A selective direct aldol reaction in aqueous media catalyzed by zinc-proline

The Zn-proline complex is shown to catalyze the aldol reaction of acetone and a wide range of arenecarbaldehydes in aqueous media, accepting even deactivated arenecarbaldehydes such as methoxybenzaldehydes in good yields. Enantiomeric excesses of up to 56% could be obtained with 5 mol-% of the catalyst at room temperature, and up to 66% at -15 degrees C. The aldol reaction is regio- and stereoselective with hydroxyacetone (moderate yields) and dihydroxyacetone (excellent donor, 80-90% yields with 5 mmol-equiv.). Plausible mechanisms for the reaction are discussed. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.