Thiophilic Lewis acids (e.g. zinc iodide) have a pronounced catalytic effect on the formal [2+2] cycloaddition of alkynyl silyl sulfides 2 and Schiff bases 3 to give 2-azetidinethiones 4. In contrast to uncatalyzed reactions, a large number of functionalities is tolerated and yields are significantly improved.
The first preparation of enolizable silylimines is reported. The “in situ” trapping of these species with lithium enolates of esters gives rise in fairly good yields to N-unsubstituted 4-alkyl-β-lactams.