Stepwise cycloadditions of mesoionic systems: thionation of thioisomünchnones by isothiocyanates.

An unusual thionation strategy of mesoionic compounds with aryl isothiocyanates enables a facile synthesis of 1,3-thiazolium-4-thiolate systems. The mechanistic pathway of such a transformation most likely involves a stepwise 1,3-dipolar cycloaddition, which is supported by theoretical calculations performed with a two-layer hybrid method (B3LYP/6-31G(d):PM3).