Cycloaddition of dithiocarbonimidate 1 to the ketene generated in situ from phenoxyacetylchloride in the presence of triethylamine affords the 4-dithioalkyl-2-azetidinones 2 in good yields. Facile conversion of 2 to the 4-unsubstituted ß-lactams 3 can be accomplished by desulfurization with Raney-Nickel. Formation of several interesting products during attempts to bring a carbonyl function at C-4 in 2 has also been described.