efficient entry into cis monobactams starting from β -hydroxyesters is reported. This preparation is based on the stereoselective “electrophilicamination” of β-hydroxyester dianions and on Miller's biomimetic synthesis of the β-lactam nucleus. By this route, keyintermediates for the preparation of pharmacologically important cis aztreonam 2 and carumonam 3 were prepared.