Organic Chemistry International

Volume 2013 (2013), Article ID 631723, 8 pages

http://dx.doi.org/10.1155/2013/631723

Research Article

Synthesis of Pyrazine Substituted 1,3,4-Thiadiazole Derivatives and Their Anticonvulsant Activity

Kikkeri P. Harish,¹ Kikkeri N. Mohana,¹ and Lingappa Mallesha²

¹Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore 570 006, India

²PG Department of Chemistry, JSS College of Arts, Commerce and Science, Ooty Road, Mysore 570025, India

Received 1 April 2013; Revised 13 September 2013; Accepted 13 September 2013

Academic Editor: Ashraf Aly Shehata

Copyright © 2013 Kikkeri P. Harish et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The synthesis of new pyrazine substituted 1,3,4-thiadiazole derivatives was carried out in good yield by the reaction of pyrazine substituted 1,3,4-thiadiazoles with various sulfonyl chlorides. A chemical structure of all the new compounds was confirmed by ¹H NMR and mass spectral data. The new compounds were screened for their anticonvulsant activity against maximal electroshock (MES) seizure method. Rotarod method was employed to determine the neurotoxicity. Few compounds showed significant changes in anticonvulsant activity. The same compounds showed no neurotoxicity at the maximum dose administered (100 mg/kg).

1. Introduction

Epilepsy has been recognized as a neurological disorder, affecting a large section of people across the world. The word epilepsy usually describes a group of common chronic neurological disorders characterized by recurrent unprovoked seizures due to excessive neuronal firing or synchronous neuronal activity in the brain [1, 2]. Seizures may vary from the briefest lapses of attention or muscle jerks to severe and prolonged convulsions [3]. The maximal electroshock (MES) test is a predictor of compounds that are active against seizures [4]. The anticonvulsant drug design is based on the presumption that at least one phenyl or similar aromatic group in close proximity to two electron donor atoms in the compound is required for the activity in MES [5, 6]. Newer drugs such as flupirtine [7], topiramate [8], zonisamide [9], and vigabatrin [10] have emerged as promising anticonvulsants.

Pyrazines and its derivatives play an important role in the drug discovery realm. In particular the structural analogue of purines derivatives presents various pharmacological activities such as antibacterial [11], anti-inflammatory [12], antidepressant [13], and antiproliferative activities [14]. Thiadiazoles exhibit a broad spectrum of biological effectiveness such as antiparkinsonism [11], antihistaminic [15], and antiasthmatic [16]. Thiazolidin-4-one derivatives are also known to exhibit diverse bioactivities such as anticonvulsant [17], antidiarrheal [18], antihistaminic [19], antidiabetic [20], cardioprotective [21], and anticancer [22]. Similarly, 2,5-disubstituted 1,3,4-thiadiazoles also display wide spectrum of activities such as antibacterial [23] and anticonvulsant [24]. In the present study, a series of new pyrazine substituted 1,3,4-thiadiazole derivatives 7(a–o) have been synthesized and their anticonvulsant effects are determined through maximal electroshock (MES) seizure test.

2. Materials and Methods

2.1. Chemistry

Melting range was determined by Veego Melting Point VMP III apparatus. Elemental analyses were recorded on VarioMICRO superuser V1.3.2 Elementar. ¹H NMR spectra were recorded on Bruker DRX-500 spectrometer at 400 MHz using DMSO-d₆ as solvent and TMS as an internal standard. Mass spectral data were obtained by LC/MSD Trap XCT. Silica gel column chromatography was performed using Merck 7734 silica gel (60–120 mesh) and Merck-made TLC plates.

2.1.1. Preparation of 1,3,4-Thiadiazole-2-ylamine (2)

Thiosemicarbazide (1, 50.0 g, 0.5486 mol) was taken in 100 mL formic acid and the reaction was stirred at room temperature for 1 hr. The reaction was cooled and 100 mL conc. hydrochloric acid was added. Reaction completion was monitored by TLC. The reaction was cooled to 0°C and basified with ammonium hydroxide solution. The solid formed was filtered, washed with water, and dried to yield the above compound as off white solid (Yield: 45.50 g, 82%). ¹H-NMR (400 MHz, DMSO-d₆): δ 4.21 (s, br, 2H), 9.31 (s, 1H). MS (ESI) m/z: 102.14.

2.1.2. Preparation of 5-Bromo-1,3,4-thiadiazol-2-amine (3)

To a stirred solution of 2 (40.0 g), sodium acetate (64.89 g) in 200 mL acetic acid (4.0 vol) at 10°C, bromine was added drop wise (37.92 g). The reaction was stirred at room temperature for 3 hours. The reaction was concentrated and basified with saturated sodium bicarbonate solution. The compound was extracted with ethyl acetate and the ethyl acetate layer was washed with water followed by brine, dried over sodium sulphate. The crude product was crystallized using ethyl acetate to yield the above compound as off white solid (Yield: 53.4 g, 75%). ¹H-NMR (400 MHz, DMSO-d₆): δ 4.21 (s, br, 2H). MS (ESI) m/z: 181.10.

2.1.3. Preparation of tert-Butyl 4-(5-amino-1,3,4-thiadiazole-2-yl)piperazine-1-carboxylate (4)

To a stirred solution of 3 (25.0 g, 0.1388 mol), potassium carbonate (57.57 g, 0.4165 mol) in dimethyl formamide (250 mL) and mono boc piperazine (31.03 g, 0.1666 mol) were added and stirred the reaction for 10 hr at room temperature. The reaction was quenched into ice water (2.0 L) and stirred at room temperature for 1 hr. Solid formed was filtered and dried to yield the above compound as off white solid (Yield: 33.6 g, 85%). ¹H-NMR (400 MHz, DMSO-d₆): δ 1.41 (s, 9H), 3.10 (t, J = 4.80 Hz, 4H), 3.77 (t, J = 5.36 Hz, 4H), 4.21 (s, 2H). MS (ESI) m/z: 286.03.

2.1.4. Preparation of tert-Butyl-4-(5-(pyrazine-2-carboxamido)-1,3,4-thiadiazol-2-yl)piperazine-1-carboxylate (5)

To a stirred solution of 4 (25.0 g, 0.0876 mol), in dichloromethane (250 mL), triethyl amine (26.59 g, 0.2628 mol), pyrazine-2-carboxylic acid (13.04 g, 0.1051 mol) and TBTU (33.75 g, 0.1051 mol) were added. The reaction was stirred overnight at room temperature and the reaction completion was monitored by TLC. The reaction was washed with saturated bicarbonate solution followed by 1.0 N HCl, water, and brine. The organic layer was dried over sodium sulphate, concentrated, and crystallized using dichloromethane to yield the above compound as off white solid (Yield: 28.80 g, 84%). ¹H-NMR (400 MHz, DMSO-d₆): δ 1.41 (s, 9H), 3.23 (t, J = 3.76 Hz, 4H), 3.75 (t, J = 4.40 Hz, 4H), 8.83 (d, J = 2.00 Hz, 1H), 8.94 (d, J = 2.00 Hz, 1H), 9.25 (s, 1H), 9.72 (s, 2H).

2.1.5. Preparation of N-(5-(Piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide hydrochloride (6)

To a stirred solution of 5 (20 g, 0.0687 mol) in 1,4 dioxane at 0–5°C, 4 N HCl in dioxane (80.0 mL) was added. The reaction was stirred at room temperature for 10 hr. The solid was filtered, washed with diethyl ether, and packed in air tight container to yield the above compound as pale yellow hygroscopic solid (16.73 g, 90%). ¹H-NMR (400 MHz, DMSO-d₆): δ 3.22 (t, J = 3.75 Hz, 4H), 3.73 (t, J = 4.30 Hz, 4H), 8.83 (d, J = 2.00 Hz, 1H), 8.94 (d, J = 2.00 Hz, 1H), 9.27 (s, 1H), 9.72 (s, 2H). MS (ESI) m/z: 292.05.

2.1.6. General Procedure for the Synthesis of Pyrazine Substituted 1,3,4-Thiadiazole Derivatives 7 (a–o)

To a mixture of 6 (1.0 eq) and triethyl amine (4.5 eq) in dichloromethane (10 volume), substituted sulphonyl chloride (2.4 eq) was added at 5–10°C and stirred overnight at room temperature. Reaction completion was confirmed through TLC. The reaction was poured into separating funnel, washed with water followed by brine solution, and dried over anhydrous sodium sulphate. Crude product was purified by crystallization using ethyl acetate to yield the thiadiazole substituted sulfonamide as off white to pale yellow solids.

(1) N-(5-(4-((1,ľ-Biphenyl)sulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7a ). Yield: 70% (White solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.18 (t, J = 4.10 Hz, 4H), 3.67 (t, J = 5.02 Hz, 4H), 7.40–7.52 (m, 3H), 7.64–7.87 (m, 6H), 8.81 (d, J = 2.35 Hz, 1H), 8.90 (d, J = 2.45 Hz, 1H), 9.26 (s, 1H), 12.2 (s, 1H). MS (ESI) m/z: 508.6. Anal. Calcd. for C₂₃H₂₁N₇O₃S₂: C, 54.42; H, 4.17; N, 19.32; Found: C, 54.13; H, 4.32; N, 19.52%.

(2) N-(5-(4-((2,4-Dimethylphenyl)sulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7b ). Yield: 72% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 2.34 (s, 3H), 2.37 (s, 3H), 3.21 (t, J = 4.01 Hz, 4H), 3.68 (t, J = 4.90 Hz, 4H), 7.22 (s, 1H), 7.42 (d, J = 6.35 Hz, 2H), 8.70 (d, J = 2.25 Hz, 1H), 8.81 (d, J = 2.46 Hz, 1H), 9.16 (s, 1H), 12.3 (s, 1H). MS (ESI) m/z: 460.60. Anal. Calcd. for C₁₉H₂₁F₆N₇O₃S₂: C, 49.66; H, 4.61; N, 21.34; Found: C, 49.75; H, 4.68; N, 21.46%.

(3) N-(5-(4-(Methylsulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7c ). Yield: 72% (White solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 2.35 (s, 3H), 3.23 (t, J = 3.86 Hz, 4H), 3.72 (t, J = 4.31 Hz, 4H), 8.83 (d, J = 2.00 Hz, 1H), 8.94 (d, J = 2.00 Hz, 1H), 9.27 (s, 1H), 12.1 (s, br, 1). MS (ESI) m/z: 370.40. Anal. Calcd. for C₁₂H₁₅N₇O₃S₂: C, 39.01; H, 4.09; N, 26.54; Found: C, 38.97; H, 3.96; N, 26.74%.

(4) N-(5-(4-((2,5-Dichlorothiophen-3-yl)sulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7d ). Yield: 74% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.31 (t, J = 4.08 Hz, 4H), 3.61 (t, J = 3.84 Hz, 4H), 7.43 (s, 1H), 8.83 (d, J = 2.20 Hz, 1H), 8.95 (d, J = 2.24 Hz, 1H), 9.28 (s, 1H), 12.40 (s, 1H). MS (ESI) m/z: 505.50. Anal. Calcd. for C₁₅H₁₃Cl₂N₇O₃S₃: C, 35.58; H, 2.59; N, 19.36; Found: C, 35.35; H, 2.75; N, 19.54%.

(5) N-(Naphthalene-2-ylsulfonyl)-N-(5-(4-(naphthalene-2-ylsulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7e ). Yield: 75% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.14 (t, J = 4.96 Hz, 4H), 3.57 (t, J = 5.36 Hz, 4H), 7.50–7.54 (m, 2H), 7.67–7.79 (m, 5H), 7.85–7.87 (m, 1H), 7.88–7.91 (m, 1H), 8.09–8.24 (m, 4H), 8.48 (s, 1H), 8.80 (d, J = 2.32 Hz, 1H), 8.92 (d, J = 2.44 Hz, 1H), 9.23 (s, 1H). MS (ESI) m/z: 686.80. Anal. Calcd. for C₃₂H₂₇N₇O₅S₃: C, 55.43; H, 3.75; N, 14.60; Found: C, 55.54; H, 3.65; N, 14.42%.

(6) N-(5-(4-((3,5-Dimethylisoxazol-4-yl)sulfonyl)piperazine-1-yl)1,3,4-thiadiazole-2-yl) pyrazine-2-carboxamide ( 7f ). Yield: 73% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 2.34 (s, 3H), 2.62 (s, 3H), 3.22 (t, J = 4.92 Hz, 4H), 3.58 (t, J = 5.28 Hz, 4H), 8.82 (d, J = 2.36 Hz, 1H), 8.93 (d, J = 2.44 Hz, 1H), 9.26 (s, 1H), 12.30 (s, 1H). MS (ESI) m/z: 451.00. Anal. Calcd. for C₁₆H₁₈N₈O₄S₂: C, 42.66; H, 4.03; N, 24.87; Found: C, 42.78; H, 4.30; N, 24.75%.

(7) N-(5-(4-((3,5-Bis(trifluoromethyl)phenyl)sulfonyl) piperazine-1-yl)-1,3,4-thiazol-2-yl) pyrazine-2-carboxamide ( 7g ). Yield: 72% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.31 (t, J = 4.08 Hz, 4H), 3.88 (t, J = 4.96 Hz, 4H), 8.15 (s, 1H), 8.23 (s, br, 2H), 8.71 (d, J = 2.16 Hz, 1H), 8.74 (s, 1H), 8.91 (d, J = 2.28 Hz, 1H), 9.47 (s, 1H). MS (ESI) m/z: 566.40. Anal. Calcd. for C₁₉H₁₅ F₆N₇O₃S₂: C, 40.21; H, 2.66; N, 17.28; Found: C, 40.28; H, 2.54; N, 17.43%.

(8) N-(5-(4-Tosylpiperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7h ). Yield: 71% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 2.24 (s, 3H), 3.25 (t, J = 4.82 Hz, 4H), 3.78 (t, J = 5.08 Hz, 4H), 7.75 (d, J = 8.58 Hz, 2H), 8.12 (d, J = 8.13 Hz, 2H), 8.82 (d, J = 2.26 Hz, 1H), 8.93 (d, J = 2.34 Hz, 1H), 9.23 (s, 1H), 12.27 (s, 1H). MS (ESI) m/z: 444.50. Anal. Calcd. for C₁₈H₁₉N₇O₃S₂: C, 48.53; H, 4.30; N, 22.01; Found: C, 48.67; H, 4.42; N, 22.31%.

(9) N-(5-(4-((4-(tert-Butyl)phenyl)sulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7i ). Yield: 73% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 1.14 (s, 9H), 3.41 (t, J = 4.72 Hz, 4H), 3.93 (t, J = 5.28 Hz, 4H), 7.72 (d, J = 7.68 Hz, 2H), 7.99 (d, J = 7.83 Hz, 2H), 8.80 (d, J = 2.27 Hz, 1H), 8.91 (d, J = 2.44 Hz, 1H), 9.21 (s, 1H), 12.23 (s, 1H). MS (ESI) m/z: 486.50. Anal. Calcd. for C₂₁H₂₅N₇O₃S₂: C, 51.73; H, 5.17; N, 20.11; Found: C, 51.53; H, 5.32; N, 20.32%.

(10) N-(5-(4-((3-(Trifluoromethyl)phenyl)sulfonyl) piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide (7j ). Yield: 73% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.15 (t, J = 4.40 Hz, 4H), 3.60 (t, J = 5.20 Hz, 4H), 7.90–7.94 (m, 1H), 8.00 (s, 1H), 8.09–8.14 (m, 2H), 8.78 (d, J = 2.26 Hz, 1H), 8.85 (d, J = 2.44 Hz, 1H), 9.23 (s, 1H), 12.23 (s, 1H). MS (ESI) m/z: 498.50. Anal. Calcd. for C₁₈H₁₆F₃N₇O₃S₂: C, 43.28; H, 3.23; N, 19.63; Found: C, 43.43; H, 3.43; N, 19.73%.

(11) N-(5-(4-((4-Iodophenyl)sulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7k ). Yield: 71% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.06 (t, J = 3.80 Hz, 4H), 3.45 (t, J = 4.10 Hz, 4H), 7.68 (d, J = 7.05 Hz, 2H), 7.86 (d, J = 6.75 Hz, 2H), 8.78 (d, J = 2.22 Hz, 1H), 8.92 (d, J = 2.34 Hz, 1H), 9.24 (s, 1H), 12.28 (s, 1H). MS (ESI) m/z: 556.80. Anal. Calcd. for C₁₇H₁₆IN₇O₃S₂: C, 36.63; H, 2.89; N, 17.59; Found: C, 36.53; H, 2.75; N, 17.64%.

(12) N-(5-(4-((Cyclopropane)sulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7l ). Yield: 75% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 0.7-0.8 (m, 4H), 1.0–1.02 (m, 1H), 3.26 (t, J = 3.70 Hz, 4H), 3.72 (t, J = 4.28 Hz, 4H), 8.83 (d, J = 2.20 Hz, 1H), 8.94 (d, J = 2.32 Hz, 1H), 9.23 (s, 1H), 9.70 (s, 1H). MS (ESI) m/z: 394.40. Anal. Calcd. for C₁₄H₁₇N₇O₃S₂: C, 42.52; H, 4.33; N, 24.79; Found: C, 42.43; H, 4.54; N, 24.86%.

(13) N-(5-(4-((2-Nitro-4-(trifluoromethyl)phenyl)sulfonyl) piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide (7m ). Yield: 75% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.31 (t, J = 4.08 Hz, 4H), 3.88 (t, J = 4.96 Hz, 4H), 8.25 (s, br, 2H), 8.65 (s, 1H), 8.81 (d, J = 2.40 Hz, 1H), 8.93 (d, J = 2.68 Hz, 1H), 9.26 (s, 1H), 12.30 (s, 1H). MS (ESI) m/z: 543.50. Anal. Calcd. for C₁₈H₁₅F₃N₈O₅S₂: C, 39.71; H, 2.78; N, 20.58; Found: C, 39.79; H, 2.65; N, 20.76%.

(14) N-(5-(4-((4-Bromophenyl)sulfonyl)piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide ( 7n ). Yield: 72% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.16 (t, J = 3.90 Hz, 4H), 3.55 (t, J = 4.20 Hz, 4H), 7.71 (d, J = 7.25 Hz, 2H), 7.83 (d, J = 7.65 Hz, 2H), 8.73 (d, J = 2.29 Hz, 1H), 8.90 (d, J = 2.34 Hz, 1H), 9.28 (s, 1H), 12.18 (s, 1H). MS (ESI) m/z: 509.40. Anal. Calcd. for C₁₇H₁₆BrN₇O₃S₂: C, 40.01; H, 3.16; N, 19.21; Found: C, 39.98; H, 3.32; N, 19.32%.

(15) N-(5-(4-((4-(Trifluoromethyl)phenyl)sulfonyl) piperazine-1-yl)-1,3,4-thiadiazole-2-yl)pyrazine-2-carboxamide (7o ). Yield: 73% (Off white solid); ¹H-NMR (400 MHz, DMSO-d₆): δ 3.16 (t, J = 4.40 Hz, 4H), 3.68 (t, J = 5.20 Hz, 4H), 7.10 (dd, J = 1.84, 7.42 Hz, 2H), 8.09–8.14 (m, 2H), 8.71 (d, J = 2.25 Hz, 1H), 8.82 (d, J = 2.44 Hz, 1H), 9.27 (s, 1H), 12.21 (s, 1H). MS (ESI) m/z: 498.40. Anal. Calcd. for C₁₈H₁₆F₃N₇O₃S₂: C, 43.28; H, 3.23; N, 19.63; Found: C, 43.38; H, 3.42; N, 19.60%.

2.2. Anticonvulsant Evaluation

2.2.1. Animals

Male wistar rats procured from the National Institute of Nutrition, Hyderabad (190–220 g), were used in the present study. The animals were kept in individual cages for one week to acclimatize for the laboratory conditions. They were allowed to free access of water and food.

All the experimental procedures were carried out in accordance with Committee for the Purpose of Control and Supervision of Experiments on Animals (CPCSEA) guidelines. The study was reviewed and approved by the Institutional Animal Ethics Committee, G Pulla Reddy College of Pharmacy, Hyderabad, India.

2.2.2. Maximal Electroshock Seizure Model (MES)

Maximal electroshock seizure model was used in the present study to evaluate the anticonvulsant activity of the compounds on male wistar rats. Seizures were induced in rats by delivering electro shock of 150 mA for 0.2 s by means of a convulsiometer through a pair of ear clip electrodes. The test compounds (100 mg/kg) were administered by oral route in the form of solution (the compounds were dissolved in 1% sodium carboxymethyl cellulose), 30 minutes before the maximal electroshock seizure test. The animals were observed closely for 2 minutes. The percentage of inhibition of seizure relative to control was recorded and calculated [25]. Phenytoin (100 mg/kg) was used as a standard drug.

2.2.3. Neurotoxicity Screening

Acute neurological toxicity in mice was evaluated by rotarod test [25]. The mice were trained to stay on the accelerating rotarod that rotates at 10 revolutions per minute. The rod diameter was 3.2 cm. Trained animals were administered with the test compounds at the dose of 100 mg/kg. Neurotoxicity was indicated by the inability of the animal to maintain equilibrium on the rod for at least one minute in each of the three trails. Phenytoin was used as a standard drug.

2.2.4. Statistical Analysis

In the present study, data were analyzed by one way analysis of variance (ANOVA) followed by dunnett test to compare difference between the groups.

3. Results and Discussion

3.1. Chemistry

New pyrazine substituted 1,3,4-thiadiazole derivatives 7(a–o) were synthesized according to Scheme 1. Readily available starting materials and simple synthesizing procedures make this method very attractive and convenient for the synthesis of various thiadiazoles. Formation of products was confirmed by recording their elemental analyses, ¹H NMR and mass spectra. The ¹H NMR, spectra of 7g showed piperazine ring in the region of δ, 3.31–3.88. The mass spectra of 7g showed molecular ion peak at m/z 566.4 which is in agreement with the molecular formula, C₁₉H₁₅F₆N₇O₃S₂. The elemental analyses data showed good agreement between the experimentally determined values and the theoretically calculated values within ± 0.4 %. The chemical structures and physical data of all the synthesized compounds are tabulated in Table 1.

Table 1: Chemical structure and melting range of pyrazine substituted 1,3,4-thiadiazole derivatives 7(a–o).

Compound
R
Structure
M.p. (°C)

7a

244–246

7b

136–138

7c

170–172

7d

200–202

7e

138–140

7f

228–230

7g

260–262

7h

160–162

7i

220–222

7j

166–168

7k

252–254

7l

88–90

7m

226–228

7n

234–236

7o

170–172

Scheme 1

3.2. Biological Activity

In the present study, the anticonvulsant activity of the synthesized compounds 7(a–o) was evaluated by MES model at the dose of 100 mg/kg and the results are summarized in Table 2. Compounds 7d and 7g demonstrated significant protective effect on MES induced seizure and the effect of 7d and 7g was similar to that of standard (phenytoin). Similarly, compounds 7f, 7j, 7k, 7m, 7n, and 7o also showed good protective effect. Compounds 7a, 7b, 7c, 7e, 7h, 7i, and 7l have relatively lower anticonvulsant potencies. All the compounds were examined for their neurotoxicity using rotarod method given in the dose of 100 mg/kg. Except compounds 7c, 7h, and 7l, none of the compounds showed neurotoxicity. These compounds showed 25% toxicity compared to standard at 2 h of oral administration (Table 3).

Table 2: Effect of compounds in the maximal electroshock seizure test.

Treatment

% Protection

7a
6.51
48.70

7b
7.38
37.20

7c
8.17
9.43

7d
2.00
74.52

7e
6.85
45.41

7f
3.47
58.60

7g
1.75
74.88

7h
8.10
10.00

7i
8.05
10.62

7j
2.53
68.10

7k
4.70
50.31

7l
8.12
9.96

7m
2.10
69.43

7n
4.49
55.34

7o
2.96
65.20

Standard
1.98
75.88

Control (vehicle)
8.21
—

= Extension/Flexion [Decrease in ratio of extension phase (in seconds)/flexion phase (in seconds)].

% Protection = (control‒test)/(control) * 100.

Table 3: Neurotoxicity screening of the compounds 7(a–o).

Compound
Neurotoxicity screen

0.5 h
1 h
2 h
4 h

7a
0/4
0/4
0/4
0/4

7b
0/4
0/4
0/4
0/4

7c
0/4
0/4
1/4
1/4

7d
0/4
0/4
0/4
0/4

7e
0/4
0/4
0/4
0/4

7f
0/4
0/4
0/4
0/4

7g
0/4
0/4
0/4
0/4

7h
0/4
0/4
1/4
1/4

7i
0/4
0/4
0/4
0/4

7j
0/4
0/4
0/4
0/4

7k
0/4
0/4
0/4
0/4

7l
0/4
0/4
1/4
1/4

7m
0/4
0/4
0/4
0/4

7n
0/4
0/4
0/4
0/4

7o
0/4
0/4
0/4
0/4

Standard
0/4
0/4
0/4
0/4

The data in the table represent the ratio between the numbers of the animals that exhibited neurotoxicity against the number of tested animals.

The structural activity relationship study of these compounds indicate that the introduction of a piperazine group of pyrazine ring and 3,5-bis(trifluoromethyl) positions of the benzenesulfonyl moiety showed the best anticonvulsant activity in 7g. Compound 7d contains halogen and thiophene group showed good anticonvulsant activity in the MES model. Both compounds did not exhibit neurotoxicity at the highest administered dose. Compounds with phenyl ring in 7h exhibited more anticonvulsant activity in comparison to methyl group (7c). Compounds 7c, 7h, and 7l contribute 25% of neurotoxicity at 2 h. Among the synthesized compounds 7(a–o), all the compounds showed activity in the range of 9.43–74.88% in comparison to phenytoin which completely inhibited the convulsions produced by electro-convulsometer, but compound 7g having electron withdrawing groups showed excellent anticonvulsant activity.

4. Conclusions

In conclusion, a series of new pyrazine substituted 1,3,4-thiadiazole derivatives 7(a–o) were synthesized in good yield, characterized by different spectral studies, and their anticonvulsant activity have been evaluated. Various thiadiazole derivatives with electron withdrawing groups showed potent anticonvulsant activity. Among the synthesized compounds, 7d and 7g showed excellent anticonvulsant activity. Compounds having thiadiazole ring showed better biological activities than compounds having oxadiazole ring.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors thank Dr. B. Veeresh, B. Madhava Reddy, and Nagula Naresh Kumar, Department of Pharmacology and Toxicology, G Pullareddy College of Pharmacy, Hyderabad, India, for carrying out Anticonvulsant activity.

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