Stereochemistry-60 : Kinetic control of asymmetric induction during oxazolidine formation from (-)-ephedrine and aromatic aldehydes

Kinetically controlled oxazolidine formation was observed with aromatic aldehydes substituted by electron-withdrawing groups. The stereoselectivity is solvent dependent: non-stereoselective ring closure occurred in chloroform while a high diastereodifferentiation was observed in methanol. The first oxazolidine showing an unambiguous 2R configuration was synthesized from p-bromobenzaldehyde and (-)-ephedrine in alcohol medium. A mechanism involving a nucleophilic assistance by alcoholic solvents is suggested in order to clarify the differences in stereoselectivity.