Addition reactions of thiazol-5(4H)-ones—III : Cycloaddition reactions of mesoionic thiazol-5-ones

In contrast with the behaviour of mesoionic oxazol-5-ones, corresponding thiazol-5-ones give stable cycloadducts with electron-deficient alkenes. However, the thiazolones undergo cycloaddition-extrusion reactions with alkynes and with heterocumulenes, in many cases giving products identical with those from corresponding oxazolones. Examples of the influence of thiazolone substituents on the course of these cycloaddition reactions are provided by comparison of results of reactions of differently-substituted mesoionic thiazol-5-ones with phenyl isocyanate, phenyl isothiocyanate, or dimethyl furmarate.