Synthesis of coumarin-based 1,3,4-oxadiazol-2ylthio-N-phenyl/benzothiazolyl acetamides as antimicrobial and antituberculosis agents

Article abstract of DOI:10.1007/s00044-012-0026-x

In an attempt to find new agents to fight against microbial infections, a series of coumarin-based 1,3,4-oxadiazol-2ylthio-N-phenyl/benzothiazolyl acetamides was synthesized starting from coumarin-3-carboxylic acid ethyl ester obtained through Knoevenagel and Pinner reaction. In vitro antimicrobial activity against several bacteria (S. aureus, B. cereus, E. coli, P. aeruginosa, K. pneumoniae, S. typhi, P. vulgaris, S. flexneri), fungi (A. niger, A. fumigatus, A. clavatus, C. albicans) and antimycobacterial activity against Mycobacterium tuberculosis H37Rv strain was assessed. This study shows to what extent the presence of various electron withdrawing/donating substituents on the phenyl or benzothiazole ring affects the activity profiles of the newer molecules. The relationship between activity profiles (MICs, 3.12-25 μg/mL) and the lipophilic character (LogP) of the prepared products is also discussed and the MIC values of the active conjugates seem to correlate to some extent with the lipophilicity profiles. Two (5e and 6c) of the final analogues displayed promising antimycobacterial activity at 12.5 μg/mL of MIC, half fold potent to the standard drug pyrazinamide (6.25 μg/mL). Compounds were characterized by IR, ¹H NMR, ¹³C NMR spectroscopy and elemental analysis.

Full text of DOI:10.1007/s00044-012-0026-x

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2013) 22:195–210
DOI 10.1007/s00044-012-0026-x
ORIGINAL RESEARCH
Synthesis of coumarin-based 1,3,4-oxadiazol-2ylthio-
N-phenyl/benzothiazolyl acetamides as antimicrobial
and antituberculosis agents
•
Rahul V. Patel Premlata Kumari
•
•
Dhanji P. Rajani Kishor H. Chikhalia
Received: 22 November 2011 / Accepted: 9 March 2012 / Published online: 21 March 2012
Ó Springer Science+Business Media, LLC 2012
Abstract In an attempt to find new agents to fight against
microbial infections, a series of coumarin-based 1,3,4-ox-
adiazol-2ylthio-N-phenyl/benzothiazolyl acetamides was
synthesized starting from coumarin-3-carboxylic acid ethyl
ester obtained through Knoevenagel and Pinner reaction.
In vitro antimicrobial activity against several bacteria
(S. aureus, B. cereus, E. coli, P. aeruginosa, K. pneumo-
niae, S. typhi, P. vulgaris, S. flexneri), fungi (A. niger,
A. fumigatus, A. clavatus, C. albicans) and antimycobac-
terial activity against Mycobacterium tuberculosis H37Rv
strain was assessed. This study shows to what extent the
presence of various electron withdrawing/donating sub-
stituents on the phenyl or benzothiazole ring affects the
activity profiles of the newer molecules. The relationship
between activity profiles (MICs, 3.12–25 lg/mL) and the
lipophilic character (LogP) of the prepared products is also
discussed and the MIC values of the active conjugates
seem to correlate to some extent with the lipophilicity
profiles. Two (5e and 6c) of the final analogues displayed
promising antimycobacterial activity at 12.5 lg/mL of
MIC, half fold potent to the standard drug pyrazinamide
Keywords Coumarin ꢀ 1,3,4-Oxadiazole ꢀ
Benzothiazole ꢀ Antimicrobial activity ꢀ
Antituberculosis activity
Introduction
During the past two decades, the growing population
of immunocompromised individuals, patients with
malignancies and transplant recipients has resulted in an
increase in severe opportunistic microbial infections
(Nathan, 2004). On the other hand, tuberculosis (TB), a
disease long considered substantially eradicated in the
developed countries, has resurged dramatically in the last
decades, establishing itself as one of the infectious dis-
eases resulting in around two million human deaths
worldwide (Raviglione, 2003; NIAID, http://www3.niaid.
nih.gov/topics/tuberculosis/; TAACF, http://www.taacf.
org/about-TB-background.htm). According to the World
Health Organization (WHO), in 2010, there were 8.8
million (range 8.5–9.2 million) incident cases of TB, 1.1
million (range 0.9–1.2 million) deaths from TB among
HIV-negative people and an additional 0.35 million
(range 0.32–0.39 million) deaths from HIV-associated TB
(WHO, 2011). These health problems demand urgency to
develop new authentically genuinely broad-spectrum and
low-toxicity antimicrobial and antitubercular drugs as
strains that resist the current medications are spreading
across the globe quite fast.
1
(6.25 lg/mL). Compounds were characterized by IR, H
NMR, 13C NMR spectroscopy and elemental analysis.
R. V. Patel (&) ꢀ P. Kumari
Department of Applied Chemistry, S.V. National Institute
of Technology, Surat 395007, Gujarat, India
e-mail: rahul.svnit11@gmail.com
Coumarin and its derivatives have gained great thera-
peutic importance in the field of medicinal chemistry since
they display a fascinating array of pharmacological prop-
erties (Keating and O’Kennedy, 1997; Li et al., 2011;
Hishmat et al., 2008; De Souza et al., 2005; Yee et al.,
2005). Moreover, it has been reported in the literature that
D. P. Rajani
Microcare Laboratory, Surat 395001, Gujarat, India
K. H. Chikhalia
Department of Chemistry, School of Science, Gujarat
University, Ahmedabad 380025, Gujarat, India
123

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Med Chem Res (2013) 22:195–210
compounds bearing coumarin-based 1,3,4-oxadiazole ring
is well acknowledged to possess significant antimicrobial
activity (Bhat et al., 2011; Patel et al., 2010; Biradar and
Manjunath, 2004; Desai and Mehta, 2002). Looking at the
pharmacological importance of above-mentioned scaffolds
and in continuous interest in oxadiazole derivatives as
antimicrobial agents (Chikhalia et al., 2009), we were
interested in identifying newer structural elements con-
sisting of both coumarin and oxadiazole nucleus to unearth
novel antibacterial, antifungal and antimycobacterial
agents. Furthermore, various analogues carrying benzothi-
azole ring system have been proved to be effective to
possess promising antituberculosis activities (Bhusari
et al., 2000), whereas imidazo[2,1-b][1,3,4]thiadiazoles-
based benzothiazole derivatives are evaluated for their in
vitro antituberculosis activity (Hegde et al., 2006) using
BACTEC method and the analogues were found to reveal
excellent inhibition (88–99 %) of mycobacteria at 6.25 lg/
mL of MIC. Moreover, recent literature is enriched with
progressive findings about the different biological activities
of benzothiazole derivatives (Rana et al., 2007).
recorded on a Shimadzu 8400-S FT–IR spectrophotometer
(Shimadzu India Pvt. Ltd., Mumbai, India) using KBr
pellets. Thin layer chromatography was performed on
object glass slides (2 9 7.5 cm) coated with silica gel-G
1
and spots were visualized under UV irradiation. H NMR
and 13C NMR spectra were recorded on a Varian 400 MHz
model spectrometer (Varian India Pvt. Ltd., Mumbai,
India) using DMSO as a solvent and TMS as internal
1
standard with H resonant frequency of 400 MHz and 13C
1
resonant frequency of 100 MHz. The H NMR and 13C
NMR chemical shifts were reported as parts per million
(ppm) downfield from TMS (Me4Si) and were performed at
Centre for Excellence, Vapi, India. The splitting patterns
are designated as follows: s singlet, br s broad singlet, d
doublet, m multiplet. Elemental analyses (C, H, N) were
performed using a Heraeus Carlo Erba 1180 CHN analyser
(Hanau, Germany).
General synthetic procedure for 2-chloro-N-
(substituted)phenyl acetamides (2a–k)
We have previously developed some 1,3,5-triazine-
based analogues bearing coumarin nucleus and identified
their antimicrobial efficacies (Patel et al., 2011a) and the
noticeable results directed us to assess the antimicrobial
potency of some additional 1,3,5-triazine-based derivatives
bearing 3-(5-mercapto-1,3,4-oxadiazol-2-yl)-2H-chromen-
2-one (Patel et al., 2011b) and results suggested that such
coumarin-based 1,3,4-oxadiazolyl derivatives are impor-
tant starting tool for the antimicrobial drug discovery
process. These findings have given impetus to our antimi-
crobial drug research and we thought that structural mod-
ification of the existed molecules to a different way
involving coumarin and 1,3,4-oxadiazole core could mod-
ulate the antimicrobial profiles in a more better way. Fur-
thermore, the physicochemical parameter like LogP refers
to calculated hydrophobicity of the compounds and
according to the thumb rule for LogP values, to a drug like
molecule it must be lower than ‘‘5’’ to by-pass the cell
barrier. To describe the uptake, distribution, biotransfor-
mation and excretion of organic chemicals in biological
systems, partition or distribution coefficient is critical ele-
ments (Jepson et al., 1993) and hence, in this context, the
newer molecules with appreciable lipophilic character are
presented here to produce remarkable bioactivities.
Chloroacetyl chloride (0.06 mol) was added dropwise to a
mixture of the appropriate amine (0.05 mol) and K2CO3
(0.06 mol) in acetone (50 mL) at room temperature. The
reaction mixture was refluxed for 4–8 h, then, after cooling
to room temperature, it was slowly poured into 100 mL of
ice water. A solid was formed thereafter. The precipitate
was separated by filtration and washed successively with
water. The product was dried under vacuum to obtain
2a–k. The progress of the reaction was monitored by thin
layer chromatography using toluene–acetone (8:2) solvent
system.
2-Chloro-N-phenyl acetamide (2a)
White solid, yield: 78 %., mp 135–138 °C. IR (KBr,
cm-1): 3,281 (NH), 1,680 (C=O). 1H NMR (400 MHz,
DMSO-d6): d ppm, 8.56 (br s, 1H, –NH), 7.66 (m, 2H),
7.40 (m, 2H), 7.14 (m, 1H), 4.22 (s, 2H, –CH2–Cl).
2-Chloro-N-(4-chlorophenyl)acetamide (2b)
Pale yellow solid, yield: 83 %, mp 168–170 °C. IR (KBr,
cm-1): 3,292 (NH), 1,672 (C=O). 1H NMR (400 MHz,
DMSO-d6): d 8.35 (br s, 1H, –NH), 7.63 (m, 2H), 7.45
(m, 2H), 7.19 (m, 1H), 4.21 (s, 2H, –CH2–Cl).
Materials and methods
N-(4-Bromophenyl)-2-chloroacetamide (2c)
Melting points were determined in open capillaries on a
Veego electronic apparatus VMP-D (Veego Instrument
Corporation, Mumbai, India) and are uncorrected. IR
spectra (4,000–400 cm-1) of synthesized compounds were
Grey solid, yield: 67 %, mp 180–184 °C. IR (KBr, cm-1):
3,307 (NH), 1,668 (C=O). H NMR (400 MHz, DMSO-
1
d6): d 8.32 (br s, 1H, –NH), 7.54–7.47 (m, 3H), 7.22
(m, 1H), 4.25 (s, 2H, –CH2–Cl).
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197
2-Chloro-N-(4-fluorophenyl)acetamide (2d)
N-(4-Acetamidophenyl)-2-chloroacetamide (2k)
While solid, yield: 62 %, mp 131–132 °C. IR (KBr,
cm-1): 3,299 (NH), 1,670 (C=O). 1H NMR (400 MHz,
DMSO-d6): d 8.21 (br s, 1H, –NH), 7.51 (dd, J = 7.1,
4.4 Hz, 2H), 7.27 (t, J = 8.2 Hz, 2H), 4.12 (s, 2H,
–CH2–Cl).
Pale yellow solid, yield: 72 %, mp 240–243 °C. IR (KBr,
cm-1): 3,290 (NH), 1,671 (C=O). 1H NMR (400 MHz,
DMSO-d6): d 8.21 (br s, 1H, –NH), 8.33 (s, 1H, –NH),
7.63–7.56 (m, 2H), 7.39–7.35 (m, 2H), 4.20 (s, 2H, –CH2–
Cl), 1.91 (s, 3H, –CH3).
2-Chloro-N-(4-iodophenyl)acetamide (2e)
General synthetic procedure for 2-amino-6-substituted
benzothiazoles (3a–k)
While solid, yield: 85 %, mp 192–196 °C. IR (KBr, cm-1):
3,314 (NH), 1,678 (C=O). H NMR (400 MHz, DMSO-
1
A mixture of 0.1 mol of 4-substituted aniline and 0.1 mol
of potassium thiocyanate (KCNS) in 100 mL glacial acetic
acid (AcOH) was cooled in an ice bath and stirred for 10–
20 min, and then 0.1 mol bromine in glacial acetic acid
was added dropwise at such a rate to keep the temperature
below 10 °C throughout the addition. The reaction mixture
was stirred at room temperature for 2–4 h, the hydrobro-
mide (HBr) salt thus separated out was filtered, washed
with acetic acid, dried, dissolved in hot water and basified
to pH 11.0 with ammonia solution (NH4OH) and the
resulting precipitate was filtered, washed with water and
dried to get the desired product (3a–k). The progress of the
reaction was monitored by thin layer chromatography
using toluene–acetone (8:2) solvent system.
d6): d 8.44 (br s, 1H, –NH), 7.64 (d, J = 7.1 Hz, 2H), 7.42
(d, J = 7.4 Hz, 2H), 4.20 (s, 2H, –CH2–Cl).
2-Chloro-N-(4-nitrophenyl)acetamide (2f)
Yellow solid, yield: 75 %, mp 185–186 °C. IR (KBr,
cm-1): 3,283 (NH), 1,669 (C=O). 1H NMR (400 MHz,
DMSO-d6): d 8.41 (br s, 1H, –NH), 7.53–7.41 (m, 4H),
4.18 (s, 2H, –CH2–Cl).
2-Chloro-N-(4-cyanophenyl)acetamide (2g)
While solid, yield: 88 %, mp 182–184 °C. IR (KBr, cm-1):
3,270 (NH), 1,669 (C=O). H NMR (400 MHz, DMSO-
1
d6): d 8.32 (br s, 1H, –NH), 7.67 (d, J = 7.3 Hz, 2H), 7.58
(d, J = 7.4 Hz, 2H), 4.14 (s, 2H, –CH2–Cl).
Benzo[d]thiazol-2-amine (3a)
Light brown solid, yield: 78 %, mp 128–130 °C. IR (KBr,
cm-1): 3,371 (NH), 1,578 (C=N). 1H NMR (400 MHz,
DMSO-d6): d ppm, 7.28 (s, 2H, NH2, D2O exchangeable),
7.24–7.51 (m, 4H, Ar–H).
2-Chloro-N-p-tolylacetamide (2h)
While solid, yield: 80 %, mp 164–166 °C. IR (KBr, cm-1):
3,295 (NH), 1,671 (C=O). H NMR (400 MHz, DMSO-
1
6-Chlorobenzo[d]thiazol-2-amine (3b)
d6): d 8.30 (br s, 1H, –NH), 7.52–7.49 (m, 2H), 7.34–7.36
(m, 2H), 4.21 (s, 2H, –CH2–Cl), 2.19 (s, 3H, –CH3).
White solid, yield: 72 %, mp 202–203 °C. IR (KBr, cm-1):
3,391 (NH), 1,546 (C=N). H NMR (400 MHz, DMSO-
1
2-Chloro-N-(4-methoxyphenyl)acetamide (2i)
d6): d 7.52 (d, J = 1.7 Hz, 1H, H-7), 7.35 (d, J = 7.3 Hz,
1H, H-4), 7.21 (dd, J = 7.9, 2.1 Hz, 1H, H-5), 6.72 (s, 2H,
NH2, D2O exchangeable).
While solid, yield: 76 %, mp 121–122 °C. IR (KBr, cm-1):
3,311 (NH), 1,668 (C=O). H NMR (400 MHz, DMSO-
1
d6): d 8.27 (br s, 1H, –NH), 7.60 (d, J = 7.5 Hz, 2H), 7.44
(d, J = 7.1 Hz, 2H), 4.24 (s, 2H, –CH2–Cl), 3.77 (s, 3H,
–OCH3).
6-Bromobenzo[d]thiazol-2-amine (3c)
Light yellow solid, yield: 76 %, mp 216–219 °C. IR (KBr,
cm-1): 3,410 (NH), 1,589 (C=N). 1H NMR (400 MHz,
DMSO-d6): d 7.72 (d, J = 1.5 Hz, 1H, H-7), 7.39–7.46 (m,
2H, H-4 and H-5), 6.72 (s, 2H, NH2, D2O exchangeable).
2-Chloro-N-(4-ethoxyphenyl)acetamide (2j)
While solid, yield: 68 %, mp 189–193 °C. IR (KBr, cm-1):
3,319 (NH), 1,664 (C=O). H NMR (400 MHz, DMSO-
1
6-Fluorobenzo[d]thiazol-2-amine (3d)
d6): d 8.39 (br s, 1H, –NH), 7.55 (d, J = 7.3 Hz, 2H), 7.38
(d, J = 7.4 Hz, 2H), 4.22 (s, 2H, –CH2–Cl), 3.97 (q,
J = 5.9 Hz, 2H, –O–CH2–), 1.49 (t, J = 5.8 Hz, 3H,
–CH3).
Off while solid, yield: 80 %, mp 181–184 °C. IR (KBr,
cm-1): 3,368 (NH), 1,584 (C=N). 1H NMR (400 MHz,
DMSO-d6): d 7.59 (dd, J = 2.4, 8.3 Hz, 1H, H-7), 7.34
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6-Methoxybenzo[d]thiazol-2-amine (3i)
(dd, J = 4.8, 8.6 Hz, 1H, H-4), 7.16 (dt, J = 2.2, 8.8 Hz,
1H, H-5), 7.09 (s, 2H, NH2, D2O exchangeable).
Off white solid, yield: 71 %, mp 169–170 °C. IR (KBr,
cm-1): 3,278 (NH), 1,571 (C=N). 1H NMR (400 MHz,
DMSO-d6): d 7.62–7.53 (m, 2H, H-4, H-7), 7.20 (dd,
J = 8.2, 1.9 Hz, 1H, H-5), 5.42 (s, 2H, NH2, D2O
exchangeable), 3.80 (s, 3H, OCH3).
6-Iodobenzo[d]thiazol-2-amine (3e)
Off while solid, yield: 70 %, mp 206–208 °C. IR (KBr,
cm-1): 3,290 (NH), 1,573 (C=N). 1H NMR (400 MHz,
DMSO-d6): d 7.79 (d, J = 1.9 Hz, 1H, H-7), 7.51 (d,
J = 7.1 Hz, 1H, H-4), 7.16 (dd, J = 8.1, 2.2 Hz, 1H, H-5),
6.92 (s, 2H, NH2, D2O exchangeable).
6-Ethoxybenzo[d]thiazol-2-amine (3j)
Off white solid, yield: 74 %, mp 162–165 °C. IR (KBr,
cm-1): 3,390 (NH), 1,585 (C=N). 1H NMR (400 MHz,
DMSO-d6): d 7.53–7.37 (m, 2H, H-4, H-7), 7.27 (dd,
J = 8.6, 1.6 Hz, 1H, H-5), 5.53 (s, 2H, NH2, D2O
exchangeable), 4.08 (q, J = 6.2 Hz, 2H, OCH2CH3), 2.21
(t, J = 6.4 Hz, 3H, CH2CH3).
6-Nitrobenzo[d]thiazol-2-amine (3f)
Yellow solid, yield: 74 %, mp 248–251 °C. IR (KBr,
cm-1): 3,402 (NH), 1,569 (C=N). 1H NMR (400 MHz,
DMSO-d6): d 7.62 (d, J = 1.9 Hz, 1H, H-7), 7.29–7.37
(m, 2H, H-4 and H-5), 6.22 (s, 2H, NH2, D2O
exchangeable).
N-(2-Aminobenzo[d]thiazol-6-yl)acetamide (3k)
While solid, yield: 61 %, mp 141–143 °C. IR (KBr, cm-1):
3,401 (NH), 1,573 (C=N). H NMR (400 MHz, DMSO-
1
2-Aminobenzo[d]thiazole-6-carbonitrile (3g)
d6): d 8.13 (s, 1H, –NH), 7.67–7.54 (m, 2H, H-4, H-7), 7.30
(dd, J = 8.6, 1.6 Hz, 1H, H-5), 7.15 (s, 2H, NH2, D2O
exchangeable), 2.04 (s, 3H, CH3).
A mixture of 0.1 mol of 4-aminobenzonitrile and 0.2 mol
of potassium thiocyanate (KSCN) in 100 mL glacial acetic
acid was cooled in an ice bath and stirred for 10–20 min,
and then 0.1 mol of bromine in glacial acetic acid was
added dropwise at such a rate to keep the temperature
below 10 °C throughout the addition. The reaction mixture
was stirred at room temperature for 2–4 h, the HBr salt thus
separated out. In the reaction mixture 150 mL water was
added. The solid 4-cyano-2-thiocyanatoaniline thus
obtained was filtered, dried and recrystallized from ethanol.
Then in 0.05 mol of this intermediate, 10 mL of concen-
trated HCl and 20 mL of water was added and the mixture
was refluxed for 2 h. The progress of the reaction was
monitored by thin layer chromatography using toluene–
acetone (9:1) solvent system. After the completion of the
reaction, the solution was cooled and the product was fil-
tered, washed with water and recrystallized from ethanol to
give 3g (Sawhney and Boykin, 1979). Light yellow solid,
yield: 61 %, mp 206–207 °C. IR (KBr, cm-1): 3,376 (NH),
1,576 (C=N). 1H NMR (400 MHz, DMSO-d6): d ppm, 7.70
(d, J = 1.8 Hz, 1H, H-7), 7.55 (d, J = 7.4 Hz, 1H, H-4),
7.26 (dd, J = 8.4, 2.0 Hz, 1H, H-5), 7.18 (s, 2H, NH2, D2O
exchangeable).
General synthetic procedure for N-(benzo[d]thiazol-2-
yl)-2-chloroacetamides (4a–k)
Chloroacetyl chloride (0.06 mol) was added dropwise to a
mixture of the appropriate 2-amino-6-substituted benzo-
thiazole (3a–k) (0.05 mol) and K2CO3 (0.06 mol) in ben-
zene (50 mL) at room temperature. The reaction mixture
was refluxed for 6–12 h, then, after cooling to room tem-
perature, it was slowly poured into 100 mL of ice water. A
solid was formed thereafter. The precipitate was separated
by filtration and washed successively with water. The
product was dried under vacuum to obtain 4a–k. The
progress of the reaction was monitored by thin layer
chromatography using toluene–acetone (8:2) solvent
system.
N-(Benzo[d]thiazol-2-yl)-2-chloroacetamide (4a)
Light brown solid, yield: 73 %, mp 135–138 °C. IR (KBr,
cm-1): 3,283 (NH), 1,679 (CO). 1H NMR (400 MHz,
DMSO-d6): d ppm, 8.43 (s, 1H, –NH), 7.19–7.38 (m, 4H,
Ar–H), 4.23 (s, 2H, CH2–Cl).
6-Methylbenzo[d]thiazol-2-amine (3h)
2-Chloro-N-(6-chlorobenzo[d]thiazol-2-yl)acetamide (4b)
While solid, yield: 88 %, mp 207–210 °C. IR (KBr, cm-1):
3,290 (NH), 1,689 (CO). H NMR (400 MHz, DMSO-d6):
Light yellow solid, yield: 77 %, mp 144–145 °C. IR (KBr,
cm-1): 3,393 (NH), 1,570 (C=N). 1H NMR (400 MHz,
DMSO-d6): d 7.53–7.39 (m, 3H, H-4, H-5, H-7), 5.89
(s, 2H, NH2, D2O exchangeable), 1.96 (s, 3H, CH3).
1
d 8.56 (s, 1H, –NH), 7.64 (d, J = 1.6 Hz, 1H, H-7), 7.45
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Med Chem Res (2013) 22:195–210
199
(d, J = 7.6 Hz, 1H, H-4), 7.27 (dd, J = 8.1, 1.8 Hz, 1H,
H-5), 4.17 (s, 2H, –CH2–Cl).
DMSO-d6): d 8.55 (s, 1H, NH), 7.71–7.64 (m, 2H, H-4, H-
7), 7.33 (dd, J = 8.3, 2.0 Hz, 1H, H-5), 4.24 (s, 2H, –CH2–
Cl), 3.80 (s, 3H, OCH3).
N-(6-Bromobenzo[d]thiazol-2-yl)-2-chloroacetamide (4c)
2-Chloro-N-(6-ethoxybenzo[d]thiazol-2-yl)acetamide (4j)
Light yellow solid, yield: 72 %, mp 195–197 °C. IR (KBr,
cm-1): 3,310 (NH), 1,671 (CO). 1H NMR (400 MHz,
DMSO-d6): d 8.43 (s, 1H, –NH), 7.60 (d, J = 1.7 Hz, 1H, H-
7), 7.35–7.43 (m, 2H, H-4 and H-5), 4.06 (s, 2H, –CH2–Cl).
Off white solid, yield: 84 %, mp 179–181 °C. IR (KBr,
cm-1): 3,287 (NH), 1,679 (CO). 1H NMR (400 MHz,
DMSO-d6): d 8.40 (s, 1H, NH), 7.60–7.47 (m, 2H, H-4, H-
7), 7.34 (dd, J = 8.5, 1.8 Hz, 1H, H-5), 4.26 (s, 2H, –CH2–
Cl), 4.18 (q, J = 5.8 Hz, 2H, OCH2CH3), 2.19 (t,
J = 6.7 Hz, 3H, CH2CH3).
2-Chloro-N-(6-fluorobenzo[d]thiazol-2-yl)acetamide (4d)
Off while solid, yield: 60 %, mp 177–179 °C. IR (KBr,
cm-1): 3,277 (NH), 1,665 (CO). 1H NMR (400 MHz,
DMSO-d6): d 8.39 (s, 1H, –NH), 7.70 (dd, J = 2.3, 8.5 Hz,
1H, H-7), 7.38 (dd, J = 4.5, 8.2 Hz, 1H, H-4), 7.23 (dt,
J = 2.4, 8.9 Hz, 1H, H-5), 4.25 (s, 2H, –CH2–Cl).
N-(6-Acetamidobenzo[d]thiazol-2-yl)-2-chloroacetamide
(4k)
White solid, yield: 52 %, mp 227–229 °C. IR (KBr, cm-1):
3,277 (NH), 1,670 (CO). H NMR (400 MHz, DMSO-d6):
1
2-Chloro-N-(6-iodobenzo[d]thiazol-2-yl)acetamide (4e)
d 8.57 (s, 1H, NH), 8.33 (s, 1H, –NH), 7.62–7.51 (m, 2H,
H-4, H-7), 7.27 (dd, J = 8.4, 1.9 Hz, 1H, H-5), 4.21 (s, 2H,
–CH2–Cl), 2.04 (s, 3H, CH3).
Off while solid, yield: 79 %, mp 197–200 °C. IR (KBr,
cm-1): 3,256 (NH), 1,680 (CO). 1H NMR (400 MHz,
DMSO-d6): d 8.46 (s, 1H, –NH), 7.58 (d, J = 2.2 Hz, 1H,
H-7), 7.45 (d, J = 7.4 Hz, 1H, H-4), 7.29 (dd, J = 8.5,
1.9 Hz, 1H, H-5), 4.16 (s, 2H, –CH2–Cl).
Synthesis of ethyl 2-oxo-2H-chromene-3-carboxylate (2)
Salicylaldehyde (1) (10 g, 0.045 mol) and diethylmalonate
(7.34 g, 0.045 mol) were dissolved in ethanol (150 mL) to
give clear solution. Piperidine (18 mL) was added and the
mixture was refluxed for 5 h. After the completion of the
reaction (monitored by thin layer chromatography in tolu-
ene–ethyl acetate solvent system), the content was con-
centrated to small volume. Then the reaction mixture was
poured onto crushed ice and the resulted solid was filtered,
dried and recrystallized from ethanol to afford (3) as a
white solid, yield: 86 %, mp 124–125 °C. IR (KBr, cm-1):
1,745 (C=O, ester), 1,725 (CO, coumarin), 1,677 (C=O),
1,231 (C–O). 1H NMR (400 MHz, DMSO-d6): d ppm, 8.27
(1H, s, H-4, coumarin), 7.52–7.29 (4H, m, Ar–H), 3.56 (q,
J = 5.9 Hz, 2H, –COOCH2–), 1.29 (t, J = 6.6 Hz, 3H,
–COOCH2CH3).
2-Chloro-N-(6-nitrobenzo[d]thiazol-2-yl)acetamide (4f)
Yellow solid, yield: 78 %, mp 218–220 °C. IR (KBr, cm-1):
3,278 (NH), 1,671 (CO). 1H NMR (400 MHz, DMSO-d6): d
8.55 (s, 1H, –NH), 7.60 (d, J = 1.8 Hz, 1H, H-7), 7.35–7.46
(m, 2H, H-4 and H-5), 4.28 (s, 2H, –CH2–Cl).
2-Chloro-N-(6-cyanobenzo[d]thiazol-2-yl)acetamide (4g)
Light yellow solid, yield: 69 %, mp 225–227 °C. IR (KBr,
cm-1): 3,298 (NH), 1,685 (CO). 1H NMR (400 MHz,
DMSO-d6): d 8.37 (s, 1H, NH), 7.55 (d, J = 1.6 Hz, 1H,
H-7), 7.43 (d, J = 7.7 Hz, 1H, H-4), 7.18 (dd, J = 8.6,
2.1 Hz, 1H, H-5), 4.20 (s, 2H, –CH2–Cl).
Synthesis of 2-oxo-2H-chromene-3-carbohydrazide (3)
2-Chloro-N-(6-methylbenzo[d]thiazol-2-yl)acetamide (4h)
2-Oxo-2H-chromene-3-carboxylate (5.0 g, 0.023 mol) and
hydrazine hydrate 99 % (1.15 g, 0.023 mol) were dis-
solved in ethanol (100 mL) to give clear solution and
refluxed for 10 h. The content was concentrated to half of
the volume and allowed to cool. The solid mass thus
obtained (reaction was monitored by thin layer chroma-
tography in toluene–acetone solvent system) on cooling
was retained by filtering and washed with small amount of
ice–cooled ethanol to afford 3 as white solid (Bhat et al.,
2008), yield: 88 %, mp 137–140 °C. IR (KBr, cm-1):
3,385, 3,290 (NHs), 1,721 (CO, coumarin), 1,687 (C=O,
Light yellow solid, yield: 53 %, mp 175–177 °C. IR (KBr,
cm-1): 3,311 (NH), 1,673 (CO). 1H NMR (400 MHz,
DMSO-d6): d 8.45 (s, 1H, NH), 7.63–7.46 (m, 3H, H-4,
H-5, H-7), 4.20 (s, 2H, –CH2–Cl), 1.81 (s, 3H, CH3).
2-Chloro-N-(6-methoxybenzo[d]thiazol-2-yl)acetamide
(4i)
Off white solid, yield: 75 %, mp 169–172 °C. IR (KBr,
cm-1): 3,291 (NH), 1,667 (CO). 1H NMR (400 MHz,
123

200
Med Chem Res (2013) 22:195–210
amide). 1H NMR (400 MHz, DMSO-d6): d ppm, 8.24 (1H,
s, H-4, coumarin), 8.12 (m, 1H, CONHNH2), 7.62–7.39
(4H, m, Ar–H), 4.78 (s, 2H, NH2, D2O exchangeable).
146.17, 143.13, 141.87, 138.90, 136.45, 133.99, 131.24,
128.78, 125.93, 123.59, 122.21, 119,85, 116.77, 114.89
(14C, Ar–C), 37.13 (1C, S-CH2). Anal. Calcd for
C19H13N3O4S: C 60.15, H 3.45, N 11.08; found: C 59.97,
H 3.59, N 10.94).
Synthesis of 3-(5-mercapto-1,3,4-oxadiazol-2-yl)-2H-
chromen-2-one (4)
N-(4-Chloro-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5b)
To a solution of the 2-oxo-2H-chromene-3-carbohydrazide,
3 (2.5 g, 0.01 mol) in ethanol (50 mL) at 0 °C, carbon
disulphide (0.01 mol) and potassium hydroxide (0.01 mol)
were added and the reaction mixture was refluxed until the
evolution of H2S gas ceased. Excess solvents were evap-
orated under reduced pressure and the residue was dis-
solved in water and then acidified with dilute hydrochloric
acid (10 %) to pH 6. The precipitate was filtered off, dried
and crystallized from ethanol to give 4 (Patel et al., 2010).
The completion of reaction was monitored by thin layer
chromatography in n-hexane–ethyl acetate (8:2) solvent
system. Yield 72 %, mp 166–168 °C. IR (KBr, cm-1):
2,591 (SH), 1,726 (CO, coumarin), 1,629, 1,531 (2C=N,
oxadiazole), 1,043 (C–O–C, oxadiazole). 1H NMR
(400 MHz, DMSO-d6): d ppm, 14.51 (br s, 1H, SH), 8.21
(1H, s, H-4, coumarin), 7.58–7.37 (m, 4H, Ar–H).
Yellow solid, yield 66 %, mp 265–266 °C. IR (KBr,
cm-1): 3,311 (NH), 1,718 (CO, coumarin), 1,681 (C=O),
1,623, 1,524 (2C=N, oxadiazole), 1,075 (C–O–C, oxadi-
1
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.51 (s,
1H, –NH), 8.29 (s, 1H, H-4, coumarin), 7.82 (dd, J = 7.8,
1.3 Hz, 1H), 7.63–7.54 (m, 3H, Ar–H), 7.43–7.28 (m, 4H),
4.23 (s, 2H, S-CH2); 13C NMR (100 MHz, DMSO-d6): d
ppm, 172.20 (1C, C-2, oxadiazole), 168.22 (1C, C=O),
163.90 (1C, C-5, oxadiazole), 162.39 (1C, C=O, couma-
rin), 151.24, 148.70, 147.62, 145.91, 142.79, 139.17,
136.98, 135.02, 133.20, 130.38, 127.49, 124.33, 120.68,
118.77 (14C, Ar–C), 31.79 (1C, S-CH2). Anal. Calcd for
C19H12ClN3O4S: C 55.14, H 2.92, N 10.15; found: C
55.29, H 2.78, N 9.96.
General synthetic procedure for 2-(5-2-oxo-2H-
chromen-3-yl)-1,3,4-oxadiazol-2-ylthio)-N-
phenylacetamides (5a–k)
N-(4-Bromo-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5c)
Yellow solid, yield 71 %, mp 255–257 °C. IR (KBr,
cm-1): 3,294 (NH), 1,726 (CO, coumarin), 1,677 (C=O),
1,621, 1,544 (2C=N, oxadiazole), 1,068 (C–O–C, oxadi-
To a 0.05 mol of 3-(5-mercapto-1,3,4-oxadiazol-2-yl)-2H-
chromen-2-one (4) in 25 mL of acetone, 0.05 mol of 2-
chloro-N-(substituted)phenyl acetamide (2a–k) in acetone
and 0.05 mol of K2CO3 was added and the reaction mix-
ture was refluxed for 6–12 h, then, after cooling to room
temperature, it was slowly poured into 100 mL of ice water
and the resulting solid was separated by filtration and
washed successively with water. The product was dried
under vacuum to obtain 5a–k (Amir et al., 2011). The
progress of the reaction was monitored by thin layer
chromatography using n-hexane–ethyl acetate (8:2) solvent
system.
1
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.44 (s,
1H, –NH), 8.25 (s, 1H, H-4, coumarin), 7.83 (dd, J = 7.7,
1.6 Hz, 1H), 7.67–7.55 (m, 3H, Ar–H), 7.46–7.34 (m, 2H),
4.26 (s, 2H, S-CH2), 3.74 (s, 2H, –N–CH2); 13C NMR
(100 MHz, DMSO-d6): d ppm, 169.30 (1C, C-2, oxadiaz-
ole), 166.37 (1C, C=O), 164.02 (1C, C-5, oxadiazole),
163.11 (1C, C=O, coumarin), 148.92, 146.77, 145.90,
144.27, 142.33, 139.88, 137.65, 134.99, 132.14, 129.83,
126.73, 124.32, 123.10, 120.93 (14C, Ar–C), 33.22 (1C, S-
CH2). Anal. Calcd for C19H12BrN3O4S: C 49.80, H 2.64, N
9.17; found: C 49.94, H 2.72, N 9.11.
2-(5-(2-Oxo-2H-chromen-3-yl)-1,3,4-oxadiazol-2-ylthio)-
N-phenylacetamide (5a)
N-(4-Fluoro-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5d)
Yellow solid, yield 67 %, mp 258–260 °C. IR (KBr,
cm-1): 3,308 (NH), 1,723 (CO, coumarin), 1,688 (C=O),
1,630, 1,534 (2C=N, oxadiazole), 1,062 (C–O–C, oxadi-
Yellow solid, yield 68 %, mp 270–272 °C. IR (KBr,
cm-1): 3,311 (NH), 1,729 (CO, coumarin), 1,680 (C=O),
1,630, 1,531 (2C=N, oxadiazole), 1,060 (C–O–C, oxadi-
1
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.42 (s,
1
1H, –NH), 8.21 (s, 1H, H-4, coumarin), 7.59–7.31 (m, 6H,
Ar–H), 4.32 (s, 2H, S-CH2), 7.19 (d, J = 14.6 Hz, 1H),
4.10 (s, 2H); 13C NMR (100 MHz, DMSO-d6): d ppm,
170.24 (1C, C-2, oxadiazole), 167.82 (1C, C=O), 164.99
(1C, C-5, oxadiazole), 161.87 (1C, C=O, coumarin),
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.40 (s,
1H, –NH), 8.29 (s, 1H, H-4, coumarin), 7.79 (dd, J = 7.3,
1.8 Hz, 1H), 7.62–7.28 (m, 5H, Ar–H), 7.16 (t, J = 7.2 Hz,
2H), 4.20 (s, 2H, S-CH2); 13C NMR (100 MHz, DMSO-
d6): d ppm, 171.16 (1C, C-2, oxadiazole), 166.29 (1C,
123

Med Chem Res (2013) 22:195–210
201
C=O), 163.73 (1C, C-5, oxadiazole), 161.14 (1C, C=O,
coumarin), 153.28, 150.37, 157.88, 146.04, 143.66, 141.27,
138.73, 135.98, 133.22, 129.74, 126.90, 124.23, 121.93,
118.92 (14C, Ar–C), 36.61 (1C, S-CH2). Anal. Calcd for
C19H12FN3O4S: C 57.43, H 3.04, N 10.57; found: C 57.31,
H 2.97, N 10.43.
coumarin), 148.19, 147.81, 145.17, 143.55, 141.78, 137.68,
136.02, 133.42, 130.28, 126.77, 124.35, 121.79, 117.69
(13C, Ar–C), 104.98 (1C, C:N), 96.91 (1C, –C–C:N),
33.90 (1C, S-CH2). Anal. Calcd for C20H12N4O4S: C
59.40, H 2.99, N 13.85; found: C 59.53, H 3.07, N 13.71.
2-[5-(2-Oxo-2H–2H-chromen-3-yl)-[1,3,4]oxadiazol-2-
ylsulfanyl]-N-p-tolyl-acetamide (5h)
N-(4-Iodo-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5e)
Light yellow solid, yield 74 %, mp 254–255 °C. IR (KBr,
cm-1): 3,321 (NH), 1,727 (CO, coumarin), 1,675 (C=O),
1,636, 1,541 (2C=N, oxadiazole), 1,070 (C–O–C, oxadi-
Light yellow solid, yield 65 %, mp 248–249 °C. IR (KBr,
cm-1): 3,308 (NH), 1,731 (CO, coumarin), 1,683 (C=O),
1,628, 1,538 (2C=N, oxadiazole), 1,063 (C–O–C, oxadi-
1
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.55 (s,
1
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.53 (s,
1H, –NH), 8.19 (s, 1H, H-4, coumarin), 7.67–7.26 (m, 6H,
1H, –NH), 8.35 (s, 1H, H-4, coumarin), 7.70–7.32 (m, 8H,
Ar–H), 4.20 (s, 2H, S-CH2), 1.33 (s, 3H, CH3); 13C NMR
(100 MHz, DMSO-d6): d ppm, 172.56 (1C, C-2, oxadiaz-
ole), 168.27 (1C, C=O), 163.88 (1C, C-5, oxadiazole),
161.99 (1C, C=O, coumarin), 150.92, 148.70, 147.32,
144.58, 142.31, 138.70, 134.88, 131.28, 129.83, 125.69,
124.10, 121.28, 118.93, 115.77 (14C, Ar–C), 33.98 (1C, S-
CH2), 19.87 (1C, CH3). Anal. Calcd for C20H15N3O4S: C
61.06, H 3.84, N 10.68; found: C 61.21, H 3.70, N 10.53.
Ar–H), 7.18 (d, J = 8.1 Hz, 2H), 4.27 (s, 2H, S-CH2); 13
C
NMR (100 MHz, DMSO-d6): d ppm, 168.96 (1C, C-2,
oxadiazole), 167.09 (1C, C=O), 163.36 (1C, C-5, oxadi-
azole), 162.56 (1C, C=O, coumarin), 146.12, 144.87,
143.04, 141.93, 137.99, 136.07, 134.16, 130.82, 128.36,
125.97, 123.23, 120.94, 117.65, 115.79 (14C, Ar–C), 34.09
(1C, S-CH2). Anal. Calcd for C19H12IN3O4S: C 45.16, H
2.39, N 8.32; found C 45.03, H 2.49, N 8.21,
N-(4-Methoxy-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5i)
N-(4-Nitro-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5f)
Yellow solid, mp 252–255 °C, yield 69 %. IR (KBr,
cm-1): 3,298 (NH), 1,727 (CO, coumarin), 1,683 (C=O),
1,633, 1,530 (2C=N, oxadiazole), 1,067 (C–O–C, oxadi-
Yellow solid, yield 61 %, mp 278–280 °C. IR (KBr,
cm-1): 3,273 (NH), 1,727 (CO, coumarin), 1,680 (C=O),
1,637, 1,547 (2C=N, oxadiazole), 1,087 (C–O–C, oxadi-
1
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.57 (s,
1
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.47 (s,
1H, –NH), 8.32 (s, 1H, H-4, coumarin), 7.77 (d,
J = 7.1 Hz, 2H), 7.57–7.30 (m, 4H, Ar–H), 6.93 (d,
J = 7.4 Hz, 2H), 4.23 (s, 2H, S-CH2), 3.86 (s, 3H,
–OCH3); 13C NMR (100 MHz, DMSO-d6): d ppm, 169.66
(1C, C-2, oxadiazole), 168.13 (1C, C=O), 163.80 (1C, C-5,
oxadiazole), 161.92 (1C, C=O, coumarin), 147.67, 146.13,
143.87, 142.13, 140.72, 136.89, 135.60, 132.17, 129.99,
127.65, 126.27, 124.70, 122.27, 119.57 (14C, Ar–C), 55.76
(1C, OCH3), 35.80 (1C, S-CH2). Anal. Calcd for
C20H15N3O5S: C 58.67, H 3.69, N 10.26; found: C 58.78,
H 3.61, N 10.37.
1H, –NH), 8.22 (s, 1H, H-4, coumarin), 7.74 (d,
J = 7.6 Hz, 2H), 7.58–7.26 (m, 6H, Ar–H), 4.23 (s, 2H, S-
CH2); 13C NMR (100 MHz, DMSO-d6): d ppm, 169.24
(1C, C-2, oxadiazole), 168.11 (1C, C=O), 164.73 (1C, C-5,
oxadiazole), 163.01 (1C, C=O, coumarin), 146.23, 144.32,
143.76, 140.93, 137.88, 136.12, 133.56, 131.82, 128.77,
126.21, 125.10, 122.94, 120.32, 119.26 (14C, Ar–C), 36.05
(1C, S-CH2). Anal. Calcd for C19H12N4O6S: C 53.77, H
2.85, N 13.20; found: C 53.64, H 2.89, N 13.09.
N-(4-Cyano-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5g)
N-(4-Ethoxy-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5j)
Yellow solid, yield 67 %, mp 243–245 °C. IR (KBr,
cm-1): 3,279 (NH), 1,717 (CO, coumarin), 1675 (C=O),
1621, 1531 (2C=N, oxadiazole), 1067 (C–O–C, oxadiaz-
Yellow solid, yield 63 %, mp 260–262 °C. IR (KBr,
cm-1): 3,289 (NH), 1,729 (CO, coumarin), 1,680 (C=O),
1,632, 1,539 (2C=N, oxadiazole), 1,057 (C–O–C, oxadi-
1
ole). H NMR (400 MHz, DMSO-d6): d ppm, 8.39 (s, 1H,
1
–NH), 8.30 (s, 1H, H-4, coumarin), 7.78 (d, J = 7.3 Hz,
2H), 7.66 (d, J = 7.4 Hz, 2H), 7.53–7.27 (m, 4H, Ar–H),
4.26 (s, 2H, S-CH2); 13C NMR (100 MHz, DMSO-d6): d
ppm, 170.79 (1C, C-2, oxadiazole), 166.87 (1C, C=O),
165.21 (1C, C-5, oxadiazole), 162.29 (1C, C=O,
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.52 (s,
1H, –NH), 8.26 (s, 1H, H-4, coumarin), 7.80 (d,
J = 7.3 Hz, 2H), 7.62–7.29 (m, 4H, Ar–H), 6.88 (d,
J = 7.8 Hz, 2H), 4.17 (s, 2H, S-CH2), 3.90 (q, J = 6.7 Hz,
2H, –O–CH2–), 1.23 (t, J = 5.7 Hz, 3H, CH3); 13C NMR
123

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Med Chem Res (2013) 22:195–210
(100 MHz, DMSO-d6): d ppm, 170.24 (1C, C-2, oxadiaz-
ole), 167.80 (1C, C=O), 164.16 (1C, C-5, oxadiazole),
162.54 (1C, C=O, coumarin), 149.87, 147.69, 146.27,
143.88, 140.24, 137.66, 136.71, 133.23, 132.98, 128.99,
127.53, 124.33, 121.93, 118.77 (14C, Ar–C), 65.61 (1C,
OCH2CH3), 35.32 (1C, S-CH2), 22.31 (1C, OCH2CH3).
Anal. Calcd for C21H17N3O5S: C 59.57, H 4.05, N 9.92;
found: C 59.71, H 3.91, N 10.04.
13C NMR (100 MHz, DMSO-d6): d ppm, 170.24 (1C, C-2,
oxadiazole), 167.82 (1C, C=O), 164.99 (1C, C-5, oxadi-
azole), 161.87 (1C, C=O, coumarin), 155.98 (1C, C-2,
benzothiazole), 148.92, 146.34, 143.67, 140.96, 138.65,
137.03, 134.99, 131.26, 128.60, 126.63, 125.55, 123.20,
120.12, 119.99 (14C, Ar–C), 36.16 (1C, S-CH2). Anal.
Calcd for C20H12N4O4S2: C 55.04, H 2.77, N 12.84; found:
C 55.22, H 2.59, N 12.96.
N-(4-Acetylamino-phenyl)-2-[5-(2-oxo-2H-chromen-3-yl)-
[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (5k)
N-(6-Chloro-benzothiazol-2-yl)-2-[5-(2-oxo-2H-chromen-
3-yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (6b)
Yellow solid, yield 65 %, mp 239–241 °C. IR (KBr,
cm-1): 3,299 (NH), 1,727 (CO, coumarin), 1,685 (C=O),
1,623, 1,530 (2C=N, oxadiazole), 1,072 (C–O–C, oxadi-
Yellow solid, yield 74 %, mp 261–263 °C. IR (KBr,
cm-1): 3,285 (NH), 1,728 (CO, coumarin), 1,673 (C=O),
1,657 (C=N, benzothiazole), 1,635, 1,530 (2C=N, oxadi-
1
1
azole). H NMR (400 MHz, DMSO-d6): d ppm, 8.59 (s,
azole), 1,073 (C–O–C, oxadiazole). H NMR (400 MHz,
1H, –NH), 8.42 (s, 1H, –NH), 8.22 (s, 1H, H-4, coumarin),
7.73 (d, J = 7.6 Hz, 2H), 7.62–7.28 (m, 6H, Ar–H), 4.26
(s, 2H, S-CH2), 1.86 (s, 3H, –CH3); 13C NMR (100 MHz,
DMSO-d6): d ppm, 169.82 (1C, C-2, oxadiazole), 167.54,
166.44 (2C, C=O), 163.90 (1C, C-5, oxadiazole), 161.81
(1C, C=O, coumarin), 146.50, 144.32, 143.91, 141.27,
137.89, 136.15, 133.49, 131.92, 129.74, 126.55, 124.32,
123.29, 121.75, 118.73 (14C, Ar–C), 34.33 (1C, S-CH2),
19.65 (1C, CH3). Anal. Calcd for C21H16N4O5S: C 57.79,
H 3.70, N 12.84; found: C 57.89, H 3.61, N 12.69.
DMSO-d6): d ppm, 8.59 (s, 1H, –NH), 8.22 (s, 1H, H-4,
coumarin), 7.82 (d, J = 1.7 Hz, 1H), 7.53 (d, J = 7.3 Hz,
1H), 7.41 (dd, J = 7.5, 1.6 Hz, 1H), 7.34–7.17 (m, 4H, Ar–
H), 4.31 (s, 2H, S-CH2); 13C NMR (100 MHz, DMSO-d6):
d ppm, 171.83 (1C, C-2, oxadiazole), 169.18 (1C, C=O),
165.73 (1C, C-5, oxadiazole), 162.19 (1C, C=O, couma-
rin), 153.61 (1C, C-2, benzothiazole), 148.79, 146.21,
145.03, 143.29, 140.82, 137.65, 135.34, 131.95, 128.74,
127.52, 124.89, 121.86, 119.26, 117.60 (14C, Ar–C), 36.22
(1C, S-CH2). Anal. Calcd for C20H11ClN4O4S2: C 51.01, H
2.35, N 11.90; found: C 50.87, H 2.49, N 11.76.
General synthetic procedure for N-(benzo[d]thiazol-2-
yl)-2-(5-(2-oxo-2H-chromen-3-yl)-1,3,4-oxadiazol-2-
ylthio)acetamides (6a–k)
N-(6-Bromo-benzothiazol-2-yl)-2-[5-(2-oxo-2H-chromen-
3-yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (6c)
To a 0.05 mol of 3-(5-mercapto-1,3,4-oxadiazol-2-yl)-2H-
chromen-2-one (4) in 25 mL of acetone, 0.05 mol of N-
(benzo[d]thiazol-2-yl)-2-chloroacetamides (4a–k) in ace-
tone and 0.05 mol of K2CO3 was added and the reaction
mixture was refluxed for 8–15 h, then, after cooling to
room temperature, it was slowly poured into 100 mL of ice
water and the resulting solid was separated by filtration and
washed successively with water. The product was dried
under vacuum to obtain 6a–k (Akhtar et al., 2008). The
progress of the reaction was monitored by thin layer
chromatography using n-hexane–ethyl acetate (8:2) solvent
system.
Yellow solid, yield 71 %, mp 256–258 °C. IR (KBr,
cm-1): 3,313 (NH), 1,719 (CO, coumarin), 1,675 (C=O),
1,661 (C=N, benzothiazole), 1,631, 1,524 (2C=N, oxadi-
1
azole), 1,065 (C–O–C, oxadiazole). H NMR (400 MHz,
DMSO-d6): d ppm, 8.51 (s, 1H, –NH), 8.26 (s, 1H, H-4,
coumarin), 7.73 (d, J = 1.6 Hz, 1H), 7.56–7.19 (m, 6H,
Ar–H), 4.32 (s, 2H, S-CH2); 13C NMR (100 MHz, DMSO-
d6): d ppm, 169.82 (1C, C-2, oxadiazole), 166.69 (1C,
C=O), 165.03 (1C, C-5, oxadiazole), 163.10 (1C, C=O,
coumarin), 153.22 (1C, C-2, benzothiazole), 146.60,
144.32, 143.20, 140.92, 137.89, 136.02, 133.85, 131.92,
128.76, 126.45, 124.39, 121.56, 117.69, 116.29 (14C, Ar–
C), 37.11 (1C, S-CH2). Anal. Calcd for C20H11BrN4O4S2:
C 46.61, H 2.15, N 10.87; found: C 46.47, H 2.22, N 10.77.
N-(Benzo[d]thiazol-2-yl)-2-(5-(2-oxo-2H-chromen-3-yl)-
1,3,4-oxadiazol-2-ylthio)acetamide (6a)
N-(6-Fluoro-benzothiazol-2-yl)-2-[5-(2-oxo-2H-chromen-
3-yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (6d)
Yellow solid, yield 68 %, mp 254–256 °C. IR (KBr,
cm-1): 3,321 (NH), 1,723 (CO, coumarin), 1,682 (C=O),
1,654 (C=N, benzothiazole), 1,626, 1,533 (2C=N, oxadi-
Yellow solid, yield 62 %, mp 274–275 °C. IR (KBr,
cm-1): 3,286 (NH), 1,727 (CO, coumarin), 1,680 (C=O),
1,659 (C=N, benzothiazole), 1,629, 1,527 (2C=N, oxadi-
1
azole), 1,081 (C–O–C, oxadiazole). H NMR (400 MHz,
DMSO-d6): d ppm, 8.53 (s, 1H, –NH), 8.19 (s, 1H, H-4,
coumarin), 7.66–7.21 (m, 8H, Ar–H), 4.24 (s, 2H, S-CH2);
1
azole), 1,073 (C–O–C, oxadiazole). H NMR (400 MHz,
123

Med Chem Res (2013) 22:195–210
203
N-(6-Cyano-benzothiazol-2-yl)-2-[5-(2-oxo-2H–2H-
chromen-3-yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide
(6g)
DMSO-d6): d ppm, 8.48 (s, 1H, –NH), 8.25 (s, 1H, H-4,
coumarin), 7.79 (dd, J = 8.2, 1.6 Hz, 1H), 7.61 (dd,
J = 2.0, 8.3 Hz, 1H), 7.42 (dd, J = 5.2, 8.1 Hz, 1H), 7.30–
7.16 (m, 4H, Ar–H), 4.30 (s, 2H, S-CH2); 13C NMR
(100 MHz, DMSO-d6): d ppm, 170.52 (1C, C-2, oxadiaz-
ole), 168.09 (1C, C=O), 166.25 (1C, C-5, oxadiazole),
162.81 (1C, C=O, coumarin), 152.51 (1C, C-2, benzothi-
azole), 145.41, 144.22, 141.93, 139.24, 136.58, 134.23,
133.28, 130.92, 128.68, 126.45, 125.20, 123.20, 120.84,
119.37 (14C, Ar–C), 35.62 (1C, S-CH2). Anal. Calcd for
C20H11FN4O4S2: C 52.86, H 2.44, N 12.33 %; found: C
52.99, H 2.53, N 12.21.
Yellow solid, mp 260–262 °C, yield 63 %. IR (KBr,
cm-1): 3,318 (NH), 1,727 (CO, coumarin), 1,688 (C=O),
1,649 (C=N, benzothiazole), 1,635, 1,541 (2C=N, oxadi-
1
azole), 1,080 (C–O–C, oxadiazole). H NMR (400 MHz,
DMSO-d6): d ppm, 8.55 (s, 1H, –NH), 8.21 (s, 1H, H-4,
coumarin), 7.84 (d, J = 1.8 Hz, 1H), 7.68 (d, J = 7.5 Hz,
1H), 7.45 (dd, J = 8.7, 1.6 Hz, 1H), 7.37–7.24 (m, 4H, Ar–
H), 4.33 (s, 2H, S-CH2); 13C NMR (100 MHz, DMSO-d6):
d ppm, 172.33 (1C, C-2, oxadiazole), 168.23 (1C, C=O),
165.47 (1C, C-5, oxadiazole), 162.92 (1C, C=O, couma-
rin), 154.19 (1C, C-2, benzothiazole), 146.23, 144.34,
141.29, 140.27, 138.76, 137.20, 134.89, 132.15, 130.26,
126.79, 125.47, 122.47, 117.89 (13C, Ar–C), 106.16 (1C,
C : N), 97.57 (1C, –C–C : N), 34.13 (1C, S-CH2).
Anal. Calcd for C21H11N5O4S2: C 54.66, H 2.40, N 15.18;
found: C 54.78, H 2.31, N 15.03.
N-(6-Iodo-benzothiazol-2-yl)-2-[5-(2-oxo-2H-chromen-3-
yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (6e)
Yellow solid, yield 69 %, mp 250–253 °C. IR (KBr,
cm-1): 3,394 (NH), 1,731 (CO, coumarin), 1,676 (C=O),
1,652 (C=N, benzothiazole), 1,621, 1,539 (2C=N, oxadi-
azole), 1,078 (C–O–C, oxadiazole). 1H. 1H NMR
(400 MHz, DMSO-d6): d ppm, 8.52 (s, 1H, –NH), 8.29 (s,
1H, H-4, coumarin), 7.86 (d, J = 2.3 Hz, 1H), 7.71 (d,
J = 7.7 Hz, 1H), 7.51 (dd, J = 8.6, 1.6 Hz, 1H), 7.38–
7.23 (m, 4H, Ar–H), 4.33 (s, 2H, S-CH2); 13C NMR
(100 MHz, DMSO-d6): d ppm, 169.37 (1C, C-2, oxadi-
azole), 166.29 (1C, C=O), 163.92 (1C, C-5, oxadiazole),
161.83 (1C, C=O, coumarin), 154.70 (1C, C-2, benzo-
thiazole), 146.24, 145.20, 142.79, 140.38, 138.75, 135.90,
132.76, 131.28, 129.05, 126.77, 125.28, 122.75, 120.34,
117.85 (14C, Ar–C), 33.84 (1C, S-CH2). Anal. Calcd for
C20H11IN4O4S2: C 42.72, H 1.97, N 9.96; found: C 42.61,
H 2.11, N 9.83.
N-(6-Methyl-benzothiazol-2-yl)-2-[5-(2-oxo-2H-chromen-
3-yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (6h)
Yellow solid, yield 73 %, mp 246–247 °C. IR (KBr,
cm-1): 3,284 (NH), 1,729 (CO, coumarin), 1,675 (C=O),
1,662 (C=N, benzothiazole), 1,630, 1,522 (2C=N, oxadi-
1
azole), 1,063 (C–O–C, oxadiazole). H NMR (400 MHz,
DMSO-d6): d ppm, 8.53 (s, 1H, –NH), 8.28 (s, 1H, H-4,
coumarin), 7.77 (d, J = 7.8 Hz, 1H), 7.64–7.23 (m, 6H,
Ar–H), 4.36 (s, 2H, S-CH2), 1.39 (s, 3H, CH3); 13C NMR
(100 MHz, DMSO-d6): d ppm, 169.56 (1C, C-2, oxadiaz-
ole), 167.27 (1C, C=O), 165.28 (1C, C-5, oxadiazole),
161.90 (1C, C=O, coumarin), 153.21 (1C, C-2, benzothi-
azole), 145.87, 143.28, 140.92, 138.66, 136.54, 135.40,
132.98, 130.28, 127.86, 124.94, 123.09, 120.83, 118.73,
116.54 (14C, Ar–C), 36.02 (1C, S-CH2), 19.80 (1C, CH3).
Anal. Calcd for C21H14N4O4S2: C 55.99, H 3.13, N 12.44;
found: C 56.14, H 2.96, N 12.31.
N-(6-Nitro-benzothiazol-2-yl)-2-[5-(2-oxo-2H-chromen-3-
yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (6f)
Yellow solid, yield 68 %, mp 291–293 °C. IR (KBr,
cm-1): 3,308 (NH), 1,726 (CO, coumarin), 1,669 (C=O),
1,655 (C=N, benzothiazole), 1,620, 1,525 (2C=N, oxadi-
1
azole), 1,072 (C–O–C, oxadiazole). H NMR (400 MHz,
N-(6-Methoxy-benzothiazol-2-yl)-2-[5-(2-oxo-2H-
chromen-3-yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide
(6i)
DMSO-d6): d ppm, 8.53 (s, 1H, –NH), 8.20 (s, 1H, H-4,
coumarin), 7.91 (d, J = 1.6 Hz, 1H), 7.74 (dd, J = 7.1,
2.1 Hz, 1H), 7.59 (d, J = 7.4 Hz, 1H), 7.43 (dd, J = 7.5,
1.4 Hz, 2H), 7.34–7.26 (m, 2H, Ar–H), 4.18 (s, 2H, S-
CH2); 13C NMR (100 MHz, DMSO-d6): d ppm, 169.24
(1C, C-2, oxadiazole), 167.59 (1C, C=O), 165.14 (1C, C-5,
oxadiazole), 162.53 (1C, C=O, coumarin), 151.98 (1C, C-
2, benzothiazole), 147.45, 146.24, 144.39, 141.89, 137.69,
135.79, 134.27, 131.93, 128.73, 125.96, 123.28, 121.73,
120.95, 117.63 (14C, Ar–C), 36.70 (1C, S-CH2). Anal.
Calcd for C20H11N5O6S2: C 49.89, H 2.30, N 14.55; found:
C 50.05, H 2.13, N 14.59.
Yellow solid, yield 67 %, mp 268–270 °C. IR (KBr, cm-1):
3,297 (NH), 1,723 (CO, coumarin), 1,688 (C=O), 1,666
(C=N, benzothiazole), 1,624, 1,539 (2C=N, oxadiazole),
1
1,060 (C–O–C, oxadiazole). H NMR (400 MHz, DMSO-
d6): d ppm, 8.62 (s, 1H, –NH), 8.29 (s, 1H, H-4, coumarin),
7.83–7.72 (m, 2H), 7.48 (dd, J = 8.5, 1.8 Hz, 1H), 7.38–
7.23 (m, 4H, Ar–H), 4.27 (s, 2H, S-CH2), 3.78 (s, 3H,
OCH3); 13C NMR (100 MHz, DMSO-d6): d ppm, 170.09
(1C, C-2, oxadiazole), 168.92 (1C, C=O), 165.15 (1C, C-5,
123

204
Med Chem Res (2013) 22:195–210
oxadiazole), 162.92 (1C, C=O, coumarin), 150.92 (1C, C-2,
benzothiazole), 146.57, 143.29, 142.39, 140.92, 138.76,
135.99, 134.20, 130.94, 127.89, 126.54, 125.32, 123.95,
120.63, 118.73 (14C, Ar–C), 56.38 (1C, OCH3), 36.19 (1C,
S-CH2). Anal. Calcd for C21H14N4O5S2: C 54.07, H 3.02, N
12.01; found: C 54.22, H 3.19, N 11.88.
(Staphylococcus aureus MTCC 96, Bacillus cereus MTCC
430, Escherichia coli MTCC 739, Pseudomonas aerugin-
osa MTCC 741, Klebsiella pneumoniae MTCC 109, Sal-
monella typhi MTCC 733, Proteus vulgaris MTCC 1771,
Shigella flexneri MTCC 1457), fungi (Aspergillus niger
MTCC 282, Aspergillus fumigatus MTCC 343, Aspergillus
clavatus MTCC 1323, Candida albicans MTCC 183) using
agar streak dilution method as well as against M. tuber-
culosis H37Rv strain using BACTEC MGIT and Lowen-
stein–Jensen MIC method as reported earlier (Hawkey and
Lewis, 1994; Anargyros et al., 1990; Isenberg, 1992; Patel
et al., 2011c, d). Ciprofloxacin and ketoconazole were used
as control drugs for antibacterial and antifungal activities,
respectively, whereas isoniazid, rifampicin, ethambutol and
pyrazinamide are used as control drugs for antituberculosis
activity.
N-(6-Ethoxy-benzothiazol-2-yl)-2-[5-(2-oxo-2H-chromen-
3-yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide (6j)
Yellow solid, yield 71 %, mp 275–277 °C. IR (KBr,
cm-1): 3,316 (NH), 1,732 (CO, coumarin), 1,673 (C=O),
1,658 (C=N, benzothiazole), 1,620, 1,541 (2C=N, oxadi-
1
azole), 1,077 (C–O–C, oxadiazole). H NMR (400 MHz,
DMSO-d6): d ppm, 8.50 (s, 1H, –NH), 8.25 (s, 1H, H-4,
coumarin), 7.79–7.67 (m, 2H), 7.57 (dd, J = 8.4, 1.9 Hz,
1H), 7.38–7.26 (m, 4H, Ar–H), 4.35 (s, 2H, S-CH2), 4.14
(q, J = 6.6 Hz, 2H, OCH2CH3), 2.21 (t, J = 6.7 Hz, 3H,
CH2CH3); 13C NMR (100 MHz, DMSO-d6): d ppm,
169.87 (1C, C-2, oxadiazole), 167.03 (1C, C=O), 164.73
(1C, C-5, oxadiazole), 161.29 (1C, C=O, coumarin),
155.12 (1C, C-2, benzothiazole), 145.46, 144.20, 140.94,
137.89, 135.69, 134.32, 133.28, 130.92, 128.97, 127.64,
125.69, 123.41, 122.94, 119.73 (14C, Ar–C), 65.19 (1C,
OCH2CH3), 35.03 (1C, S-CH2), 22.19 (1C, OCH2CH3).
Anal. Calcd for C22H16N4O5S2: C 54.99, H 3.36, N 11.66;
found: C 55.16, H 3.30, N 11.52.
Results and discussion
Scheme 1 outlines the synthetic pathway followed by the
synthesis of the title compounds 5a–k and 6a–k. The sol-
vents and reagents were used as received or were dried
prior to use as needed. Salicylaldehyde and diethylmalo-
nate were reacted in the presence of piperidine in ethanol to
form ethyl-2-oxo-2H-chromene-3-carboxylate (2) which
on treatment with 99 % hydrazine hydrate yielded 2-oxo-
2H-chromonene-3-carbohydrazide (3) in good yield. The
resulted carbohydrazide moiety was cyclized using sodium
hydroxide in the presence of carbon disulphide to furnish
the corresponding oxadiazole nucleus, 3-(5-sulfanyl-1,3,4-
oxadiazol-2-yl)-2H-chromen-2–one (4). 2-Chloro-N-phe-
nyl acetamides were synthesized according to the reported
literature (Baraldi et al., 2007). Chloroacetyl chloride was
reacted with para-(substituted) phenyl amines to give the
corresponding 2-chloro-N-(substituted)phenyl acetamides
(2a–k) and the correct synthesis was confirmed using FT–
N-(6-Acetylamino-benzothiazol-2-yl)-2-[5-(2-oxo-2H-
chromen-3-yl)-[1,3,4]oxadiazol-2-ylsulfanyl]-acetamide
(6k)
Yellow solid, yield 66 %, mp 257–259 °C. IR (KBr,
cm-1): 3,289 (NH), 1,728 (CO, coumarin), 1,682 (C=O),
1,655 (C=N, benzothiazole), 1,624, 1,530 (2C=N, oxadi-
1
azole), 1,081 (C–O–C, oxadiazole). H NMR (400 MHz,
DMSO-d6): d ppm, 8.59 (s, 1H, –NH), 8.42 (s, 1H, –NH),
8.28 (s, 1H, H-4, coumarin), 7.79 (d, J = 1.6 Hz, 2H), 7.64
(dd, J = 7.8, 1.9 Hz, 2H), 7.59 (dd, J = 8.6, 1.7 Hz, 1H),
7.40–7.31 (m, 2H, Ar–H), 4.33 (s, 2H, S-CH2), 2.19 (s, 3H,
CH3); 13C NMR (100 MHz, DMSO-d6): d ppm, 170.76
(1C, C-2, oxadiazole), 168.32, 167.84 (2C, CO), 164.90
(1C, C-5, oxadiazole), 161.82 (1C, C=O, coumarin),
154.21 (1C, C-2, benzothiazole), 146.78, 145.70, 143.25,
141.28, 140.10, 138.76, 135.48, 132.97, 130.92, 127.99,
126.25, 123.76, 120.93, 116.54 (14C, Ar–C), 36.34 (1C, S-
CH2), 19.35 (1C, CH3). Anal. Calcd for C22H15N5O5S2: C
53.54, H 3.06, N 14.19; found: C 53.40, H 2.94, N 14.04.
1
IR and H NMR spectroscopy. The 2-amino-6-substituted
benzothiazoles (3a–k) were synthesized by reacting aryl
amines with potassium thiocyanates in satisfactory yields
by a known preparation method (Wang et al., 2006; Rana
et al., 2008) and were converted to the respective N-
(benzo[d]thiazol-2-yl)-2-chloroacetamides (4a–k) using
chloroacetyl chloride in benzene solvent as described in the
literature (Turan-Zitouni et al., 2003; Amin et al., 2008).
Above-mentioned acetamide derivatives (2a–k and
4a–k) were then condensed to the 3-(5-mercapto-1,3,4-
oxadiazol-2-yl)-2H-chromen-2–one (4) in acetone solvent
at reflux temperature furnished 5a–k and 6a–k. All the IR,
1H NMR, 13C NMR spectral data of compounds 5a–k and
6a–k were in accordance with assumed structures. The
purity of the synthesized compounds was monitored by
TLC and ascertained by elemental analysis.
Methods for pharmacological activity evaluations
The synthesized derivatives (5a–k, 6a–k) were exam-
ined for antimicrobial activity against several bacteria
123

Med Chem Res (2013) 22:195–210
205
Scheme 1 Synthetic procedure
for 5a–k and 6a–k. Reagents
and conditions: (a1)
O
C
a1
R
NH2
Cl
R
NH
ClCH2COCl, anhyd. K2CO3,
acetone; (a2) ClCH2COCl,
anhyd. K2CO3, benzene; (b)
KSCN, Br2, AcOH, R.T.; (c)
piperidine, reflux, EtOH; (d)
NHNH2ꢀH2O, reflux, EtOH; (e)
CS2/KOH, reflux, EtOH; (f)
Acetone, reflux
1a-k
2a-k
b
O
C
N
S
N
a2
NH
Cl
NH2
R
S
R
4a-k
3a-k
O
C
H2
C
O
C
O
O
CHO
OH
NH
c
CH3
O
d
N
2
O
O
H
O
O
O
O
1
3
2
N N
O
e
SH
O
O
4
f
O
NHC
O
C
N
4a-k
2a-k
Cl
R
NH
Cl
S
R
N N
O
N N
O
N
H
H
S
C N
O
S
C N
R
S
R
O
O
O
O
O
6a-k
5a-k
R = H, Cl, Br, F, I, NO2, CN, CH3, OCH3, OCH2CH3, NHCOCH3
Biological activity
potent analogue against Gram-negative strain S. typhi at
25 lg/mL of MIC, whereas similar to previously mentioned
analogue 5g, analogue 6g with similar cyano functional
group incorporated to the benzothiazole ring system showed
good level of inhibitory potential against Gram-negative
K. pneumoniae at 25 lg/mL of MIC. Higher potency (MIC
3.12 lg/mL) has been observed with the analogue 6c within
the class of compounds those involving 2-chloro-N-benzo-
thiazole acetamide linkage to the 1,3,4-oxadiazole core
towards Gram-positive strain B. cereus. The said analogue
was the only one showing highest antibacterial activity with
lowest MIC value in the present bioassay. Final derivative 5j
with ethoxy functional group indicated significant activity
against Gram-negative P. aeruginosa at lowest 6.25 lg/mL
of MIC and it will be appropriate to mention here that this
was the lowest MIC level observed against the mentioned
Gram-negative strain, i.e. higher activity level. Furthermore,
an analogue 6k with acetamido functionality showed similar
inhibitory efficacy (MIC 6.25 lg/mL) against the same
bacteria P. aeruginosa. Insertion of electron-withdrawing
alkoxy functional group in the form of methoxy functionality
to the benzothiazole ting system (6i) attached to the nucleus
Investigation on antibacterial screening data (Table 1)
showed that some of the compounds showed excellent
activity against all the mentioned bacteria. From the bioassay
it can be stated that the final analogues with the substitutions
of N-benzothiazolyl-2-chloroacetamides were, in most
cases, active against all the pathogenic strains studied than
the analogues bearing 2-chloro-N-phenyl acetamides. In
addition, final analogues exhibited good antibacterial activ-
ity against Gram-positive bacteria when compared to that
against Gram-negative ones.
Final analogue 5c with 2-chloro-N-phenyl acetamide
constituent bearing electron-donating halogen atom in the
form of bromine, showed higher effectiveness against Gram-
negative strain E. coli at 12.5 lg/mL of MIC, while another
analogue (5g) from the same series bearing electron-with-
drawing cyano substituent indicated excellent inhibitory
efficacy of Gram-positive bacteria S. aureus at 6.25 lg/mL
of MIC along with similar efficacy of analogue 5i bearing
electron-withdrawing methoxy functional group against the
same bacteria. The latter analogue (5i) was the only and most
123

206
Med Chem Res (2013) 22:195–210
Table 1 In vitro antibacterial activity of newly synthesized compounds
Entry
R
LogP
MIC (lg/mL)
S.a
B.c
E.c
P.a
K.p
S.t
P.v
S.f
5a
5b
5c
H
3.08
3.64
3.91
3.24
4.43
3.11
3.11
3.56
2.95
3.29
1.99
4.27
4.83
5.10
4.43
5.63
4.23
4.31
4.76
4.15
4.49
3.18
[100
50
100
25
100
25
[100
100
50
[100
100
100
100
100
100
50
[100
100
100
50
[100
100
100
100
50
[100
50
Cl
Br
50
12.5
50
12.5
50
100
50
5d
5e
F
12.5
100
100
6.25
100
6.25
25
100
100
[100
100
[100
25
I
100
100
100
50
100
50
[100
100
100
100
25
100
100
[100
[100
100
100
[100
100
25
5f
NO2
CN
25
5g
5h
5i
50
100
[100
100
100
[100
100
50
CH3
[100
50
[100
100
100
[100
[100
100
100
100
100
50
OCH3
OC2H5
NHCOCH3
H
100
100
100
100
6.25
3.12
12.5
100
100
100
100
50
5j
50
6.25
50
50
5k
6a
6b
6c
50
[100
[100
50
[100
100
50
100
50
50
Cl
100
100
50
Br
25
50
100
50
50
50
6d
6e
F
12.5
100
50
50
100
[100
12.5
100
100
25
25
I
50
[100
100
[100
100
50
100
100
100
100
25
100
100
[100
[100
50
6f
NO2
CN
100
50
6g
6h
6i
25
25
CH3
100
25
[100
100
50
100
100
100
100
0.78
–
OCH3
OC2H5
NHCOCH3
6j
12.5
50
25
6.25
12.5
0.78
–
12.5
100
0.39
–
100
[100
0.39
–
100
100
1.56
–
6k
Cip.
DMSO
100
0.39
–
100
1.56
–
1.56
–
LogP was calculated using the ChemDraw Ultra, version 11.0
Cip. ciprofloxacin, S.a Staphylococcus aureus, B.c Bacillus cereus, E.c Escherichia coli, P.a Pseudomonas aeruginosa, K.p Klebsiella pneu-
moniae, S.t Salmonella typhi, P.v Proteus vulgaris, S.f Shigella flexneri
was found essential to contribute grater activity against
Gram-negative S. typhi at 25 lg/mL of MIC. Similar
inhibitory concentration (25 lg/mL) was observed for ana-
logue 6b bearing electron-donating halogen (Cl) atom within
the series involving benzothiazole ring system against Gram-
negative strain S. flexneri. Nitro group (strong electron
withdrawing) bearing analogue (6f) within the benzothiazole
ring series showed remarkable activity against Gram-nega-
tive strain at 12.5 lg/mL of MIC. Many of the remaining
analogues were succeeded to indicate good-to-moderate
activity level of 6.25–25 lg/mL against Gram-positive
strains as well as 12.5–50 lg/mL against Gram-negative
strains, while some analogues were found to indicate poor
activity at concentration level 50–100 lg/mL.
In vitro antifungal screening results for the final ana-
logues 5a–6k are furnished in Table 2 and from the results
it can be stated that the analogues displayed comparatively
good in vitro antifungal potency (12.5–25 lg/mL) when
compared to that of against Gram-positive strains (6.25,
12.5 and 25 lg/mL) as mentioned in the previous table. In
addition, it will suffice to mention here that opposite to the
antibacterial results, in case of bioassay against fungal
strains, halogen substituents (Cl, Br, F) were found more
effective as compared to the analogues bearing alkoxy
(OCH3, OC2H5) functional groups.
Final analogues bearing benzothiazole ring system
involving substitution of electron-donating chloro (6b) and
fluoro (6d) functional groups were found to demonstrate
123

Med Chem Res (2013) 22:195–210
207
Table 2 In vitro antifungal activity of newly synthesized compounds
Entry
R
LogP
MIC (lg/mL)
A.n
A.f
A.c
C.a
5a
5b
5c
H
3.08
3.64
3.91
3.24
4.43
3.11
3.11
3.56
2.95
3.29
1.99
4.27
4.83
5.10
4.43
5.63
4.23
4.31
4.76
4.15
4.49
3.18
[100
25
[100
25
[100
100
100
100
100
100
[100
100
50
100
50
Cl
Br
50
100
50
50
5d
5e
F
50
25
I
100
100
[100
100
100
100
[100
[100
12.5
25
100
50
100
12.5
50
5f
NO2
CN
5g
5h
5i
[100
[100
100
100
[100
[100
25
CH3
[100
100
50
OCH3
OC2H5
NHCOCH3
H
5j
100
50
5k
6a
6b
6c
100
[100
100
50
[100
100
100
50
Cl
Br
100
25
6d
6e
F
12.5
100
50
50
I
100
100
[100
[100
100
100
[100
0.78
–
100
100
100
100
25
100
50
6f
NO2
CN
6g
6h
6i
100
[100
50
50
CH3
100
50
OCH3
OC2H5
NHCOCH3
6j
100
100
1.56
–
50
100
[100
1.56
–
6k
Kit.
DMSO
12.5
0.78
–
LogP was calculated using the ChemDraw Ultra, version 11.0
Kit ketoconazole, A.n Aspergillus niger, A.f Aspergillus fumigates, A.c Aspergillus clavatus, C.a Candida albicans
promising activity against two fungal species as A. niger and
A. fumigatus at 12.5 lg/mL and 25 lg/mL of MICs, respec-
tively, along with similar efficacy (25 lg/mL) of analogue 5b
from phenyl acetamide series bearing chlorine substituent
against A. fumigatus. Analogue 6k from benzothiazolyl series
bearing highly lipophilic acetamido functional group linkage
was appeared with noticeable inhibitory potential against A.
clavatus fungi at 12.5 lg/mL of MIC, while similar inhibitory
concentration (12.5 lg/mL) was observed for analogue 5f
bearing strong electron-withdrawing nitro functional group
within the seriesinvolvingsubstitution ofN-phenyl acetamide
against C. albicans fungi. Many of the remaining derivatives
were found to indicate moderate inhibition of the mentioned
panel of fungal species at 25–50 lg/mL of MIC, whereas
some derivatives were shown poor inhibition at 100 lg/mL.
In vitro TB activities of compounds 5a–6k were asses-
sed against M. tuberculosis H37Rv (Table 3). Preliminar-
ily, the analogues presented here are examined for their in
vitro antituberculosis activity using BACTEC MGIT
method at concentration level of 6.25 lg/mL and it is seen
from the results that none of the analogues showed activity
at such level. Hence, all the analogues were further tested
for the same activity using Lowenstein–Jensen MIC
method. From the secondary bioassay results is can be
stated that both the class of analogues either involving
phenyl or benzothiazolyl-acetamide linkages shown equal
degree of lowest MICs in the range 12.5–62.5 lg/mL. Two
of the final analogues halo substituents as 5e bearing iodo
substituent to the phenyl acetamide moiety and 6b bearing
bromo substituent to the benzothiazolyl acetamide moiety
123

208
Med Chem Res (2013) 22:195–210
Table 3 In vitro antituberculosis activity of newly synthesized compounds
Entry
R
BACTEC MGIT methoda
L–J MIC methoda
MIC (lg/mL)
% Inhibition
MIC (lg/mL)
% Inhibition
5a
5b
5c
5d
5e
5f
H
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
[6.25
0.20
–
500
100
62.5
25
95
96
97
99
99
94
95
94
97
98
95
95
96
99
98
97
96
98
95
98
97
96
Cl
–
Br
–
F
–
I
–
12.5
500
200
500
50
NO2
–
5g
5h
5i
CN
–
CH3
–
OCH3
OC2H5
NHCOCH3
H
–
5j
–
25
5k
6a
6b
6c
6d
6e
6f
–
250
500
62.5
12.5
50
–
Cl
–
Br
–
F
–
I
–
100
250
100
200
25
NO2
–
6g
6h
6i
CN
–
CH3
–
OCH3
OC2H5
NHCOCH3
Isoniazid
Rifampicin
Ethambutol
Pyrazinamide
–
6j
–
50
6k
–
100
99
99
99
99
0.25
3.12
6.25
a
Each value is the mean of three independent experiment
linked to the 1,3,4-oxadiazole core via sulphur linkage
showed higher mycobacterial inhibitory efficacies (99 %)
at 12.5 lg/mL of MIC. The said two analogues were found
to exhibit half fold activity against the mentioned myco-
bacterial strain when compared to that of standard drug
pyrazinamide (6.25 lg/mL). In addition, analogue 5d with
fluorine atom and 5j with ethoxy functional group insertion
to the series involving N-phenyl acetamide moiety along
with analogue (6i) from benzothiazole series bearing
methoxy substituent showed appreciable activity against
mycobacterial strain H37Rv at 25 lg/mL of MIC with
98–99 % of inhibition. The said profile of these analogues
can be compared to that observed in the antimicrobial
bioassay; however, many of the final analogues exhibited
moderate activity at 50–62.5 lg/mL of MIC, whereas some
were found inactive with the MIC level [100 lg/mL.
The relationship between lipophilicity (LogP) and MICs
of final analogues was discussed, in which
(a) Among the most potent analogues tested, in case of
B. cereus, E. coli, P. aeruginosa, P. vulgaris, K. pneumo-
niae, S. typhi bacteria and A. clavatus, A. fumigatus fungi,
compounds with higher lipophilicity showed highest
activity and the activity is found to decrease with the
decrease in the lipophilicity value (LogP).
(b) In case of active analogues against S. aureus,
S. flexneri bacteria and A. niger, C. albicans fungi as
well as mycobacteria (H37Rv), the above-mentioned
123

Med Chem Res (2013) 22:195–210
209
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relationship between lipophilicity of the potent ana-
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(c) In case of active analogues against A. clavatus fungi,
the opposite trend was observed with lipophilicity
versus MIC values as the least lipophilic analogue
exhibited highest activity and the activity was found
to decrease with increase of lipophilicity of the
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Acknowledgments The authors are thankful to Applied Chemistry
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their deep gratitude to Microcare Laboratory, Surat, India for carrying
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