Recyclization of diethoxymethyl substituted benzimidazo-fused thiazolium salts

3-Diethoxymethyl-3-hydroxy-3,9-dihydro-2H-benz[4,5]imidazo[2,1-b]thiazol-4-ium salts were synthesized by reaction of chloro oxiranes or isomeric α-chloro ketones with 2-mercaptobenzimidazole. The acetal group of the aforementioned salts undergoes hydrolysis in acidic medium to liberate aldehyde which would react with the NH-moiety of mercaptobenzimidazole, thus leading to cycle expansion with the formation of 3,3,4-trihydroxy-3,4-dihydro-2H-benz[4,5]imidazo[2,1-b][1,3]thiazinium salts. In DMSO solution, the 3,3,4-trihydroxythiazine moiety of the latter compounds is oxidized into 3,4-dioxothiazine one existing in enol form.