Enantioselective reduction of ketones with borane, catalyzed by (S)-(−)-proline or (S)-(+)-prolinol.

The enantioselective reduction of ketones by borane in the presence of catalytic amounts of (S)-(−)-proline or (S)-(+)-prolinol as chiral auxiliaries has been investigated. The alcohols possess all the (R) configuration, and are obtained in good enantiomeric excess. A mechanistic rationale is proposed, involving an oxazaborolidine formed from (S)-(+)-prolinol and borane.