NEW CHIRAL BOROHYDRIDES. 3. PREPARATION AND ASYMMETRIC REDUCING PROPERTIES OF POTASSIUM 9-0 -(1,2-ISOPROPYLIDENE-5-DEOXY-α-D-XYLOFURANOSYL)-9-BORATABICYCLONONANE

Article abstract of DOI:10.1016/S0957-4166(00)82314-9

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 thecorresponding alcohols in 65 percent ee, 76 percent ee, and 80 percent ee, respectively.The optical induction for unhindered aliphatic ketones was low, such as 36 percent for 2-heptanone, 46 percent ee for 3-methyl-2-butanone.The reductions of alkyl aromatic ketones provide high optical inductions, such as 70 percent ee for acetophenone, 82 percent ee for butyrophenone, 89 percent ee for isobutyrophenone, 99 percent ee for pivalophenone, and 93 percent ee for 2'-methylacetophenone.For some functionalized ketones, the reduction of 2-chloroacetophenone yields the corresponding alcohol in 92 percent ee, whereas the reduction of 3-acetylpyridine, methyl benzoylformate, and 4-phenyl-3-butyn-2-one provide the corresponding alcohols in 62 percent ee, 60 percent ee and 52 percent ee, respectively.