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Direct Catalytic Asymmetric Aldol Reaction

โœ Scribed by Yoshikawa, Naoki; Yamada, Yoichi M. A.; Das, Jagattaran; Sasai, Hiroaki; Shibasaki, Masakatsu


Book ID
118276867
Publisher
American Chemical Society
Year
1999
Tongue
English
Weight
439 KB
Volume
121
Category
Article
ISSN
0002-7863

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โœฆ Synopsis


The direct catalytic asymmetric aldol reaction using aldehydes and unmodified ketones is described for the first time herein. This reaction was first found to be promoted by 20 mol % of anhydrous (R)-LLB (L ) lanthanum, L ) lithium, B ) (R)-binaphthol moiety) at -20 ยฐC, giving a variety of aldol products in ee's ranging from 44 to 94%. This asymmetric reaction has been greatly improved by developing a new heteropolymetallic asymmetric catalyst [(R)-LLB, KOH, and H 2 O]. Using 3-8 mol % of this catalyst, a variety of direct catalytic asymmetric aldol reactions were again found to proceed smoothly, affording aldol products in ee's ranging from 30 to 93% and in good to excellent yields. Interestingly, the use of this new heteropolymetallic asymmetric catalyst has realized a diastereoselective and enantioselective aldol reaction using cyclopentanone for the first time. It is also noteworthy that a variety of aldehydes, including hexanal, can be utilized for the current direct catalytic asymmetric aldol reaction. Chiral aldehydes containing R-hydrogen including (S)-hydrocinnamaldehyde-R-d have been found to produce the corresponding aldol products with negligible racemization (0-4%) at the R-position. One of the aldol products has been successfully converted to the key synthetic intermediates of epothilone A and bryostatin 7. The possible structure of the heteropolymetallic catalyst is also discussed. Finally, mechanistic studies have revealed a characteristic reaction pathway, namely that the reaction is kinetically controlled and the rate-determining step is the deprotonation of the ketone. This is consistent with the fact that the reaction rate is independent of the concentration of the aldehyde.


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In the synthesis of complex molecular targets, the ability to control the stereoselectivity of the aldol reaction has raised this process to a level of prominence shared by few reactions. In most cases, however, the stoichiometric transformation of the active methylene partner into its enolate or an