Lewis Acid Mediated Enantioselective Synthesis of Oxacephams

Significance

Reported is the enantioselective, Lewis acid mediated formation of benzo-5-oxa-cephams using tin tetrachloride and a BINOL based ligand. A plausible mechanism is proposed involving the formation of a planar N-acyliminium anion, which then reacts with the phenol. The enantioselectivity is rationalized in a preliminary manner as a kinetic resolution of the product after the reaction is complete. However, only anecdotal evidence is given for this resolution and work is in progress to further prove this mechanism. Although the chiral BINOL-based ligand is used in stoichiometric amounts it is chromatographically recoverable.