Photolysis of alkyl cobaloximes with maleic anhydride and PhSSPh proceeds via radical alkene addition and subsequent PhSSPh trapping. The product trans 3-phenylthio 4-alkyl succinic anhydrides can be converted by oxidation and thermal syn elimination into 3-substituted maleic anhydrides, which are known to be regioselectively and chemoselectively reduced to produce gamma-hydroxybutenolides or butenolides.