Highly trans-selective synthesis of beta-lactams by tandem Lewis base-catalyzed Mannich-type addition and cyclization was established; that is, reaction of benzylideneanilines and trimethylsilyl enolates derived from S-ethyl thioates proceeded smoothly to afford the corresponding beta-lactams in good to high yields with high trans-selectivity by using Lewis base catalyst such as lithium phenoxide or tetrabutyl ammonium phenoxide in DMF at -45 degrees C or THF at -78 degrees C, respectively.