Asymmetric cyanohydrin synthesis catalyzed by synthetic dipeptides, 2

In the asymmetric addition of hydrogen cyanide to benzaldehyde to give mandelonitrile, a cyanohydrin, the highest enantiomeric excess (e.e.) ever reported (90%) was obtained with (2d) whereas the corresponding linear dipeptide gave a very low e. e. This high e. e. decreased with the reaction time, because of the racemization of the product. All cyclic dipeptides used containing an (S)‐histidine residue preferred the (R)‐product. A scheme for the interaction between the catalyst and the reactants to give the (R)‐product is proposed.