A novel series of 2,5-disubstituted 1,3,4-thiadiazoles as potential anticonvulsant agent

Article abstract of DOI:10.1016/j.cclet.2010.04.012

In pursuit for better antiepileptic drug and the importance of semicarbazones and 2,5-disubstituted 1,3,4-thiadiazoles as anticonvulsant pharmacophore, a series of novel N-({5-[(6-methyl-1-benzofuran-3-yl)methyl]-1,3,4-thiadiazol-2-yl}carbamothioyl)-2/3/4-substitutedbenzamide were designed, synthesized and evaluated for their anticonvulsant activity. The findings of the present studies confirmed that the pharmacophore model with four binding sites is crucial for anticonvulsant activity. Structure-activity relationships among synthesized compounds were also established.

Full text of DOI:10.1016/j.cclet.2010.04.012

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 1149–1152
www.elsevier.com/locate/cclet
A novel series of 2,5-disubstituted 1,3,4-thiadiazoles
as potential anticonvulsant agent
a
b
Harish Rajak a, , Chinmay K. Behera , Rajesh S. Pawar ,
*
Pradeep K. Singour b, Murli Dhar Kharya c
a SLT Institute of Pharmaceutical Sciences, Guru Ghasidas University, Bilaspur-495 009, (C.G.), India
b VNS Institute of Pharmacy, Neelbud, Bhopal-462 044, (M.P.), India
c Department of Pharmaceutical Sciences, Dr. H.S. Gour University, Sagar-470 003, (M.P.), India
Received 29 December 2009
Abstract
In pursuit for better antiepileptic drug and the importance of semicarbazones and 2,5-disubstituted 1,3,4-thiadiazoles as
anticonvulsant pharmacophore, a series of novel N-({5-[(6-methyl-1-benzofuran-3-yl)methyl]-1,3,4-thiadiazol-2-yl}carba-
mothioyl)-2/3/4-substitutedbenzamide were designed, synthesized and evaluated for their anticonvulsant activity. The findings
of the present studies confirmed that the pharmacophore model with four binding sites is crucial for anticonvulsant activity.
Structure–activity relationships among synthesized compounds were also established.
# 2010 Harish Rajak. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: 1,3,4-Thiadiazoles; Semicarbazones; Anticonvulsant activity
About 50 million people worldwide have epilepsy, with almost 90% of these people being in developing countries.
Epilepsy also affects about 4% of individuals over their lifetime. Despite the development of numerous new
anticonvulsants, more than 30% of people with epilepsy do not have seizure control and other do so only at the
disbursement of noteworthy dose-related toxicity and unusual side effects [1]. Thus there is an immense need for the
development of new anticonvulsant drugs with greater efficacy and minimal toxicity.
In the recent years, aryl semicarbazones have established as structurally novel class of compounds for their
antiepileptic activity [2,3]. On the other hand, several research groups have reported 1,3,4-thiadiazole analogues for
their anticonvulsant activity [4,5].
The conformational studies of the clinically established anticonvulsant drugs such as phenytoin, lamotrigine and
phenobarbitone has led to the proposal of a general model for anticonvulsant activity [6]. This semicarbazones based
pharmacophore model is consist of following four essential binding sites: (i) An aromatic hydrophobic binding site
(A); (ii) A hydrogen bonding domain (HBD); (iii) An electron donor group (D) and (iv) Another hydrophobic–
hydrophobic site regulating the pharmacokinetic properties of the anticonvulsant (C) (Fig. 1).
The title compounds were prepared using the synthetic strategy described in Scheme 1. 6-Methylbenzofuran-3-
acetic acid 1 was synthesized by the alkali hydrolysis of 7-methyl-4-bromomethyl coumarins [7]. The reaction of
* Corresponding author.
E-mail address: harishdops@yahoo.co.in (H. Rajak).
1001-8417/$ – see front matter # 2010 Harish Rajak. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.04.012

[(Fig._1)TD$FIG]
1150
H. Rajak et al. / Chinese Chemical Letters 21 (2010) 1149–1152
Fig. 1. Presence of pharmacophoric structural requirements in test compounds: (A) hydrophobic aromatic system, (HBD) hydrogen binding
domain, (D) electron donor moiety, and (C) hydrophobic–hydrophilic site.
[(Scheme_1)TD$FIG]
Scheme 1. Synthesis of 2,5-disubstituted-1,3,4-thiadiazole derivatives. Reaction conditions: (a) POCl3, reflux for 30 min, KOH; (b) Benzene, reflux
for 1 h; (c) NH4SCN, reflux for 40 min; (d) Benzene, reflux for 6 h.
compound 1 with thiosemicarbazide 2 in presence of POCl3 resulted in the formation of 5-(6-methylbenzofuran-3-
ylmethyl)-[1,3,4]thiadiazol-2-ylamine 3 [8]. The substituted benzoyl chlorides 5 were prepared by refluxing
substituted benzoic acid 4 with thionyl chloride in the presence of benzene. The substituted benzoylisothiocyanates 6
were synthesized by reaction of compound 5 with ammonium thiocyanate [9]. The equimolar quantities of compound
3 (0.01 mol) and compound 6 (0.01 mol) were refluxed in the presence of benzene for 6–8 h followed by filtration and
recrystallization with benzene to yield the title compounds 7(a–j) [10].
The anticonvulsant activity [11,12] of the test compounds was evaluated by maximal electroshock seizure (MES)
and subcutaneous pentylenetrtrazole (scPTZ) models using male albino mice (swiss, 18–25 g) and rat (wistar 100–
150 g). Phenytoin, carbamazepine and sodium valproate were used as the standard drugs for the comparison. Acute
neurological toxicity was determined in the rotorod test [13]. Procedures employed for evaluation of anticonvulsant
activity and neurotoxicity were reviewed and approved by the University Animal Ethical Committee.
The results indicates that 50% of the compounds i.e., 7(b), 7(d), 7(g), 7(h) and 7(j) were active in the MES
screening as compared to 40% of the compounds i.e., 7(c), 7(e), 7(g) and 7(i) in the ScPTZ test. Thus the compounds
exhibited some MES selectivity indicating their effectiveness in generalized seizures of the tonic–clonic (grandmal)
type. The majority of the compounds showed activity after 4 h, indicating that the synthesized compounds are slow
acting anticonvulsants. In the present studying N-({5-[(6-methyl-1-benzofuran-3-yl)methyl]-1,3,4-thiadiazol-2-
yl}carbamothioyl)-4-hydroxybenzamide 7(g) come out as the most active compound, showing considerable activity in
maximal electroshock seizure (at 100 mg/kg after 0.5 h and at 300 mg/kg after 4.0 h) and subcutaneous
pentylenetrtrazole model (at 300 mg/kg after 4.0 h) without any neurotoxicity (up to 300 mg/kg after 4.0 h) (Tables 1
and 2).

H. Rajak et al. / Chinese Chemical Letters 21 (2010) 1149–1152
1151
Table 1
Anticonvulsant activity and minimal motor impairment of 2,5-disubstituted 1,3,4-thiadiazoles.
Compound
Intraperitoneal injection in micea
MES screening
scPTZ screening
NT screening
0.5 h
0.5 h
4.0 h
0.5 h
4.0 h
4.0 h
7(a)
7(b)
7(c)
7(d)
7(e)
7(f)
7(g)
7(h)
7(i)
–
–
–
300
–
–
–
–
–
–
–
–
300
300
–
–
–
300
–
–
–
300
–
–
–
–
300
–
–
–
–
–
–
–
300
–
–
100
–
300
300
–
–
300
–
–
–
–
100
–
–
–
300
–
100
–
7(j)
–
300
–
300
Phenytoin
30
30
30
100
–
–
100
300
–
300
–
100
100
–
100
300
–
Carbamazepine
Na valproate
300
a
30, 100 and 300 mg/kg of doses were administered i.p. in mice. The data of the table indicates the minimal dose whereby biological activity was
demonstrated in half or more of the mice. The activity was measured after 0.5 and 4.0 h of dose administration of test compounds. The sign - (dash)
represents an absence of activity at maximum dose administered (300 mg/kg).
In order to synthesize a diverse range of title compounds, various substitutions i.e., hydroxy, nitro, methoxy and
chloro groups were employed around the phenyl ring on the right hand side of the molecules. On critical overview of
synthesized compounds, it has been found that compounds bearing the groups like hydroxy, methoxy on distant phenyl
ring possess high potency in MES and scPTZ tests. Whereas replacement of these groups with nitro and chloro groups
on the distant phenyl ring has resulted in compounds with decrease in anticonvulsant activity. On comparison of
results, it has been found that antiepileptic activity of test compounds changes on varying p-substituted group on aryl
moiety as follows: hydroxy > methoxy > nitro > chloro group. The biological inactiveness of compound 7(a) in
anticonvulsant screening might be due to lack of any electronegative substitution on the phenyl ring. Compounds with
substituted phenyl ring were found to possess considerable activity in comparison to compounds with naphthyl or
benzoylphenyl group. Replacement of the phenyl ring i.e., 7(b)–7(h) with naphthyl or benzoylphenyl group i.e., 7(i)
and 7(j) in the structure of synthesized compounds caused decrease in anticonvulsant activity. It might be due to the
increase in the lipophilic nature of aromatic hydrophobic binding site beyond optimum lipophilicity required for
biological activity. The considerable anticonvulsant activity of compounds 7(b)–7(h) may also be attributed to the
presence of phenyl substitution which might be responsible for additional van der Waals bonding to the binding site.
The structure of the title compounds fulfilled all the structural requirements i.e., presence of hydrophobic aromatic ring
system, N as electron donor system and (6-methyl-1-benzofuran-3-yl)methyl-1,3,4-thiadiazol-2-yl moiety as
hydrophobic–hydrophilic portion responsible for metabolism. These new data might be expedient in the future
development of semicarbazones as novel anticonvulsants.
Table 2
Anticonvulsant evaluation of compounds after oral administration in rats.
Compound
Oral administration in ratsa
MES screening
NT screening
0.25 h
0.25 h
0.5 h
1 h
2 h
4 h
0.5 h
1 h
2 h
4 h
7(e)
7(g)
7(h)
0
0
0
1
0
0
0
0
0
1
0
1
1
1
1
0
0
0
0
0
0
0
0
0
1
0
0
0
0
1
a
The compounds were administered in a dose of 30 mg/kg. The data in table indicates the number of rats out of four, which were protected.

1152
H. Rajak et al. / Chinese Chemical Letters 21 (2010) 1149–1152
References
[1] T.R. Brown, G.L. Holmes, N. Engl. J. Med. 344 (2001) 1145.
[2] R. Thirumurugan, D. Sriram, A. Saxena, et al. Bioorg. Med. Chem. 14 (2006) 3106.
[3] N. Siddiqui, A. Rana, S.A. Khan, et al. Bioorg. Med. Chem. Lett. 17 (2007) 4178.
[4] H. Rajak, R. Deshmukh, S. Kashaw, et al. Arch. Pharm. 342 (2009) 453.
[5] V. Jatav, P. Mishra, S. Kashaw, et al. Eur. J. Med. Chem. 43 (2008) 1945.
[6] S.N. Pandeya, I. Ponnilavarasan, A. Pandey, et al. Pharmazie 54 (1999) 923.
[7] Y. Fall, L. Santana, M. Teijeira, et al. Heterocycles 41 (1995) 647.
[8] V.B. Jadhav, M.V. Kulkarni, V.P. Rasal, et al. Eur. J. Med. Chem. 43 (2008) 1721.
[9] A. Rana, N. Siddiqui, S.A. Khan, et al. Eur. J. Med. Chem. 43 (2008) 1114.
[10] Compound 7(d): mp (8C) 218–220; yield 62%; IR (cmꢀ1) (KBr) 3035.3 (aromatic C–H str), 1601.3 and 1502.7 (aromatic C–C str), 746.8 (C–S
of 1,3,4-thiadiazole nucleus), 1644.1 (C N of 1,3,4-thiadiazole nucleus), 1684.2 (C O str of amide), 3436.6 (N–H str of amide), 1612.4
(C N group), 833.6 ( para-disubstituted C–H def), 722.8 (C–Cl str), 2894.8 (Alkane C–H str), 1092.5 (C S str); 13C NMR (75 MHz, DMSO-
d6, TMS): d 140.3 (C-20), 115.8 (C-30), 126.4 (C-40), 154.1 (C-50), 112.5 (C-60), 133.6 (C-70), 123.8 (C-80), 121.6 (C-90), 164.3 (C-2), 156.4 (C-
5), 177.5 (NHCSNHNCO), 168.9 (NHCSNHNCO), 128.8 (C-2000 and C-6000), 129.2 (C-3000 & C-5000), 137.3 (C-4000), 131.6 (C-1000), 21.2 (CH3),
24.4000 (CH2 connecting benzofuran & thiadiazole ring); 1H NMR (300 MHz, DMSO-d6, TMS): d 7.5–7.9 (m, 4H, ArH), 2.3 (s, 3H, CH3), 4.2 (s,
2H, CH2), 6.9–7.4 (m, 4H, benzofuran protons), 6.5 (s, 1H, NHCSNHCO), 9.3 (s, 1H, NHCSNHCO); ESI-MS (methanol) m/z 443.9 ([M+H]+).
Compound 7(g): mp (8C) 263–265; yield 65%; IR (cmꢀ1) (KBr) 3040.6 (aromatic C–H str), 1602.5 and 1503.8 (aromatic C–C str), 741.7 (C–S
of 1,3,4-thiadiazole nucleus), 1648.1 (C N of 1,3,4-thiadiazole nucleus), 1680.2 (C O str of amide), 3439.4 (N–H str of amide), 1616.8
(C N group), 828.9 (C–H def disubstituted benzene ring), 2890.6 (alkane C–H str), 1091.7 (C S str), 3447.3 (O–H str of alcoholic group); 13
C
NMR (75 MHz, DMSO-d6, TMS): d 140.3 (C-20), 115.7 (C-30), 126.4 (C-40), 154.1 (C-50), 112.6 (C-60), 133.7 (C-70), 123.9 (C-80), 121.6 (C-
90), 164.2 (C-2), 156.5 (C-5), 177.6 (NHCSNHNCO), 168.9 (NHCSNHNCO), 128.9 (C-2000 and C-6000), 115.8 (C-3000 and C-5000), 160.8 (C-4000),
126.2 (C-1000), 21.3 (CH3), 24.3 (CH2 connecting benzofuran & thiadiazole ring); 1H NMR (300 MHz, DMSO-d6, TMS): d 6.9–7.8 (m, 4H,
ArH), 2.3 (s, 3H, CH3), 4.2 (s, 2H, CH2), 6.9–7.4 (m, 4H, benzofuran protons), 6.4 (s, 1H, NHCSNHCO), 9.4 (s, 1H, NHCSNHCO), 5.3 (s, 1H,
ArOH); ESI-MS (methanol) m/z 425.5.([M+H]+).
[11] R.I. Krall, J.K. Penry, B.G. White, et al. Epilepsia 19 (1978) 409.
[12] R.J. Porter, B.J. Hessie, J.J. Cereghino, et al. Fed. Proc. 44 (1985) 2645.
[13] H.G. Vogel, Drug Discovery and Evaluation—Pharmacological Assays, Springer, Germany, 2002, p. 487.