Effect of Calix[4]arene as a Hydrophobic Substituent on Proline Catalysis of Direct Asymmetric Aldol Reactions in the Presence of Water

Abstract: A new organocatalyst trans-4-hydroxy-L-proline-derived calix[4]arene was synthesized and its catalyst performance for the direct asymmetric aldol reactions between cyclohexanone and different aromatic aldehydes was investigated. The effect of a series of reaction conditions such as solvent, water and additives were evaluated in detail, and it was observed that the addition of water had a big effect on the enantioselectivities. Specifically, high anti-diastereoselectivity (anti/syn = 92 : 8) and high enantioselectivity (ee 88%) were obtained from the reaction between cyclohexanone and 4-fluorobenzaldehyde in the presence of water.