Chiral monoaminoalcohols and diaminoalcohols of squaric acid: new catalysts for the asymmetric reduction of ketones by borane

Two series of new chiral ligands, squaric acid aminoalcohols and C2-symmetric squaric acid diaminoalcohols have been synthesized. The chiral oxazaborolidines formed in situ from these ligands have been used in the enantioselective borane reduction of prochiral ketones and diketones to afford alcohol products with up to 99% enantiomeric excesses. The structures of the ligands have an obvious effect on the ee of the resulting alcohols.