Chiral terpene auxiliaries II. Spiroborate esters derived from α-pinene—new catalysts for asymmetric borane reduction of prochiral ketones

New spiroborate esters derived from (1R,2R,3S,5R)-3-aminopinan-2-ol and ethylene glycol, propane-1,3-diol, pinacol, catechol, (1S,2S,3R,5S)-pinane-2,3-diol, and (1R,2R,3S,5R)-pinane-2,3-diol were used as catalysts in the borane reduction of acetophenone and other prochiral aryl alkyl ketones producing the corresponding alcohols in high yields. The influence of the spiroborate structure on the enantioselectivity and configuration of the product alcohols was examined.