Abstract
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 CF 3 SO 2 Cl followed by an internal alkylation. The biologically active isopenam 11 was synthesized from thio-S-ester 6 via intermediate 9. The key step involves chlorination of the thiol moiety in monocyclic β-lactam 7 by CF 3 SO 2 Cl followed by an internal cyclization.