Origin of the Diastereoselectivity of the Heterogeneous Hydrogenation of a Substituted Indolizine.

In this work, the stereoselective heterogeneous hydrogenation of a tetrasubstituted indolizine was studied. Partial hydrogenation products were obtained in three steps from a substituted pyridine-2-carboxaldehyde prepared from commercial pyridoxine hydrochloride. The hydrogenation of the indolizine ring was shown to be diastereoselective, forming and . Theoretical calculations ( and DFT) were used to rationalize the unusual stereoselectivity for , and a keto-enol tautomerism under kinetic control has been proposed as the source of diastereoselectivity.