of 3,5-
Drimethoxyphenyl), 7.49 (d, J= 8 Hz, 2H, C2 and C6H of 4-Cl-
Phenyl), 7.67 (d, J= 6.8 Hz, 2H, C3,C5H of 4-Cl-Phenyl), 8.27 (s, 1H,
Pyrazole-H), 13.89 (s, 1H, pyrazole-NH); LC-MS: 383.0 (M + 1). Anal.
Calcd For C19H15ClN4O3: C, 59.61; H, 3.95; N, 14.64; Found: C,
59.53; H, 3.91; N, 14.48.
Antioxidant activity.
Briefly, 1 mL of 0.1mM methanolic
2-(4-(4-Chlorophenyl)-1H-pyrazol-3-yl)-5-(2,4-dimethoxyphenyl)-
1,3,4-oxadiazole (5d). Off white solid (yield 0.29 g, 58.7%).
solution of DPPH was added to 3 mL of the synthesized samples
at different concentrations in methanol (10, 20, 50, 75, 100mg/
mL). The samples were kept in the dark for 30 min; after which,
the absorbance was measured at 517 nm in UV spectrophotometer
(Systronics 2202). In its radical form, DPPH absorbs at 517 nm,
but upon reduction by an antioxidant or a radical species, its
absorption decreases. Lower absorbance of the reaction mixture
indicates higher free radical scavenging activity. BHT, which is a
good antioxidant, is taken as a standard in this study.
The capability to scavenge the DPPH radical was calculated using the
following equation:DPPH. Scavenging effect (%) = [(Ac ꢀ As)/Ao) 100]
where Ac is the absorbance of the control reaction and As is the
absorbance in the presence of sample.
1
mp 146–148ꢁC; IR υ (KBr) 3467, 3086, 2925, 1617 cmꢀ1; H
NMR (400 MHz) (DMSO-d6): d 3.84 (s, 3H, 4-Methoxy), 3.87
(s, 3H, 2-Methoxy), 6.72 (d, J = 8.4 Hz, 1H, C5H of 2,4-
Dimethoxyphenyl), 6.77(s, 1H, C3H of 2,4-Dimethoxyphenyl),
7.48 (d, J = 7.6 Hz, 2H, C2 and C6H of 4-Cl-Phenyl), 7.66
(d, J = 7.2 Hz, 2H, C3,C5H of 4-Cl-Phenyl), 7.78 (d, J = 8 Hz,
1H, C6H of 2,4-Dimethoxyphenyl), 8.29 (s, 1H, Pyrazole-H),
13.84 (brs, 1H, pyrazole-NH); MS: 383.1 (M + 1).
2-(4-(4-Chlorophenyl)-1H-pyrazol-3-yl)-5-(4-fluorophenyl)-1,3,4-
oxadiazole (5e). White solid (yield 0.31 g, 62.7%). mp 246–247ꢁC;
IR υ (KBr) 3446, 3142, 1604 cmꢀ1; 1H NMR (400 MHz) (DMSO-d6):
d 7.46–7.50 (m, 4H, C2,C6H of 4-Cl-Phenyl, and C2,C6H of 4-F-
Phenyl), 7.70 (d, J=7.2Hz, 2H, C3,C5H of 4-Cl-Phenyl), 8.05
(t, J= 13.2 Hz, 2H, C3,C5H of 4-F-Phenyl), 8.29 (s, 1H, Pyrazole-H),
13.91 (s, 1H, pyrazole-NH); 13C NMR (125 MHz) (DMSO-d6): d
117.23 (d, J= 22.5 Hz), 120.43, 121.76, 128.66, 129.72 (d, J=10Hz),
130.69, 130.81, 130.96, 132.34, 133.45, 160.61, 163.28, 164.60
(d, J= 248.75 Hz). DEPT-135: d 117.22 (d, J= 22 Hz), 128.66, 129.71
(d, J= 8.75 Hz), 130.70, 130.96. MS: 341.0 (M + 1).
Acknowledgments. We acknowledge the Head, SAIF, IIT, Chennai,
for NMR spectra. We are thankful to IICT, Hyderabad for providing
MS and LC-MS facility. We also acknowledge SAIF, CDRI,
Lucknow, for providing elemental analysis data. We are very much
thankful to the Head, Department of microbiology, Maratha
Mandal Dental College, Belgaum, Karnataka, for antitubercular
activity studies.
2-(4-(4-Chlorophenyl)-1H-pyrazol-3-yl)-5-(4-cyanophenyl)-1,3,4-
oxadiazole (5f). White solid (yield 0.31 g, 62.7ꢀ%1). mp 255–
257ꢁC; IR υ (KBr) 3438, 3135, 2231, 1602 cm
;
1H NMR
(400 MHz) (DMSO-d6): d 7.47 (d, J = 7.6 Hz, 2H, C2 and C6H
of 4-Cl-Phenyl), 7.68 (d, J = 7.6 Hz, 2H, C3,C5H of 4-Cl-
Phenyl), 8.08 (d, J = 7.6 Hz, 2H, C2,C6H of 4-CN-Phenyl), 8.14
(d, J = 7.6 Hz, 2H, C3,C5H of 4-CN-Phenyl), 8.30 (s, 1H,
pyrazole-H), 13.94 (s, 1H, pyrazole-NH). Anal. Calcd For
C18H10ClN5O: C, 62.17; H, 2.90; N, 20.14; Found: C, 62.03;
H, 2.87; N, 19.97.
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