Studies on the Design and Synthesis of New Monocyclic β-Lactams Containing Substructures of Penicillin G

The studies on design and synthesis of new monocyclic β-lactam esters 4(R/S)-(1'-methoxycarbonylpropyl2'(R/S)-thio)-3(R)-phenylacetamidoazetidin-2-one (3a) and 4(R/S)-(1'-methoxycarbonyl-2'-methyl-propyl-2'thio)-3(R)-phenylacetamidoazetidin-2-one (3b) were described. Compounds 3a and 3b were specifically designed to retain all penicillin substructures except the bicyclic system, which would be conceived by cleaving the C(3)–N(4) bond of penicillin G. Compounds 3a and 3b are of particular interest in the context of the structural elucidation of monocyclic β-lactams originated from penicillin. Key intermediates, β-mercapto esters 6a and 6b, were synthesized from conjugate acids 4a and 4b using three-step synthetic sequences, respectively, and 4(S)-acetoxy-3(S)-phenylacetamidoazetidin-2-one (7) was obtained from the degradation of penicillin G. Reactions of 6a and 6b with 7, thus obtained, provided the target compounds 3a and 3b, respectively.