New enantiomerically pure fused 2 or bridged 3 polycyclic beta-lactam systems are regio- and stereoselectively prepared via intramolecular nitrene-alkene cycloaddition of 2-azetidinone-tethered alkenyl-aldehydes 1. The regioselectivity of the cycloaddition can be tuned by moving the alkene substituent from N1 to C3 on the 2-azetidinone ring. (C) 1999 Elsevier Science Ltd. All rights reserved.