1,2-Benzisothiazoles. Part IV. Preparation of the 3-methyl derivative from o-mercaptoacetophenone oxime: a re-examination

Abstract

Treatment of o-mercaptoacetophenone oxime with polyphosphoric acid gives a mixture of 2-methylbenzothiazole, resulting from Beckmann rearrangement of the oxime prior to cyclisation, and 3-methyl-1,2-benzisothiazole. The former product always predominates, but the proportion depends upon the temperature of the cyclisation. Similar treatment of 4′-chloro-, 5′-chloro-, and 5′-nitro-2′-mercaptoacetophenone oxime gives mainly the substituted benzothiazole in each case. Italian workers had reported previously that the cyclisation of o-mercaptoacetophenone oxime and related compounds gave entirely the 3-methyl-1,2-benzisothiazole derivative. It seems probable that their starting material, which they believed to be o-mercaptoacetophenone oxime, was 2-imino-5-methyl-3,1,4-benzoxathiazepine (14). This compound is decomposed to 3-methyl-1,2-benzisothiazole and cyanic acid by heating either in polyphosphoric acid or in a suitable inert solvent.