Enantioselective direct aldol addition of acetone to aliphatic aldehydes

The asymmetric direct aldol addition of acetone to aliphatic aldehydes catalyzed by D-proline, L-proline, and its derivatives was studied. While excellent results could be obtained in neat acetone using alpha-branched aldehydes, unbranched and beta-branched aldehydes gave moderate results. Two dipeptide derivatives, L-Pro-L-Try-CH(2)OH and L-Pro-L-Trp-OCH(3), were prepared and tested in this reaction and both were found to be able to induce enantioselectivities. The ee-values in the case of some aldehydes approached that obtained with L-proline. Immobilization of L-proline on a polystyrene resin by its carboxylic group provided a catalyst which is able to induce enantioselectivity, can be easily removed from the reaction mixture, and reused without a significant decrease in the enantioselectivity of the beta-hydroxyketones obtained in the cross-aldol additions.