Synthesis and evaluation of antibacterial and antitubercular activities of some novel imidazo[2,1-b][1,3,4]thiadiazole derivatives

Article abstract of DOI:10.1007/s00044-012-0080-4

A series of 2,5,6-trisubstituted imidazo[2,1-b][1,3,4]thiadiazoles were synthesized, structures of the compounds were elucidated and evaluated for antitubercular activity against Mycobacterium tuberculosis H37Rv using microplate alamar blue assay (MABA) method, antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Vibrio cholera, and Escherichia coli by broth micro dilution assay method. Some compounds exhibited significant antibacterial and antitubercular activities. Compounds 10, 14, and 15 emerged as the most active molecules, showed significant antimicrobial activity and may serve as leads for further optimization. Graphical Abstract: Synthesis, spectral studies, and antibacterial and antitubercular activities of a novel series of imidazo[2,1-b][1,3,4]thiadiazole derivatives are described[Figure not available: see fulltext.].

Full text of DOI:10.1007/s00044-012-0080-4

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2013) 22:869–878
DOI 10.1007/s00044-012-0080-4
ORIGINAL RESEARCH
Synthesis and evaluation of antibacterial and antitubercular
activities of some novel imidazo[2,1-b][1,3,4]thiadiazole
derivatives
•
S. D. Joshi K. Manish Aravind Badiger
•
Received: 1 August 2011 / Accepted: 2 May 2012 / Published online: 19 May 2012
Ó Springer Science+Business Media, LLC 2012
Abstract A series of 2,5,6-trisubstituted imidazo[2,1-b]
[1,3,4]thiadiazoles were synthesized, structures of the
compounds were elucidated and evaluated for antituber-
cular activity against Mycobacterium tuberculosis H37Rv
using microplate alamar blue assay (MABA) method,
antibacterial activity against Staphylococcus aureus,
Bacillus subtilis, Vibrio cholera, and Escherichia coli by
broth micro dilution assay method. Some compounds
exhibited significant antibacterial and antitubercular
activities. Compounds 10, 14, and 15 emerged as the most
active molecules, showed significant antimicrobial activity
and may serve as leads for further optimization.
three million deaths annually worldwide (Valadas and
Antunes, 2005). According to world health organization
(WHO), it is estimated that one-third of the world’s pop-
ulation is currently infected with the bacillus and about 30
million people will be infected within next 20 years if
control is not further strengthened (Duncan, 1997). The
worsening situation has prompted the WHO to declare
tuberculosis a global public health crisis (Qian and
Montellano, 2006).
In addition; HIV-1 and 2 infections, which impairs the
immune system and allows large number of people already
infected with the TB to progress to active disease. The
pathogenic synergy of tuberculosis with HIV enhances the
overall incidence of TB in HIV-positive patients by 50
times relative to the rate for HIV-negative individuals
(Corbett et al., 2003).
Keywords Imidazo[2,1-b][1,3,4]thiadiazole ꢀ
Antitubercular ꢀ Antibacterial activities
Therefore, there is an urgent need to develop new
chemical entities with new structural classes and with a
novel mechanism of action other than isoniazid (INH),
rifampicin (RIF), and pyrazinamide (PZA). In this regard,
since the last decade search for new antitubercular sub-
stances has ranked among the priority areas of chemo-
therapeutic research.
Introduction
Tuberculosis (TB) is a chronic necrotizing bacterial
infection with a wide variety of manifestations caused by
Mycobacterium tuberculosis and is responsible for over
Imidazo[2,1-b][1,3,4]thiadiazoles are useful targets in
the search for antimicrobials as they have been associated
with a wide variety of interesting biological properties.
Among the numerous heterocyclic moieties of biologi-
cal and pharmacological interest, the imidazo[2,1-b]
[1,3,4]thiadiazole ring is endowed with various activities,
such as anticancer (Terzioglu and Gursoy, 2003), antitu-
bercular (Kolavi et al., 2006), antibacterial (Gadad et al.,
2000), anticancer (Karki et al., 2011; Noolvi et al., 2011),
leishmanicidal (Foroumadi et al., 2005), anticonvulsant-
analgesic (Khazi et al., 1996), anti-inflammatory (Jadhav
et al., 2008), and antisecretory (Andreani et al., 2000)
This paper is dedicated to Professor G. S. Gadaginamath on the
occasion of his 68th birthday.
S. D. Joshi (&) ꢀ K. Manish
Department of Pharmaceutical Chemistry, S.E.T’s College
of Pharmacy, S. R. Nagar, Dharwad, Karnataka, India
e-mail: shrinivasdj@rediffmail.com
A. Badiger
Department of Pharmaceutical Chemistry, Shree Dhanvantary
Pharmacy College, Surat, Gujarat, India
123

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Med Chem Res (2013) 22:869–878
activities. Recently, these derivatives have attracted the
interest of researchers as potential antitubercular agents.
Some derivatives of the imidazo[2,1-b][1,3,4]thiadiazoles
have exhibited good activity against M. tuberculosis H37Rv
(Gadad et al., 2004).
Methods and materials
Chemistry
Synthesis of the basic nucleus imidazo[2,1-b][1,3,4]thiadia-
zole is brought about by the condensation of 5-amino-1,3,
4-thiadiazole-2-sulphonamide 1 with 7-diethylamino-3-bro-
moacetyl-2H-chromen-2-one 2 under reflux in dry ethanol
(Scheme 1). Mannich reaction of imidazo[2,1-b][1,3,4]thia-
diazole 3 with different cyclic secondary amines (morpholine,
piperidine, and pyrrolidine) and formaldehyde in methanol
The purpose of the present study was to explore and
develop the novel molecules with improved potential for
treating tuberculosis. In this research paper, we have
reported the design, synthesis and preliminary antimyco-
bacterial and antibacterial evaluation of a novel class of
imidazo[2,1-b][1,3,4]thiadiazole derivatives.
H3COC
CHO
a
CH3COCH2COOCH2CH3
+
O
O
N(C2H5)2
OH
(C2H5)2N
b
BrH2COC
O
O
N(C2H5)2
2
Br
+
N
BrH2COC
O
N
N
c
N
CH2CO
NH2
+
H2NO2S
NH2
S
H2NO2S
O
N(C2H5)2
S
O
O
N(C2H5)2
1
2A
2
-HBr
O
N
O
d
N
N
e
N
N
N
N
N
H2NO2S
S
N
H2NO2S
S
N
H2NO2S
S
N
3
O
N(C2H5)2
O
N(C2H5)2
O
O
N(C2H5)2
O
N
4
5
i
f
N
N
SCN
h
N
N
H2NO2S
S
N
g
N(C2H5)2
O
H2NO2S
NO
S
N
O
6
9
O
N(C2H5)2
O
N
N
Br
N
N
H2NO2S
S
N
H2NO2S
S
N
8
O
N(C2H5)2
O
7
O
N(C2H5)2
O
Scheme 1 Synthesis of imidazothiadiazole derivatives 3–9. a Piper-
idine, stir, 30 min, b bromine, acetic acid, reflux, 2 h, c ethanol reflux,
24 h, Na2CO3, d morpholine, formaldehyde, methanol, reflux, 18 h,
e piperidine, formaldehyde, methanol, reflux, 18 h, f pyrroldine,
formaldehyde, methanol, reflux, 18 h, g bromine, acetic acid,
CH3COONa, stir, 1 h, h sodium nitrite, acetic acid, stir, rt, 1 h,
reflux, 3 h, i potassium thiocyanate, acetic acid, stir, rt, 3 h
123

Med Chem Res (2013) 22:869–878
871
with catalytic amount of acetic acid yielded corresponding
derivatives 4, 5 and 6. Imidazo[2,1-b][1,3,4]thiadiazole 3 was
subjected to electrophilic substitution reaction at the fifth
position with bromine in the presence of sodium acetate in
acetic acid to obtain 5-bromo derivative 7. Nitroso derivative
8 was obtained by refluxing the imidazothiadiazole 3 with
sodium nitrite solution. While imidazothiadiazole 3 furnished
thiocyanato derivative 9 on treating with bromine and potas-
sium thiocyanate in glacial acetic acid.
heterocyclization to yield thiazolidinone derivative 14.
Reaction of 10 with aminoguanidine hydrochloride pro-
duced guanylhydrazone 15 (Scheme 2).
Experimental
Chemicals used in the synthesis of the titled compounds were
purchased from Sigma-Aldrich, S.D. Fine-Chem Limited,
and Spectrochem Pvt. Ltd. The melting points of synthesized
compounds were determined on Shital scientific industries
digital melting point apparatus and are uncorrected; infra-red
spectra were recorded on a Bruker spectrophotometer by
using KBr pellets. 1H and 13C-NMR spectra were recorded
on Bruker 300 MHz and Bruker AVANCE III 500 MHz
instruments using CDCl3 or DMSO-d6 as solvent and TMS
as internal standard, chemical shifts are expressed as d values
(ppm). Mass spectra (MS) were taken in JEOL GCMATE II
GC–mass spectrometer and Schimadzu QP 20105 GC–mass
spectrometer. Microanalysis of compounds were also per-
formed on Leco Tru Spec CHNS analyzer for the determi-
nation of percentage of C, H, and N. All new compounds
exhibited spectral data consistent with the proposed struc-
tures and values of microanalysis are within 0.4 % of the
theoretical values. Analytical thin-layer chromatography
Vilsmeier Haack reaction of imidazothiadiazole 3 in
DMF and POCl3 furnished formyl derivative 10. During
this reaction, the DMF also reacted with the NH2 function
of the 2-sulfonamido group converting it to N,N-(dimeth-
ylaminomethino)sulfonamide group. The formyl group has
been utilized to synthesize corresponding alcohol, nitrile,
thiazolidinone, and guanyl hydrazone derivatives. The
reduction of aldehyde 10 by NaBH4 in methanol at room
temperature yielded the corresponding carbinol 11 in good
yield. The condensation of aldehyde 10 with hydroxyl-
amine hydrochloride in pyridine gave corresponding oxime
12, which on dehydration with thionyl chloride produced
the nitrile 13 in good yield. The aldehyde 10 when treated
with aniline gave the corresponding Schiff base, which on
refluxing with thioglycolic acid in benzene underwent
N
N
H2NO2S
S
N
3
O
N(C2H5)2
O
a
CH2OH
CHO
N
b
N
N
N
S
N
S
N
(CH3)2N-CH=N-SO2
(CH3)2N-CH=N-SO2
O
N(C2H5)2
N(C2H5)2
O
O
O
11
10
f
O
NH
H
e
N
CH=N-N-C-NH2
S
N
N
N
N
c
S
N
(CH3)2N-CH=N-SO2
S
N
(CH3)2N-CH=N-SO2
O
N(C2H5)2
O
O
N(C2H5)2
O
14
15
CH=N-OH
CN
N
N
N
N
d
S
N
(CH3)2N-CH=N-SO2
S
N
(CH3)2N-CH=N-SO2
O
N(C2H5)2
O
N(C2H5)2
O
O
13
12
Scheme 2 Synthesis of imidazothiadiazole derivatives 10–15. a
phosphorous oxy chloride, DMF, stir, 00 C, 30 min, stir, rt, 2 h, stir,
600 C, 2 h, Na2CO3, b sodium borohydride, methanol, stir, 7 h,
c hydroxylamine hydrochloride, pyridine, reflux 8 h, d thionyl
chloride, reflux, 10 min, Na2CO3, e aniline, ethanol, acetic acid,
reflux, 4 h, thioglycolic acid, benzene, reflux, 8 h, f ethanol, amino-
guanidine hydrochloride, reflux, 1 h
123

872
Med Chem Res (2013) 22:869–878
(TLC) was performed on precoated TLC sheets of silica gel
60 F254 (Merck, Darmstadt, Germany), visualizing by long-
and short-wavelength ultraviolet (UV) lamps.
(coumarin-C10), 115.48 (coumarin-C6), 113.08 (coumarin-
C8), 110.59 (imidazole-C5), 106.09 (thiadiazole-C2), 30.57
(coumarin-2CH2), 12.84 (coumarin-2CH3); MS (70 eV) m/z
420 (M ? 1) 404, 340, 139, 101, 80, 62, 48. Anal. Calcd. For
C17H17N5O4S2: C, 48.67; H, 4.08; N, 16.70. Found: C, 48.49;
H, 4.07; N, 16.65.
Synthesis of 5-amino-1,3,4-thiadiazole-2-sulphonamide (1)
N-(5-sulfamoyl-1,3,4-thiadiazol-2-yl)acetamide (0.03 mol,
6.60 g) in concentrated hydrochloric acid (30 mL) was
refluxed for 6 h. The reaction mixture was cooled and basified
with sodium bicarbonate solution. The solid that separated
was filtered, washed with water, dried, and recrystallized from
water (yield 65 %). mp 220–222 °C. IR (KBr) 3430, 3326,
1604, 678 cm-1; 1H-NMR (DMSO-d6) d; 7.78 (s, 2H, NH2),
8.05 (s, 2H, SO2NH2). Anal.: C2H4N4O2S2.
Synthesis of 5-(morpholin-4-ylmethyl)-6-[7-(diethylamino)-
2-oxo-2H-chromen-3-yl]imidazo[2,1-b][1,3,4]thiadiazole-
2-sulfonamide (4)
A mixture of 6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]
imidazo[2,1-b][1,3,4]thiadiazole-2-sulfonamide (3) (0.005 mol,
2.09 g), morpholine (0.006 mol, 0.52 g), formaldehyde
(1 mL) and acetic acid (1 mL) in methanol (20 mL) was
refluxed for 18 h (monitored by TLC). The solution was
diluted with water, extracted with chloroform (3 9 30 mL);
the combined chloroform extract was washed with water
(3 9 30 mL) and dried over anhydrous sodium sulfate. The
solution was evaporated to dryness in vacuum and the resi-
due was recrystallized from methanol (yield 62.30 %). mp
160–162 °C. IR (KBr) 3273, 2967, 1719, 1614, 1416, 1355,
1171, 641 cm-1; 1H-NMR (DMSO-d6); 7.95–7.96 (m, 2H,
coumarin-C4–H, C5–H), 7.19–7.20 (d, J: 4.0 Hz, 1H, cou-
marin-C6–H), 6.76 (s, 1H, coumarin-C8–H), 4.20 (s, 2H,
CH2), 3.85–4.00 (t, J: 4.0 Hz, 4H, morpholine-C2–H, C6–H),
3.22–3.45 (m, 4H, 2CH2), 2.82–2.96 (t, J: 4.0 Hz, 4H,
morpholine-C3–H, C5–H), 2.47 (s, 2H, SO2NH2), 1.02–1.15
(m, 6H, 2CH3). 13C-NMR; 171.82 (C=O), 156.92 (thiadia-
zole-C7a), 150.66 (coumarin-C9), 147.46 (imidazole-C6),
146.44 (coumarin-C7), 140.06 (coumarin-C4), 131.55 (cou-
marin-C3), 126.42 (coumarin-C5), 117.12 (coumarin-C10),
114.74 (coumarin-C6), 113.88 (coumarin-C8), 111.42
(imidazole-C5), 106.78 (thiadiazole-C2), 68.46 (morpho-
line-C3 & C5), 54.32 (morpholine-C2 & C6), 52.32 (N–CH2),
39.22 (coumarin 2CH3), 31.56 (coumarin 2CH2); MS
(70 eV) m/z 517 (M ? 1), 516 (M?), 445, 433, 419, 80, 62,
48; Anal. Calcd. For C22H26O5N6S2: C, 50.95; H, 5.05; N,
16.20. Found: C, 50.90; H, 5.06; N, 16.16.
Synthesis of 7-diethylamino-3-bromoacetyl-2H-chromen-
2-one (2)
A mixture of 4-N,N-diethyl amino salisaldehyde (0.015 mol,
2.88 g) and ethylacetoacetate (0.015 mol, 1.90 mL) in
piperidine (1 mL) was stirred for 30 min. The solid that
separated was filtered, washed with alcohol, and dried. The
crude product was recrystallized from ethanol. To the yellow
colored solid thus obtained (0.01 mol) in chloroform
(10 mL) was added bromine (0.00095 mol, 0.13 g in 5 mL
chloroform) drop wise at room temperature. After the addi-
tion, the mixture was refluxed for 2 h. The reaction mixture
was pouredinto ice cold water; separated solid was collected,
washed with water, dried, and recrystallized from ethanol
(yield 65 %). mp 164–166 °C. IR (KBr) 1713, 1679, 1614,
1508, 622 cm-1. Anal.: C15H16BrNO3.
Synthesis of 6-[7-(diethylamino)-2-oxo-2H-chromen-
3-yl]imidazo[2,1-b][1,3,4]thiadiazole-2-sulfonamide (3)
A mixture of equimolar quantities of 5-amino-1,3,4-thiadia-
zole-2-sulphonamide (1) (0.03 mol, 5.40 g) and 7-diethyla-
mino-3-bromoacetyl-2H-chromen-2-one (2) (0.03 mol, 10.14 g)
was refluxed in dry ethanol for 24 h. The excess of solvent
was distilled off and the solid hydrobromide that separated
was collected by filtration, suspended in water and neutral-
ized by aqueous sodium carbonate solution to get free base 3.
It was filtered, washed with water, dried, and recrystallized
from ethanol (yield 64.60 %). mp 192–194 °C. IR (KBr)
Synthesis of 5-(piperidin-1-ylmethyl)-6-[7-(diethylamino)-
2-oxo-2H-chromen-3-yl]imidazo[2,1-b][1,3,4]thiadiazole-
2-sulfonamide (5)
3315, 3293, 1711, 1603, 1496, 1353, 1180, 652 cm-1
;
A mixture of 6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]
imidazo[2,1-b][1,3,4]thiadiazole-2-sulfonamide (3) (0.005 mol,
2.09 g), piperdine (0.006 mol, 0.51 g), formaldehyde
(1 mL), and acetic acid (1 mL) in methanol (20 mL)
was refluxed for 18 h (monitored by TLC). The solution
was diluted with water, extracted with chloroform
(3 9 30 mL); the combined chloroform extract was
washed with water (3 9 30 mL) and dried over anhydrous
sodium sulfate. The solution was evaporated to dryness in
1H-NMR (DMSO-d6) d; 8.56 (s, 1H, imidazole-C5–H),
7.55–7.58 (m, 2H, coumarin-C4–H, C5–H), 6.71 (d, J:
4.0 Hz, 1H, coumarin-C6–H), 6.57 (s, 1H, coumarin-C8–H),
3.22–3.45 (m, 4H, 2CH2), 2.50 (s, 2H, SO2NH2), 1.06–1.46
(m, 6H, 2CH3); 13C-NMR (DMSO-d6) d; 172.11 (C=O),
153.29 (thiadiazole-C7a), 151.15 (coumarin-C9), 148.09
(imidazole-C6), 145.70 (coumarin-C7), 140.39 (coumarin-
C4), 130.40 (coumarin-C3), 128.23 (coumarin-C5), 119.82
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Med Chem Res (2013) 22:869–878
873
Synthesis of 5-bromo-6-[7-(diethylamino)-2-oxo-2H-
chromen-3-yl]imidazo[2,1-b][1,3,4]thiadiazole-2-
sulfonamide (7)
vacuum and the residue was recrystallized from methanol
(yield 64.70 %). mp 162–164 °C. IR (KBr) 3223, 2960,
1
2981, 1716, 1614, 1456, 1354, 1173, 643 cm-1; H-NMR
(DMSO-d6); 7.95–7.96 (m, 2H, coumarin-C4–H, C5–H),
7.19–7.20 (d, J: 3.99 Hz, 1H, coumarin-C6–H), 6.76 (s, 1H,
coumarin-C8–H), 4.00 (s, 2H, CH2), 3.22–3.45 (m, 4H,
2CH2), 2.52 (m, 4H, piperidine-C2–H, C6–H), 2.47 (s, 2H,
SO2NH2), 1.61–1.81 (m, 6H, piperidine-C3–H, C4–H, C5–
H), 1.02–1.15 (m, 6H, 2CH3). 13C-NMR; 170.28 (C=O),
157.56 (thiadiazole-C7a), 151.38 (coumarin-C9), 148.24
(imidazole-C6), 147.26 (coumarin-C7), 140.28 (coumarin-
C4), 132.82 (coumarin-C3), 127.76 (coumarin-C5), 118.10
(coumarin-C10), 115.82 (coumarin-C6), 114.22 (coumarin-
C8), 110.20 (imidazole-C5), 106.84 (thiadiazole-C2), 54.22
(piperdine-C2 & C6), 51.72 (N–CH2), 26.52 (piperdine-C3
& C5), 25.90 (piperdine-C4), 39.48 (coumarin 2CH3), 30.46
(coumarin 2CH2); MS (70 eV) m/z 519 (M ? 1), 518
(M?), 445, 439, 419, 80, 62, 48; Anal. Calcd. For
C23H28N6S2O4: C, 53.47; H, 5.46; N, 16.27. Found: C,
53.38; H, 5.45; N, 16.21.
To a well stirred solution of imidazo[2,1-b][1,3,4]thiadi-
azole-2-sulfonamide (3) (0.01 mol, 4.19 g) in glacial acetic
acid (10 mL) and anhydrous sodium acetate (0.02 mol)
was added bromine (0.01 mol) drop wise with stirring at
room temperature. After the addition, stirring was contin-
ued for 2 h. The reaction mixture was poured into ice cold
water and basified with ammonia solution. The separated
solid was collected, washed with water, dried, and recrys-
tallized from ethanol (yield 61.00 %). mp 142–145 °C. IR
(KBr) 3319, 3269, 2974, 1715, 1599, 1375, 1489, 1180,
1
643, 585 cm-1; H-NMR (DMSO-d6); 7.60–7.62 (m, 2H,
coumarin-C4–H, C5–H), 6.76–6.78 (d, J: 7.99 Hz, 1H,
coumarin-C6–H), 6.56 (s, 1H, coumarin-C8–H), 3.22–3.45
(m, 4H, 2CH2), 2.47 (s, 2H, SO2NH2), 1.11–1.18 (m, 6H,
2CH3). 13C-NMR; 168.15 (C=O), 158.44 (thiadiazole-C7a),
150.22 (coumarin-C9), 149.15 (imidazole-C6), 146.30
(coumarin-C7), 130.22 (coumarin-C3), 141.44 (coumarin-
C4), 126.44 (coumarin-C5), 118.44 (coumarin-C10), 116.56
(coumarin-C6), 114.12 (coumarin-C8), 110.24 (imidazole-
C5), 107.46 (thiadiazole-C2), 39.24 (coumarin 2CH3),
30.78 (coumarin 2CH2); MS (70 eV) m/z 499 (M ? 1), 498
(M?), 419, 101, 80, 62, 48; Anal. Calcd. For
C17H16O4N5S2Br: C, 40.97; H, 3.24; N, 14.05. Found: C,
40.88; H, 3.23; N, 14.01.
Synthesis of 5-(pyrrolidin-1-ylmethyl)-6-[7-(diethylamino)-
2-oxo-2H-chromen-3-yl]imidazo[2,1-b][1,3,4]thiadiazole-
2-sulfonamide (6)
A mixture of 6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]
imidazo[2,1-b][1,3,4]thiadiazole-2-sulfonamide (3) (0.005 mol,
2.09 g), pyrrolidine (0.006 mol, 0.42 g), formalin (1 mL)
and acetic acid (1 mL) in methanol (20 mL) was refluxed
for 18 h (monitored by TLC). The solution was diluted
with water, extracted with chloroform (3 9 30 mL); the
combined chloroform extract was washed with water
(3 9 30 mL) and dried over anhydrous sodium sulfate. The
solution was evaporated to dryness in vacuum and the res-
idue was recrystallized from methanol (yield 54.00 %). mp
182–184 °C. IR (KBr) 3266, 2985, 1722, 1610, 1418, 1352,
Synthesis of 5-nitroso-6-[7-(diethylamino)-2-oxo-2H-
chromen-3-yl]imidazo[2,1-b][1,3,4]thiadiazole-2-
sulfonamide (8)
To a well stirred solution of imidazothiadiazole (3)
(0.01 mol, 4.19 g) in acetic acid (20 mL) was added
sodium nitrite solution (0.021 mol, in 4 mL water) drop
wise at room temperature. After the addition, stirring was
continued for 30 min and the mixture was refluxed for 3 h.
The reaction mixture was poured into ice cold water,
separated solid was collected, washed with water dried and
recrystallized from methanol (yield 80.70 %). mp
200–202 °C. IR (KBr) 3275, 1746, 1638, 1566, 1420,
1
1178, 645 cm-1; H-NMR (DMSO-d6); 7.90–7.94 (m, 2H,
coumarin-C4–H, C5–H), 7.20–7.21 (d, J: 3.99 Hz, 1H, cou-
marin-C6–H), 6.68 (s, 1H, coumarin-C8–H), 4.04 (s, 2H,
CH2), 3.20–3.50 (m, 8H, 2CH2 & pyrrolidine-C2 & C5–H),
2.42 (s, 2H, SO2NH2), 2.13–2.60 (m, 4H, pyrrolidine-C3 &
C4–H), 1.00–1.22 (m, 6H, 2CH3). 13C-NMR; 171.56 (C=O),
156.22 (thiadiazole-C7a), 152.25 (coumarin-C9), 150.26
(imidazole-C6), 148.54 (coumarin-C7), 142.85 (coumarin-
C4), 134.78 (coumarin-C3), 128.54 (coumarin-C5), 117.29
(coumarin-C10), 116.14 (coumarin-C6), 113.78 (coumarin-
C8), 111.29 (imidazole-C5), 105.22 (thiadiazole-C2), 52.44
(pyrrolidine-C2 & C5), 48.72 (N–CH2), 24.66 (pyrrolidine-
C3 & C4), 39.54 (coumarin 2CH3), 31.52 (coumarin 2CH2);
MS (70 eV) m/z 503 (M?), 419, 404, 340, 139, 101, 80, 62,
48. Anal. Calcd. For C22H26N6S2O4: C, 52.57; H, 5.21; N,
16.72. Found: C, 52.50; H, 5.20; N, 16.69.
1
1336, 1158, 648 cm-1; H-NMR (DMSO-d6); 7.96–7.98
(m, 2H, coumarin-C4–H, C5–H), 7.19- 7.20 (d, J: 3.99 Hz,
1H, coumarin-C6–H), 6.76 (s, 1H, coumarin-C8–H),
3.22–3.45 (m, 4H, 2CH2), 2.48 (s, 2H, SO2NH2), 1.11–1.18
(m, 6H, 2CH3). 13C-NMR; 169.22 (C=O), 156.24 (thiadi-
azole-C7a), 150.78 (coumarin-C9), 149.26 (imidazole-C6),
147.10 (coumarin-C7), 141.40 (coumarin-C4), 133.16
(coumarin-C3), 127.20 (coumarin-C5), 119.46 (coumarin-
C10), 116.22 (coumarin-C6), 115.20 (coumarin-C8), 111.20
(imidazole-C5), 107.60 (thiadiazole-C2), 39.22 (coumarin
2CH3), 30.16 (coumarin 2CH2); MS (70 eV) m/z 449
123

874
Med Chem Res (2013) 22:869–878
(M ? 1), 448 (M?), 419, 369, 101, 80, 62, 48; Anal. Calcd.
For C17H16N6O5S2: C, 45.53; H, 3.60; N, 18.74. Found: C,
45.46; H, 3.61; N, 18.70.
(d, J: 8.79 Hz, 1H, coumarin-C6–H), 6.63 (s, 1H, couma-
rin-C8–H), 3.26–3.35 (m, 4H, 2CH2), 3.04 (m, 6H, 2CH3),
1.06–1.15 (m, 6H, 2CH3); 13C-NMR; 182.15 (CHO),
172.10 (C=O), 160.10 (NH=CH), 153.21 (thiadiazole-C7a),
151.15 (coumarin-C9), 148.09 (imidazole-C6), 145.70
(coumarin-C7), 140.59 (coumarin-C4), 131.40 (coumarin-
C3), 126.53 (coumarin-C5), 119.82 (coumarin-C10), 115.48
(coumarin-C6), 113.08 (coumarin-C8), 111.55 (imidazole-
C5), 106.59 (thiadiazole-C2), 39.51 (coumarin 2CH3),
30.57 (coumarin 2CH2), 12.84 (formamide 2CH3); MS
(70 eV) m/z 502 (M?), 474, 459, 431, 101, 62, 48. Anal.
Calcd. For C21H22O5N6S2: C, 50.19; H, 4.41; N, 16.72.
Found: C, 50.02; H, 4.42; N, 16.68.
Synthesis of 5-thiocyanato-6-[7-(diethylamino)-2-oxo-2H-
chromen-3-yl]imidazo[2,1-b][1,3,4]thiadiazole-2-
sulphonamide (9)
To a well stirred solution of imidazothiadiazole (3)
(0.01 mol, 4.19 g) in glacial acetic acid (30 mL) and
potassium thiocyanate (0.02 mol) was added bromine
(0.01 mol) in glacial acetic acid (5 mL) drop wise with
stirring at room temperature. The stirring was further
continued for 3 h at room temperature. The reaction mix-
ture was poured into ice water, the separated solid was
collected, washed with water, dried, and recrystallized
from methanol (yield 68.00 %). mp 124–126 °C. IR (KBr)
3313, 2968, 2153, 1716, 1569, 1487, 1345, 1182,
Synthesis of [6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]
imidazo[2,1-b][1,3,4]thiadiazole-2-[N-(dimethylamino-
methino)] sulfonamido-5-yl]methanol (11)
1
650 cm-1; H-NMR (DMSO-d6); 7.60–7.62 (m, 2H, cou-
5-Formyl-6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]im-
idazo[2,1-b][1,3,4]thiadiazole-2-[N-(dimethylaminometh-
ino)] sulfonamide 10 (0.001 mol, 0.50 g) was added in
small portions to a stirred and cooled solution of sodium
borohydride (0.001 mol, 0.36 g) in dry methanol (25 mL).
The mixture was stirred at room temperature for 7 h
(monitored by TLC) and poured over ice water. The solid
separated was collected by filtration, washed with cold
methanol, dried, and crystallized from DMF. (yield
67.00 %). mp 210–212 °C. IR (KBr) 3354, 2942, 2866,
marin-C4–H, C5–H), 6.76–6.78 (d, J: 8.79 Hz, 1H, cou-
marin-C6–H), 6.36 (s, 1H, coumarin-C8–H), 3.22–3.45 (m,
4H, 2CH2), 2.47 (s, 2H, SO2NH2), 1.04–1.16 (m, 6H,
2CH3). 13C-NMR; 170.44 (C=O), 155.12 (thiadiazole-C7a),
151.22 (coumarin-C9), 149.12 (SCN) 148.42 (imidazole-
C6), 146.20 (coumarin-C7), 141.22 (coumarin-C4), 132.10
(coumarin-C3), 126.12 (coumarin-C5), 119.12 (coumarin-
C10), 115.46 (coumarin-C6), 114.14 (coumarin-C8), 112.12
(imidazole-C5), 106.12 (thiadiazole-C2), 39.14 (coumarin
2CH3), 31.16 (coumarin 2CH2); MS (70 eV) m/z 477
(M ? 1), 476 (M?), 419, 101, 80, 62, 48; Anal. Calcd. For
C18H16O4N6S3: C, 45.37; H, 3.38; N, 17.63. Found: C,
45.30; H, 3.39; N, 17.58.
1
2766, 1718, 1618, 1495, 1360, 1166, 658 cm-1; H-NMR
(DMSO-d6); 8.25 (s, 1H, CH=N), 7.58–7.92 (m, 2H, cou-
marin-C4–H, C5–H), 6.44 (d, 1H, J: 8.99 Hz, coumarin-C6–
H), 6.24 (s, 1H, coumarin-C8–H), 5.02 (s, 2H, CH2),
3.16–3.34 (m, 4H, 2CH2), 3.08 (m, 6H, 2CH3), 2.58 (br s,
1H, OH, D2O exchangeable), 1.15–1.45 (m, 6H, 2CH3);
13C-NMR; 171.54 (C=O), 162.44 (NH=CH), 152.24 (thia-
diazole-C7a), 152.56 (coumarin-C9), 149.22 (imidazole-C6),
146.48 (coumarin-C7), 142.28 (coumarin-C4), 130.88
(coumarin-C3), 125.78 (coumarin-C5), 120.54 (coumarin-
C10), 116.54 (coumarin-C6), 114.20 (coumarin-C8), 112.26
(imidazole-C5), 107.58 (thiadiazole-C2), 46.98 (CH2OH),
38.54 (coumarin 2CH3), 31.48 (coumarin 2CH2), 13.78
(formamide 2CH3); MS (70 eV) m/z 505 (M?), 475, 462,
434, 101, 62, 48. Anal. Calcd. For C21H24O5N6S2: C, 49.99;
H, 4.79; N, 16.66. Found: C, 49.90; H, 4.78; N, 16.68.
Synthesis of 5-formyl-6-[7-(diethylamino)-2-oxo
-2H-chromen-3-yl]imidazo[2,1-b][1,3,4]thiadiazole-
2-[N-(dimethylaminomethino)] sulfonamide (10)
Vilsmeier–Haack reagent was prepared by adding phos-
phoryl chloride (3 mL) in dimethylformamide (20 mL) at
0 °C with stirring. Then, imidazothiadiazole (3) (0.01 mol,
4.1 g) was added to the reagent and stirred 0 °C for
30 min. The mixture was further stirred for 2 h at room
temperature and at 60 °C for additional 2 h. The reaction
mixture was then poured in sodium carbonate solution and
stirred at 90 °C for 2 h. After cooling, the mixture was
diluted with water, extracted with chloroform and the
collective extract was washed with water and dried over
anhydrous sodium sulfate. The residue obtained after the
removal of chloroform was recrystallized from ethanol
(yield 78.00 %). mp 198–200 °C. IR (KBr) 2926, 2853,
2742, 1714, 1681, 1622, 1489, 1336, 1156, 652 cm-1; 1H-
NMR (DMSO-d6); 10.06 (s, 1H, CHO), 8.69 (s, 1H,
CH=N), 7.64–7.80 (m, 2H, coumarin-C4–H, C5–H), 6.78
Synthesis of 6-[7-(diethylamino)-2-oxo-2H-chromen-3-
yl]imidazo[2,1-b][1,3,4]thiadiazole-2-[N-(dimethyla-
minomethino)] sulfonamido-5-carbaldehydeoxime (12)
A
mixture of 5-formyl-6-[7-(diethylamino)-2-oxo-2H-
chromen-3-yl]imidazo[2,1-b][1,3,4] thiadiazole-2-[N-(dim-
ethylaminomethino)]sulfonamide 10 (0.001 mol, 0.50 g)
123

Med Chem Res (2013) 22:869–878
875
Synthesis of 2-[-6-[7-(diethylamino)-2-oxo-2H-chromen-
and hydroxylamine hydrochloride (0.0012 mol, 0.83 g) was
refluxed in pyridine (15 mL) for 8 h (monitored by TLC), the
cooled mixture was then poured over ice and the precipitate
was collected by filtration, washed with water, aq alcohol,
dried, and recrystallized from DMF. (yield 65.00 %). mp
228–230 °C. IR (KBr) 3282, 2948, 2852, 2771, 1720, 1628,
1488, 1366, 1160, 652 cm-1; 1H-NMR (DMSO-d6); 9.86 (br
s, 1H, OH, D2O exchangeable), 8.69 (s, 1H, CH=N), 8.46 (s,
1H, CH=N), 7.44–7.84 (m, 2H, coumarin-C4–H, C5–H), 6.45
(d, J: 8.90 Hz, 1H, coumarin-C6–H), 6.20 (s, 1H, coumarin-
C8–H), 3.02–3.45 (m, 4H, 2CH2), 2.98 (m, 6H, 2CH3),
1.10–1.22 (m, 6H, 2CH3); 13C-NMR; 170.24 (C=O), 161.28
(NH=CH), 154.30 (CH=N), 153.87 (thiadiazole-C7a),
151.46 (coumarin-C9), 148.56 (imidazole-C6), 145.92
(coumarin-C7), 141.54 (coumarin-C4), 131.54 (coumarin-
C3), 124.46 (coumarin-C5), 121.46 (coumarin-C10), 117.22
(coumarin-C6), 115.24 (coumarin-C8), 111.46 (imidazole-
C5), 107.41 (thiadiazole-C2), 37.21 (coumarin 2CH3), 31.85
(coumarin 2CH2), 12.54 (formamide 2CH3); MS (70 eV)
m/z 518 (M?), 475, 474, 447, 101, 62, 48. Anal. Calcd. For
C21H23O5N7S2: C, 48.73; H, 4.48; N, 18.94. Found: C, 48.69;
H, 4.47; N, 18.89.
3-yl]imidazo[2,1-b][1,3,4]thiadiazole-
2-[N-(dimethylaminomethino)] sulfonamido]-3-phenyl-1,
3-thiazolidin-4-one (14)
5-Formyl-6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]im-
idazo[2,1-b][1,3,4]thiadiazole-2-[N-(dimethylaminometh-
ino)] sulfonamide 10 (0.001 mol, 0.50 g) and aniline
(1.1 g, 0.012 mol) were refluxed in dry ethanol (40 mL)
with catalytic amount of acetic acid for 4 h. The solid that
separated was filtered, washed with alcohol, and dried. The
crude product was recrystallized from DMF, the yellow
colored solid thus obtained (0.01 mol) was refluxed with
thioglycolic acid (0.012 mol), in dry benzene (25 mL) for
8 h. The colorless solid that separated was collected by
filtration, dried, and recrystallized from aq DMF. (yield
49.00 %). mp 252–254 °C. IR (KBr) 2990, 2888, 2752,
1
1726, 1710, 1612, 1495, 1374, 1162, 658 cm-1; H-NMR
(DMSO-d6); 8.72 (s, 1H, CH=N), 7.64–7.89 (m, 2H, cou-
marin-C4–H, C5–H) 7.96 (s, 1H, thiazolidine-CH), 6.78 (d,
J: 8.99 Hz, 1H, coumarin-C6–H), 6.59 (s, 1H, coumarin-
C8–H), 5.88 (s, 2H, CH2), 3.33–3.55 (m, 4H, 2CH2), 3.22
(m, 6H, 2CH3), 1.08–1.22 (m, 6H, 2CH3); 13C-NMR;
171.22 (C=O), 166.84 (thiazolidine-C=O), 161.65
(NH=CH), 152.54 (thiadiazole-C7a), 151.62 (coumarin-
C9), 150.12 (imidazole-C6), 144.54 (coumarin-C7), 141.32
(coumarin-C4), 131.58 (coumarin-C3), 126.94 (coumarin-
C5), 120.44 (coumarin-C10), 119.23 (coumarin-C6), 117.75
(coumarin-C8), 111.46 (imidazole-C5), 109.79 (thiadia-
zole-C2), 40.26 (thiazolidine-C2), 37.68 (thiazolidine-C5),
36.26 (coumarin 2CH3), 31.82 (coumarin 2CH2), 11.89
(formamide 2CH3); MS (70 eV) m/z 652 (M?), 609, 581,
474, 101, 62, 48. Anal. Calcd. For C29H29O5N7S3: C,
53.44; H, 4.48; N, 15.04. Found: C, 53.40; H, 4.47; N,
15.00.
Synthesis of 6-[7-(diethylamino)-2-oxo-2H-chromen-
3-yl]imidazo[2,1-b][1,3,4]thiadiazole-2-[N-(dimethyla-
minomethino)] sulfonamido-5-carbonitrile (13)
The 6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]imidazo-
[2,1-b][1,3,4]thiadiazole-2-[N-(dimethylaminomethino)]
sulfonamido-5-carbaldehydeoxime 12 (0.001 mol, 0.51 g)
was suspended in thionyl chloride (8 mL) and refluxed for
10 min. Poured over cold water, neutralized by sodium
carbonate solution, the solid that separated was collected
by filtration was washed repeatedly with water, dried,
and recrystallized from benzene. (yield 58.00 %). mp
238–240 °C. IR (KBr) 2990, 2856, 2746, 2210, 1722,
Synthesis of 5-guanylhydrazone-6-[7-(diethylamino)-
2-oxo-2H-chromen-3-yl]imidazo[2,1-b][1,3,4]thiadiazole-
2-[N-(dimethylaminomethino)] sulfonamide (15)
1
1612, 1492, 1368, 1155, 656 cm-1; H-NMR (DMSO-d6);
8.44 (s, 1H, CH=N), 7.76–8.10 (m, 2H, coumarin-C4–H,
C5–H), 6.95 (d, J: 8.99 Hz, 1H, coumarin-C6–H), 6.26 (s,
1H, coumarin-C8–H), 3.12–3.52 (m, 4H, 2CH2), 2.96 (m,
6H, 2CH3), 1.16–1.26 (m, 6H, 2CH3); 13C-NMR; 172.63
(C=O), 162.32 (NH=CH), 152.54 (thiadiazole-C7a), 150.26
(coumarin-C9), 149.34 (imidazole-C6), 145.24 (coumarin-
C7), 142.64 (coumarin-C4), 130.24 (coumarin-C3), 125.56
(coumarin-C5), 121.88 (coumarin-C10), 118.92 (CN),
118.34 (coumarin-C6), 116.44 (coumarin-C8), 112.87
(imidazole-C5), 108.62 (thiadiazole-C2), 36.56 (coumarin
2CH3), 32.76 (coumarin 2CH2), 12.36 (formamide 2CH3);
MS (70 eV) m/z 500 (M?), 474, 457, 429, 101, 62, 48.
Anal. Calcd. For C21H21O4N7S2: C, 50.49; H, 4.24; N,
19.63. Found: C, 50.40; H, 4.23; N, 19.68.
5-formyl-6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]im-
idazo[2,1-b][1,3,4]thiadiazole-2-[N-(dimethylaminomethi-
no)] sulfonamide 10 (0.001 mol, 0.50 g) was dissolved in
100 mL of ethanol and treated with (0.01 mol) of amino-
guanidine hydrochloride prepared from treating the sus-
pension of aminoguanidine bicarbonate in ethanol with
excess of 37 % hydrochloric acid. The reaction mixture
was refluxed for 1 h and the resulting precipitate was
collected by filtration and dried. The HCl salt thus obtained
was suspended in ice cold water and basified with aqueous
ammonia solution. The free base 15 thus formed was fil-
tered, dried, and recrystallised from ethanol. (yield
123

876
Med Chem Res (2013) 22:869–878
61.00 %). mp 263–265 °C. IR (KBr) 3425 (b), 3122, 2948,
2844, 2736, 1714, 1615, 1484, 1340, 1162, 650 cm-1; 1H-
NMR (DMSO-d6); 12.20 (s, 1H, =N–NH), 8.78 (s, 1H,
5-CH=N), 8.58 (s, 1H, CH=N–SO2), 7.55–7.99 (m, 2H,
coumarin-C4–H, C5–H), 6.71 (d, t: 8.90 Hz, 1H, coumarin-
C6–H), 6.52 (s, 1H, coumarin-C8–H), 3.18–3.41 (m, 4H,
2CH2), 3.09 (m, 6H, 2CH3), 2.02 (s, 2H, NH2), 1.09–1.20
(m, 6H, 2CH3); 13C-NMR; 172.56 (C=O), 163.81 (NH-
C=NH), 162.44 (NH=CH), 154.94 (CH=N), 151.32 (thia-
diazole-C7a), 150.75 (coumarin-C9), 149.71 (imidazole-
C6), 144.28 (coumarin-C7), 142.69 (coumarin-C4), 130.47
(coumarin-C3), 127.29 (coumarin-C5), 121.85 (coumarin-
C10), 118.54 (coumarin-C6), 117.22 (coumarin-C8), 110.29
(imidazole-C5), 108.82 (thiadiazole-C2), 36.42 (coumarin
2CH3), 31.26 (coumarin 2CH2), 11.24 (formamide 2CH3);
MS (70 eV) m/z 559 (M?), 516, 488, 474, 101, 62, 48.
Anal. Calcd. For C22H26O4N10S2: C, 47.30; H, 4.69; N,
25.07. Found: C, 47.26; H, 4.68; N, 25.02.
Table 1 In vitro antitubercular activities (MIC, lg/mL) of imi-
dazo[2,1-b][1,3,4]thiadiazoles 3–15
Compound
Mycobacterium
tuberculosis H37Rva
3
3.125
25
4
5
25
6
25
7
6.25
50
8
9
12.5
1.6
10
11
3.125
25
12
13
25
14
0.8
15
0.8
Pyrazinamide
Streptomycin
a
3.125
6.25
Biological activities
MIC was evaluated by the alamar blue dye using microplate alamar
blue assay method
Minimum Inhibitory Concentration (MIC) values were
determined for newly synthesized compounds against
M. tuberculosis strain H37Rv using the Micro plate Alamar
Blue assay (MABA) (Franzblau et al. 1998. Pyrazinamide
and streptomycin were used as standard drugs. The
96-wells plate received 100 ll of the Middlebrook 7H9
broth and serial dilution of compounds were made directly
on plate. The final drug concentrations tested were 0.2, 0.4,
0.8, 1.6, 3.125, 6.25, 12.5, 25, 50, and 100 lg mL-1. Plates
were covered and sealed with parafilm and incubated at
37 °C for five days. After this, 25 ll of freshly prepared 1:1
mixture of almar blue reagent and 10 % tween 80 was
added to the plate and incubated for 24 h. A blue color in
the well was interpreted as no bacterial growth and pink
color was scored as growth. The MIC was defined as
lowest drug concentration which prevented the color
change from blue to pink. Table 1 reveals the antituber-
cular activity (MIC) of compounds 3–15.
tested compound that yield no visible growth on the plate.
To insure that the solvent had no effect on the bacterial
growth, a control was performed with the test medium
supplemented with DMSO at the same dilutions as used in
the experiments and DMSO had no effect on the micro-
organisms in the concentrations studied. Table 2 reveals
the antibacterial activity (MIC values) of compounds 3–15.
Results and discussion
Structure of imidazothiadiazole 3 was established by the
absence of tN–H band in the IR spectra and appearance of
imidazole proton (C5–H) at d 8.56 in 1H-NMR spectra. The
1H-NMR spectra of the compounds 4, 5, and 6 showed the
absence of imidazole proton and a singlet was observed
around d 4.00, which was assigned to methylene protons
and other aliphatic and aromatic protons resonated in the
expected region. For morpholine derivatives 4, two triplets
were observed. Formation of Mannich bases was further
confirmed by their 13C NMR and mass spectra.
The MIC determination of the tested compounds was
investigated in side-by-side comparison with norfloxacin
against Gram-positive (Staphylococcus aureus, Bacillus
subtilis) and Gram-negative bacteria (Vibrio cholera,
Escherichia coli) by broth microdilution method (Yajko
et al., 1995). Serial dilutions of the test compounds and
reference drugs were prepared in Mueller–Hinton agar.
Drugs (10 mg) were dissolved in dimethylsulfoxide
(DMSO, 1 mL). Further progressive dilutions with melted
Mueller–Hinton agar were performed to obtain the required
concentrations of 1, 2, 4, 8, 16, 31.25, 62.5, 125, 250, and
500 lg mL-1. The tubes were inoculated with 105
cfu mL-1 (colony forming unit/mL) and incubated at 37°C
for 18 h. The MIC was the lowest concentration of the
All the electrophillic substitution reactions carried out
on imidazothiadiazole afforded the regioselectively
expected 5-substituted derivatives 7, 8, 9, and 10. IR
spectrum of compound 8 displayed the absorption band at
1566 cm-1 for NO, while compound 9 showed band at
2153 cm-1 for SCN. IR spectra of 10 displayed the sharp
band for carbonyl stretching frequency tC=O at 1681 cm-1
and the singlet due to C5-imidazole proton was absent and
a signal for aldehyde proton was observed at d 10.06 in the
123

Med Chem Res (2013) 22:869–878
877
at d 12.20 and d 8.78 assigned to guanyl NH and 5-CH=N
protons, respectively.
Table 2 In vitro antibacterial activities (MIC, lg/mL) of imi-
dazo[2,1-b][1,3,4]thiadiazoles 3–15a
The synthesized compounds were evaluated for their in
vitro antitubercular activity against M. tuberculosis H37Rv
by using microplate alamar blue assay method. The
investigation of antimycobacterial screening data (Table 1)
revealed that all the tested compounds showed good
mycobacterial inhibition. The tested compounds showed
activities against mycobacteria with MIC values ranging
from 0.8 to 50 lg/mL. The activity is considerably affected
by substitutents at position-5 on the imidazo[2,1-
b][1,3,4]thiadiazole nucleus. Compounds 4, 5, 6, 12, and
13 exhibited moderate antitubercular activity at a MIC of
20 lg/mL. Compound 10 having formyl group at 5th
position, showed improved activity at MIC value of 1.6 lg/
mL. It has been observed that compounds 14 and 15 having
a thiazolidinone and gaunylhydrazone groups respectively
at 5th position, showed highest antimycobacterial activity
at a MIC of 0.8 lg/mL.
Compound
Gram-negative Organisms Gram-positive Organisms
V. cholera
E. coli
S. aureus
B. subtilis
3
31.25
31.25
31.25
8
31.25
31.25
31.25
8
62.5
62.5
62.5
62.5
62.5
125
62.5
16
62.5
62.5
62.5
62.5
62.5
125
62.5
16
4
5
6
7
16
16
8
31.25
31.25
4
31.25
31.25
4
9
10
11
8
8
31.25
62.5
62.5
16
31.25
62.5
62.5
16
12
16
16
13
16
16
14
1
1
15
1
1
16
16
Norfloxacillin
1
16
2
2
In order to determine the antimicrobial activity of the
synthesized compounds, two Gram-positive, two Gram-
negative bacterial species were screened using the broth
dilution technique. Results of antibacterial activity of the
compounds are given in Table 2. The investigation of
antibacterial screening data revealed that all the tested
compounds showed moderate to good bacterial inhibition.
All the compounds showed significant antibacterial activity
against Gram-negative bacteria and moderate activity
against Gram-positive bacteria. Compounds showed anti-
microbial activity at MIC values of 1–125 lg/mL. Among
the Mannich bases the pyrrolidine derivative 6 was found
to have more activity against V. cholera and E. coli at a
MIC of 8 lg/mL. Compound 10 has exhibited very good
activity against all the bacterial strains; however, com-
pounds 11 and 12 were highly active against V. cholera
and E. coli. The thiazolidinone derivative 14 and gau-
nylhydrazone derivative 15 showed superior activity
against V. cholera and E. coli at a MIC of 1 lg/mL.
a
MIC was evaluated by the broth microdilution assay method
1H-NMR spectrum, thus confirming the formation of imi-
dazothiadiazole-5-carbaldehyde. All the 5-substituted
1
derivatives showed the absence of C5–H in their H-NMR
spectra confirming the substitution at 5th position. The
structures of all the compounds were finally ascertained by
the 13C-NMR and mass spectra.
The compound 11 showed the absence of carbonyl
stretching frequency tC=O and the presence of broad band
at 3354 cm-1 for tO–H in IR spectrum. The H-NMR dis-
1
played the absence of signal due to aldehyde proton and the
methylene protons resonated as singlet at d 5.02 and -OH
proton was observed as broad singlet at d 2.58. The
absence of tC=O band and the presence of tO–H band in the
IR spectrum of the product confirmed the formation of
oxime 12 and signal due to aldehydic proton was absent
and new signals for the azomethine and the –OH proton
(D2O exchangeable) resonated at d 9.86 and d 8.69,
1
respectively in the H-NMR spectrum, thus substantiating
Conclusion
the formation of aldoxime 12. The product obtained after
treatment of this aldoxime with thionyl chloride showed a
sharp band at 2210 cm-1 in the IR spectrum attributed for
tCN and there was no band due to tO–H. The absence of
both the -OH and azomethine protons in the 1H-NMR
spectrum confirmed the formation of nitrile 13. The com-
pound 14 showed the carbonyl stretching frequency tC=O at
The present research study reports the successful synthesis,
antibacterial, and antitubercular studies of new series of
imidazo[2,1-b][1,3,4]thiadiazole derivatives carrying bio-
logically active entities. Their screening results revealed
that all the compounds showed moderate to very good
activities against pathogenic strains. The preliminary in
vitro antibacterial and antitubercular screening of these
5-substituted-6-[7-(diethylamino)-2-oxo-2H-chromen-3-yl]
imidazo[2,1-b][1,3,4]thiadiazole-2-sulfonamides 4–15 has
revealed that coumarin substitution at 6th position and
different groups at 5th position, have emerged as potential
1
1710 cm-1, the H-NMR of the same displayed two sing-
lets at d 7.96 and d 5.88, respectively. These data con-
firmed the presence of thiazolidinone ring at 5-position.
1H-NMR spectrum of 15 displayed a D2O exchangeable
peak due to NH2 at d 2.02. The presence of new resonances
123

878
Med Chem Res (2013) 22:869–878
of 2-(1-methyl-5-nitro-1H-imidazol-2-yl)-5-substituted-1,3,4-thia-
diazole derivatives. Eur J Med Chem 40:1346–1350
compounds endowed with moderate to good antibacterial
and antitubercular activities. From first examination of the
antimicrobial activity results, it is evident that compounds
10, 14, and 15 containing 5-formyl, 5-thiazolidinone, and
5-gaunylhydrazone groups, respectively showed increased
activity. The possible improvement of antitubercular
activity of basic imidazo[2,1-b][1,3,4]thiadiazole structure
through modulation of ring substitutents and/or additional
functional groups warrants further investigations. Due to
the better activity against tested mycobacteria and micro-
organisms, compounds 10, 14, and 15 have been selected
for further development and studies to acquire more
information about structure–activity relationships are in
progress in our laboratories. In summary we have identified
a novel series of substituted imidazo[2,1-b][1,3,4]thiadia-
zole derivatives which may be developed into potential
class of antitubercular and antimicrobial agents.
Franzblau SG, Witzig RS, McLaughlin JC, Torres P, Madico G,
Hernandez A, DegnanMT, Cook MB, Quenzer VK, Ferguson RM,
Gilman RH (1998) Rapid, low-technology MIC determination
with clinical Mycobacterium tuberculosis isolates by using the
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Synthesis and antibacterial activity of some 5-guanylhydrazone/
thiocyanato-6-arylimidazo[2,1-b][1,3,4]thiadiazole-2-sulfona-
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antitubercular activity of a series of 2-sulfonamido/trifluoro-
methyl-6-substituted imidazo[2,1-b]-1,3,4-thiadiazole deriva-
tives. Bioorg Med Chem 12:5651–5659
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Synthesis and anti- inflammatory evaluation of methylene
bridged benzofuranyl imidazo[2,1-b][1,3,4]thiadiazoles. Eur J
Med Chem 43:1721–1729
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Chiruvella KK, Raghavan SC (2011) Synthesis and biological
evaluation of novel 2-aralkyl-5-substituted-6-(40-fluorophenyl)-
imidazo[2,1-b][1,3,4]thiadiazole derivatives as potent anticancer
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Geneva
Acknowledgments We thank Dr. V. H. Kulkarni, Principal, Shri.
H. V. Dambal, President, S. E. T’s College of Pharmacy, Dharwad,
India, for providing necessary facilities. We are thankful to Dr. G. S.
Gadaginamath for his valuable suggestions. We are grateful to Dr.
K. G. Bhat, Maratha Mandal’s Dental College, Hospital and Research
Centre, Belgaum, India for providing the facilities for antibacterial
and antitubercular activity. We also wish to thank Director, SAIF,
Indian Institute of Technology, Chennai, India and Director, USIC,
Karnataka University Dharwad, India for providing NMR and Mass
spectral data.
Noolvi MN, Patel HM, Singh N, Gadad AK, Cameotra SS, Badiger A
(2011) Synthesis and anticancer evaluation of novel 2-cyclopro-
pylimidazo[2,1-b][1,3,4]-thiadiazole derivatives. Eur J Med Chem
46:4411–4418
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