New chiral borohydrides. 3. Preparation and asymmetric reducing properties of potassium 9- O -(1,2-isopropylidene-5-deoxy-α-D-xylofuranosyl)-9-borata-bicyclo[3.3.1]nonane

Abstract

A stable, new chiral borohydride, potassium 9- O- (1,2- O-isopropylidene-5- deoxy-α-D-xylofuranosyl)-9-boratabicyclo[3.3.1]nonane (K xylide, 1D) using a xylose derivative as a chiral auxiliary was prepared by the treatment of the corresponding borinic ester 3D with potassium hydride in THF. The reagent provided high optical induction for asymmetric reduction of prochiral ketones, such as relatively hindered ketones, alkyl aromatic ketones, and α-haloketones. Thus, the reduction of 4-methyl-2-pentanone, 3,3-dimethyl-2-butanone, and 2,2-dimethylcyclopentanone provided the corresponding alcohols m 65 % ee, 76 %ee, and 80 % ee, respectively. The optical induction for unhindered aliphatic ketones was low, such as 36 % ee for 2-heptanone, 46 % ee for 3-methyl-2-butanone. The reductions of alkyl aromatic ketones provide high optical inductions, such as 70 % ee for acetophenone, 82 % ee for butyrophenone, 89 % ee for isobutyrophenone, 99 % ee for pivalophenone, and 93 % ee for 2′-methylacetophenone. For some functionalized ketones, the reduction of 2-chloroacetophenone yields the corresponding alcohol in 92% ee, whereas the reduction of 3-acetylpyridine, methyl benzoylformate, and 4-phenyl-3-butyn-2-one provide the corresponding alcohols in 62% ee, 60 % ee and 52 % ee, respectively.