Reinvestigation of reactions of thiazolium and benzothiazolium N-phenacylides with electron-deficient acetylenes

Abstract

Reactions of thiazolium and benzothiazolium N-phenacylides with dimethyl acetylenedicarboxylate (DMAD) have been reexamined. The thiazolium N-phenacylides 5, generated in situ from 4-methyl- or 5-phenyl-3-phenacylthiazolium bromides 13 with triethylamine, reacted with DMAD in dry DMF to give the thiazole ring-opened products 15, in which two molecules of DMAD had been incorporated. The reactions when conducted in both aqueous DMF and in dry DMF in the presence of lithium perchlorate gave the hemithioacetals 6 which could be transformed into the 1 : 2 reaction products 15 of the ylides and DMAD. Benzothiazolium N-phenacylides 8 similarly reacted with DMAD to afford the 1 : 2 reaction products 19 or the hemithioacetals 20. The reaction mechanism is discussed.