4-hydroxyphenylglycine-based oxazaborolidines for enantio-selective reductions of ketones

Amino alcohols 6 featuring the diphenylhydroxymethyl group are prepared from the commercial amino acid (R)-4-hydroxyphenylglycine 3. Oxazaborolidines generated in situ from the amino alcohols 6 and borane-dimethyl sulfide serve as catalysts in enantioselective reductions of acetophenone. The para-substituents of the phenyl ring at the stereogenic carbon atom have a substantial influence on the enantioselectivity. When mediated by the nonaflate 6d, the reduction of ketones 7, 9, 11, and 13 leads to the carbinols 8, 10, 12, and 14 in 90 to 96 % ee.