A facile access to thiazolidin-4-ones from α-enolic dithioesters and α-halohydroxamates in situ derived active 1,3-dipolar aza-oxyallyl cations was achieved under mild conditions. These five-membered heterocycles with Z-exocyclic double bonds and diverse substituents can be readily obtained via a regio- and stereoselective formal [3+2] cycloaddition pathway.