An efficient method for one-pot synthesis of beta-lactams from aldimines and ketene silyl acetals by tandem Lewis base-catalyzed Mannich-type addition and cyclization, namely reaction of benzylideneanilines and trimethylsilyl enolates derived from esters or thioesters was established by using a Lewis base catalyst such as lithium acetate, N-lithio-2-pyrrolidone, potassium salt of phthalimide or lithium methoxide in DMF at room temperature to afford the corresponding beta-lactams in good to high yields with moderate trans-selectivities.