A novel tandem approach to trisubstituted gamma-sultams has been developed involving N-alkylation of methanesulfonanilides with EWG-substituted allyl bromides followed by intramolecular Michael addition. A series of various isothiazolidine 1,1-dioxides have been prepared under mild transition-metal-free conditions in high yields and trans-diastereoselectivity (confirmed by X-ray crystallography). The dependence of reactivity on electronic properties of substrates has been investigated.