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Asymmetric borane reduction of ketones catalyzed by N ‐hydroxyalkyl‐ l ‐menthopyrazoles
Author(s) -
Kashima Choji,
Tsukamoto Yoshihiro,
Higashide Kohei,
Nakazono Hiroyuki
Publication year - 2000
Publication title -
journal of heterocyclic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.321
H-Index - 59
eISSN - 1943-5193
pISSN - 0022-152X
DOI - 10.1002/jhet.5570370449
Subject(s) - borane , chemistry , yield (engineering) , enantioselective synthesis , catalysis , ring (chemistry) , medicinal chemistry , ethanol , boric acid , organic chemistry , pyrazole , materials science , metallurgy
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.