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A Further Contribution to the Study of Sagittamide A: Synthesis of a Pivotal Intermediate Belonging to a Rare L-Series
Molecules 2012, 17(8), 9010-9022; doi:10.3390/molecules17089010
Article

Chiral Zn(II)-Bisamidine Complex as a Lewis-Brønsted Combined Acid Catalyst: Application to Asymmetric Mukaiyama Aldol Reactions of α-Ketoesters

 and *
Received: 11 July 2012 / Revised: 21 July 2012 / Accepted: 21 July 2012 / Published: 30 July 2012
(This article belongs to the Special Issue Stereoselective Synthesis)
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Abstract

Focusing on the steric and electronic properties of the resonance-stabilized amidine framework, a cationic metal-bisamidine complex was designed as a conjugated combined Lewis-Brønsted acid catalyst. The chiral Zn(II)-bisamidine catalyst prepared from the 2,2'-bipyridyl derived bisamidine ligand, ZnCl2, and AgSbF6 promoted asymmetric Mukaiyama aldol reaction of α-ketoester and α,α-disubstituted silyl enol ether to afford the α-hydroxyester having sequential quarternary carbons in good yield, albeit with low enantioselectivity. Addition of 1.0 equivalent of the fluoroalcohol having suitable acidity and bulkiness dramatically increased the enantioselectivity (up to 68% ee). DFT calculations suggested that this additive effect would be caused by self-assembly of the fluoroalcohol on the Zn(II)-bisamidine catalyst.
Keywords: Lewis-Brønsted combined acid; bisamidine; asymmetric Mukaiyama aldol reaction; α-ketoester Lewis-Brønsted combined acid; bisamidine; asymmetric Mukaiyama aldol reaction; α-ketoester
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Gotoh, R.; Yamanaka, M. Chiral Zn(II)-Bisamidine Complex as a Lewis-Brønsted Combined Acid Catalyst: Application to Asymmetric Mukaiyama Aldol Reactions of α-Ketoesters. Molecules 2012, 17, 9010-9022.

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