A Ring-Enlargement Reaction Yielding 1,2,5-Benzothiadiazonin-6-one 1,1-Dioxides

At room temperature or under reflux in MeCN, 3-amino-2H-azirines 2 and 3,4-dihydro-2H-1,2-benzothiazin-3-one 1,1-dioxide ( 4 ) give 1,2,5-benzothiadiazonin-6-one 1,1-dioxides 5 in fair-to-good yield (Scheme 2). The structure of this novel type of heterocyclic compounds has been established by X-ray crystallography of 5a (Fig.). A ring expansion via a zwitterionic intermediate of type A ' is proposed as the reaction mechanism of the formation of 5 .