Synthesis of substituted benzo[d]thiazol-2-ylcarbamates as potential anticonvulsants

Article abstract of DOI:10.1007/s00044-012-0434-y

A series of substituted benzo[d]thiazol-2-ylcarbamates 4a-g and 5a-g were synthesized and evaluated for anticonvulsant activity. The structures of the synthesized compounds were confirmed on the basis of their physical and spectral data. The compounds were evaluated for anticonvulsant activity using PTZ-induced convulsion and maximal electroshock models. The target compounds have shown significant activity in these models.

Full text of DOI:10.1007/s00044-012-0434-y

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res
DOI 10.1007/s00044-012-0434-y
ORIGINAL RESEARCH
Synthesis of substituted benzo[d]thiazol-2-ylcarbamates
as potential anticonvulsants
•
•
•
Ashvini Navale Smita Pawar Meenakshi Deodhar
Amol Kale
Received: 1 August 2012 / Accepted: 15 December 2012
Ó Springer Science+Business Media New York 2013
Abstract A series of substituted benzo[d]thiazol-2-ylc-
arbamates 4a–g and 5a–g were synthesized and evaluated
for anticonvulsant activity. The structures of the synthe-
sized compounds were confirmed on the basis of their
physical and spectral data. The compounds were evaluated
for anticonvulsant activity using PTZ-induced convulsion
and maximal electroshock models. The target compounds
have shown significant activity in these models.
population showed an annual incidence rate of 27.27 per
100,000 per year. (Banerjee et al., 2010). There is a contin-
uing demand for new anticonvulsant agents as several of the
currently available antiepileptic drugs (AEDs) have been
associated with severe side effects and fail to control seizures
in about 30 % of epileptic patients. (Mc Namara, 2006;
Kramer, 2001; Kwan and Brodie, 2000). Therefore, intensive
research efforts are being devoted to find new anticonvulsant
compounds with promising activity.
Keywords Benzothiazole Á Carbamate Á
Benzothiazole is a bicyclic ring system with electron rich
sulfur and nitrogen atom. Rana et. al (2000) in their review
on benzothiazole have listed various pharmacological
actions displayed by benzothiazole like antimicrobial, anti-
tubercular, anticonvulsant, antitumor, antihyperglycemic,
and HIV-1 integrase inhibition. Biologist’s attention was
drawn to the benzothiazole containing compounds with the
discovery of riluzole. Du et. al 2007 determined the role of
riluzole and lamotrigine on AMPA receptor localization.
In the recent years, several new drugs have been added
to the list of therapeutic agents against epilepsy. (Fig. 1)
Among these felbamate (Lemke and Williams, 2008), to-
piramate (Liberatore et al., 2007), RWJ 333369 & mep-
robamate are carbamate derivatives with a broad spectrum
of anticonvulsant activity.
Anticonvulsant activity
Introduction
Epilepsy is not a disease, but a syndrome of different cerebral
disorders of central nervous system (CNS), and is character-
ized by paroxysmal, excessive, and hypersynchronous dis-
charges of large numbers of neurons. (Wasteralin et al.,
1989). It is a common neurological condition, affecting
0.5–1 % of the population worldwide (45–100 million peo-
ple) and is characterized by recurrent seizure attacks. The
prevalence of epilepsy in developing countries is usually
higher than in developed countries. The incidence and prev-
alence rates are surprisingly similar to those in developed
countries. A recent study conducted in Kolkata’s urban
These literature findings prompted us to synthesize
benzo[d]thiazol-2-ylcarbamate derivatives 4a–g and 5a–g
in the hope to develop compounds having significant
anticonvulsant activity.
Experimental
A. Navale Á S. Pawar (&) Á M. Deodhar Á A. Kale
Department of Pharmaceutical Chemistry, P.D.E.A’S Seth
Govind Raghunath Sable College of Pharmacy,
Saswad, Pune, Maharashtra 412301, India
The target compounds 4a–g and 5a–g were synthesized via
two step pathway indicated in Scheme 1. All the chemicals
used in the synthesis were of laboratory grade. Melting
e-mail: smitapawar4477@yahoo.co.in
123

Med Chem Res
O
O
CO
CO
NH2
NH2
O
O
CO
CO
NH2
NH2
F3CO
S
H
F
NH2
N
Flebamate
2 - Fluorofebamate
Riluzole
H2N
O
O
O
O
O
SO2NH2
S
O
HO
O
O
NH2
O
Cl
H2N
O
O
Topiramate
O
Carisbamate
meprobamate
Fig. 1 Some examples of anticonvulsant agents
points were determined in open capillary on Veego (model:
VMP-D) electronic apparatus and are uncorrected. The IR
spectra of the synthesized compounds were recorded on
Shimadzu 8400-S FT-IR Spectrophotometer using potas-
sium bromide. The 1H NMR spectra were recorded in
CDCl3 or DMSO using NMR Varian-Mercury 300 MHz
spectrometer and chemical shifts are given in units as parts
per million, downfield from Tetra Methyl Silane (TMS) as
an internal standard. Mass spectra were taken on Schimatzu
GC–MS spectrometer. To monitor the reactions, purity of
reactants and products, thin layer chromatography was
performed on precoated aluminum sheets (silica gel 60
collected and dried over sodium sulfate. The solvent was
removed under reduced pressure to yield the product which
was recrystallized from toluene: methanol mixture.
Biology
The synthesized compounds were evaluated for anticon-
vulsant activity using PTZ and electroshock-induced con-
vulsion models. Albino male Swiss mice (20–25 g) were
housed (5 animals per cage) under the standard laboratory
conditions (light period of 12 h/day, temperature
25 2 °C, and humidity 55 5 %) with free access to
food (standard pellets chow, Lipton, India) and water. Food
but not water was deprived overnight and during the
experiment. Animals were dosed, one at a time.
F254, 6 9 2.5 cm) using benzene: methanol solvent sys-
tems. The physical and spectral data are given in Tables 1
and 2, respectively.
General procedure for substituted benzo[d]thiazol-2-yl
Pentylenetetrazole (PTZ)-induced seizure
carbamate 4a–g and 5a–g
(Vogel et al., 2002)
The target compounds 4a–g and 5a–g were synthesized as
per scheme 1. Substituted 2-amino benzothiazole (3a–g)
(0.1 mmol), synthesized as per reported procedure [Liber-
atore et al., 2007] was dissolved in a 80 ml of dry
dichloromethane containing pyridine (0.1 mmol). To this
solution was added ethyl chloroformate (for 4a–g) or
phenyl chloroformate (0.1 mmol) (for 5a–g) and the
reaction mixture was refluxed for 6 h. The reaction mixture
was then diluted with dichloromethane (200 ml) and
washed with portion of 1 M sodium bicarbonate solution,
followed by water and then brine. The organic layer was
The method employs PTZ to induce clonic convulsion in
mice. Animals were divided into three groups, control,
standard, and test group, each containing six animals. The
control group received 0.1 % CMC suspension while the
standard group received the standard drug (phenytoin) in
20 mg/kg dose. The test group was administered test
compound 4a–g and 5a–g in a dose of 35 mg/kg, PTZ
(60 mg/kg) was administered to the mice 60 min after the
administration of test compounds and the standard. Mice
were then observed over a period of 30 min. The absence
of an episode of clonic spasm of at least 5 s duration
123

Med Chem Res
H
N
NH2
NH4SCN
HCl, H2O
R
R
NH2
S
2
1
Stir
Br2 in CHCl3
N
R
NH2
S
3a-g
O
C
O
C
C2H5
O
Cl
O
Cl
Reflux
6 hrs
CH2Cl2
CH2Cl2
Pyridine
Reflux
6 hrs
Pyridine
N
S
N
S
NH
O
C2H5
R
NH
O
R
O
O
4a-g
5a-g
R = H, 6-CH3 , 6 -C2H5 , 6 -OCH3, 6 - Cl ,6-NO2,4 - CH3
a
b
c
d
e
f
g
Scheme 1 Synthetic procedure for preparation of the target compound
indicated the compound’s ability to abolish the effect of
PTZ on seizure threshold.
percent protection and duration of hind limb extension
were calculated.
Maximal electroshock (MES) seizures
(Vogel et al., 2002)
Result and discussion
Convulsion in mice was produced using Electroconvuls-
ometer. Similar to the above procedure, animals were
divided into three groups, control, standard, and test group,
each containing six animals. The control group received
0.1 % CMC suspension while the standard group received
the standard drug (phenytoin) in 20 mg/kg dose. The test
group was administered test compound 4a–g and 5a–g in a
dose of 35 mg/kg. An alternating current (54 mA) was
delivered via crocodile ear clip for 0.3 s 30 min after the
administration of standard and the test compounds.. The
Chemistry
All the synthesized substituted ethyl benzo[d]thiazol-2-yl)
carbamate 4a–g and substituted phenyl benzo[d]thiazol-2-yl)
carbamate 5a–g were high melting solids (Table 1). These
compounds were found to be soluble in DMSO. The solid
state spectra (KBr, cm-1) of (4a–g) (Table 2) reveal char-
acteristic aromatic stretch at 2,900–3,000 cm-1, and N–H
stretch at 3,100–3,300 cm-1. The C=N group present in the
benzothiazole ring reveals peaks at 1,600–1,500 cm-1, while
123

Med Chem Res
Table 1 Physical data of compounds 4a–g and 5a–g
O
S
N
NH
C
O
R1
R
Compound
R
R1
Yield
M.P. (°C)
Rfa
4a
4b
4c
4d
4e
4f
H
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C2H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
80
78
65
75
80
80
75
75
90
90
85
80
80
75
209–212
248–251
210–212
225–227
235–237
200–202
205–207
230–233
247–250
210–212
225–228
270–272
200–202
230–232
0.52
0.45
0.45
0.55
0.5
6-CH3
6-C2H5
6-OCH3
6-Cl
6-NO2
4-CH3
H
0.65
0.63
0.45
0.6
4g
5a
5b
5c
5d
5e
5f
6-CH3
6-C2H5
6-OCH3
6-Cl
0.75
0.5
0.7
6-NO2
4-CH3
0.65
0.7
5g
a
Solvent system: benzene : methanol
the carbonyl (C=O) peak of carbamate is seen at 1,700–
1,750 cm-1 and C–O stretch at 1,000–1,100 cm-1. The peak
6-methoxy benzo[d] thiazol-2yl carbamate 4d, phenyl benzo
[d]thiazole-2-yl carbamate 5a, phenyl 6-methylbenzo[d]thia-
zol-2-ylcarbamate 5b, phenyl 6-nitrobenzo[d]thiazol-2-ylcar-
bamate 5f completely prevented the onset of clonic convulsions
thereby providing 100 % protection in PTZ model. In the
maximal electroshock- induced convulsions model also, com-
pound 4b and 5b displayed activity very near to that of stan-
dard. The two compounds increased the latency of onset of
clonus to more than 10 min. The compound ethyl 6-meth-
ylbenzo[d]thizol-2-ylcarbamate 4b and phenyl 6-meth-
ylbenzo[d]thiazol-2-ylcarbamate 5b has shown significant
activity (Figs. 2, 3).
1
for C–N stretch is observed at 1,350–1,280 cm -1. The H
NMR (d) spectra of the (4a–g) were recorded in DMSO. A
singlet of secondary amide proton at around 12 ppm and a
quartet of methylene proton was observed at around 4 ppm.
A doublet of methyl protons appeared around 2 ppm. The
aromatic protons were observed at 7–8 ppm as multiplet.
IR spectra of (5a–g) show the stretching vibrations for
secondary amide between 3,093 cm-1 and 3,141 cm-1
.
The carbonyl C=O stretching was observed between 1,722
and 1,758 cm-1, C=N at 1,574 cm-1; C–N at 1,282 cm-1
and C–O at 1,053 cm-1. The 1H NMR spectra of 5a–g was
recorded in DMSO. The spectra reveal a peak by secondary
amide proton at 12.65 ppm. The aromatic protons showed
peaks at 7–8.5 ppm as multiplet. Therefore, the spectral
data confirms the structure assigned to these compounds.
SAR
The compound phenyl 6-methylbenzo[d]thiazol-2-ylcar-
bamate 5b has been found to be active in both biological
models. The compounds 4e and 5d were found to be least
active in both models. So we may state that electron
withdrawing group in benzothiazole ring shows significant
activity and electron donating groups like chloro and
methoxy show least activity.
Biological activity
The synthesized compounds were tested for anticonvulsant
activity using PTZ and electroshock-induced convulsion
models. Phenytoin was used as standard. The results are
reported in Tables 3 and 4.
Conclusion
All target compounds 4a–g and 5a–g exhibited significant
anticonvulsant activity for both PTZ and MES models. While
ethyl 6-methyl benzo[d]thizol-2-ylcarbamate 4b, ethyl
The target compounds (4a–g and 5a–g), carbamate deriva-
tives of benzothiazole were synthesized using simple scheme
123

Med Chem Res
Table 2 Spectral data of compounds 4a–g and 5a–g
Compound IR (cm-1
)
1H NMR(DMSO-d6) d ppm*
MS (m/z)
4a
4b
4c
ArCH(2963), NH(3141), C=O (1722), 12(1H,s,CONH)7-8 (4H, m,ArCH), 4.2 (q, 2H, J = 7.1), 224(M?2),222(M?),177(M-
C=N(1606), C–N(1280), C–O(1060) 2.2(t, 3H, J = 6.2). 47),150(M-72)
ArCH(2962), NH(3165), C=O (1748), 12(1H,s,CONH),7-8 (3H, m,ArCH), 4.2 (q, 2H, J = 6.6), 238(M?2),236(M?),
C=N(1610), C–N(1288), C–O(1057)
2.2(3H, s,CH3), 2.35(t, 3H, J = 6.6).
(M?45),164(M-72)
ArCH(2963), NH(3086), C=O (1749), 11.90(s,1H,CONH),7-8 (3H, m,ArCH), 4.12 (q, 2H,
C=N(1611), C–N(1300), C–O(1082)
252(M?2),250(M?),205(M-
45),178(M-72)
J = 6.9) 1.24 (t, 3H, J = 15.2 Hz), 2.59(2H, q,CH2),
1.30(t, 3H, J = 6.5)
4d
4e
ArCH(2983), NH(3093), C=O (1748), 11.59(1H,s,CONH),7-8 (3H, m,ArCH), 4.12 (q,
C=N(1574), C–N(1283), C–O(1053) 2H,J = 7.2)1.30(t, 3H, J = 6.2), 3.73(3H, s,CH3)
252(M?),207(M-45)
180(M-72)
ArCH(2849), NH(3066), C=O (1606), 12.19(1H,s,CONH),7-8 (3H, m,ArCH), 4.12 (q, 2H,
C=N(1500), C–N(1300), C–O(1150), J = 7.2) 1.17 (t, 3H, J = 6.2).
Cl(700)
256(M?),210(M-46)
183(M-73)
4f
ArCH(2849), NH(3066), C=O (1606), 12.06(1H,CONH,s),7-8 (3H, m,ArCH), (q, 2H, J = 7.3), 267(M?),222(M-45),
=N(1500), C–N(1300), C–O(1150),
Cl(700)
1.12(t, 3H, J = 6.6).
195(M-72)
4g
5a
5b
5c
5d
5e
5f
ArCH(2910), NH(3072), C=O (1720), 12(1H,s,CONH),7-8 (3H, m,ArCH), 4.20 (q, 2H, J = 7.2) 236(M?),191(M-45),
164(M-72)
C=N(1605), C–N(1209), C–O(1037)
1.24(t, 3H, J = 6.5),2.45(3H, s,CH3)
ArCH(2983), NH(3093), C=O (1748), 12.62(1H,s,CONH),7-8 (9H, m,ArCH)
C=N(1574), C–N(1283), C–O(1053)
270(M?),176(M-64),
148(M-122)
ArCH(2922), NH(3086), C=O (1733), 12.62(1H,s,CONH),7-8 (8H, m,ArCH), 2.40(3H, s,CH3)
C=N(1574), C–N(1288), C–O(1150)
284.90(M?) 191(M-
93),164(M-120)
ArCH(2965), NH(3173), C=O (1758), 12(1H,s,CONH),7-8 (8H, m,ArCH), 2.75(q, 2H, J = 7.2) 298(M?),205.26(M-
C=N(1631), C–N(1250), C–O(1039) 1.20 (t, 3H, J = 6.1) 93),178(M-120)
ArCH(2927), NH(3091), C=O (1733), 12.30(1H,s,CONH),7-8 (8H, m,ArCH), 3.75(3H, s,CH3), 300(M?),207(M-93),
C=N(1582), C–N(1274), C–O(1045)
180(M-120)
ArCH(2983), NH(3141), C=O (1722), 12.60(1H,s,CONH),7-8 (8H, m,ArCH)
C=N(1564), C–N(1280), C–O(1060)
304(M ?),212(M-92),
184(M-120)
ArCH(2983), NH(3141), C=O(1722), 12.80(1H,s,CONH),7-8 (8H, m,ArCH)
C=N(1564), C–N(1280), C–O(1060)
315(M?),221(M-94)
195(M-120)
5g
ArCH(2983), NH(3141), C=O (1722), 12.70(1H,s,CONH),7-8 (8H, m,ArCH), 2.60(3H, s,CH3)
C=N(1564), C–N(1280), C–O(1060)
284(M?),190(M-94),
164(M-120)
* CONH proton at 12 d ppm was found to be D2O exchangeable
Table 3 Observation table for
Group n = 6
Onset of clonic convulsions (sec) Protection (%) Death/recovery (%)
pentylenetetrazole (PTZ)-
induced convulsion in mice
Control
59.166 0.04216
0
All died
No death
16.67
Standard(Phenytoin) 20 mg/kg Absent
100
4a (35 mg/kg)
4b (35 mg/kg)
4c (35 mg/kg)
4d (35 mg/kg)
4e (35 mg/kg)
4f (35 mg/kg)
4g (35 mg/kg)
5a (35 mg/kg)
5b (35 mg/kg)
5c (35 mg/kg)
5d (35 mg/kg)
5e (35 mg/kg)
5f (35 mg/kg)
5g (35 mg/kg)
86.66 0.4216***
Absent
83.33
100
No death
16.67
83 0.5164***
Absent
83.33
100
No death
33.34
71.83 0.7032***
80.5 0.03416***
85.166 0.3073***
Absent
66.66
83.33
83.33
100
16.67
16.67
No death
No death
16.67
Absent
100
Data were analyzed by one-way
ANOVA followed by Dunnett’s
test. Values were expressed as
Mean S.E.M
86 0.5164***
80.5 0.3416***
70.83 0.4014***
Absent
83.33
66.66
66.66
100
33.34
33.34
*** P \ 0.001, as compared
with control (0.1 % CMC
suspension) group
No death
16.67
75.166 0.1667***
83.33
123

Med Chem Res
Table 4 Observation table of
maximal electroshock-induced
convulsion
Group
n = 6
Duration of tonic
flexion sec.
Duration of tonic
extension sec.
Latency of onset of
clonus min.
Death/
recovery
Control
0
20.66 0.8433
3.16 0.0833
13.39 0.1381**
11.18 0.0628**
12.55 0.1537**
10.415 0.1061**
9.56 0.1071**
5.40 0.1506**
8.37 0.1776**
10.36 0.1364**
11.47 0.1464**
13.28 0.3741**
7.59 0.1117**
4.675 0.1186**
11.20 0.071**
10.34 0.166**
9.58 0.1291**
All died
Standard
5.34 0.1261**
7.29 0.1020**
6.24 0.0988**
8.12 0.1765**
6.40 0.1350**
9.27 0.1596**
7.44 0.1650**
7.33 0.1418**
9.54 0.1518**
6.74 0.2737**
5.2916 0.1635**
11.08 0.1183**
8.34 0.06534**
6.32 0.1375**
5.48 0.1510**
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
No death
No death
No death
No death
No death
No death
No death
No death
No death
No death
No death
No death
No death
No death
No death
4a (35 mg/kg)
4b (35 mg/kg)
4c (35 mg/kg)
4d (35 mg/kg)
4e (35 mg/kg)
4f (35 mg/kg)
4g (35 mg/kg)
5a (35 mg/kg)
5b (35 mg/kg)
5c (35 mg/kg)
5d (35 mg/kg)
5e (35 mg/kg)
5f (35 mg/kg)
5g (35 mg/kg)
Data were analyzed by one-way
ANOVA followed by Dunnett’s
test. Values were expressed as
Mean S.E.M
** P \ 0.01, significant as
compared with control (0.1 %
CMC suspension) group
data, all the compounds have displayed significant protection
against the two types of convulsions. Compounds 4b, 4d, 5a,
5b, and 5f have shown 100 % protection and no animal death
has been seen. Hence, we may conclude that the reported
series of substituted benzo[d]thiazol-2-ylcarbamates holds
promise for the development as potential anticonvulsant
agents after further optimization.
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Fig. 3 Graph of latency of onset of clonus versus treatment given to
the groups
and were evaluated for their anticonvulsant activity using two
popular models, that is PTZ and maximal electroshock-
induced convulsions. As evident from the biological activity
123