Remarkable Lewis acid mediated enhancement of enantioselectivity during free-radical reductions by simple chiral non-racemic stannanes

Additions of 1 equiv. of achiral and chiral Lewis acids to free-radical reduction reactions involving (1S,2S,5R)-menthyldiphenyltin hydride 1, bis[(1S,2S,5R)-menthyl]-phenyltin hydride 2, bis[1S,2S,5R)-menthyl][8-(N,N-dimethylamino)naphthyl]tin hydride 3, bis[(1R,2S,5R-menthyl]{1-(S)-N,N-dimethylaminoethyl]phenyl}tin hydride 4 or 3 alpha-dimethylstannyl-5 alpha-cholestane 5 result in remarkable increases in enantioselectivity.