Synthesis and antioxidant evaluation of some new 2-benzoylamino-5-hetaryl-1,3,4-oxadiazoles

Article abstract of DOI:10.1007/s11164-015-2121-3

A series of new functionalized 2-benzoylamino-5-hetaryl-1,3,4-oxadiazoles were efficiently synthesized via the reaction of the versatile key intermediates, 2-benzoylamino-5-cyanomethyl-1,3,4-oxadiazole (1) and N-(5-(5-amino-3-phenylamino)-1H-pyrazol-4-yl)

Full text of DOI:10.1007/s11164-015-2121-3

Res Chem Intermed
DOI 10.1007/s11164-015-2121-3
Synthesis and antioxidant evaluation of some new
2-benzoylamino-5-hetaryl-1,3,4-oxadiazoles
Samir Bondock1,2
Hassan A. Etman2
Shymaa Adel2
•
•
Received: 17 March 2015 / Accepted: 26 May 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A series of new functionalized 2-benzoylamino-5-hetaryl-1,3,4-oxa-
diazoles were efficiently synthesized via the reaction of the versatile key intermediates,
2-benzoylamino-5-cyanomethyl-1,3,4-oxadiazole (1) and N-(5-(5-amino-3-pheny-
lamino)-1H-pyrazol-4-yl)-1,3,4-oxadiazol-2-yl)-benzamide (13), with some appropri-
ate electrophilic reagents. The structures of the newly synthesized compounds were
established on the basis of elemental analyses, spectral data, and by alternative syn-
thesis wherever possible. The mechanisms of the studied reactions are discussed. Also,
we evaluate the antioxidant activity of some representative examples of the newly
prepared compounds. Among the synthesized compounds, 2-benzoylamino-5-cyano-
methyl-1,3,4-oxadiazole (1) and N-(5-(7-methyl-5-oxo-2-(phenylamino)-4,5-dihy-
dropyrazolo[1,5-a]pyrimidin-3-yl)-1,3,4-oxadiazol-2-yl)benzamide
(17)
showed
excellent antioxidant activity and exhibited high protection against DNA damage
induced by the Bleomycin iron complex.
Keywords 1,3,4-Oxadiazole Á Pyrazole Á Pyrazolo[1,5-a]pyrimidine Á
Imidazo[1,2-b]pyrazole Á Antioxidant activity
Introduction
1,3,4-Oxadiazole constitutes an important scaffold for the development of new drugs
[1]. Compounds having 1,3,4-oxadiazole skeletons possessed a wide range of
therapeutic activities like antifungal [2], antioxidant [3–5], antimicrobial [6],
antitubercular [7], anti-inflammatory [8, 9], antimalarial [10], antihypoglycemic
&
Samir Bondock
Bondock@mans.edu.eg
1
2
Chemistry Department, Faculty of Science, King Khalid University, Abha 9004, Saudi Arabia
Chemistry Department, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
123

S. Bondock et al.
[11], and anticancer [12–14] activities as well as potential antihypertensive agents
[15], antiviral [16] and anticonvulsant [17] properties. The pharmacological activity
of 1,3,4-oxadiazoles can be ascribed to the presence of toxophoric –N=C–O– linkage
which may react with the nucleophilic centers of the microbial cell [18]. They have
also received interest in medicinal chemistry as surrogates (bioisosteres) for
carboxylic acids, esters and carboxamides [19]. On the other hand, the heterocyclic
moieties including pyrazoles [20], thiophenes [21], pyrazolo[1,5-a]pyrimidines [22]
and imidazo[1,2-b]pyrazoles [23] have received considerable interest owing to their
wide spectrum of biological activities. Merging of these two bioactive components;
1,3,4-oxadiazole and heterocyclic moiety, in the same molecule is a fascinating
approach to discover novel potent drugs, due to the possible synergistic effect.
In view of the above-mentioned facts and in continuation of our ongoing program
[24–32] aim to find a new class of biheterocycles with potential chemotherapeutic
activities, we planned to synthesize and study the antioxidant properties of hitherto
unreported 2-benzoylamino-5-hetaryl-1,3,4-oxadiazoles.
Results and discussion
Chemistry
The synthetic pathway to achieve the target compounds is depicted in Schemes 1, 2,
3 and 4. The key intermediate 2-benzoylamino-5-cyanomethyl-1,3,4-oxadiazole (1)
was prepared by cyclodesulfurization of 1-cyanoacetyl-4-benzoylthiosemicarbazide
with mercuric oxide in boiling ethanol as previously reported by us [12]. It is well
known that the reaction of hydrozonyl nitrile with a-halocarbonyl compounds
represents the simplest method to obtain the 4-aminopyrazoles [33]. In this context,
we investigated the reaction of 1 with arene diazonium salt and a-bromoester. Thus,
coupling of 1 with p-tolyl diazonium chloride in pyridine at 0 °C gave the
hydrozonyl nitrile derivative 2.
Alkylation of hydrazonyl nitrile 2 with ethyl bromoacetate in boiling dimethyl-
formamide containing a catalytic amount of triethylamine afforded a single product
identified as 2-(2-((5-benzamido-1,3,4-oxadiazol-2-yl)(cyano)methylene)-1-p-tolyl-
hydrazinyl) acetate (3). The base catalyzed Thorpe-Ziegler cyclization of 3 led to
the formation of ethyl 4-amino-3-(5-benzamido-1,3,4-oxadiazol-2-yl)-1-p-tolyl-1H-
pyrazole-5-carboxylate (4). Compounds 3 and 4 were established on the basis of
microanalyses and spectral data. The IR spectrum of 4 showed absorption bands at
3345–3285, 3197, 1739, 1703 cm-1 due to NH2, NH, ester C=O, and amidic C=O
function, respectively. Its 1H–NMR spectrum revealed triplet and quartet signal at d
1.26 and 4.35 ppm due to the ethoxy protons besides three singlet signals at 2.32,
6.35, and 12.06 due to CH3, NH2, and NH protons, and multiplet signal around 7.16-
8.12 ppm owing to nine aromatic protons. The 13C–NMR spectrum of 4 revealed 18
carbon types, the most important signals being displayed at d 14.2, 20.7, 59.8, 160.4,
and 164.8 ppm due to two methyl carbons, OCH2, and two carbonyl carbons,
respectively. Its mass spectrum showed a molecular ion peak at m/z = 432 (M?)
which fits with a molecular formula (C22H20N6O4).
123

Synthesis and antioxidant evaluation of some new…
Scheme 1 Synthetic route to pyrazolyl-, tetrahydrobenzothienyl- and acroyl oxadiazoles 4–6
The Gewald reaction of 1 with elemental sulfur and cyclohexanone in a warming
mixture of ethanol and dimethylformamide (1:1) containing morpholine as a basic
catalyst gave N-(5-(2-amino-4,5,6,7-tetrahydrobenzo[b] thiophen-3-yl)-1,3,4-oxa-
diazol-2-yl)benzamide (5). The IR spectrum of 5 displayed absorption bands at
3446–3388, 3195, 1675 cm-1 due to NH2, NH, and amidic C=O functions,
respectively. Also, its mass spectrum showed the molecular ion peak at m/z = 340
(M?), corresponding to the molecular formula (C17H16N4O2S).
The reaction of enaminones with activated nitriles has been reported as an
efficient synthetic route to attain 2-pyridones [34]. In this context, we investigated
the reaction of activated nitrile moiety of 1 with enaminone. Thus, treatment of 1
with enaminone, E-3-(dimethylamino)-1-phenylprop-2-en-1-one [35], in boiling
dimethylformamide containing a catalytic amount of triethylamine afforded, a
single product, identified as N-(5-(1-cyano-4-(dimethylamino)-2-phenylbuta-1,3-
dienyl)-1,3,4-oxadiazol-2-yl)benzamide (6) instead of N-(5-(2-oxo-6-phenyl-1,2-
dihydropyridin-3-yl)-1,3,4-oxadiazol-2-yl)benzamide (9) as expected (Scheme 1).
The identity of structure 6 was based on the IR spectrum which showed absorption
bands at 3222, 2213, 1678, 1625 cm-1 due to NH, nitrile, amidic C=O, and C=C
functions, respectively. Its mass spectrum showed the molecular ion peak at
m/z = 385 (M?) corresponding to the molecular formula (C22H19N5O2).
We next explored the synthetic use of 1 to obtain some novel pyrano
[2,3-c]pyrazole and chromene based 1,3,4-oxadiazole. Thus, the Knoevenagel
123

S. Bondock et al.
Scheme 2 Synthetic route to Pyrano[2,3-c]pyrazole 11 and Chromene 12 bearing oxadiazole
condensation of 1 with p-anisaldehyde in refluxing ethanolic triethylamine solution
furnished the cyanoolefin derivative 10 [12]. Treatment of cyanoolefin 10 with each
of 1-phenyl-3-methyl-2-pyrazolin-5-one and 5,5-dimethyl-1,3-cyclohexanedione in
refluxing ethanol containing a catalytic amount of piperidine afforded the respective
pyrano[2,3-c]pyrazole 11 and chromene 12 derivatives (Scheme 2). Compounds 11
and 12 were unambiguously synthesized by an alternative route involving the one-
pot three components condensation reaction of 1 with, each of, p-anisaldehyde and
1-phenyl-3-methyl-2-pyrazolin-5-one, p-anisaldehyde and 5,5-dimethyl-1,3-cyclo-
hexanedione, in refluxing ethanolic piperidine solution.
The analytical and spectral data are consistent with the structure proposed for
compounds 11 and 12. In general, the IR spectra of these products revealed
absorption bands at 3451–3418 and 3380–3324 cm-1 for NH2 group and lack of
absorption bands for CN group. The mass spectra of 11 and 12 showed the
molecular ion peak at m/z 520 (M?, 12.5 %) and 486 (M?, 8 %), respectively. The
1H–NMR spectrum of 11 displayed four singlet signals at d 2.32, 3.83, 4.84,
12.31 ppm assignable to methyl, methoxy, pyran-H4, amidic NH protons, respec-
tively, besides a complex multiplet at d 7.12–8.12 ppm due to the amino and
aromatic protons. Formation of compounds 11 and 12 could be explained via a
mechanism starting with the Michael addition of an active methylene group of
pyrazolone and dimedone to the ylidenic bond in cyanoolefin 10 to give an acyclic
intermediate, which cyclized in situ by the nucleophilic attack of the enolic (OH)
group on the cyanocarbon, followed by tautomerization to give the final products.
123

Synthesis and antioxidant evaluation of some new…
Scheme 3 Synthetic route to pyrazolo[1,5-a]pyrimidines 15, 17 and 19 bearing oxadiazole
5-Aminopyrazoles are versatile precursor and have been extensively used as
building blocks for the synthesis of several polysubstituted condensed pyrazoles of
potent pharmacological activity [36]. It was thus of interest to investigate the
reactivity of the second key intermediate N-(5-(5-amino-3-phenylamino)-1H-
pyrazol-4-yl)-1,3,4-oxadiazol-2-yl)-benzamide (13) [12] towards a variety of
bifunctional electrophilic reagents. The general literature procedure [37] for the
synthesis of pyrazolo[1,5-a]pyrimidines involves cyclocondensation of aminopyra-
zoles with 1,3-bielectrophilic reagents. Thus, cyclocondensation reaction of
compound 13 with acetylacetone in boiling acetic acid produced a single product
123

S. Bondock et al.
Scheme 4 Synthetic route to imidazo[1,2-b]pyrazoles 22 and 24 bearing oxadiazoles
formulated as N-(5-(5,7-dimethyl-2-(phenylamino)pyrazolo[1,5-a]pyrimidin-3-yl)-
1,3,4-oxadiazol-2-yl)benzamide (15).
The structure 15 was confirmed on the basis of elemental analysis and spectral
data. The IR spectrum displayed absorption bands at 3332, 3265 and 1664 cm-1 due
1
to two NH and an amidic C=O functions. The H–NMR spectrum (DMSO-d6)
exhibited three sharp singlet signals at d 2.61, 2.75 and 6.95 ppm assignable to two
unequivalent methyl protons and fused pyrimidine (H5) protons, respectively,
besides a multiplet signal at d 7.05–7.94 ppm region owing to aromatic protons, and
two broad singlet signals, exchangeable with D2O, at d 10.65 and 11.36 ppm due to
two NH protons. The mass spectrum showed a molecular ion peak at m/z = 425,
corresponding to molecular formula (C23H19N7O2).
Analogously, compound 13 was reacted with ethyl acetoacetate in boiling acetic
acid to give an isolable product, formulated as N-(5-(7-hydroxy-5-methyl-2-
(phenylamino)pyrazolo[1,5-a]pyrimidin-3-yl)-1,3,4-oxadiazol-2-yl)benzamide (17).
Structure 17 was elucidated by elemental analysis, spectral data, and an alternative
synthesis route. Thus, treatment of 13 with acetoacetanilide in boiling acetic acid
gave product identical in all aspects (m.p., mixed m.p., and spectra) with 17.
The formation of compound 17 is assumed to proceed via initial attacks of the
exocyclic amino group of 13 on the keto group of ethyl acetoacetate followed by
elimination of water to give the non-isolable intermediate 16 that underwent
intramolecular cyclization via elimination of ethanol molecule.
Moreover, treatment of 13 with a-cyanocinnamonitrile in boiling ethanol
containing a catalytic amount of piperidine, under reflux, gave isolable product
evidence by mechanism which could be formulated N-(5-(7-amino-6-cyano-5-
phenyl-2-(phenylamino)pyrazolo[1,5-a]pyrimidin-3-yl)-1,3,4-oxadiazol-
2-yl)benzamide (19).
The reaction seems to proceed through Michael addition reaction of the exocyclic
amino group to b-carbon of cinnamonitrile to give intermediate 18 which underwent
intramolecular cyclization via the addition of the endocyclic NH group to the nitrile
functional group and subsequent oxidation to give the final product 19. More
evidences for structure 19 came from its independent synthesis, treatment of 20,
which was prepared via condensation of 13 with benzaldehyde, with malononitrile
123

Synthesis and antioxidant evaluation of some new…
in boiling ethanol containing a catalytic amount of piperidine gave a product
identical in all aspects (m.p., mixed m.p., and spectra) with 19 (Scheme 3).
Literature survey revealed that there have been few reports on the synthesis of
imidazo[1,2-b]pyrazoles [23, 38]. The interest in the synthesis of azapentalene ring
system is attributed to the fact that they are useful in both theoretical chemistry and
material sciences [39]. Thus, from a structure–activity viewpoint, the insertion of
1,3,4-oxadiazole moiety into the azapentalene ring system may offer derivatives
with enhanced or new utility.
In this context, we studied the regioselectivity of the cyclocondensation of
5-aminopyrazole with each of chloroacetyl chloride and phenacycl bromide as a
possible synthetic route to attain imidazo[1,2-b]pyrazoles. Thus, cyclocondensation
of 13 with chloroacetyl chloride in dioxane containing a catalytic amount of
triethylamine at room temperature afforded a single product for which two probable
isomeric structures were considered 21 and 22 (Scheme 4). The structure 21 was
ruled out on the basis of its analytical and spectral data. The structure 22 was
elucidated for the reaction product where, its 1H–NMR spectrum (DMSO-d6)
showed a doublet signal at d 4.05 ppm (J = 6.6 Hz) assignable to a CH2 protons
attached to NH of fused pyrazole ring, besides a multiplet signal at d 7.32–7.93 ppm
due to aromatic protons and three broad singlet signals at d 10.25, 11.50 and
12.36 ppm due to three NH protons. The IR spectrum showed the presence of
absorption bands at 1690 and 1663 cm-1 due to cyclic C=O and amidic C=O
functions, respectively. Moreover, its mass spectrum showed the molecular ion peak
at m/z = 401.
Similarly, the reaction of 13 with phenacyl bromide in dioxane, under reflux, in
the presence of triethylamine afforded a product of a molecular formula
(C26H19N7O2). Two theoretically possible structures were considered (cf. structure
23 and 24) (Scheme 4). The structure 24 was considered most likely than 23 based
on its spectroscopic data. The IR spectrum of 24 showed absorption bands at
3330–3160 cm-1 region due to three NH functions, at 1661 cm-1 characteristic to
1
amidic C = O function. Its H NMR spectrum (DMSO-d6) displayed a doublet
signal at d 6.24 ppm (J = 6.6 Hz) assignable to cyclic methine proton adjacent to
NH of fused pyrazole, a multiplet signal at d 7.17–7.87 ppm due to aromatic protons
and three broad singlet signals at d 10.25, 11.37 and 12.66 ppm due to three NH
protons. The mass spectrum showed a molecular ion peak at m/z = 461 (M?),
corresponding to a molecular formula (C26H19N7O2).
Biological evaluation
Antioxidant activity by means of ABTS method
The antioxidant activity of the newly synthesized compounds was evaluated by the
ABTS antioxidant activity method as described previously by Lissi and coworkers
[40]. Some of the 5-hetaryloxadiazoles exhibited an antioxidant effect as shown in
Table 1. The results of antioxidant activity of 2-benzoylamino-5-hetaryl-1,3,4-
oxadiazoles compared with the control (L-ascorbic acid) revealed that the antioxidant
123

S. Bondock et al.
Table 1 The antioxidant
activity of the synthesized
compounds by mean of ABTS
method
Compound no.
Absorbance of samples
Compound no.
% inhibitiona
Control of ABTS
0.534
0.052
0.121
0.277
0.336
0.528
0.507
0.483
0.445
0.449
0.197
0.340
0.220
0.0
Ascorbic acid
90.26
77.34
48.12
37.07
1.12
1
2
4
5
6
5.05
11
12
15
17
19
20
9.55
16.66
15.91
63.10
36.32
58.80
a
The % inhibition values were
the mean of three
replicates 0.02
Table 2 Pro-oxidant effects of
compounds 1 and 17 on ferric
Bleomycin-induced DNA
damage
Compound no.
Bleomycin-dependent DNA damage
Absorbance of sample
Control
0.0
Ascorbic acid
0.098 0.001
0.101 0.001
0.094 0.001
1
17
activity of compounds 1 and 17 was found to be the highest. On the other hand,
compounds 2, 4, 19, and 20 showed a moderate antioxidant activity, and the rest of
the investigated compounds showed a weak activity. From the structure activity
relationship (SAR) point of view, the presence of pyrazole or pyrazolopyrimidine
moieties at the 5-position of the 1,3,4-oxadiazole ring system enhanced the
antioxidant activity. While the introduction of tetrahydrobenzothiophene or pyra-
no[2,3-c]pyrazole derivatives at the 5-position of oxadiazole diminished the
antioxidant activity as in the case of compounds 5 and 11.
The most potent antioxidant active compounds 1 and 17 were tested for
Bleomycin-dependent DNA damage (Table 2). Damage of DNA in the presence of
a Belomycin-Fe complex has been adopted as a sensitive and specific method to
examine potential pro-oxidant agents [41]. If the samples to be tested are able to
reduce the Bleomycin-Fe3? to Bleomycin-Fe2?, DNA degradation in this system
will be stimulated, resulting in a positive test for pro-oxidant activity. DNA
degradation is accompanied by the formation of a product similar to malondialde-
hyde. L-Ascorbic acid can also reduce Fe3? to Fe2? as a reducing agent. Table 2
shows that compound 1 and 17 have an excellent antioxidant activity and ability to
protect DNA from the induced damage by Belomycin. These higher activities may
123

Synthesis and antioxidant evaluation of some new…
be attributed to the presence of a hydroxyl group attached to an aromatic ring in 17
or the secondary amino group in 1, those can act as scavenging of free radicals.
Conclusion
In conclusion, the aim of the present study was to synthesize and evaluate the
antioxidant activity of some novel 2-benzoylamino-5-hetaryl-1,3,5-oxadiazoles with
the hope of discovering new structure leads to serve as potent antioxidant agent. Our
aim has been verified by the synthesis of different groups of structure hybrids
comprising basically the 1,3,4-oxadiazole fragment attached to either pyrazole,
benzothiophene, pyrano[2,3-c]pyrazole, chromene, pyrazolo[1,5-a]pyrimidines or
imidazo[1,2-b]pyrazoles counter parts. The obtained results clearly revealed that
1,3,4-oxadiazoles 1 and 17 linked to pyrazolo[1,5-a]pyrimidines showed excellent
antioxidant activity and exhibited high protection against DNA damage induced by
Bleomycin iron complex.
Experimental
The chemicals used for the synthesis of the compounds were obtained from Aldrich
and Sigma Chemical Company without further purification. The solvents used were of
analytical grade. Melting points were measured on an electrothermal Gallenkamp
melting point apparatus. Elemental analyses were carried out at the Microanalytical
Unit, Cairo University, Giza, Egypt; the results were in satisfactory agreement with
the calculated values. The IR spectra were recorded in KBr disks on a Mattson 5000
1
FTIR spectrometer (not all frequencies are reported). The H–NMR spectra were
acquired using a Bruker WP 300 spectrometer at 300 MHz using TMS as an internal
standard and DMSO-d6 as solvent. The Mass spectra were performed using a
Finnigan Incos 500 mass spectrometer at 70 eV. N-(5-(cyanomethyl)-1,3,4-oxadiazol-
2-yl)benzamide (1), N-(5-(1-Cyano-2-(4-methoxyphenyl)vinyl)-1,3,4-oxadiazol-2-yl)
benzamide (10) and N-(5-(3-amino-5-(phenylamino)-1H-pyrazol-4-yl)-1,3,4-oxa-
diazol-2-yl)benzamide (11) were prepared according to previously reported procedure
[12].
Synthesis of N-(5-[cyano(p-tolylhydrazono)methyl]-1,3,4-oxadiazol-2-yl)benz-amide
(2) A solution of the p-tolyl diazonium chloride (0.002 mol) was added dropwise
to a solution of compound 1 (0.46 g, 0.002 mol) in pyridine (20 mL) at 0–5 °C
with stirring. The reaction mixture was stirred further for 6 h at 0 °C. The
resulting solid was collected by filtration, dried and recrystallized from ethanol to
give compound 2.
Orange powder; Yield 73 %; m.p. 239–240 °C; IR (KBr): m/cm-1 = 3230
1
(amidic NH), 3195 (NH), 2215 (CN), 1716 (CO), 1604 (C=N). H–NMR (DMSO-
d6): dppm = 2.28 (s, 3H, CH3), 7.19-8.06 (m, 9H, Ar–H), 12.21 (bs, 1H, amidic NH),
12.45 (s, 1H, NH-hydrazone); 13C–NMR (DMSO-d6): dppm = 20.1 (CH3), 111.3
(CN), 114.3 (C=N), 117.6 (2CHAr), 125.8 (2CHAr), 127.3 (2CHAr), 128.8 (2CHAr),
123

S. Bondock et al.
132.4 (CHAr), 133.5 (CAr), 136.3 (CAr), 140.5 (CAr), 145.9 (Oxadiazole-C2), 146.5
(Oxadiazole-C5), 164.5 (C=O). MS (EI, 70 eV) m/z (%): 346 (M?, 15.6), 333 (6.2),
317 (5.6), 306 (0.4), 289 (10.3), 274 (8.2), 259 (0.9), 244 (10.4), 227 (10.3), 105
(100.0), 91 (11.6), 77 (37.8), 65 (4.3). Anal. Calcd. for C18H14N6O2 (346.34): C
62.42, H 4.07, N 24.27 %. Found: C 62.48, H 4.11, N 24.31 %.
Synthesis of ethyl 2-(2-((5-benzoylamino-1,3,4-oxadiazol-2-yl)(cyano) methylene)-
1-p-tolylhydrazinyl)acetate (3) A mixture of compound 2 (0.34 g, 0.001 mol) and
ethyl bromoacetate (0.16 mL, 0.001 mol) in DMF (20 mL) containing three drops
of triethylamine was refluxed for 3 h. The reaction mixture was allowed to cool and
then poured onto 100 mL ice-cold water. The resulting solid was collected by
filtration, dried and recrystallized from ethanol to give compound 3.
Orange powder; Yield 67 %; m.p. 260–261 °C; IR (KBr): m/cm-1 = 3245 (NH),
1
3192 (NH), 1745 (CO, ester), 1713 (CO, amidic), 1602 (C=N). H–NMR (DMSO-
d6): dppm = 1.23 (t, J = 7.0 Hz, 3H, CH3), 2.35 (s, 3H, CH3), 4.12 (q, J = 7.0 Hz,
2H, OCH2), 4.42 (s, 2H, NCH2CO), 7.15–7.98 (m, 9H, Ar–H), 11.98 (s, 1H,
NHCO); 13C-NMR (DMSO-d6): dppm = 14.1 (CH3), 20.6 (CH3), 54.6 (NCH2), 61.5
(OCH2), 110.6 (CN), 115.1 (2CHAr), 116.2 (C=N), 127.3 (2CHAr), 128.4 (CAr),
128.9 (2CHAr), 132.3 (CHAr), 133.3 (2CHAr), 133.8 (CAr), 146.0 (Oxadiazole-C2),
147.3 (Oxadiazole-C5), 151.2 (CAr), 164.5 (C = O), 169.8 (C=O). MS (EI, 70 eV)
m/z (%): 432 (M?, 5.2), 407 (11.1), 393 (10.3), 390 (4.1), 373 (1.2), 346 (16.3), 327
(0.3), 318 (0.3), 300 (0.5), 282 (0.5), 272 (0.2), 265 (1.3), 242 (6.2), 225 (0.6), 217
(1.2), 200 (0.3), 105 (100.0), 91 (11.5), 77 (36.1), 65 (4.1). Anal. Calcd. for
C22H20N6O4 (432.43): C 61.10, H 4.66, N 19.43 %. Found: C 61.14, H 4.72, N
19.48 %.
Synthesis of ethyl 4-amino-3-(5-benzoylamino-1,3,4-oxadiazol-2-yl)-1-p-tolyl-1H-
pyrazole-5-carboxylate (4) A mixture of compound 3 (0.43 g, 0.001 mol) in
ethanolic sodium ethoxide solution [prepared by dissolving (0.2 g, 0.001 mol) of
sodium metal in (25 mL) absolute ethanol) was refluxed for 6 h. The reaction
mixture was allowed to cool down and then poured onto 50 mL ice-cold water
containing few drops of dilute HCl. The resulting product was collected by
filtration, dried and recrystallized from ethanol to give compound 4.
Orange powder; Yield 67 %; m.p. 119–120 °C; IR (KBr): m/cm-1 = 3345-3285
1
(NH2), 3197 (NH), 1739 (CO, ester), 1703 (CO, amidic), 1600 (C=N). H–NMR
(DMSO-d6): dppm = 1.26 (t, J = 7.5 Hz, 3H, CH3), 2.32 (s, 3H, CH3), 4.35 (q,
J = 7.5 Hz, 2H, OCH2), 6.35 (s, 2H, NH2), 7.16–8.12 (m, 9H, Ar–H), 12.06 (s, 1H,
NHCO); 13C–NMR (DMSO-d6): dppm = 14.2 (CH3), 20.7 (CH3), 59.8 (OCH2),
118.6 (Pyrazole-C5), 124.9 (2CHAr), 127.8 (2CHAr), 128.9 (2CHAr), 131.2 (2CHAr),
132.4 (CHAr), 132.6 (Pyrazole-C4), 133.0 (Pyrazole-C3), 133.6 (CAr), 136.7 (CAr),
140.4 (CAr), 145.7 (Oxadiazole-C2), 147.7 (Oxadiazole-C5), 160.4 (C=O), 164.8
(C=O). MS (EI, 70 eV) m/z (%): 432 (M?, 6.2), 407 (9.1), 393 (6.3), 390 (7.1), 373
(11.2), 346 (16.3), 327 (10.3), 318 (8.3), 300 (4.5), 282 (8.5), 272 (10.2), 265 (10.3),
242 (20.2), 225 (9.6), 217 (1.2), 200 (0.3), 105 (100.0), 91 (11.5), 77 (36.1), 65
(4.1). Anal. Calcd. for C22H20N6O4 (432.43): C 61.10, H 4.66, N 19.43 %. Found: C
61.12, H 4.70, N 19.38 %.
123

Synthesis and antioxidant evaluation of some new…
Synthesis of N-(5-(2-amino-4,5,6,7-tetrahydrobenzo[b]thiophen-3-yl)-1,3,4-oxadiazol-
2-yl)benzamide (5) A mixture of compound 1 (0.23 g, 0.001 mol), cyclohexanone
(0.10 g, 0.001 mol) and elemental sulfur (0.10 g, 0.003 mol) in ethanol (20 mL)
and DMF (5 mL) containing morpholine (0.09 g, 0.001 mol) was heated on water
bath at 50 °C for 6 h. After the reaction mixture was cooled, the solid product
obtained was filtered off, dried well and recrystallized from ethanol to give
compound 5.
Green crystals; Yield 81 %; m.p. 179–180 °C; IR (KBr): m/cm-1 = 3446–3388
(NH2), 3195(NH), 1675(CO), 1600(C=N). 1H–NMR (DMSO-d6): dppm = 1.35–2.233
(m, 8H, Cyclohexane), 6.22 (s, 2H, NH2), 7.24–7.98 (m, 5H, Ar–H), 12.01 (s, 1H,
NHCO); 13C–NMR (DMSO-d6): dppm = 21.1, 23.8, 26.3, 27.1 (4CH2), 80.5
(Thiophene-C3), 121.3 (Thiophene-C4), 127.6 (2CHAr), 128.3 (2CHAr), 130.1
(Thiophene-C5), 131.0 (CAr), 132.5 (CHAr), 143.9 (Oxadiazole-C2), 154.5 (Oxa-
diazole-C5), 162.9 (Thiophene-C2), 165.0 (C=O). MS (EI, 70 eV) m/z (%): 342
(M??2, 2.1), 340 (11.2), 339 (4.1), 327 (3.1), 313 (5.2), 299 (10.1), 285 (11.2), 278
(20.2), 250 (40.3), 236 (30.8), 235 (20.1), 208 (50.3), 180 (10.3), 160 (12.3), 152
(16.8), 128 (2.7), 123 (1.0), 111 (1.7), 96 (11.5), 83 (2.9), 64 (100). Anal. Calcd. for
C17H16N4O2S (340.4): C 59.98, H 4.74, N 16.46 %. Found: C 59.93, H 4.76, N
16.49 %.
Synthesis of N-(5-(1-cyano-4-(dimethylamino)-2-phenylbuta-1,3-dienyl)-1,3,4-oxa-
diazol-2-yl)benzamide (6) A mixture of compound 1 (0.46 g, 0.002 mol) and
3-(dimethylamino)-1-phenylprop-2-en-1-one (0.35 g, 0.002 mol) in DMF (10 mL)
and TEA (0.2 mL) was refluxed for 6 h. The reaction mixture was left to cool and
then poured onto ice-cold water (25 mL). The formed precipitate was filtered off,
dried and recrystallized from ethanol to give compound 6.
Green crystals; Yield 68 %; m.p. 210–211 °C; IR (KBr): m/cm-1 = 3222 (NH),
1
2213 (CN), 1678 (CO), 1625 (C=C). H–NMR (DMSO-d6): dppm = 3.21 (s, 3H,
CH3), 3.34 (s, 3H, CH3), 6.14 (d, J = 12.6 Hz, 1H, CH=), 6.36 (d, J = 12.6 Hz,
1H, CH =), 7.14–8.05 (m, 10H, Ar–H), 11.98 (s, 1H, NHCO); 13C–NMR (DMSO-
d6): dppm = 41.2 (N(CH3)2), 93.8 (C=), 98.3 (C=), 117.4 (CN), 127.3 (2CHAr),
128.0 (2CHAr), 129.2 (2CHAr), 130.0 (2CHAr), 130.6 (CHAr), 131.4 (CHAr), 132.8
(CAr), 135.6 (CAr), 143.9 (Oxadiazole-C2), 145.2 (Oxadiazole-C5), 149.6 (C=N),
154.4 (C=), 164.9 (C=O). MS (EI, 70 eV) m/z (%): 386 (M??1, 1.5), 385 (9.7), 369
(2.3), 356 (1.8), 345 (4.1), 329 (3.2), 321 (2.2), 316 (10.7), 315 (1.7), 298 (2.3), 291
(1.5), 280 (1.4) 279 (2.6), 264 (3.9), 238 (3.0), 240 (4.7), 224 (2.1), 213 (1.1), 212
(8.5), 105 (100), 77 (67.8). Anal. Calcd. for C22H19N5O2 (385.42): C 68.56, H 4.97,
N 18.17 %. Found: C 68.60, H 5.02, N 18.24 %.
General procedure for the Synthesis of pyrano[2,3-c]pyrazole 11 and chromene 12
derivatives To a solution of compound 1 (0.23 g, 0.001 mol) and p-anisaldehyde
(0.12 mL, 0.001 mol), in ethanol (25 mL) containing a few drops of piperidine,
either 1-phenyl-3-methyl-2-pyrazolin-5-one (0.17 g, 0.001 mol) or 5,5-dimethyl-
1,3-cyclohexanedione (0.14 g, 0.001 mol) was added. The reaction mixture, in each
case, was heated under reflux for 24 h. The solid products formed upon pouring
onto an ice-cold-water mixture (25 mL) containing few drops of hydrochloric acid
123

S. Bondock et al.
was collected by filtration, dried, and recrystallized from DMF to afford compounds
11 and 12, respectively.
N-(5-(6-Amino-4-(4-methoxyphenyl)-3-methyl-1-phenyl-1,4-di-hydropyrano[2,3-c]
pyrazol-5-yl)-1,3,4-oxadiazol-2-yl)benzamide (11) Pale yellow powder; Yield
83 %; m.p. 230–231 °C; IR (KBr): m/cm-1 = 3451–3418 (NH2), 3190 (NH), 1666
(C=O), 1610 (C=N). 1H–NMR (DMSO-d6): dppm = 2.32 (s, 3H, CH3), 3.83 (s, 3H,
OCH3), 4.84 (s, 1H, pyran-H-4), 7.12–8.12 (m, 16H, Ar–H, NH2), 12.31 (s, 1H, NH
amidic); 13C–NMR (DMSO-d6): dppm = 13.4 (CH3), 33.4 (Pyran-C4), 55.2 (OCH3),
91.9 (Pyran-C3), 98.2 (Pyrazole-C4a), 117.1 (2CHAr), 119.5 (2CHAr), 124.8 (CHAr),
127.5 (2CHAr), 127.8 (2CHAr), 128.8 (2CHAr), 129.1 (2CHAr), 129.5 (CHAr), 131.1
(CAr), 132.2 (CHAr), 137.0 (CAr), 138.0 (CAr), 142.1 (Pyrazole-C2), 142.3 (Pyrazole-
C3), 145.6 (Oxadiazole-C2), 149.3 (Oxadiazole-C5), 161.4 (CAr), 164.8 (C=O). MS
(EI, 70 eV) m/z (%): 520 (M?, 12.5), 445 (10.1), 428 (5.2), 415 (8.2), 400 (10.2),
384 (6.2), 370 (11.6), 361 (9.1), 346 (14.8), 345 (20.2), 331 (0.2), 318 (4.1), 292
(8.1), 279 (6.3), 264 (4.1), 241 (8.3), 226 (10.2), 211 (11.1), 200 (20.6), 186 (4.4),
171 (10.4), 158 (2.9), 143 (2.3), 127 (1.9), 115 (3.8), 105 (100), 88 (1.8), 77 (43.0),
63 (1.9). Anal. Calcd. for C29H24N6O4 (520.19): C 66.91, H 4.65, N 16.14 %.
Found: C 66.82, H 4.61, N 16.11 %.
N-(5-(2-Amino-4-(4-methoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chro-
men-3-yl)-1,3,4-oxadiazol-2-yl)benzamide (12) Pale yellow powder; Yield 79 %;
m.p. 120–121 °C; IR (KBr): m/cm-1 = 3380–3324 (NH2), 2999 (NH), 1668 (C=O),
1645 (C=O), 1599 (C=N).1H–NMR (DMSO-d6): dppm = 1.29 (s, 6H, 2CH3), 1.69
(s, 2H, CH2), 2.45 (s, 2H, CH2), 3.79 (s, 3H, OCH3), 3.88 (s, 1H, CH), 6.10 (s, 2H,
NH2), 6.75–8.27 (m, 9H, Ar–H), 12.78 (s, 1H, NH); 13C–NMR (DMSO-d6):
dppm = 26.8 (CH3), 28.2 (CH3), 31.7 (Cq), 39.6 (CH2), 40.4 (CH), 44.2 (CH2), 55.2
(OCH3), 91.3 (Chromene-C3), 111.5 (C=), 114.7 (2CHAr), 127.0 (2CHAr), 127.8
(2CHAr), 128.9 (2CHAr), 130.2 (CAr), 132.4 (CHAr), 138.6 (CAr), 145.2 (Oxa-
diazole-C2), 154.2 (Chromene-C2), 155.3 (Oxadiazole-C5), 157.2 (CAr-O), 160.5
(O–C=), 164.8 (C=O), 196.2 (C=O). MS (EI, 70 eV) m/z (%): 486 (M?,8.0), 457
(5.4), 456 (0.7), 455 (8.0), 441 (11.7), 432 (6.5), 415 (100), 400 (7.7), 379 (9.4), 370
(18.2), 361 (13.6), 346 (32.8), 324 (9.3), 316 (8.9), 295 (41.6), 281 (24.4), 273
(14.1), 253 (8.9), 250 (14.4), 236 (8.3), 211 (13.2), 197 (13.6), 178 (6.3), 164 (11.3),
149 (8.8), 135 (6.2), 120 (4.1), 111 (19.3). Anal. Calcd. for C27H26N4O5 (486.52): C
66.65, H 5.39, N 11.52 %. Found: C 66.69, H 5.43, N 11.58 %.
Synthesis of N-(5-(5,7-dimethyl-2-(phenylamino)pyrazolo[1,5-a]pyrimidin-3-yl)-1,3,4-
oxadiazol-2-yl)benzamide (15) A mixture of compound 13 (0.36 g, 0.001 mol) and
acetylacetone (0.13 g, 0.001 mol) in acetic acid (20 mL) was refluxed for 3 h. The
reaction mixture was left to cool and then poured into ice-cold water. The resulting
solid product was collected and recrystallized from DMF to give compound 15.
Grey powder; Yield 76 %; m.p.214–215 °C; IR (KBr): m/cm-1 = 3332, 3265
(2NH), 1664 (amidic CO), 1596 (C=N). 1H–NMR (DMSO-d6): dppm = 2.61 (s, 3H,
CH3), 2.75 (s, 3H, CH3), 6.95 (s, 1H, Pyrimidine-H5), 7.05–7.94 (m, 10H, Ar–H),
10.65 (s, 1H, NHPh), 11.36 (s, 1H, NHCOPh); 13C–NMR (DMSO-d6): dppm = 15.7
(CH3), 19.4 (CH3), 98.1 (Pyrazole-C4), 107.9 (Pyrimidine-C5), 118.5 (2CHAr),
123

Synthesis and antioxidant evaluation of some new…
119.5 (CHAr), 127.9 (2CHAr), 128.8 (2CHAr), 129.1 (2CHAr), 130.1 (Pyrazole-C3),
131.2 (CAr), 132.4 (CHAr), 144.3 (Oxadiazole-C2), 145.7 (CAr), 149.5 (Pyrimidine-
C4), 151.2 (Oxadiazole-C5), 156.3 (Pyrimidine-C2), 163.6 (Pyrimidine-C6), 164.5
(C=O). MS (EI, 70 eV) m/z (%): 425 (M?, 6.1), 415 (3.2), 392 (5.2), 381 (1.2), 369
(8.1), 352 (8.4), 338 (1.1), 322 (12.4), 321 (10.3), 296 (1.2), 281 (4.5), 266 (20.3),
265 (100), 237 (1.8), 221 (1.5), 196 (1.3), 186 (1.9), 174 (2.7), 161 (1.1), 146 (1.9),
137 (16.02), 118 (1.9), 103 (6.6), 92 (2.6), 77 (8.2), 65 (5.3). Anal. Calcd. for
C23H19N7O2 (425.44): C 64.93, H 4.50, N 23.05 %. Found: C 65.02, H 4.54, N
23.13 %.
Synthesis of N-(5-(7-methyl-5-oxo-2-(phenylamino)-4,5-dihydro-pyrazolo[1,5-a]pyrim-
idin-3-yl)-1,3,4-oxadiazol-2-yl)benzamide (17) A mixture of compound 13 (0.36 g,
0.001 mol) and ethyl acetoacetate (0.13 g, 0.001 mol) or acetoacetanilide (0.177 g,
0.001 mol) in glacial acetic acid (20 mL) was refluxed for 3 h. The reaction mixture
was left to cool down and then poured onto ice-cold water (25 mL). The resulting
solid was collected by filtration, dried, and recrystallized from DMF to give
compound 17.
Grey powder; Yield 71 %; m.p. 229–230 °C; IR (KBr): m/cm-1 = 3460 (OH),
3340, 3215 (2NH), 1661 (amidic CO), 1600 (C=N). 1H–NMR (DMSO-d6):
dppm = 2.36 (s, 3H, CH3), 6.90 (s, 1H, pyrimidine-H5), 7.24–7.95 (m, 10H, Ar–H),
10.19 (s, br, 1H, NH), 11.47 (s, 1H, NHCOPh), 12.63 (s, 1H, OH); 13C–NMR
(DMSO-d6): dppm = 19.6 (CH3), 89.9 (Pyrimidine-C5), 98.4 (C=), 118.2 (2CHAr),
119.8 (CHAr), 127.0 (2CHAr), 128.5 (2CHAr), 129.4 (2CHAr), 131.0 (CAr), 132.4
(CHAr), 133.5 (C=N), 144.0 (Oxadiazole-C2), 145.7 (CAr), 148.5 (C=), 151.2
(Oxadiazole-C5), 164.9 (C=O), 167.1 (C=N), 174.5 (Pyrimidine-C4). MS (EI,
70 eV) m/z (%) = 427 (M?, 8.3), 419 (1.6), 393 (1.4), 388 (1.9), 360 (8.1), 346
(19.8), 323 (11.5), 309 (6.5), 259 (3.1), 283 (32.0), 226 (32.8), 256 (37.4), 240
(71.8), 237 (15.7), 215 (35.8), 209 (13.4), 189 (12.8), 184 (100.0), 160 (29.8), 146
(16.2), 129 (5.4), 118 (30.6), 103 (93.6), 91 (21.9), 77 (86.6), 68 (12.5). Anal. Calcd.
for C22H17N7O3 (427.42): C 61.82, H 4.01, N 22.94 %. Found: C 61.88, H 4.07, N
23.02 %.
Synthesis of N-(5-(3-(benzylideneamino)-5-(phenylamino)-1H-pyrazol-4-yl)-1,3,4-
oxadiazol-2-yl)benzamide (20) Equimolar amounts of compound 13 (0.36 g,
0.001 mol) and benzaldehyde (0.11 g, 0.001 mol) in ethanol (20 mL) containing a
catalytic amount of piperidine was refluxed for 3 h. The reaction mixture was left to
cool and then poured onto ice-cold water (15 mL). The obtained solid product was
collected by filtration, dried, and recrystallized from DMF to give compound 20.
Yellow powder; Yield 62 %; m.p. 299–300 °C; IR (KBr): m/cm-1 = 3440, 3315,
3181 (3NH), 1650 (C=O), 1600 (C=N). 1H–NMR (DMSO-d6): dppm = 6.82 (s, 1H,
CH=), 7.17–7.92 (m, 16H, Ar–H, NH), 11.85 (s, br, 1H, NH). MS (EI, 70 eV) m/z
(%): 449 (M?, 12.1), 434 (7.9), 432 (3.2), 408 (1.0), 403 (2.2), 389 (1.6), 369 (5.2),
359 (0.2), 344 (0.15), 343 (0.2), 329 (0.3), 313 (0.9), 304 (41.6), 289 (100), 266
(2.0), 265 (10.8), 262 (1.8), 261 (2.1), 238 (12.7), 231 (2.7), 211 (5.5), 200 (17.1),
185 (7.6), 174 (16.6), 171 (2.4), 170 (1.2), 156 (5.9), 146 (6.1), 128 (7.0), 118 (8.7),
123

S. Bondock et al.
104 (33.6), 91 (17.2), 84 (2.9), 77 (50.9). Anal. Calcd. for C25H19N7O2 (449.46): C
66.81, H 4.26, N 21.81 %. Found: C 66.78, H 4.24, N 21.72 %.
Synthesis of N-(5-(7-amino-6-cyano-5-phenyl-2-(phenylamino)pyrazolo[1,5-a]pyrim-
idin-3-yl)-1,3,4-oxadiazol-2-yl)benzamide (19) Method A: Equimolar amounts of
compound 13 (0.36 g, 0.001 mol) and a-cyanocinnamonitrile (0.15 g, 0.001 mol) in
ethanol (20 mL) containing a catalytic amount of piperidine was refluxed for 4 h.
The reaction mixture was left to cool down and then poured onto ice-cold water
(15 mL). The solid obtained was collected by filtration, dried, and recrystallized
from DMF to give compound 19.
Method B: Equimolar amounts of Schiff base 20 (0.45 g, 0.001 mol) and
malononitrile (0.08 g, 0.001 mol) in ethanol (10 mL) containing a catalytic amount
of piperidine was refluxed for 4 h. The reaction mixture was left to cool and then
poured into ice-cold water (10 mL). The obtained product was collected by filtration
and recrystallized from DMF to give compound 19.
Brown powder; Yield 73 %; m.p. 229–230 °C; IR (KBr): m/cm-1 = 3435–3395
(NH2), 3292, 3197 (2NH), 2191 (CN), 1653 (C=O), 1600 (C=N). 1H–NMR
(DMSO-d6): dppm = 5.91 (s, 2H, NH2), 7.32–7.90 (m, 15H, Ar–H), 9.38 (s, 1H,
NH), 11.58 (s, 1H, NH); 13C–NMR (DMSO-d6): dppm = 74.2 (Pyrimidine-C5), 97.8
(C=), 114.8 (CN), 118.2 (2CHAr), 119.1 (CHAr), 121.4 (2CHAr), 127.5 (2CHAr),
128.7 (2CHAr), 129.0 (2CHAr), 129.8 (CHAr), 130.2 (2CHAr), 131.4 (CAr), 132.2
(CHAr), 133.1 (C=N), 137.8 (CAr), 144.0 (Oxadiazole-C2), 145.0 (C=), 145.7 (CAr),
150.8 (Oxadiazole-C5), 155.4 (Pyrimidine-C6), 164.4 (C=O), 169.8 (Pyrimidine-
C4). MS (EI, 70 eV) m/z (%): 513 (M?, 9.5), 455 (11.4), 442 (1.5), 431 (8.6), 419
(9.6), 393 (10.7), 382 (12.2), 372 (4.8), 356 (11.8), 336 (13.2), 303 (25.0), 277
(14.4), 261 (17.6), 235 (23.2), 219 (36.7), 208 (34.8), 185 (18.98), 178 (28.0), 166
(32.0), 141 (39.7), 136 (23.9), 118 (100.0), 109 (27.5), 89 (35.8), 83 (34.4), 65
(88.3). Anal. Calcd. for C28H19N9O2 (513.51): C 65.49, H 3.73, N 24.55 %. Found:
C 65.53, H 3.76, N 24.62 %.
General procedure for the Synthesis of imidazol[1,2-b]pyrazole derivatives 2 and
224 To a solution of compound 13 (0.36 g, 0.001 mol) in dioxane (25 mL)
containing two drops of triethylamine, either chloroacetyl chloride (0.08 ml,
0.001 mol), or phenacyl bromide (0.2 g, 0.001 mol) was added. The reaction
mixture, in each case, was heated under reflux for 6 h. The solid products formed
upon pouring onto an ice-cold water (15 mL) containing few drops of hydrochloric
acid was collected by filtration, dried, and recrystallized from a mixture of ethanol/
DMF (3:1) to afford compounds 22 and 24, respectively.
N-(5-(3-Oxo-6-(phenylamino)-2,3-dihydro-1H-imidazo[1,2-b]pyrazol-7-yl)-1,3,4-oxa-
diazol-2-yl)benzamide (22) Yellow powder; Yield 79 %; m.p. 246–247 °C; IR
(KBr): m/cm-1 = 3326, 3198, 3154 (3NH), 1690 (CO, cyclic), 1663 (C=O, amidic),
1584 (C=N). 1H–NMR (DMSO-d6): dppm = 4.05 (d, J = 6.6 Hz, 2H, CH2),
7.32–7.93 (m, 10H, Ar–H), 10.25 (br, s, 1H, NH), 11.50 (br, s, 1H, NH), 12.36 (br, s,
1H, NH); 13C–NMR (DMSO-d6): dppm = 47.7 (NCH2), 83.1 (Pyrazole-C4), 118.5
(2CHAr), 119.5 (CHAr), 127.8 (2CHAr), 128.9 (2CHAr), 129.7 (2CHAr), 131.1 (CAr),
132.6 (CHAr), 140.6 (CAr), 144.3 (Oxadiazole-C2), 145.3 (Pyrazole-C3), 147.0
123

Synthesis and antioxidant evaluation of some new…
(Oxadiazole-C5), 153.8 (Imidazole-C2), 164.1 (C=O), 165.9 (C=O). MS (EI, 70 eV)
m/z (%): 401 (M?, 4.3), 344 (1.5), 285 (1.7), 253 (100), 225 (2.2), 214 (4.3), 181
(12.6), 117 (8.3), 105 (6.5), 99 (14.2), 91 (11.9), 84 (18.9), 73 (33.0), 69 (15.5).
Anal. Calcd. for C20H15N7O3 (401.38): C 59.85, H 3.77, N 24.43 %. Found: C
59.91, H 3.83, N 24.47 %.
N-(5-(3-Phenyl-6-(phenylamino)-1H-imidazo[1,2-b]pyrazol-7-yl)-1,3,4-oxadiazol-
2-yl)benzamide (24) Orange powder; Yield 82 %; m.p. 230–231 °C; IR (KBr): m/
cm-1 = 3330, 3200, 3160 (3NH), 1661 (CO, amidic), 1587 (C=N). 1H–NMR
(DMSO-d6): dppm = 6.24 (d, J = 6.6 Hz, 1H, CH=), 7.17–7.87 (m, 15H, Ar–H),
10.25 (s, 1H, NH), 11.37 (s, 1H, NH), 12.66 (s, 1H, NH); 13C–NMR (DMSO-d6):
dppm = 78.5 (Pyrazole-C4), 118.3 (2CHAr), 119.1 (CHAr), 121.4 (2CHAr), 123.3
(Imidazole-C4), 126.4 (CHAr), 127.6 (2CHAr), 128.5 (2CHAr), 129.0 (2CHAr), 130.2
(2CHAr), 130.9 (Imidazole-C5), 131.3 (CAr), 132.4 (CHAr), 134.3 (CAr), 138.4
(Imidazole-C2), 138.9 (Pyrazole-C3), 143.2 (CAr), 144.5 (Oxadiazole-C2), 149.6
(Oxadiazole-C5), 164.8 (C=O). MS (EI, 70 eV) m/z (%): 461 (M?, 5.5), 425 (1.3),
382 (1.6), 353 (1.0), 340 (12.2), 295 (1.8), 268 (1.1), 248 (8.4), 227 (24.4), 188
(1.5), 158 (1.5), 122 (1.8), 115 (5.5), 95 (100.0), 88 (6.8), 76 (36.5), 65 (1.9). Anal.
Calcd. for C26H19N7O2 (461.47): C 67.67, H 4.15,N 21.25 %. Found: C 67.72, H
4.22, N 21.24 %.
Antioxidant screening using ABTS method [40]
The antioxidant activity was evaluated from the bleaching of ABTS derived radical
cations. The radical cations derived from ABTS [2,20-azino-bis-(3-ethylbenzothia-
zoline-6-sulfonic acid)] was prepared by reaction of ABTS (60 lL) with MnO2
(3 mL, 25 mg/mL) in 5 mL aqueous phosphate buffer (pH = 7). After shaking the
solution for a few minutes, it was centrifuged and filtered. The absorbance A
(control) of the resulting green–blue solution (ABTS radical solution) was recorded
at kmax = 734 nm. The absorbance A (test) was measured upon the addition of
(20 lL of 1 mg/mL) solution of the tested sample in spectroscopic grade MeOH/
buffer (1:1 v/v) to the ABTS solution. The inhibition ratio (%) was calculated using
the following equation:
ð%Þ Inhibition ¼ ½AðcontrolÞ À AðtestÞ=Aðcontrolފ  100:
Ascorbic acid (20 lL, 2 mM) solution was used as standard antioxidant (positive
control). Blank sample was run using solvent without ABTS and using MeOH/
phosphate buffer (1:1) instead of a sample. Negative control was run with ABTS
and MeOH/phosphate buffer (1:1) instead of test compounds. The results of
antioxidant activity were depicted in Table 1.
Bleomycin-dependent DNA damage
The assay was done with minor modifications following a method described earlier
by Aeschbach et al. [42] and Chan and Tang [43]. The reaction mixture (0.5 mL)
contained DNA (0.5 g/mL), Bleomycin sulfate (0.05 mg/mL), and MgCl2 (5 mM),
123

S. Bondock et al.
FeCl3 (50 mM) and the samples were dissolved in DMSO to be tested at a
concentration (20 lg of 1 mg/mL). L-Ascorbic acid was used as a positive control.
The reaction was terminated by addition of EDTA (0.05 mL, 0.1 M). The color was
developed by adding thiobarbituric acid (TBA) (0.5 mL, 1 %, w/v) and HCl
(0.5 mL, 25 %, v/v) followed by heating at 80 °C for 10 min. After centrifugation,
the extent of DNA damage was measured by the increase in absorbance at 532 nm
(Table 2).
References
1. C.S. de Oliveira, B.F. Lira, J.M. Barbosa-Filho, J.G.F. Lorenzo, P.F. de Athayde-Filho, Molecules
17, 10192–10231 (2012)
2. F. Liu, M. Wang, X. Teng, P. Zhang, L. Jiang, Res. Chem. Intermed. 40, 1575–1581 (2014)
3. L. Ma, Y. Xiao, C. Li, Z.-L. Xie, D.-D. Li, Y.-T. Wang, H.-T. Ma, H.-L. Zhu, M.-H. Wang, Y.-H. Ye,
Bioorg. Med. Chem. 21, 6763–6770 (2013)
4. G.M. Reddy, A. Muralikrishna, V. Padmavathi, A. Padmaja, T.K. Tilak, C.A. Rao, Chem. Pharm.
Bull. 61, 1291–1297 (2013)
5. R. Maheshwari, P. Chawla, S.A. Saraf, Med. Chem. Res. 20, 1650–1655 (2011)
6. N.C. Desai, G.M. Kotadiya, Med. Chem. Res. 23, 4021–4033 (2014)
7. G.V.S. Kumar, Y. Rajendraprasad, B.P. Mallikarjuna, S.M. Chandrashekar, C. Kistayya, Eur. J. Med.
Chem. 45, 2063–2074 (2010)
8. A. Kumar, C.S. Rajput, Eur. J. Med. Chem. 44, 83–90 (2009)
9. T. Chandra, N. Garg, S. Lata, K.K. Saxena, A. Kumar, Eur. J. Med. Chem. 45, 1772–1776 (2010)
10. K. Mogilaiah, B.H. Ramesh, R.B. Rao, Indian J. Heterocycl. Chem. 10, 109–112 (2000)
11. S.L. Goankar, K.M.L. Rai, B. Prabhuswamy, Eur. J. Med. Chem. 41, 841–846 (2006)
12. S. Bondock, S. Adel, H.A. Etman, F.A. Badria, Eur. J. Med. Chem. 48, 192–199 (2012)
13. B.N.P. Kumar, K.N. Mohana, L. Mallesha, B. Veeresh, Med. Chem. Res. 23, 3363–3373 (2014)
14. K. Liu, X. Lu, H.J. Zhang, J. Sun, H.L. Zhu, Eur. J. Med. Chem. 47, 473–478 (2012)
15. K.A. Ali, E.A. Ragab, T.A. Farghaly, M.M. Abdalla, Acta Pol. Pharm. Drug Res. 68, 237–247 (2011)
16. Z. Li, P. Zhan, X. Liu, Mini Rev. Med. Chem. 11, 1130–1142 (2011)
17. H. Rajak, R. Deshmukh, R. Veerasamy, A.K. Sharma, P. Mishra, M.D. Kharya, Bioorg. Med. Chem.
Lett. 20, 4168–4172 (2010)
18. B. Rigo, D. Couturier, J. Heterocycl. Chem. 22, 287–288 (1985)
`
¨
19. J. Bostrom, A. Hogner, A. Llinas, E. Wellner, A.T. Plowright, J. Med. Chem. 55, 1817–1830 (2012)
20. A.G. El-Sehemi, S. Bondock, Y.A. Ammar, Med. Chem. Res. 23, 827–838 (2014)
21. T. Nasr, S. Bondock, S. Eid, Eur. J. Med. Chem. 84, 491–504 (2014)
22. S. Bondock, W. Fadaly, M.A. Metwally, Eur. J. Med. Chem. 45, 3692–3701 (2010)
23. S. Bondock, A.E. Tarhoni, A.A. Fadda, J. Heterocycl. Chem. 52, 346–351 (2015)
24. S. Bondock, H. Gieman, A. El-Shafei, J. Saudi Chem. Soc. (2015). doi:10.1016/j.jscs.2015.01.005. in
press
25. S. Bondock, H. Gieman, Res. Chem. Intermed. (2014). doi:10.1007/s11164-014-1899-8. in press
26. S. Bondock, Res. Chem. Intermed. (2014). doi:10.1007/s11164-014-1672-z. in press
27. T. Nasr, S. Bondock, M. Youns, Eur. J. Med. Chem. 76, 539–548 (2014)
28. S. Bondock, T. Naser, Y.A. Ammar, Eur. J. Med. Chem. 62, 270–279 (2013)
29. S. Bondock, W. Khalifa, A.A. Fadda, Eur. J. Med. Chem. 46, 2555–2561 (2011)
30. S. Bondock, W. Fadaly, M.A. Metwally, Eur. J. Med. Chem. 44, 4813–4818 (2009)
31. S. Bondock, R. Rabie, H.A. Etman, A.A. Fadda, Eur. J. Med. Chem. 43, 2122–2129 (2008)
32. S. Bondock, W. Khalifa, A.A. Fadda, Eur. J. Med. Chem. 42, 948–954 (2007)
33. A.O. Abdelhamid, H.M. Abdelaziz, J. Heterocycl. Chem. 45, 1719–1728 (2008)
34. S.M. Al-Mousawi, M.A. El-Apasery, M.H. Elnagdi, Molecules 15, 58–67 (2010)
35. F. Al-Omran, N. Al-Awadi, M.M. Abdel Khalik, K. Kaul, A. Abu El-Khair, M.H. Elngadi, J. Chem.
Res. (S), 84–85 (1997)
123

Synthesis and antioxidant evaluation of some new…
36. H.F. Anwar, M.H. Elnagdi, Arkivoc 1, 198–250 (2009)
37. S.M. Al-Mousawi, M.A. Mohammad, M.H. Elnagdi, J. Heterocycl. Chem. 38, 989–991 (2001)
38. A.S. Shawali, M.A. Mosselhi, F.M.A. Eltalbawy, T.A. Farghaly, N.M. Tawfik, Tetrahedron 64,
5524–5530 (2008)
39. P. Langer, J. Wuckelt, M. Doring, J. Org. Chem. 65, 729–734 (2000)
40. E.A. Lissi, B. Modak, R. Torres, J. Esocbar, A. Urzua, Free Radic. Res. 30, 471–477 (1999)
41. J.M.C. Gutteridge, D.A. Rowley, B. Halliwell, Biochem. J. 199, 263–265 (1981)
42. R. Aeschbach, J. Loliger, B.C. Scott, A. Murcia, J. Butler, B. Halliwell, O.I. Aruoma, Food Chem.
Toxicol. 32, 31–36 (1994)
43. T.Y. Chan, P.L. Tang, J. Pineal Res. 20, 187–191 (1996)
123