Asymmetric borane reduction of ketones catalyzed by N-hydroxyalkyl-l-menthopyrazoles

The equimolar mixture of N-(hydroxyalkyl)pyrazoles and borane formed boric ester complex, in which the remaining borane was stabilized by the adjacent nitrogen of thr pyrazole ring. The borane complex derived from the chiral pyrazoles such as 3-phenyl-l-menthopyrazole reduced p-methylacetophenone (2 1) enantioselectively. When (2'S)-2 -(2'-phenyl-2'-hydroxyethyl)-3-phenyl-l -menthpyrazole ((2'S)-10b) was used, 21 was reduced into (S)-p-methylwpheny l-l-ethanol (22) in moderate chemical and optical yields. Due to the inconv enience of the preparation and the lower optical yield, the use of N-(alpha -hydroxyalkyl)pyrazoles was unpromising for the enantioselective reduction of ketones by borane.