Studies on Mesoionic Compounds. XIV. Synthesis of Mesoionic 1, 3-Thiazolium-4-thiolates

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Two approaches to 3-alkyl-2, 5-diphenyl-1, 3-thiazolium-4-thiolate (4) are described : (1) exchange of the exocyclic oxygen atom of the mesoionic 4-olate (6) with sulfur via O-methylation and (2) base-catalyzed thiation of the N-alkyl-1, 3-thiazolium compound (3). The mesoionic thiazoles underwent cycloaddition with dimethyl acetylenedicarboxylate to give thermally stable bicyclic compounds 8.