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

✍ Scribed by Choji Kashima; Yoshihiro Tsukamoto; Kohei Higashide; Hiroyuki Nakazono

Publisher
Journal of Heterocyclic Chemistry
Year
2000
Tongue
English
Weight
607 KB
Volume
37
Category
Article
ISSN
0022-152X

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✦ Synopsis

Abstract

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 (21) enantioselectively. When (2′S)‐2‐(2′‐phenyl‐2′‐hydroxyethyl)‐3‐phenyl‐l‐menthopyrazole ((2′S)‐10b) was used, 21 was reduced into (S)‐p‐methylphenyl‐1‐ethanol (22) in moderate chemical and optical yields. Due to the inconvenience of the preparation and the lower optical yield, the use of N‐(α‐hydroxyalkyl)pyrazoles was unpromising for the enantioselective reduction of ketones by borane.

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