Asymmetric Aldol Reaction Organocatalyzed by (<i>S</i>)-Proline-Containing Dipeptides: Improved Stereoinduction under Solvent-Free Conditions

The organocatalytic activity of the methyl ester of (S)-proline-(S)-phenylalanine, (S,S)-2, in the asymmetric aldol reaction between cyclohexanone and acetone with various aromatic aldehydes under solvent-free conditions in a ball mill has been evaluated. α,α-Dipeptide (S,S)-2 catalyzed the stereoselective formation of the expected aldol products, with higher diastereo- and enantioselectivity relative to similar reactions in solution, up to 91:9 anti:syn diastereomeric ratio and up to 95% enantiomeric excess.