Facile synthesis of novel bicyclic β-lactams: analogues of C-fused penicillin type ring systems

Treatment of 2-methyl/benzylthioethanoic acid 1 and α,β-unsaturated imines 2 furnished cis and trans-3-methyl/benzylthio-β-lactams 3 and 4 whereas, trans-β-lactams 4 were further transformed into corresponding cis-3-substituted-3-methyl/benzylthio-β-lactams 5–7. The access to novel five-membered ring bicyclic-β-lactams 8 and 9 (in the ratio of 1:4, respectively) was achieved by halogen-mediated cyclization of cis-3-subsituted-β-lactams 3, 5–7 leading 5-endo cyclization rather than 4-exo ring closure process. The molecular structure of 9e has been established with the help of single crystal X-ray analysis (antipodal). The cis or trans configuration of the C-3 substituted group was assigned with respect to C4–H.