Synthesis and antimicrobial studies of novel methylene bridged benzisoxazolyl imidazo[2,1-b][1,3,4]thiadiazole derivatives

Article abstract of DOI:10.1016/j.ejmech.2008.12.019

Novel methylene bridged benzisoxazolyl imidazo[2,1-b][1,3,4]thiadiazoles (3a-f) were synthesized from benzisoxazolyl-3-acetic acid and thiosemicarbazide. Reaction of 3 with bromine in glacial acetic acid in the presence of anhydrous sodium acetate yielded

Full text of DOI:10.1016/j.ejmech.2008.12.019

European Journal of Medicinal Chemistry 44 (2009) 2828–2833
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Original article
Synthesis and antimicrobial studies of novel methylene bridged
benzisoxazolyl imidazo[2,1-b][1,3,4]thiadiazole derivatives
*
Ravi S. Lamani, Nitinkumar S. Shetty, Ravindra R. Kamble, Imtiyaz Ahmed M. Khazi
Department of Chemistry, Karnatak University, Dharwad 580 003, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel methylene bridged benzisoxazolyl imidazo[2,1-b][1,3,4]thiadiazoles (3a–f) were synthesized from
benzisoxazolyl-3-acetic acid and thiosemicarbazide. Reaction of 3 with bromine in glacial acetic acid in
the presence of anhydrous sodium acetate yielded the corresponding 5-bromo derivatives (4a–f). While
compound 3 furnished the 5-nitroso derivatives (5a–f) on refluxing with sodium nitrite solution. Thi-
ocyanato derivatives (6a–f) were obtained by treating the imidazothiadiazole 3 with bromine and
potassium thiocyanate in glacial acetic acid at room temperature. The structures of the newly synthe-
sized compounds were confirmed by IR, NMR, mass and elemental analysis. All the compounds were
screened for their antibacterial and antifungal activities. Some of the compounds displayed very good
antibacterial and antifungal activity.
Received 20 September 2008
Received in revised form
11 November 2008
Accepted 15 December 2008
Available online 25 December 2008
Keywords:
2-Amino-1,3,4-thiadiazole
Imidazo[2,1-b][1,3,4]thiadiazoles
Antibacterial activity
Antifungal activity
Ó 2008 Elsevier Masson SAS. All rights reserved.
1. Introduction
In view of the high degree of bioactivity shown by the above two
heterocyclic compounds and in continuation of our search for
After the discovery of the broad spectrum anthelmintic ‘‘tetra-
misole’’ [1], the synthesis and biological activities of many condensed
imidazo[2,1-b][1,3,4]thiazoles [2–5] were reported. In addition,
compounds closely related to tetramisole have been investigated. In
this regard, many derivatives containing the imidazo[2,1-
b][1,3,4]thiadiazole ring system, which is bioisosteric with the imi-
dazo[2,1-b][1,3,4]thiazole system of tetramisole are being explored.
Consequently, a large number of imidazo[2,1-b][1,3,4]thiadiazole
derivatives have been reported to possess diverse pharmacological
properties such as anticancer [6], antitubercular [7], antibacterial
[8], antifungal [9], anticonvulsant, analgesic [10] and antisecretory
[11] activities. Moreover, much interest has also been focused on the
anti-inflammatory [12], cardiotonic [13], diuretic [14] and herbicidal
[15] activities displayed by compounds incorporating this heterocy-
clic system.
biological active molecules [18–21], it was envisaged to construct
a system, which combines both these systems in a single molecular
frame work to explore the additive effects towards their biological
activities. Hence we report herein the synthesis of hitherto unre-
ported benzisoxazolyl imidazothiadiazoles and their bromo,
nitroso and thiocycanato derivatives and evaluation of their anti-
bacterial and antifungal activities.
2. Chemistry
1,2-Benzisoxazole-3-acetic acid required for the present work
has been prepared by the reaction of the 4-hydroxycoumarin and
hydroxylamine hydrochloride in dry methanol by adopting the well
known procedure developed by Gianelli [22]. Reaction of 1,2-ben-
zisoxazole-3-acetic acid with thiosemicarbazide in refluxing
phosphorous oxychloride afforded the desired 2-amino-5-benzo[-
d]isoxazol-3-ylmethyl-[1,3,4]thiadiazole (2), which upon conden-
Benzisoxazole derivatives are known to possess important bio-
logical activities [16] and are useful in different therapies. The 1,2-
benzisoxazole moiety is isosteric with indole and can mimic/bind
to biologically important enzymes in a manner similar to indole
derivatives. Out of many biologically active compounds, zonisamide
[17] is widely prescribed as an antiepileptic drug.
sation with
a-haloaryl ketones under reflux in dry ethanol yielded
the imidazothiadiazoles 3 in good yields. It is well established that
this reaction proceeds via the intermediate iminothiadiazole [8]
which under reflux temperature spontaneously undergoes dehy-
drocyclisation to form the desired fused heterocycle (Scheme 1).
The electronic and steric factors at 5th position of 2-amino-5-
substituted-1,3,4-thiadiazole are crucial in determining the course
of its reaction with substituted
electronegative groups imparts less nucleophilic character to the
a-haloarylketones. The strongly
* Corresponding author. Tel.: þ91 836 2215286; fax: þ91 836 2771275.
E-mail address: drimkorgchem@gmail.com (I.A.M. Khazi).
0223-5234/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2008.12.019

R.S. Lamani et al. / European Journal of Medicinal Chemistry 44 (2009) 2828–2833
2829
COOH
S
S
NH2
i
NH2
+
H2N
N
H
N
N
N
N
O
O
2
1
O
Br
ii
R
S
N
N
R
N
N
3a-f
O
a, R = H; b, R = Cl; c, R = NO2; d, R = Br; e, R = OMe; f, R = 3-Coumarinyl
Scheme 1. Reagents and conditions: (i) POCl3, reflux, 45 min, KOH; (ii) dry EtOH, reflux, 18 h, Na2CO3.
nitrogen at 4th position of the 1,3,4-thiadiazole. Various
ketones were prepared by the bromination of the corresponding
ketones.
a
-haloaryl
nN–H band and the absence of the carbonyl stretching band of the
carboxylic acid. Further the 1H NMR spectra displayed a D2O
exchangeable peak due to NH2 at
d 7.14. Structures of imidazo-
Thus obtained benzisoxazolyl imidazo[2,1-b][1,3,4]thiadiazole
derivatives (3) were subjected to electrophilic substitution reaction
to yield the corresponding 5-substituted derivatives (Scheme 2).
Imidazothiadiazoles 3 on treatment with bromine in glacial acetic
acid in the presence of anhydrous sodium acetate yielded the cor-
responding 5-bromo derivatives (4a–f). Nitroso derivatives (5a–f)
were obtained by refluxing the imidazothiadiazoles 3 with sodium
nitrite solution. While imidazothiadiazoles furnished thiocyanato
derivatives (6a–f) on treating with bromine and potassium thio-
cyanate in glacial acetic acid at room temperature.
thiadiazole derivatives (3a–f) were established by the absence of
nN–H band in the IR spectra and appearance of imidazole proton
(C5–H) around
spectra of these compounds further confirmed the assigned
structures.
All the electrophilic substitution reactions carried out on imi-
dazothiadiazole derivatives (3a–f) afforded the expected 5-
substituted derivatives. The structures of the so obtained 5-bromo
derivatives (4a–f), 5-nitroso derivatives (5a–f) and 5-thiocyanato
derivatives (6a–f) were confirmed by their analytical and spectral
data. All the 5-substituted derivatives showed the absence of C5–H
in their 1H NMR spectra confirming the substitution at 5th position.
The IR spectra of compounds 5 displayed the absorption bands at
1529–1540 cmꢀ1 for NO while compounds 6 showed bands at
2138–2184 cmꢀ1 for SCN. The structures of all the compounds were
finally ascertained by the 13C NMR and mass spectral data.
d
8 in the 1H NMR spectra. The 13C NMR and mass
3. Results and discussion
The formation of 2-aminothiadiazole (2) by the reaction
between 1,2-benzisoxazole-3-acetic acid and thiosemicarbazide
was confirmed by its IR spectrum, which showed the presence of
N
S
R
N
N
3a-f
N
O
iii
ii
i
S
N
N
R
N
N
N
S
N
SCN
6a-f
O
R
O
N
S
N
R
Br
N
N
4a-f
N
O
NO
N
5a-f
a, R = H; b, R = Cl; c, R = NO2; d, R = Br; e, R = OMe; f, R = 3-Coumarinyl
Scheme 2. Reagents and conditions: (i) Br2/AcOH, sod. acetate; (ii) NaNO2, AcOH, reflux, 2 h; (iii) Br2 KSCN, AcOH.

2830
R.S. Lamani et al. / European Journal of Medicinal Chemistry 44 (2009) 2828–2833
4. Antimicrobial activity
activity against all the bacterial strains, however compounds 3b
and 3d are highly active against Escherichia coli-ATCC 35218 when
compared to Ampicilin. The high activity is attributed to the pres-
ence of electron withdrawing chloro and bromo functional groups.
Antifungal results indicated that compounds 3d, 3f and 6e have
shown good activity against Candida albicans. Compound 3d
showed very good activity comparable to that of standard while
compounds 3a, 3e and 6f have shown good activity against Asper-
gilus fumigates.
The newly synthesized compounds were screened for their
antibacterial and antifungal screening using agar well diffusion
method [23].
The antibacterial activity of the test compounds was evaluated
against two Gram-positive bacteria, Staphylococcus aureus-ATCC
25923, Bacillus subtilis-ATCC 6633 and Gram-negative bacteria
Pseudomonas aeruginosa-ATCC 10145, Escherichia coli-ATCC 35218.
Ampicillin was used as standard drug.
Antifungal activity was screened against two fungal strain,
Candida albicans and Aspergilus fumigatus using Clotrimazole as
standard drug. Dimethylformamide was used as solvent control.
The bacterial cultures were inoculated on Mueller Hinton Agar
(Merck) and fungal culture was inoculated on Potato Dextrose Agar.
Media (20 mL) were poured into each sterilized Petri dish (99 mm).
Media plates were inoculated with liquid cultures homogeneously
by spread plate method.
5. Conclusion
We have synthesized novel methylene bridged benzisoxazolyl
imidazothiadiazoles and their bromo, nitroso and thiocyanato
derivatives. The results of antibacterial screening reveals that
among all the compounds screened five compounds showed good
bacterial inhibition while six of the newly synthesized compounds
displayed good antifungal activity.
All the compounds were dissolved in dimethylsulfoxide (DMSO)
to get a concentration of 100
mg. Each sample (100
mL) was loaded
6. Experimental protocols
into the wells of agar plates directly. Plates inoculated with the
bacteria were incubated at 37 ꢁC for 24 h and the fungal culture was
incubated at 25 ꢁC for 72 h. All determinations were done in trip-
Melting points were determined in open capillaries and are
uncorrected. The IR spectra were recorded on Nicolet Impact 410 FT
IR spectrophotometer using KBr pellets. 1H and 13C NMR were
recorded on Bruker 300-MHz FT NMR spectrometer in CDCl3 and
DMSO-d6 with TMS as internal standard. Mass spectrum was recor-
ded on Finnigan MAT (Model MAT8200) spectrometer and elemental
analyses were carried out using Heraus CHN rapid analyzer.
The purity of the compounds was checked by thin layer chroma-
tography (TLC) on silica gel plate using hexane and ethyl acetate.
licates. The standard antibiotic, Ampicillin (100
and Clotrimazole (100 g/ml) for fungal were used as positive
controls and 100 l of DMSO used as a negative control, zone of
mg/ml) for bacteria
m
m
inhibitions were recorded in mm.
Priliminary screening was conducted for all compounds at 100 mg/
mL concentration, against the above-mentioned microorganisms.
Different series of dilutions of compounds were made (0.5, 1.0, .,
5.0
mg/mL) to determine the MIC. The results are given in Table 1.
The investigation of antibacterial screening revealed that some
6.1. Preparation of Benzo[d]isoxazol-3-yl-acetic acid (1)
of the tested compounds showed moderate to good bacterial
inhibition. Particularly compounds 3b, 3d, 4b, 4e and 6b have
shown very good activity against Bacillus subtilis-ATCC 6633 and
Escherichia coli-ATCC 35218. Compound 6b has exhibited very good
1,2-Benzisoxazole-3-acetic acid was prepared according to the
literature method [22]. Colorless crystalline solid. Yield 73%, m.p
123–125 ꢁC (lit. m.p. 126 ꢁC).
Table 1
Antibacterial and antifungal activities of the compounds (2, 3a–f, 4a–f, 5a–f and 6a–f) [zone of inhibition in mm and MIC values (mean of triplicates)].
Compd. No.
B. substilis
S. aureus
E. coli
P. aeruginosa
C. albicans
A. fumigatus
2
3a
3b
3c
3d
3e
3f
4a
4b
4c
4d
4e
4f
5a
5b
5c
5d
5e
5f
6a
6b
6c
6d
6e
15 (1)
11 (2)
15 (1.5)
16 (1)
12 (2)
14 (1.5)
15 (2.5)
10 (3.5)
13 (3)
18 (1)
17 (1.5)
11 (2)
13 (2)
11 (1.5)
15 (2)
18 (1)
10 (2)
12 (1.5)
16 (1)
13 (1.5)
14 (1.5)
16 (1.5)
18 (1)
09 (2)
12 (1.5)
09 (2)
14 (1.5)
19 (1)
–
10 (3.5)
09 (3.5)
18 (1.5)
13 (3)
20 (1)
12 (3)
10 (3.5)
11 (3)
15 (2)
10 (3.5)
14 (2)
16 (2)
13 (2.5)
11 (3)
13 (3)
09 (3.5)
14 (2)
16 (2)
12 (3)
12 (3)
16 (2)
7 (4)
14 (2.5)
09 (3.5)
12 (3)
08 (3.5)
14 (2.5)
11 (3)
10 (3)
16 (2)
12 (2.5)
11 (3)
16 (2)
17 (2)
14 (2.5)
10 (3)
15 (2.5)
11 (3)
12 (3)
08 (3.5)
14 (2.5)
17 (2)
19 (1.5)
10 (3)
18 (2)
19 (1.5)
16 (2)
17 (2)
14 (3)
12 (3.5)
11 (3.5)
10 (3.5)
22 (2)
15 (3)
20 (2.5)
14 (3)
15 (3)
09 (4)
16 (2.5)
10 (4)
14 (3)
13 (3.5)
15 (3)
17 (2.5)
17 (2.5)
14 (3)
15 (3)
12 (3.5)
15 (3)
11 (3.5)
14 (3)
20 (2.5)
12 (3.5)
–
11 (3.5)
19 (2)
16 (1.5)
11 (4)
13 (3.5)
19 (2)
13 (3.5)
12 (3.5)
10 (4)
08 (4.5)
16 (2.5)
15 (2.5)
17 (2.5)
16 (2.5)
17 (2.5)
11 (4)
15 (2.5)
10 (4)
12 (3.5)
15 (2.5)
11 (4)
17 (0.5)
11 (2.5)
18 (0.5)
14 (2.5)
15 (1)
12 (2.5)
18 (0.5)
12 (2.5)
12 (2.5)
18 (0.5)
10 (3)
13 (2)
14 (1.5)
14 (1.5)
12 (2.5)
11 (3)
15 (1.5)
14 (1.5)
17 (1)
08 (2.5)
14 (2)
15 (1)
09 (3.5)
12 (4)
16 (2.5)
19 (2)
15 (2)
13 (3)
14 (3.5)
16 (2)
–
6f
15 (1)
18 (0.5)
–
Ampicilin
Clotrimazole
Control
–
–
22 (2)
DMSO
20 (2)
DMSO
DMSO
DMSO
DMSO
DMSO
MIC values are given in brackets.

R.S. Lamani et al. / European Journal of Medicinal Chemistry 44 (2009) 2828–2833
2831
6.2. Preparation of 2-amino-5-benzo[d]isoxazol-3-ylmethyl-[1,3,4]
thiadiazole (2)
(s, 1H, C5–H, imidazole). Anal. calcd. for C18H11BrN4OS: C, 52.57; H,
2.70; N, 13.62. Found: C, 52.49; H, 2.76; N, 13.58%.
1,2-Benzisoxazole-3-acetic acid (10 g, 0.06 mol) and thio-
semicarbazide (5.14 g, 0.6 mol) in phosphorous oxychloride (30 ml)
was refluxed gently for 45 min. The reaction mixture was cooled
and quenched (highly exothermic) with cold water (90 ml). The
resulting solution was refluxed for additional 4 h and filtered hot.
The filtrate was cooled and basified with aqueous potassium
hydroxide solution. The solid that separated was filtered, washed
with water, dried and recrystallized from ethanol and DMF. Brown
6.3.5. 3-[6-(4-Methoxyphenyl)imidazo[2,1-b][1,3,4]thiadiazol-2-
ylmethyl]benzo[d]isoxazole (3e)
Yellow solid (ethanol þ dioxan), yield 55%, m.p. 161–163 ꢁC; IR
(KBr) n d:
, cmꢀ1: 2922, 2852, 1595, 1501; 1H NMR (300 MHz, CDCl3)
3.80 (s, 3H, OCH3), 4.73 (s, 2H, CH2), 7.30–7.65 (m, 6H, Ar-H), 7.72 (d,
J ¼ 7.21 Hz, 2H, Ar-H), 8.02 (s, 1H, C5–H, imidazole), 13C NMR
(75 MHz, CDCl3) d: 30.1, 56,110,113,116.3,119.9,120,122,125.5,126,
130.1, 133.6, 140.4, 144.2, 156.8, 159, 163.6. Anal. calcd. for
C19H14N4O2S: C, 62.97; H, 3.89; N, 15.46. Found: C, 63.02; H, 3.82; N,
15.51%.
solid, yield 56%, m.p. 210–212 ꢁC, IR (KBr)
n
, cmꢀ1: 3283, 3112, 2767,
: 4.66 (s, 2H, CH2),
2679, 1622, 1518; 1H NMR (300 MHz, DMSO-d6)
d
7.14 (s, 1H, D2O exchangeable, NH), 7.39–7.76 (m, 4H, Ar-H). GC–MS
233. Anal. calcd. for C10H8N4OS: C, 51.71; H, 3.47; N, 24.12. Found: C,
51.79; H, 3.42; N, 24.19%.
6.3.6. 3-(2-Benzo[d]isoxazol-3-ylmethylimidazo[2,1-b][1,3,4]
thiadiazol-6-yl)-chromen-2-one (3f)
Pale yellow silky solid (ethanol þ DMF), yield 69%, m.p. 177–
179 ꢁC; IR (KBr) , cmꢀ1: 2926, 2858,1710, 1591,1524, 1501; 1H NMR
n
6.3. Preparation of 3-(6-arylimidazo[2,1-b][1,3,4]thiadiazol-2-ylm-
ethyl)benzo[d]isoxazole (3a–f)
(300 MHz, CDCl3 þ TFA)
d: 5.06 (s, 2H, CH2), 7.27–8.01 (m, 8H, Ar-
H), 8.63 (s, 1H, C4–H, coumarin), 8.77 (s, 1H, C5–H, imidazole), GC–
MS: 400.9 (m/z), Anal. calcd. for C21H12N4O3S: C, 62.99; H, 3.02; N,
13.99. Found: C, 63.06; H, 3.10; N, 13.91%.
General method. A mixture of equimolar quantities of 2-amino-5-
benzo[d]isoxazol-3-ylmethyl-[1,3,4]thiadiazole 2 (2.32 g, 0.01 mol)
and bromoacetyl compound (0.01 mol) was refluxed in dry ethanol
for 18 h. The excess of solvent was distilled off and the solid
hydrobromide salt that separated was collected by filtration, sus-
pended in water and neutralized by aqueous sodium carbonate
solution to get free base (3a–f). It was filtered, washed with water,
dried and recrystallized from suitable solvent.
6.4. Preparation of 3-(5-bromo-6-arylimidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl)benzo[d]isoxazole (4a–f)
General method. To a well stirred solution of 3-(6-Arylimi-
dazo[2,1-b][1,3,4]thiadiazol-2-ylmethyl)-benzo[d]isoxazoles 3a–f
(0.01 mol) in glacial acetic acid (5 mL) and anhydrous sodium
acetate (0.02 mol) was added bromine (0.01 mol) dropwise with
stirring at room temperature. After the addition, stirring was
continued for 2 h. The reaction mixture was poured on to ice cold
water and basified with ammonia solution. The separated solid was
collected, washed with water, dried and purified by column chro-
matography using hexane ethyl acetate mixture (7:3, v/v) as eluent.
6.3.1. 3-(6-Phenylimidazo[2,1-b][1,3,4]thiadiazol-2-ylmethyl)
benzo[d]isoxazole (3a)
Brown crystalline solid (ethanol), yield 54%, m.p. 164–166 ꢁC; IR
(KBr)
CDCl3)
J ¼ 7.83 Hz, 2H, Ar-H), 7.82 (d, J ¼ 7.39 Hz, 2H, Ar-H), 8.02 (s, 1H, C5–
H, imidazole); 13C NMR (75 MHz, CDCl3)
: 28.9, 109.7, 110.6, 120.9,
n
, cmꢀ1: 2922, 2851, 1599, 1501, 1486; 1H NMR (300 MHz,
d: 4.74 (s, 2H, CH2), 7.30–7.63 (m, 5H, Ar-H), 7.73 (d,
d
6.4.1. 3-(5-Bromo-6-phenylimidazo[2,1-b][1,3,4]thiadiazol-2-
ylmethyl)benzo[d]isoxazole (4a)
121.5, 124.4, 125.5, 128.0, 129.1, 130.9, 134.0, 146.0, 146.9, 153.5,
158.8, 164.0. Anal. calcd. for C18H12ON4S: C, 65.04; H, 3.64; N, 16.86.
Found: C, 65.10; H, 3.71; N, 16.92%.
Pale yellow amorphous solid, yield 80%, m.p. 129–131 ꢁC; IR
(KBr)
CDCl3)
n
, cmꢀ1: 2923, 2851, 1603, 1597, 1506, 786; 1H NMR (300 MHz,
d
: 4.80 (s, 2H, CH2), 7.52–7.81 (m, 7H, Ar-H), 7.92 (d,
6.3.2. 3-[6-(4-Chlorophenyl)imidazo[2,1-b][1,3,4]thiadiazol-2-
ylmethyl]benzo[d]isoxazole (3b)
J ¼ 7.71 Hz, 2H, Ar-H). Anal. calcd. for C18H11BrN4OS: C, 52.57; H,
2.70; N, 13.62. Found: C, 52.62; H, 2.74; N, 13.58%.
White crystalline solid (ethanol þ DMF), yield 51%, m.p. 152–
154 ꢁC; IR (KBr)
n
, cmꢀ1: 2924, 2854,1597, 1504; 1H NMR (300 MHz,
6.4.2. 3-[5-Bromo-6-(4-chlorophenyl)imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (4b)
CDCl3)
d
: 4.74 (s, 2H, CH2), 7.39 (d, J ¼ 8.4 Hz, 2H, Ar-H), 7.61–7.69
(m, 4H, Ar-H), 7.75(d, J ¼ 8.4 Hz, 2H, Ar-H), 7.99 (s, 1H, C5–H,
imidazole). Anal. calcd. for C18H11ClN4OS: C, 58.94; H, 3.02; N, 15.27.
Found: C, 59.00; H, 3.10; N, 15.32%.
Yellow crystalline solid, yield 75%, m.p. 141–143 ꢁC; IR (KBr)
n,
cmꢀ1: 2922, 2854, 1597, 1508, 855; 1H NMR (300 MHz, CDCl3)
4.79 (s, 2H, CH2), 7.59–7.76 (m, 4H, Ar-H), 7.80 (d, J ¼ 8.1 Hz, 2H, Ar-
H), 7.89 (d, J ¼ 8.3 Hz, 2H, Ar-H). Anal. calcd. for C18H10 BrClN4OS: C,
48.51; H, 2.26; N, 12.57. Found: C, 48.47; H, 2.31; N, 12.61%.
d
:
6.3.3. 3-[6-(4-Nitrophenyl)imidazo[2,1-b][1,3,4]thiadiazol-2-
ylmethyl]benzo[d]isoxazole (3c)
Yellow crystalline solid (ethanol þ DMF), yield 50%, m.p. 168–
6.4.3. 3-[5-Bromo-6-(4-nitrophenyl)imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (4c)
170 ꢁC; IR (KBr) , cmꢀ1: 2924, 2853,1599, 1501; 1H NMR (300 MHz,
n
CDCl3)
d
: 4.77 (s, 2H, CH2), 7.35–7.73 (m, 4H, Ar-H), 7.98 (d,
Yellow color solid yield 75%, m.p. 136–138 ꢁC; IR (KBr)
n
, cmꢀ1
:
J ¼ 8.61 Hz, 2H, Ar-H), 8.16 (s, 1H, C5–H, imidazole), 8.29 (d,
J ¼ 8.58 Hz, 2H, Ar-H). Anal. calcd. for C18H11N5 O3S: C, 57.29; H,
2.94; N, 18.56. Found: C, 57.35; H, 3.01; N, 18.62%.
2924, 2852, 1596, 1506, 856; 1H NMR (300 MHz, CDCl3)
2H, CH2), 7.52–8.18 (m, 6H, Ar-H), 8.31(d, J ¼ 8.1 Hz, 2H, Ar-H). Anal.
calcd. for C18H10BrN5O3S: C, 47.38; H, 2.21; N,15.35. Found: C, 47.42;
H, 2.18; N, 15.40%.
d: 4.80 (s,
6.3.4. 3-[6-(4-Bromophenyl)imidazo[2,1-b][1,3,4]thiadiazol-2-
ylmethyl]benzo[d]isoxazole (3d)
6.4.4. 3-[5-Bromo-6-(4-bromophenyl)imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (4d)
Brown color crystalline solid (ethanol þ dioxan), yield 51%,
m.p. 157–159 ꢁC; IR (KBr)
n
, cmꢀ1: 2924, 2854, 1511, 1474; 1H
Yellow color solid, yield 85%, m.p. 127–129 ꢁC; IR (KBr)
n
, cmꢀ1
: 4.80 (s,
2H, CH2), 7.38–7.65 (m, 4H, Ar-H), 7.76 (d, J ¼ 7.87 Hz, 2H, Ar-H),
:
NMR (300 MHz, CDCl3)
Ar-H), 7.54 (d, J ¼ 7.9 Hz, 2H, Ar-H), 7.61–7.72 (m, 4H, Ar-H), 8.01
d
: 4.74 (s, 2H, CH2), 7.38 (d, J ¼ 6.8 Hz, 2H,
2924, 2854, 1597, 1509, 856; 1H NMR (300 MHz, CDCl3)
d

2832
R.S. Lamani et al. / European Journal of Medicinal Chemistry 44 (2009) 2828–2833
7.91 (d, J ¼ 8.31 Hz, 2H, Ar-H), GC–MS: 490.7 (m/z). Anal. calcd. for
C18H10Br2N4OS: C, 44.11; H, 2.06; N, 11.43. Found: C, 44.09; H, 2.10;
N, 11.40%.
(d, J ¼ 7.8 Hz, 2H, Ar-H), 7.86 (d, J ¼ 7.71 Hz, 2H, Ar-H), 13C NMR
(75 MHz, CDCl3) d: 31.8, 106.1, 117.4, 119.4, 120.6, 122, 124.2, 124.7, 126.3,
128.1, 128.5, 130.4, 133.8, 142.3, 159.1, 163.8. Anal. calcd. for
C18H10BrN5O2S: C, 49.11; H, 2.29; N, 15.91. Found: C, 49.08; H, 2.33; N,
15.87%.
6.4.5. 3-[5-Bromo-6-(4-methoxyphenyl)imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (4e)
Yellow color solid, yield 79%, m.p. 145–147 ꢁC; IR (KBr)
n
, cmꢀ1
d: 3.81 (s,
:
6.5.5. 3-[6-(4-Methoxyphenyl)-5-nitroso-imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (5e)
2924, 2853, 1597, 1501, 856; 1H NMR (300 MHz, CDCl3)
3H, OCH3), 4.80 (s, 2H, CH2), 7.73–7.79 (m, 6H, Ar-H), 7.81 (d,
Brown crystalline solid (ethanol þ dioxan), yield 46%, m.p. 229–
J ¼ 7.5 Hz, 2H, Ar-H), 13C NMR (75 MHz, CDCl3)
d
: 29.6, 110.6, 122.9,
231 ꢁC; IR (KBr) , cmꢀ1: 2958, 2854, 1604, 1536, 1500; 1H NMR
n
123, 124.9, 125, 129.0, 129.9, 132.6, 137.0, 145.0, 147.1, 154.5, 157.8,
164.0. Anal. calcd. for C19H13BrN4O2S: C, 51.71; H, 2.97; N, 12.70.
Found: C, 51.69; H, 3.02; N, 12.75%.
(300 MHz, CDCl3) d: 3.81 (s, 3H, OCH3), 4.80 (s, 2H, CH2), 7.49–7.71
(m, 4H, Ar-H), 7.75 (d, J ¼ 7.41 Hz, 2H, Ar-H), 7.87 (d, J ¼ 7.39 Hz, 2H,
Ar-H). Anal. calcd. for C19H13N5O3S: C, 58.30; H, 3.35; N, 17.89.
Found: C, 58.26; H, 3.33; N, 17.85%.
6.4.6. 3-(2-Benzo[d]isoxazol-3-ylmethyl-5-bromo-imidazo[2,1-b]
[1,3,4]thiadiazol-6-yl)chromen-2-one (4f)
6.5.6. 3-(2-Benzo[d]isoxazol-3-ylmethyl-5-nitroso-imidazo[2,1-b]
[1,3,4]thiadiazol-6-yl)chromen-2-one (5f)
Bright yellow color solid yield 78%, m.p. 151–153 ꢁC; IR (KBr)
n,
cmꢀ1: 2923, 2854, 1714, 1596, 1504, 854; 1H NMR (300 MHz, CDCl3)
: 4.79 (s, 2H, CH2), 7.61–8.10 (m, 8H, Ar-H), 8.47 (s, 1H, C4–H,
Pale yellow solid (ethanol þ dioxan), yield 42%, m.p. 253–
d
255 ꢁC; IR (KBr) , cmꢀ1: 2959, 2854, 1715, 1603, 1524, 1501; 1H
n
coumarin), Anal. calcd. for C21H11BrN4O3S: C, 52.62; H, 2.31; N,
11.69. Found: C, 52.58; H, 2.36; N, 11.73%.
NMR (300 MHz, CDCl3)
d
: 4.80 (s, 2H, CH2), 7.49 (d, J ¼ 7.4 Hz, 1H,
Ar-H), 7.54–8.03 (m, 7H, Ar-H), 8.41 (s, 1H, C4–H, coumarin). Anal.
calcd. for C19H11N5O4S: C, 58.74; H, 2.58; N, 16.31. Found: C, 58.70;
H, 2.62; N, 16.34%.
6.5. Preparation of 3-(5-nitroso-6-arylimidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl)benzo[d]isoxazole (5a–f)
6.6. Preparation of 3-(6-aryl-5-thiocyanato-imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl)benzo[d]isoxazole (6a–f)
General method. To a well stirred solution of 3-(6-arylimi-
dazo[2,1-b][1,3,4]thiadiazol-2-ylmethyl) benzo[d]isoxazoles, 3a–f
(0.01 mol) in acetic acid (10 mL) was added sodium nitrite solution
(0.021 mol, in 5 mL water) dropwise at room temperature. After the
addition, stirring was continued for 30 min and the mixture was
refluxed for 2 h. The reaction mixture was poured in to ice cold
water, separated solid was collected, washed with water, dried and
recrystallized from suitable solvent.
General method. To a well stirred solution of 3-(6-arylimi-
dazo[2,1-b][1,3,4]thiadiazol-2-ylmethyl)benzo[d]isoxazoles, 3a–f
(0.01 mol) in glacial acetic acid (5 mL) and potassium thiocyanate
(0.02 mol) was added bromine (0.01 mol) in glacial acetic acid,
dropwise with stirring at room temperature. Then stirring was
continued for 1 h at 20–25 ꢁC and then at room temperature for
30 min. The reaction mixture was poured in to ice water the
separated solid was collected, washed with water, dried and
recrystallized from suitable solvent.
6.5.1. 3-(5-Nitroso-6-phenylimidazo[2,1-b][1,3,4]thiadiazol-2-
ylmethyl)benzo[d]isoxazole (5a)
Brown crystalline solid (chloroform þ hexane), yield 40%, m.p.
199–201 ꢁC; IR (KBr)
n
, cmꢀ1: 2956, 2850, 1603, 1529, 1501; 1H NMR
6.6.1. 3-(6-Phenyl-5-thiocyanato-imidazo[2,1-b][1,3,4]thiadiazol-
2-ylmethyl)benzo[d]isoxazole (6a)
(300 MHz, CDCl3) d: 4.79 (s, 2H, CH2), 7.59–7.71 (m, 5H, Ar-H), 7.77
(d, J ¼ 7.2 Hz, 2H, Ar-H), 7.82 (d, J ¼ 7.6 Hz, 2H, Ar-H). Anal. calcd. for
C18H11N5O2S: C, 59.83; H, 3.07; N, 19.38. Found: C, 59.87; H, 3.11; N,
19.34%.
Colorless solid (ethanol), yield 45%, m.p. 230–232 ꢁC; IR (KBr)
n,
cmꢀ1: 2916, 2851, 2138, 1569; 1H NMR (300 MHz, DMSO-d6)
d: 4.80
(s, 2H, CH2), 7.46–7.79 (m, 5H, Ar-H), 7.84 (d, J ¼ 7.8 Hz, 2H, Ar-H),
7.87 (d, J ¼ 7.69 Hz, 2H, Ar-H). Anal. calcd. for C19H11N5OS2: C, 65.88;
H, 4.07; N, 16.17. Found: C, 65.92; H, 4.10; N, 16.13%.
6.5.2. 3-[6-(4-Chlorophenyl)-5-nitroso-imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (5b)
Brown color solid (chloroform þ hexane), yield 35%, m.p. 215–
6.6.2. 3-[6-(4-Chlorophenyl)-5-thiocyanato-imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (6b)
217 ꢁC; IR (KBr) , cmꢀ1: 2957, 2850, 1606, 1531, 1504; 1H NMR
n
(300 MHz, CDCl3)
d
: 4.80 (s, 2H, CH2), 7.68–7.75 (m, 6H, Ar-H), 7.82
Colorless crystalline solid (ethanol), yield 34%, m.p. 231–233 ꢁC; IR
(d, J ¼ 7.6 Hz, 2H, Ar-H). Anal. calcd. for C18H10ClN5O2S: C, 54.62; H,
(KBr) n
, cmꢀ1: 2922, 2827, 2146, 1548; 1H NMR (300 MHz, DMSO-d6)
2.55; N, 17.69. Found: C, 54.50; H, 2.58; N, 17.72%.
d
: 4.80 (s, 2H, CH2), 7.49–7.69 (m, 4H, Ar-H), 7.75 (d, J ¼ 7.62 Hz, 2H,
Ar-H), 7.79 (d, J ¼ 7.41 Hz, 2H, Ar-H). Anal. calcd. for C19H10ClN5OS2: C,
6.5.3. 3-[6-(4-Nitrophenyl)-5-nitroso-imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]-isoxazole (5c)
59.92; H, 3.44; N, 14.71. Found: C, 59.98; H, 3.39; N, 14.75%.
Brown color solid (ethanol þ dioxan), yield 35%, m.p. 222–
6.6.3. 3-[6-(4-Nitrophenyl)-5-thiocyanato-imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (6c)
224 ꢁC; IR (KBr) , cmꢀ1: 2959, 2853, 1606, 1539, 1501; 1H NMR
n
(300 MHz, CDCl3)
d: 4.80 (s, 2H, CH2), 7.50–7.66 (m, 2H, Ar-H), 7.77
Colorless solid (ethanol þ dioxan), yield 33%, m.p. 256–258 ꢁC;
(d, J ¼ 8.04 Hz, 2H, Ar-H), 8.25 (d, J ¼ 8.8 Hz, 2H, Ar-H), 8.33 (d,
J ¼ 9 Hz, 2H, Ar-H), GC–MS: 406 (m/z). Anal. calcd. for C18H10N6O4S:
C, 53.20; H, 2.48; N, 20.68. Found: C, 53.25; H, 2.44; N, 20.72%.
IR (KBr)
d6)
n
, cmꢀ1: 2924, 2854, 2164, 1563; 1H NMR (300 MHz, DMSO,
: 4.80 (s, 2H, CH2), 7.52–7.71 (m, 6H, Ar-H), 8.0 (d, J ¼ 8.1 Hz, 2H,
d
Ar-H). Anal. calcd. for C19H10N6O3S2: C, 59.92; H, 3.44; N, 14.71.
Found: C, 59.98; H, 3.39; N, 14.75%.
6.5.4. 3-[6-(4-Bromophenyl)-5-nitroso-imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (5d)
6.6.4. 3-[6-(4-Bromophenyl)-5-thiocyanato-imidazo[2,1-b][1,3,4]
thiadiazol-2-ylmethyl]benzo[d]isoxazole (6d)
Brown crystalline solid (ethanol þ dioxan), yield 47%, m.p. 216–
218 ꢁC; IR (KBr)
n
, cmꢀ1: 2959, 2851, 1606, 1531, 1501; 1H NMR
Pale yellow crystalline solid (ethanol), yield 39%, m.p. 179–
(300 MHz, CDCl3)
d
: 4.81 (s, 2H, CH2), 7.51–7.78 (m, 4H, Ar-H), 7.80
182 ꢁC; IR (KBr) , cmꢀ1: 2927, 2854, 2154, 1534; 1H NMR (300 MHz,
n

R.S. Lamani et al. / European Journal of Medicinal Chemistry 44 (2009) 2828–2833
2833
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DMSO-d6) d: 4.80 (s, 2H, CH2), 7.55–7.68 (m, 6H, Ar-H), 7.73 (d,
J ¼ 6.8 Hz, 2H, Ar-H), 7.80 (d, J ¼ 6.8 Hz, 2H, Ar-H). 13C NMR
(75 MHz, DMSO-d6) d: 30.2, 108.7, 110.2, 110.8, 122.9, 124.5, 124.8,
125.9,128.1,130.1, 130.9, 133.0, 145.0,146.6, 152.5, 159.8, 163.9. Anal.
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[1,3,4]thiadiazol-2-ylmethyl]benzo[d]isoxazole (6e)
Yellow crystalline solid (ethanol), yield 48%, m.p. 172–174 ꢁC; IR
(KBr)
d6) : 3.79 (s, 3H, OCH3), 4.80 (s, 2H, CH2), 7.52–7.69 (m, 4H, Ar-H),
n
, cmꢀ1: 2924, 2854, 2184, 1541; 1H NMR (300 MHz, DMSO-
d
7.72 (d, J ¼ 7.4 Hz, 2H, Ar-H), 7.79 (d, J ¼ 7.6 Hz, 2H, Ar-H). GC–MS:
419 (m/z). Anal. calcd. for C20H13N5O2S2: C, 63.81; H, 4.28; N, 14.88.
Found: C, 63.84; H, 4.32; N, 14.91%.
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6.6.6. 3-(2-Benzo[d]isoxazol-3-ylmethyl-5-thiocyanato-imidazo
[2,1-b][1,3,4]thiadiazol-6-yl)chromen-2-one (6f)
Pale yellow silky solid (dioxan), yield 55%, m.p. 191–193 ꢁC; IR
(KBr)
DMSO-d6)
n
, cmꢀ1: 2924, 2856, 2161, 1720, 1569; 1H NMR (300 MHz,
d: 4.80 (s, 2H, CH2), 7.47–7.98 (m, 8H, Ar-H), 8.40 (s, 1H,
C4–H, coumarin). Anal. calcd. for C22H11N5O3S2: C, 63.76; H, 3.40; N,
13.52. Found: C, 63.81; H, 3.37; N, 14.49%.
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