Stereoselective synthesis of cis- and trans-.beta.-lactams via .alpha.-amino ester enolates and 1-aza-4-hetero-1,3-butadiene systems. Molecular structure of [cyclic] EtOC(O)C(H)(NEt2)C(H)(N(H)-tert-Bu)C(H):N(tert-Bu)ZnCl2: An unexpectedly isolated aldolate