Synthesis of benzothiazole semicarbazones as novel anticonvulsants-The role of hydrophobic domain

Article abstract of DOI:10.1016/j.bmcl.2007.05.048

A series of 1,3-benzothiazol-2-yl semicarbazones (1-15) were prepared in satisfactory yield and evaluated for their anticonvulsant, neurotoxicity and other toxicity studies. All the synthesized compounds were in good agreement with elemental and spectral data. Majority of the compounds were active in MES screen. Selected compounds were checked for their lipophilic character.

Full text of DOI:10.1016/j.bmcl.2007.05.048

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4178–4182
Synthesis of benzothiazole semicarbazones as novel
anticonvulsants—The role of hydrophobic domain
Nadeem Siddiqui,a,* Arpana Rana,a Suroor A. Khan,a Mashooq A. Bhata
and Syed E. Haqueb
aDepartment of Pharmaceutical Chemistry, Faculty of Pharmacy, Hamdard University, Hamdard Nagar, New Delhi 110062, India
bDepartment of Pharmacology, Faculty of Pharmacy, Hamdard University, Hamdard Nagar, New Delhi 110062, India
Received 23 December 2006; revised 14 May 2007; accepted 16 May 2007
Available online 21 May 2007
Abstract—A series of 1,3-benzothiazol-2-yl semicarbazones (1–15) were prepared in satisfactory yield and evaluated for their anti-
convulsant, neurotoxicity and other toxicity studies. All the synthesized compounds were in good agreement with elemental and
spectral data. Majority of the compounds were active in MES screen. Selected compounds were checked for their lipophilic
character.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Epilepsy is not a disease, but a syndrome of different
cerebral disorders of central nervous system (CNS),
and it is characterized by paroxysmal, excessive and
hypersynchronous discharges of large numbers of neu-
rons.1 Epilepsy is a common neurological condition,
affecting 0.5–1% of the population worldwide (45–100
million people).2 Studies have reported that in India
the prevalence rate of epilepsy varies from 1710 to
9780 cases per million population.3 Despite the optimal
use of available antiepileptic drugs (AEDs), many
patients with epilepsy fail to experience seizure control
and others experience the seizure control only at the
expense of significant toxic effects.4 The limitations with
conventional AEDs highlighted the need for developing
newer agents for epilepsies.
Figure 1. In recent years, aryl and heteroaryl semicar-
bazones5,6 have emerged as structurally novel
anticonvulsants.
In terms of interaction at the binding site, as proposed
previously,7,8 the pharmacophoric elements were
thought to be a lipophilic aryl ring and hydrogen-bond-
ing domain. The attachment of a second aryl ring, des-
ignated as the distal ring to the proximal aryl ring to
increase the van der waal’s bonding at the binding site
and to increase potency, has also been reported. Substi-
tutions in the aryl ring by halogens have been found to
increase potency in the MES screen.9,10 To test this
hypothesis some modifications were made in the struc-
ture of semicarbazones. The lipophilic aryl ring was
replaced with a versatile heterocyclic molecule benzo-
thiazole, which possesses preliminary anticonvulsant
properties.11 In view of these general requirements for
From the study of structures of clinically established
drugs, it can be concluded that the anticonvulsant prop-
erties have been displayed by various hydrazones
(@NANHA), amides (ACONH2) and carbamides
(ANHCOANHA). The prime need was to search for a
molecule that could complement all the above structures
in one and semicarbazido group was considered one of
them. The events leading to the development of the
semicarbazones as promising lead are indicated in
CO-NH
Dicarboximide
C=O
Amides
CO-NH2
Hydrazines
HN-NH
HN-NH
Semicarbazones
C-NH2
O
Keywords: Benzothiazoles; Semicarbazones; Anticonvulsant activity;
Hydrophobic domain.
*
Figure 1. Development of semicarbazones as template for future lead
compound.
Corresponding author. Tel.: +91 26059688x5639; e-mail: nadeems_03@
yahoo.co.in
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.05.048

N. Siddiqui et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4178–4182
4179
activity, we have designed aryl-substituted semicarbaz-
ones with benzothiazole moiety to be active in convul-
sant stimuli. These aryl semicarbazones do not possess
the dicarboximide group as found in conventional drugs
like barbiturates, hydantoins, oxazolidiones, etc., which
may be associated with toxicity and side effects. The
present work focused on synthesis and pharmacological
evaluation of 1-(4-substitutedphenyl) ethan-1-one-N-(6-
substituted-1,3-benzothiazol-2-yl) semicarbazones and
compared its potential with those of established drugs.
The lipophilicity character of the compounds was also
determined.
either sex were used as experimental animals. The test
compounds and standard drug were administered intra-
peritoneally which were suspended in Tween 80 (1%) or
in 0.5% methyl cellulose–water mixture.
MES—maximal electroshock test: Maximal electro-
shock seizure was elicited with a 60 cycle altering current
of 50 mA intensity delivered for 0.25 s via ear clip elec-
trodes. The maximal seizure typically consists of a short
period of tonic extension of the hind limbs and a final
clonic episode. Abolition of the hind limb tonic extensor
component of the seizure is defined as protection, and
results are expressed as % protection.
1-(4-Substitutedphenyl) ethan-1-one-N-(6-substituted-
1,3-benzothiazol-2-yl) semicarbazones (1–15) were
synthesized by method outlined in Scheme 1. One of the
initial attempts for the synthesis of 6-substituted-1,3-ben-
zothiazole-2-amines was made by treating arylamines
with potassium thiocyanate in a satisfactory yield by the
known protocol.12 The benzothiazole-2-yl ureas were pre-
pared by treating required benzothiazole with sodium
cyanate in the presence of glacial acetic acid. The benzo-
thiazole-2-yl-ureas were then refluxed with hydrazine
hydrate to yield hydrazine carboxamides. The final com-
pounds were synthesized by the reaction of hydrazine
carboxamides with appropriate ketones.13 Analytical
and spectral data of all the synthesized compounds were
in good agreement with the composition of synthesized
compounds. The data of physico-chemical properties of
all the compounds are given in Table 1.
NT—neurotoxicity: The rotarod test was used to evalu-
ate neurotoxicity. The animal was placed on a 3.2 cm
diameter knurled rod rotating at 6 rpm. Normal mice
can remain on a rod rotating at this speed indefinitely.
Neurological toxicity is defined as the failure of the ani-
mal to remain on the rod for 1 min. Results are
expressed as number of animals exhibiting toxicity/num-
ber of animals tested.
Assessment of liver function: The biochemical parame-
ters such as serum glutamate oxaloacetate transaminase
(SGOT) and serum glutamate pyruvate transaminase
(SGPT) were estimated by a reported method17 and
alkaline phosphate was measured by using King meth-
od.18 The total protein and total albumin were also mea-
sured according to reported methods.19,20
The anticonvulsant evaluation was undertaken using re-
ported procedures.14–16 Swiss albino mice (25–30 g) of
Histopathological study of liver: The histopathological
studies were carried out by a reported method.21 The
a
N
N
O
R
S
NH2
R
S
N
NH2
H
1-(Substituted-1,3-benzothiazol-2-yl)urea's
b
N
O
R
S
N
NHNH2
H
N-(Substituted-1,3-benzothiazol-2-yl)hydrazine
carboxamide's
O
R2
c
R1
N
O
R2
R
S
N
NHN= C
H
R1
(1-15)
1
2
R = Cl, CH , OCH ;
R = CH3, C6H5; R = H, OH, NO2, OCH3
3
3
Scheme 1. Reagents and conditions: (a) glacial acetic acid/NaOCN; (b) NaOH/EtOH, NH2NH2ÆH2O, reflux 6 h; (c) EtOH/reflux, 5 h.

4180
N. Siddiqui et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4178–4182
Table 1. Physical data of compounds (1–15)
Compound
R
R1
R2
Molecular formula
Found (Calcd)%
Yield (%) Mpa (ꢁC) Rfb(Rm)c
C
H
N
1
2
Cl
Cl
CH3
CH3
CH3
CH3
C6H5
CH3
CH3
CH3
CH3
C6H5
H
C16H13ClN4OS
C16H13ClN4O2S
C16H12ClN5O3S
55.77(55.73) 3.84(3.80) 16.30(16.25) 55
53.32(53.26) 3.60(3.63) 15.50(15.53) 60
49.34(49.30) 3.14(3.10) 18.00(17.97) 58
54.52(54.47) 4.07(4.03) 14.90(14.95) 55
62.01(61.99) 3.76(3.72) 13.72(13.77) 65
62.90(62.94) 5.00(4.97) 17.29(17.27) 60
60.02(59.98) 4.70(4.74) 16.40(16.46) 62
55.29(55.27) 4.05(4.09) 18.90(18.96) 60
61.04(61.00) 5.16(5.12) 15.84(15.81) 45
68.42(68.37) 4.62(4.69) 14.54(14.50) 50
59.84(59.88) 4.78(4.74) 17.00(16.46) 55
57.25(57.29) 4.56(4.52) 15.76(15.72) 60
52.21(52.98) 3.98(3.92) 18.21(18.17) 65
58.30(58.36) 4.94(4.90) 15.16(15.12) 55
65.60(65.65) 4.54(4.51) 13.96(13.92) 40
185
180
170
164
168
150
142
200
80
0.76(À0.50)
0.79(À0.57)
0.82(À0.65)
0.84(À0.72)
0.86(À0.78)
0.72(À0.41)
0.70(À0.36)
0.80(À0.60)
0.85(0.75)
OH
NO2
3
Cl
Cl
4
OCH3 C17H15ClN4O2S
5
Cl
H
C21H15ClN4OS
C17H16N4OS
C17H16N4O2S
C17H15N5O3S
6
CH3
CH3
CH3
CH3
CH3
H
7
OH
NO2
8
9
OCH3 C18H18N4O2S
10
11
12
13
14
15
H
C22H18N4OS
C17H16N4O2S
C17H16N4O3S
C17H15N5O4S
140
138
120
70
0.73(À0.43)
0.75(À0.47)
0.83(À0.68)
0.88(À0.86)
0.90(À0.95)
0.68(À0.32)
CH3O CH3
CH3O CH3
CH3O CH3
CH3O CH3
CH3O C6H5
H
OH
NO2
OCH3 C18H18N4O3S
C22H18N4O2S
90
100
H
a Melting point of the compounds at their decomposition.
b Solvent system benzene/acetone (9:1).
c A logarithmic function of Rf value was also calculated; Rm = log(1À1/Rf).
Table 2. Anticonvulsant and neurotoxicity screening of compounds 1–15
N
O
R2
R
S
N
H
NHN= C
R1
Compound
R
R1
R2
Intraperitoneal injection in micea
MES screen (%
protection)
Neurotoxicity screen
0.5 h
4 h
0.5 h
4 h
1
Cl
Cl
CH3
CH3
CH3
CH3
C6H5
CH3
CH3
CH3
CH3
C6H5
CH3
CH3
CH3
CH3
C6H5
—
H
100
50
83.3
50
(À)
X
(À)
X
2
OH
NO2
OCH3
H
3
Cl
Cl
33.3
66.6
50.0
33.3
66.6
100
83.3
33.3
83.3
66.6
100
83.3
100
100
33.3
66.6
33.3
33.3
66.6
100
X
X
4
X
X
5
Cl
X
X
6
CH3
CH3
CH3
CH3
CH3
CH3O
CH3O
CH3O
CH3O
CH3O
—
H
X
X
7
OH
NO2
OCH3
H
X
X
8
(À)
(À)
X
(À)
(À)
X
9
10
83.3
33.3
66.6
50.0
100
11
12
H
(À)
X
(À)
X
OH
NO2
OCH3
H
13
14
(À)
(À)
(À)
(À)
(À)
(À)
(À)
(À)
83.3
100
15
Phenytoin
—
100
a Dose of 30 mg/kg was administered. The animals were examined 0.5 and 4 h after administration. The dash (À) indicates an absence of activity. X
denotes not tested.
rats were scarified under light ether anaesthesia after
24 h of last dosage; the livers were removed and washed
N
with normal saline. Small pieces of liver tissue were pro-
cessed and embedded in paraffin wax. Sections of 5–
6 lm in thickness were cut, stained with haematoxylin
and eosin and then studied under an electron
microscope.
D
O
A
C
S
N
N
N
H
H
R
HBD
Figure 2. Proposed elements for anticonvulsant activity in basic
structure of compounds. (A) Aryl ring system, (HBD) hydrophobic
domain, (D) electron donor moiety and (C) distal aryl ring.
Candidate anticonvulsants are often evaluated initially
in the MES screens. Compounds affording protection

N. Siddiqui et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4178–4182
4181
Table 3. Enzyme estimation of selected compounds
Compound Alkaline
SGOTa SEMns SGPTb SEMns Albumin SEMns Gloublin SEMns Totalprotein SEMns
phosphatase SEM
Control
13.06 0.25
13.67 0.56
14.46 0.06*
13.69 0.06
12.78 0.45
148.67 1.50
148.11 0.54
147.89 0.95
149.00 1.90
148.56 1.34
27.67 0.840
26.78 0.78
26.17 0.75
27.90 0.48
27.67 0.56
1.67 0.009
1.67 0.100
1.70 0.028
1.66 0.010
1.68 0.030
0.13 0.01
0.13 0.05
0.13 0.03
0.14 0.56
0.14 0.38
1.80 0.01
1.82 0.05
1.79 0.02
1.81 0.04
1.80 0.02
1
8
13
15
a Denotes serum glutamate oxaloacetate transaminase.
b Denotes serum glutamate pyruvate transaminase.
* p < 0.05, ns denotes not significant. The mean level was calculated using ANOVA followed by Dunnett’s test.
in the MES test may prove to be useful in treating gen-
eralized tonic–clonic and complex partial seizure.
Neurotoxicity in mice is measured by the rotarod proce-
dure, while minimal motor impairments in rats are
detected by overt evidence of ataxia and abnormal gait
and stance. The 1-(4-substitutedphenyl) ethan-1-one-N-
(6-substituted-1,3-benzothiazol-2-yl) semicarbazone
derivatives were screened at 30 mg/kg intraperitoneally
in mice for anticonvulsant activity (Table 2). Phenytoin
was used as the standard at the dose level of 30 mg/kg.
Compounds 1, 8, 13 and 15 had shown 100% protection
at both the time intervals, that is, 0.5 and 4 h except the
compound 1 whose % protection decreased to 83.3% at
the 4 h indicating rapid onset but shorter duration of ac-
tion. Compounds 9, 11 and 14 had shown 83.3% protec-
tion at both time intervals except 11, which also had
rapid onset but shorter duration of action with 66.6%
protection at the 4 h interval. Compounds 4, 7 and 12
showed 66.6% protection in the MES screen, whereas
compounds 2, 3, 5, 6 and 10 had 50–33.3% protection
at the dose level of 30 mg/kg compared with standard
phenytoin. The selected compounds with 100–83.3%
protection were evaluated for the neurotoxicity at the
30-mg/kg dose level, none of the compound had shown
the sign of neurotoxicity.
The compound 1 had logP 3.12 and 8 had logP 3.22.
Among the compounds showing anticonvulsant activity,
compounds with more lipophilic substitution like CH3, Cl
group in benzothiazole ring are more active.
Enzyme estimation and histopathological studies of the
selected compounds with 100% protection were also
done to check the magnitude of liver toxicity. Table 3
shows the liver function tests with reference to selected
compounds. The estimation revealed that there was no
significant increase in alkaline phosphatase, SGOT,
SGPT and decrease in protein level in serum as compared
to the control level (Table 3). It was clearly indicated that
none of the compound showed any malfunctioning or
toxicity of the liver as compared to control.
The Luncas technique was used to access the livers of
mice, which were administered with test compounds at
the dose level of 30 mg/kg body weight for 15 days
and a comparison was done with the control group.
Liver samples from control group and all the experimen-
tal groups were within normal histological limits (Figs. 3
and 4).
In conclusion, the present results indicate that the com-
pounds possessed marked anticonvulsant activity as
indicated by their prevention of the effects of maximal
electroshock induced seizures in comparison with stan-
dard drug. The results further confirmed the pharmaco-
phoric model elements necessary for the anticonvulsant
The bioevaluation led to an understanding of the corre-
lation of bioactivity with structure of compounds. The
substituents like CH3, OCH3 at the aryl ring with NO2
and unsubstituted distant phenyl ring lead to highly
potent compounds having longer duration of activity.
Whereas lesser lipophilic substituent Cl at the aryl ring
with OH at distal phenyl ring had shown 50% decrease
in the potency.
The basic structure of the synthesized compounds has
all the proposed pharmacophoric elements necessary
for the anticonvulsant activity like (i) benzothiazole as
a aryl ring system (A), (ii) NHC@O acted as hydropho-
bic domain (HBD), (iii) N as electron donor moiety (D)
and (iv) distal aryl ring (C) as shown in Figure 2.
The dependence of biological activity inset of congeneric
agents on lipophilic character has been shown in many
types of drug action. In particular, some reports have
indicated that anticonvulsant activity of different types
of compounds was correlated with lipophilicity.22 The
experimental logP values were determined using the oct-
anol–water method23 for the compounds having 100%
protection against the seizure spread in anti-MES screen.
Figure 3. Microphotograph of the section of liver. Group: control,
interference: normal hepatic parenchyma with portal triad (PT),
central vein (CV) and the hepatocytes. Magnification: 100·.

4182
N. Siddiqui et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4178–4182
6. Pandeya, S. N.; Yogeeswari, P.; Stables, J. P. Eur. J. Med.
Chem. 2000, 35, 879.
7. Dimmock, J. R.; Pandeya, S. N.; Quail, J. W.; Allen, T.
M.; Kao, G. Y. Eur. J. Med. Chem. 1995, 30, 303.
8. Pandeya, S. N.; Yogesswari, P.; Stables, J. P. Eur. J. Med.
Chem. 2000, 35, 879.
9. Yogeeswari, P.; Sriram, D.; Suniljit, L. R. J.; Kumar, S. S.;
Stables, J. P. Eur. J. Med. Chem. 2002, 37, 231.
10. Dimmock, J. R.; Sidhu, K. K.; Tumber, S. D.; Basran, S.
K.; Chen, M.; Quail, J. W.; Pugazhenthi, U.; Lechler, T.;
Stables, J. P. Eur. J. Med. Chem. 1995, 30, 287.
11. Siddiqui, N.; Pandeya, S. N.; Sen, A. P.; Singh, G. S.
Pharmakeftiki 1992, 4, 121.
12. Nadkarni, A. B.; Kamath, R. V.; Khadse, G. B. Indian J.
Heterocy. Chem. 2000, 9, 309.
13. Typical procedure: To the solution of cyanate (0.5 g) in
minimum quantity of water, glacial acetic acid (5 ml) was
added. This solution was heated with respective 2-amino-
6-substituted benzothiazole (1.7 g, 0.01 mol) in alcohol,
until the contents of mixture became turbid and volume
remained half of the original volume. The contents were
added to ice cool water. The solid obtained was filtered off
and dried. Further, to the warm hydrazine hydrate (5 ml),
solution of prepared urea in alcohol, NaOH (.04 g) was
added and solid obtained was filtered off and dried. The
solution of carboxamide in glacial acetic acid (5 ml) and
ethanol (10 ml) were heated to boiling and refluxed with
aromatic ketones (1 g, 0.122 mol) for 5 h. Refluxed solu-
tion was cooled to room temperature and kept overnight.
The solid was collected out, washed with methanol, dried
and recrystallized from ethanol to get the compound 1.
FTIR (KBr) cmÀ1: 3304, 3218 (NH), 3066 (CHAAr.),
2918 (CHAaliph.), 1662 (C@O), 1577 (C@N), 1088
(CACl), 657 (CASAC); 1H NMR (DMSO-d6): (d, ppm)
2.39 (s, 3H, CH3), 6.96–7.59 (m, 8H, Ar-H), 9.18 (s, 1H,
NHN@, D2O exchangeable), 9.32 (br s, 1H, NHC@O,
D2O exchangeable).
Figure 4. Microphotograph of the section of liver. Group: 8, interfer-
ence: showing normal portal triad structures. Magnification: 100·.
activity, which will account for bioactivity of majority of
compounds. Our recent results with 4-Cl and 4-alkyl
substituted phenyl ring of benzothiazole moiety have
given impetus to the present investigation. Substitution
with NO2 at the distal aryl ring showed favoured MES
activity as compared to hydroxy (OH) substitutent with
the bigger hydrophobic aryl ring. This observation sug-
gests that distal hydrophobic centre alters the bioavail-
ability of the compounds. In the toxicity studies none
of the compounds had shown neurotoxicity and liver
toxicity.
Acknowledgments
14. Krall, R. L.; Penry, J. K.; White, B. G.; Kapferberg, H. J.;
Swinyard, E. A. Epilepsia 1978, 19, 409.
15. Silvina, M. T.; Sung, C. M.; Luis, E. B.; Guillermina, L. E.
Bioorg. Med. Chem. 2004, 12, 3857.
16. Yogeeswari, P.; Sriram, D.; Saraswat, V.; vaigunda, R. J.;
Mohan, K. M.; Murugesan, S.; Thirumurugan, R.;
Stables, J. P. Eur. J. Pharm. Sci. 2003, 20, 341.
17. Reitman, S.; Frankel, S. Am. J. Clin. Pathol. 1957, 28, 56.
18. King, E. J.; Armstrong, A. R. Can. Med. Assoc. J. 1934,
31, 376.
Authors are thankful to Dr. A. Mukherjee MD,
Pathologist, All India Institute of Medical Sciences
(AIIMS), New Delhi, for the histopathological studies
and to the University Grants Commission (UGC,
Government of India) for providing financial
assistance.
References and notes
19. Reinhold, J. G. In Total Protein Albumin and Globulin.
Standard Methods in Clinical Chemistry; Reiner, M., Ed.;
Academic Press: New York, 1953; pp 88–90.
20. Varley, H. In Practical Clinical Biochemistry; CBS
Publishers and Distributors: New Delhi, 1988; pp 236–
238.
1. Wasteralin, C. In Basic Neurochemistry: Molecular, Cel-
lular and Medical Aspects; Siegel, G., Agranoff, G., Albers,
R. W., Molinoff, P., Eds.; Raven Press: New York, 1989;
pp 797–810.
2. Bell, G. S.; Sander, J. W. In The Epidemiology of Epilepsy:
The Size of the Problem (Suppl. A), pp 306–314.
3. Gupta, Y. K.; Malhotra, J. Indian J. Physiol. Pharmacol.
2000, 44, 8.
4. Coatsworth, J. J. Studies on the clinical Efficacy of
marketed antiepileptic drugs, NINDS Monograph No.
12, 73-15, HEW publication U.S. Government printing
office, 1971.
21. Luna, L. G. In Manual of Histological Staining Methods of
the Armed Forces Institute of Pathology; McGraw-Hill:
New York, 1968; pp 567–568.
22. Hansch, C.; Bjorkroth, J. P.; Leo, A. J. Pharm. Sci. 1987,
76, 663.
23. Farrar, V. A.; Ciechanowicz-Rutkowska, M.; Grochow-
ski, J.; Serda, P.; Pilati, T.; Filippini, G.; Hinko, C. N.; El-
Assadi, A.; Moore, J. A.; Edafiogho, I. O.; Andrews, C.
W.; Cory, M.; Nicholson, J. M.; Scott, K. R. J. Med.
Chem. 1993, 36, 3517.
5. Pandeya, S. N.; Ponnilavarasan, I.; Pandey, A.; Lakhan,
R.; Stables, J. P. Pharmazie 1999, 54, 923.