BETA-LACTAMS FROM ESTER ENOLATES AND SILYLIMINES - ENANTIOSELECTIVE SYNTHESIS OF THE TRANS-CARBAPENEM ANTIBIOTICS (+)-PS-5 AND (+)-PS-6

A new synthetic route to the antibiotics (+)-PS-5 and (+)-PS-6 is described. The preparation involves a fully stereocontrolled reaction between the enantiomerically pure N-trimethylsilylimine of lactic or mandelic aldehyde and the lithium enolate of the tert-butyl butanoate or tert-butyl isovalerate, respectively. Conversion of the azetidinones obtained to 4-acetoxy derivatives via oxidative cleavage of the hydroxyethyl or hydroxybenzyl side chain and introduction of the necessary appendage in the position 4 of the azetidinone ring, followed by assemblage of the bicyclic ring system, afforded the natural trans-carbapenems (+)-PS-5 and (+)-PS-6.