BORANE REDUCTION OF THE STEROID 20-KETONE GROUP IN THE PRESENCE OF VARIOUS CHIRAL BETA-AMINO ALCOHOLS

The direction of reduction of 20-carbonyl group of 3alpha-hydroxy-5bet a-pregnan-20-one (1) with a complex of borane-methyl sulfide and chira l beta-amino alcohols, depends on the chirality of the amino alcohol. Where there is a 2-phenyl or 2-benzyl substituent, then the S enantiom er gives exclusively the steroid 20R alcohol 3. The R enantiomer of 2- amino-2-phenyl-1-ethanol gives the highest steroid 20S (2):20R (3) alc ohol ratio.