Reaction of mesoionic thiazol-4-ones with azodicarbonyl dipolarophiles. Cleavage modes of the bi- and tri-cyclic adducts

Abstract

The thiazolium-4-olates (1a–e) were treated with dimethyl and diethyl azodicarboxylate and gave, through 1,3-dipolar cycloaddition, the 3,6-sulphur bridged hexahydro-1,2,4-triazin-5-ones (2). When heated in benzene these rearranged to the blue thioxohydrazides (3), whilst when heated in methanol or ethanol they reacted with the solvent to give 2-alkoxy-5-hydrazinothiazol-4-ones (4). Reaction of the mesoionic compounds (1) with 4-phenyl-1,2,4-triazole-3,5-dione (PTAD) gave the cycloadducts (11) which reacted very readily with water or alcohols to give the thiazol-4-ones (5), analogous to (4). The structures and stereochemistry of (2), (3), and (5) were established by X-ray analyses. The mechanisms of the ring openings of (2) to (3) and (4), and of (11) to (5) are discussed.