The cis-configurated tricyclic O-3-isooxacepham 5 and bicyclic isopenam 11 were synthesized. The key step for the preparation of 5 involves chlorination of the corresponding carbanion of 3 with CF3SO2Cl followed by an internal alkylation. The biologically active isopenam II was synthesized from thio-S-ester 6 via intermediate 9. The key step involves chlorination of the thiol moiety in monocyclic beta-lactam 7 by CF3SO2Cl followed by an internal cyclization. (C) 1999 Elsevier Science Ltd. All rights reserved.