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  • Title: Asymmetric organocatalytic direct aldol reactions of ketones with alpha-keto acids and their application to the synthesis of 2-hydroxy-gamma-butyrolactones.
    Author: Xu XY, Tang Z, Wang YZ, Luo SW, Cun LF, Gong LZ.
    Journal: J Org Chem; 2007 Dec 21; 72(26):9905-13. PubMed ID: 18004868.
    Abstract:
    A variety of organocatalysts for the asymmetric direct aldol reactions of ketones with alpha-keto acids were designed on the basis of molecular recognition and prepared from proline and aminopyridines. The organic molecule 8e, derived from proline and 6-methyl-2-amino pyridine, was the best catalyst, affording excellent enantioselectivities (up to 98% ee) for the direct aldol reactions of acetone or 2-butanone with a wide range of alpha-keto acids and for the reactions of various acyclic aliphatic ketones with 3-(2-nitrophenyl)-2-oxopropanoic acid. The aldol adducts could be converted to 2-hydroxy-gamma-butyrolactones by reaction sequences of diastereoselective reduction and lactonization. Experimental and theoretical studies on the transition states revealed that the amide N-H and the pyridine N of the organocatalyst selectively form hydrogen bonds with the keto oxygen and the carboxylic acid hydroxy of the alpha-keto acid, respectively. These two hydrogen-bonding interactions are important for the reactivity and enantioselectivity of the direct asymmetric aldol condensation.
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