Palladium-Catalyzed Carbonylative [2 + 2] Cycloaddition for the Stereoselective Synthesis of Either cis- or trans-3-Alkenyl β-Lactams

A new potential alkenylketene equivalent, prepared without use of a carboxylic acid or an activated derivative, has been exploited. Palladium-catalyzed carbonylation of an allyl phosphate in the presence of an imine and a tertiary amine under CO pressure (30 kg cm⁻²) stereoselectively gave either a cis- or trans-3-alkenyl β-lactam in high yield. The stereochemical outcome strongly depends on the nature of the imine employed. Imines conjugated with a carbonyl such as a ketone or an ester stereoselectively produce cis-β-lactams at room temperature, whereas imines unconjugated with a carbonyl group exclusively afford trans-β-lactams at 70 °C.