ENANTIOSELECTIVE BORANE REDUCTION OF ACETOPHENONE CATALYZED BY OXAZABOROLIDINES DERIVED FROM CHIRAL DIETHANOLAMINES

The synthesis of oxazaborolidines of types I and 2, derived from chiral diethanolamines and their behaviour, as catalysts for the asymmetric reduction of acetophenone by borane has been investigated. Bicyclic oxazaborolidines of type 2 afforded the expected alcohol in good yield but with modest ee's (ee<40%). Monocyclic oxazaborolidines of type I gave better results. For instance, catalyst Id allowed the isolation of (S) alpha-methyl benzyl alcohol with 72% ee while oxazaborolidine 26, structurally close to 16, produced a remarkable reversal of enantiofacial selectivity. The origin of the enantioselectivity of these catalysts is discussed on the basis of transition slate models.