The photochemical reactions of a series of cyclic N-alkenyl-substituted thioimides have been examined. Irradiation of N-3-methylbut-3-enyl-5-thioxo-pyrrolidin-2-one (16) results in intramolecular [2+ 2] cycloaddition to give the highly strained thietane 17, whose structure was confirmed on the basis of its X-ray analysis. Treatment of cycloadduct 17 with dimethyl (methylthio) sulfonium tetrafluoroborate gave 2, 5, 6, 7-tetrahydropyrrolizin-3-one (20) in ...