A New Chiral Director for the Highly Diastereoselective Borane Reductionof Steroid-20-ones

Summary. The synthesis of a new chiral boroxazolidine was achieved which was used to control the stereochemistry of the borane reduction of the 20-keto group of steroids. The otherwise hardly accessible 20α-(20 S )-alcohol can thus be prepared in a yield of 91%.