EXPERIMENTAL:

All the chemicals were obtained from commercial suppliers and used after further purification. All the melting points were determined in open capillary tubes and are uncorrected. The IR spectra (ὑ in cm^-1) were recorded on a perkin-Elmer spectrophotometer in KBr pellets. ¹HMR spectra were recorded on Varian Gemini (200 MHz) spectrometer using DMSO as solvent and TMS as an internal standard. All chemical shifts values are reported in δ scale downfield from TMS. Homogeneity of the compound was checked by TLC on silica gel plates.

General procedure for the synthesis Ethyl-2-(benzothiazolylthio) acetate. 3

Conventional Method:

Dissolve the 2-Mercaptobenzothiazole (2.0 mol) in methanol and ethyl chloro acetate (2.0 mol) was added dropwise in presence of K₂CO₃ (8 g) in the mixture with stirring. The resulted mixture was refluxed for 10 hours and the reaction mixture poured into ice cold water and neutralized with dil HCl. The semisolid thus obtained was washed several times with water and left in water for 72 hours. The crystals formed were filtered, washed thoroughly with water and dried. The completion of the reaction was mentioned on T.L.C.

mp. 44-47 ^oC , Yield – 65-75 %

Green Method:

2-Mercaptobenzothiazole 1 (0.01 mole) and ethylchloroacetate (0.01 mole) 2 in ethanol and NaOH were stirred for 10 hr room temperature. The completion of the reaction was mentioned on T.L.C. The reaction mixture was poured on to the crushed ice. The solid precipitated was filtered, washed with water and recrystallised from chloroform.

The obtained product was identified from comparing product obtained by conventional method and their melting point.

mp. 45-47 ^oC , Yield- 80-90 %

IR (KBr): 3022, 1723, 1614, 696 cm^-1.

¹H-NMR: δ (ppm): 1.23 (3H, t), 4.13 (2H, q), 4.46 (2H, s), 6.79-7.87 (4H, m).

Synthesis of [(2-benzothiazolylthio) acetyl]-hydrazine 4

Conventional Method:

The Ethyl-2-(benzothiazolylthio) acetate 3 (0.01 mol), hydrazine hydrate (0.01 mol) was refluxed in the presence of ethanol for 5 h. The reaction mixture was cooled and then poured in to crushed ice. The solid product formed, was filtered, dried and recrystallized using ethanol.

mp = 165-167^oC, Yield = 62 %

Green Method:

A mixture of Ethyl-2-(benzothiazolylthio) acetate 3 (1 mol,) and hydrazine hydrate (0.4 mol) and ethanol (40 ml) was taken RBF placed in microwave oven and irradiated for 4 min. After completion of reaction (monitored by TLC), mixture was cooled and the resulting solid was filtered, dried and recrystallized from ethanol to yield compound 4

Yield: 84 %, mp = 166-167 ^oC

IR (KBr): 3022, 1722, 1612, 694 cm^-1.

¹H-NMR: δ (ppm): 4.42 (2H, s), 4.80 (2H, s), 6.70-7.80 (4H, m), 7.89 (1H, s).

Synthesis of 5-(substituted phenyl-1,3,4–oxadiazol-2-yl)-2-mercaptobenthiazole(5a-g).

Conventional Method:

The mixture of [(2-benzothiazolylthio) acetyl]-hydrazine 4 (0.01M), substituted aromatic acid (0.01M) and phosphorus-oxychloride (5ml), dichloroethane (1ml) was added and content were refluxed for 8-9 hrs. on an oil bath. Reaction was monitored by taking TLC at regular interval. After the reaction completion, excess of solvent and POCl₃ were distilled out. Reaction mass was cooled and poured into ice, left overnight. The product was obtained by filtration and purified by recrystallization from ethanol.

Green Method :

The mixture of [(2-benzothiazolylthio) acetyl]-hydrazine 4 (0.02M), substituted aromatic acid a-g (0.03M) and phosphorus-oxychloride (1ml) was ground to get homogeneous mixture and then heated in a beaker under microwave irradiation at 160W for 5-15 min. completion of reaction was monitored by TLC. The contents were cooled to room temp. and added to excess ice cold water. The solid product separated was collected by filtration and further purified by recrystllization from ethanol-DMF mixture (2:1).The melting point and yields are summarized in Table I.

Spectral details for few compounds from 5a-g are given below:

2-(benzo[d]thiazol-2-ylthio-1.3,4-oxadiazoles-2-phenyl (5a)

IR (KBr): 3027, 3080,2937, 1620, 1612, 1600, 1527, 1065, 1022, 696 cm^-1.

¹H-MNR: δ (ppm) 4.60 (2H, S), 6.65-8.22(9H, m)

2-(benzo[d]thiazol-2-ylthio-1.3,4-oxadiazoles-2-(4-Chloro phenyl) (5b):

IR (KBr) : 3025, 3077,2939, 1612, 1630, 1685, 1530, 1072, 1033,690,730 cm^-1.

¹H NMR: δ (ppm) 4.65 (2H, s), 6.60-8.28 (8H, m)

2-(benzo[d]thiazol-2-ylthio-1.3,4-oxadiazoles-2-(4-Br-phenyl) (5c):

IR (KBr) : 3029, 3081, 2936,1689, 1627, 1614,1533, 1076, 1035, 689, 722 cm^-1.

¹H NMR : δ (ppm) 4.63 (2H,S); 6.62-8.28 (8H,m).

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2-(benzo[d]thiazol-2-ylthio-1.3,4-oxadiazoles-2-(4-OMe-phenyl) (5d):

IR(KBr) : 3023,3070,2935,1683,1632,1615,1529,1074, 1031,690,725 cm^-1.

¹H NMR : δ (ppm) 4.58 (2H,S) ; 6.58-8.17 (8H,m).

2-(benzo[d]thiazol-2-ylthio-1.3,4-oxadiazoles-2-(4-Me-phenyl) (5e):

IR(KBr) : 3021,3075,2933,1680,1629,1616,1528,1073, 1030, 690,735 cm^-1.

¹H NMR : δ (ppm) 4.56 (2H,S); 6.49-7.94 (8H,m).

2-(benzo[d]thiazol-2-ylthio-1.3,4-oxadiazoles-2-(2-Cl-phenyl) (5f):

IR(KBr) : 3033,3085, 2936,1685, 1635,1615, 1529,1075, 1032, 692, 737 cm^-1.

¹H NMR : δ (ppm) 4.70 (2H,S); 6.66-8.34 (8H,m).

2-(benzo[d]thiazol-2-ylthio-1.3,4-oxadiazoles-2-(3-NO₂-phenyl) (5g):

IR(KBr) : 3032,3082,201688,1637,1613,1530,1077,1033, 694,740 cm^-1.

¹H NMR : δ (ppm) 4.68 (2H,S); 6.67-8.32 (8H,m).

Table- I. Physical Data of Compounds 5a-g

Sr .No.	Ar	Molecular Formula	% Yield	mp in ^oC
5a	Ph	C₁₆H₁₁N₃OS₂	88	210-212
5b	4-Cl-Ph	C₁₆H₁₀N₃OClS₂	89	205-207
5c	4-Br-Ph	C₁₆H₁₀N₃OBrS₂	87	212-215
5d	4-OMe-Ph	C₁₇H₁₃N₃O₂S₂	85	196-198
5e	4-Me-Ph	C₁₇H₁₃N₃OS₂	86	200-202
5f	2-Cl-Ph	C₁₆H₁₀N₇OClS₂	90	198-201
5g	3-NO₂-Ph	C₁₆H₁₀N₄O₃S₂	87	170-173

CONCLUSION:

The heterocyclic moieties of 5- substituted Oxadiazole derivatives of 2-marcapto benzothiazole even though have many biological activities but their synthesis involves non-green approach along with some hazardous outputs. The present work tried to synthesis heterocyclic compound carrying such biological activities and minimizes these along with minimal time consumption by the use of green method like stirring, grinding, and microwave chemistry.

ACKNOWLEDGEMENT:

The authors are grateful to the authorities of Research center, Shri Madhavarao Patil Mahavidyalaya, Murum for providing the Laboratory facilities and also thankful to IICT Hyderabad and NCL Pune for proving spectral data.

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Received on 11.07.2012 Modified on 14.08.2012

Asian J. Research Chem. 5(9): September, 2012; Page 1172-1175