Asymmetric Reduction with Chiral Organoboranes Based on Alpha-Pinene

摘要

Since our compilation in 1986, Itsuno described the catalytic nature of his aminoalcohol-borane mixture for the reduction of O-alkyl ketoximes. E.J.Corey and his co-workers isolated and characterized the catalyst and made modifications to prepare very effective borane-base catalysts (CBS catalysts) for chiral reduction. Though catalytic activity of a reagent is very much desirable, stoichiometric reducing agents with certain characteristics may be competitive or preferable. Such reducing agents should reduce most classes of ketones with a chiral auxiliary that is economical, with both enantiomers readily available, and easily recovered and recycled. The reagents should be easily prepared, and the reaction conditions should be simple. The enantiomeric excess of the products should be high. The nature of the reagent and its mode of action should be known with predictability of configuration for the product alcohols so that modifications of the reagent can be made. With these conditions in mind, we developed a number of chiral reducing agents. Alpha-Pinene proved to be a chiral auxiliary satisfying most of these conditions.