Atypical Mode of [3 + 2]-Cycloaddition: Pseudo-1,3-dipole Behavior in Reactions of Electron-Deficient Thioamides with Benzynes

Thioamides bearing electron-withdrawing groups on the thiocarbonyl carbon atom react with benzynes [generated by the hexadehydro-Diels–Alder cycloisomerization] in an unprecedented fashion. Namely, the dihydrobenzothiazole products are consistent with a pathway involving initial formation of a stabilized ammonium ylide by a rare type of [3 + 2]-cycloaddition reaction. The fate of this species depends upon the nature of the R group(s) attached to the ylide nitrogen atom. The demonstration of new modes of reactivity represents the major advance arising from this study.