Hydrophobically Directed Aldol Reactions: Polystyrene-Supported L-Proline as a Recyclable Catalyst for Direct Asymmetric Aldol Reactions in the Presence of Water (Eur. J. Org. Chem. 28/2007)

The cover picture shows the aldol reaction between ketones and arylaldehydes carried out by using a polystyrene-supported L-proline catalyst. This material furnishes aldol products in high yields and stereoselectivities. Screening of solvents showed that these reactions take place only in the presence of water. This solvent effect, coupled with the high stereoselectivities observed, has been explained by the formation of a hydrophobic core in the inner surface of the resin with the hydrophilic proline moiety in the resin/water interface. Such a microenvironment promotes the aldol reaction and increases the stereoselectivity. Recycling investigations have shown that this material can be reused, without loss in conversion and stereoselectivity for, at least, five cycles. This catalyst can be considered a better mimic of natural Class I aldolase enzymes that use an enamine-mediated mechanism in water. Details are discussed in the article by M. Gruttadauria et al. on p. 4688 ff. This work is dedicated to Professor Domenico Spinelli on the occasion of his 75th birthday.