Photochemical synthesis of triazolo[3,4-b]-1,3(4H)-benzothiazines: a detailed mechanistic study on photocyclization/photodesulfurisation of triazole-3-thiones

Irradiation of 4-(2-halobenzyl)-5-substituted-1,2,4-triazole-3-thiones under base mediated (CH3CN/2 M NaOH) condition afforded triazolo[3,4-b]-1,3(4H)-benzothiazines and desulfurized triazoles. Benzophenone sensitized photolysis of triazole-3-thiones gave desulfurized triazoles exclusively. The mechanism of the photocyclization/photodesulfurization and the involvement of singlet and triplet energy levels are discussed.