Mechanistic aspects of the synthesis of 3-aminopyrroles from substituted 2-methyl-1,2-thiazolium salts or 3-aminothioacrylamides

Abstract

The mechanism of the synthesis of 3-aminopyrroles 3 by ring transformation–desulfurisation of substituted 2-methyl-1,2-thiazolium salts 2 has been investigated. 3-Alkylideneaminothioacrylamides 4 and 2H-1,3-thiazines 5 could be synthesised by base treatment of 1,2-thiazolium bromides 2 and proved to be intermediates in the ring transformation–desulfurisation. Hitherto unknown bis(2-aminothiocarbonylvinyl)amines 6 are observed as by-products. When 3-phenacylaminothioacrylamide 1c is oxidised with hydrogen peroxide a novel pyrrole formation occurs to give 2-benzoylthiopyrrole 8a, where the sulfur atom is retained in the product. An erroneously reported 1,2-thiazolium-2-methanide 7 has been reassigned as 3-(4-nitrobenzylideneamino)thioacrylamide 4a on the basis of an X-ray crystal structure determination.