Abstract
The cis-configurated isocephem 26 as well as isodethiaoxacephems 21 and 33 were synthesized. The key step involves chlorination of the corresponding carbanions of 23, 18, and 31 with CF 3 SO 2 Cl followed by internal alkylation. β-Lactams 3, 21, 26, and 33 were found to possess biological activity against several pathogenic microorganisms in vitro. Electronic activation of the lactam moiety in isodethiaoxacephem 33 remarkably enhanced its biological activity.