Dithiolane-isocyanate imminium methylides which are a new type of azomethine methylide- derived 1, 3-dipole undergo efficient and regioselective cycloaddition to conjugated carbonyls and thiocarbonyls to yield predominantly 1, 3-oxazolidine-and thiazolidine-2- thiones formed from the initial cycloadducts via loss of thiirane.