(-)-Ephedrine as an auxiliary for the asymmetric synthesis of atropisomeric amides by dynamic resolution under thermodynamic control

N,N-Dialkyl-2-formylbenzamides and N,N-dialkyl-2-formyl-1-naphthamides condense with (-)-ephedrine in refluxing toluene to give oxazolidines both as single diastereoisomers with respect to the new stereogenic centre and as single conformers with respect to the slowly-rotating Ar-CONR₂ bond. In the naphthamide series, removal of the ephedrine auxiliary by hydrolysis returns the starting aldehyde (isolated by reduction to the more stable alcohol) in enantiomerically enriched form (up to 94% ee). Overall, the two-step sequence amounts to a dynamic resolution under thermodynamic control.