Highly Enantioselective and Regioselective Carbonyl Reduction of Cyclic α,β-Unsaturated Ketones Using TarB-NO₂ and Sodium Borohydride

Asymmetric 1,2-reduction of alpha,beta-unsaturated ketones using TarB-NO(2) and NaBH(4) is reported. Simple cycloalkenones give products in low enantiomeric excess. However, cycloalkenones with alpha-substituents, such as halides, alkyl, and aryl, have been enantioselectively reduced with this system to yield chiral allylic alcohols in enantiomeric excess up to 99%. The starting materials for TarB-NO(2) are inexpensive, and the boronic acid can be easily recovered in high yield by a simple acid extraction.