Synthesis of 1,2-benzisothiazoles by the oxidative ring contraction of 2-aryl- and 4-aryl-3,4-dihydro-2H-1,3-benzothiazines

Abstract

Sodium periodate oxidation of 6,7-dimethoxy-2-aryl-3,4-dihydro-2H-1,3-benzothiazines (1a-e) gave 5,6-dimethoxy-1,2-benzisothiazole (4). The N-substituted analogues (6a-c) and the 4-aryl isomers of the latter (11a, b) furnished 2-substituted 1,2-benzisothiazolidine 1-oxides (7a-c) and their 3-aryl analogues (12a, b), respectively. The observed conversions of the 1,3-benzothiazines to 1,2-benzisothiazole and to 1,2-benzisothiazolidines are new ring transformation reactions of 3,4-dihydro-1,3-benzothiazines, representing a new route for the synthesis of 1,2-benzisothiazoles and their hydrogenated derivatives. © 1989.