Abstract The cycloaddition of diazoalkanes to CS double bonds furnishes 1, 3, 4- thiadiazolines which give rise to thiocarbonyl ylides and N 2 in a cycloreversion. Thiocarbonyl ylides are nucleophilic 1, 3-dipoles which are interceptible by thiones or electron-deficient alkenes and alkynes as dipolarophiles affording 1, 3-dithiolanes, dihydrothiophenes and thiolanes, respectively. In the absence of reaction partners, ...