CHIRAL SYNTHESIS VIA ORGANOBORANES .43. SELECTIVE REDUCTIONS .58. REAGENT-CONTROLLED DIASTEREOSELECTIVE REDUCTION OF (-ALPHA-CHIRAL AND (-)-ALPHA-CHIRAL KETONES WITH (+)-B-CHLORODIISOPINOCAMPHEYLBORANE AND (-)-B-CHLORODIISOPINOCAMPHEYLBORANE())

Asymmetric reduction of (+)- and (-)-alpha-chiral ketones with (+)- an d (-)-B-chlorodiisopinocampheylborane provides the product alcohols in very high diastereomeric excess, with the matched pairs providing >10 0:1 selectivity and the mismatched pairs showing 4:1-15:1 selectivity. The high selectivity achieved even in the mismatched pairs reveals th e power of the reagent to control the stereochemical outcome. The rate s of the reaction of the matched pairs are faster than those of the mi smatched pairs, In all the cases studied thus far, the (-)-reagent ((d )Ipc(2)BCl) and (S)ketone or the (+)-reagent ((l)Ipc(2)BCl) and (R)-ke tone constitute matched pairs and the (-)-reagent and (R)-ketone or th e (+)-reagent and (S)-ketone constitute mismatched pairs, A possible m echanism for the reductions is discussed.