Primary amine catalyzed direct asymmetric aldol reaction assisted by water

L-Valine was found to be an active catalyst in the asymmetric direct aldol reaction. The aldol reaction of a variety of aromatic aldehydes with acetone was catalyzed by 20 mol % Of L-valine at 35 degrees C with the aldol products obtained in moderate to good yields (48-83%,) and enantiomeric excesses (42-72%). The reaction was more efficient catalytically with best results observed in the presence of 1 mol equiv of water, with respect to the aldehyde, in either DMSO or DMF as solvent. The effect of water concentration on the reaction rate and enantioselectivity was also investigated. Thus, with increasing water concentration in DMSO there was decreasing enantioselectivity. However, the reaction in the presence Of L-phenylalanine showed a lower level of reactivity and enantioselectivity to afford the aldol in 25% with 31% ee. In marked contrast, reaction with L-phenylglycine resulted in the negligible formation of the aldol (< 5%). Our results, suggest a new strategy in the design of new bioorganic catalysts for direct asymmetric aldol reactions. (c) 2005 Elsevier Ltd. All rights reserved.