Photochemical Ring-expansion Reaction of 1,2-Benzisothiazolinones

The photochemical reaction of a series of 2-aryl-1,2-benzisothiazol-3(2H)-ones (1) under deaerated conditions was found to give dibenzo[b,f][1,4]thiazepin-11(10H)-ones (2). A mechanism through a biradical species is proposed for the photoreaction. When the photolysis of 1 was carried out in the presence of oxygen, 2-aryl-1,2-benzisothiazol-3(2H)-one 1-oxides (11) were formed together with compounds 2.