Diastereoselective synthesis of highly constrained spiro-β-lactams via the Staudinger reaction using an unsymmetrical bicyclic ketene

A rigid bicyclic ketene was used to generate highly constrained polycyclic spiro-β-lactams through the Staudinger reaction. Depending on the imine component, high diastereoselectivity was observed in the process, leading to mainly the cis diastereoisomer in the case of aromatic imines. This results from an anti addition of the imine to the ketene, followed by a conrotatory ring closure in which the heteroatom at the 6-position of the scaffold rotates outward because of torquoelectronic effects.