Selective Reductions. 52. Efficient Asymmetric Reduction of Alpha-AcetylenicAlpha-FIuoroalkyl Ketones with Either Beta-Chlorodiisopinocampheylborane or Beta-

摘要

The preparation of fluorinated compounds, especially optically active derivativeshas gained considerable attention due to their importance and increasing range of application. The development of excellent chiral reducing agents has made the synthesis of optically active secondary alcohols a relatively simple affair. However, the asymmetric reduction of fluoroketones is yet to receive appropriate attention. Several years ago we introduced Beta-chlorodiisopinocampheylborane (1) (Aldrich: DIP-Chloride) as an excellent chiral reducing agent for the reduction of aralkyl ketones and a-hindered ketones. When we tested 1 for a representative set of ten classes of ketones, it proved inefficient, among other classes, for the reduction of a-acetylenic ketones. However, another organoborane reagent derived from a-pinene, Alpine-Borane (2), introduced by Midland prior to our synthesis of 1, is a superior reagent for the reduction of a-acetylenic ketones. The mechanism of reductions by both 1and 2 appear to be very similar. Later studies showed that 1 is an efficient reagent for the reduction of hindered acetylenic ketones6 and a-perfluoroalkyl ketones 7 wheareas 2 reacts only slowly with these ketones and provides the alcohols in relatively poor ee. reprints, (jg).