Stereoselective synthesis of constrained norbornane-derived spiro-β-lactams

New enantiopure polycyclic norbornane-derived spiro-β-lactams were synthesized by means of a Staudinger ketene-imine reaction between unsymmetrical bicyclic chiral ketenes, generated from differently substituted norbornane carboxylic acids, and (E)-N-benzyl-N-(phenylmethylene)amine, with high yields and moderate to good stereoselectivities. The diastereoisomeric results were rationalized taking into account the increasing steric encumbrance present on the norbornane skeleton and the stability of the products. The configurations of the newly formed stereocenters of spiro-β-lactams were assigned on the basis of 2D NMR experiments and X-ray analysis. Spiro-β-lactams were subjected to acid hydrolysis obtaining the corresponding norbornane-derived β-amino acids.