1,2-Benzisothiazoles. Part III. 3-Substituted derivatives

Abstract

Ethyl (1,2-benzisothiazol-3-yl)cyanoacetate exists as a mixture of two tautomers which decomposes to (1,2-benz-isothiazol-3-yl)acetonitrile (85%) when heated in dimethyl sulphoxide. Treatment of diethyl (1,2-benzisothiazol-3-yl)malonate with ethanolic sodium ethoxide gives ethyl (1,2-benzisothiazol-3-yl)acetate, which affords 3-methyl-1,2-benzisothiazole on successive hydrolysis and decarboxylation. The methyl group in 3-methyl-1,2-benzisothiazole is resistant to oxidation. Attempted Vilsmeier–Haack formylation of 3-methyl-1,2-benzisothiazole with dimethylformamide and phosphoryl chloride gives a mixture of N²-(3-benzo[b]thienyl)-N¹N¹-dimethylformamidine and N²-(2-formyl-3-benzo[b]thienyl)-N¹N¹-dimethylformamidine. In contrast, 2-methylbenzothiazole gives (benzothiazol-2-yl)malonaldehyde or a related enamine.