NMR MEASUREMENTS AND SEMIEMPIRICAL CALCULATIONS IN A 1ST APPROACH TO ELUCIDATE THE MECHANISM OF ENANTIOSELECTIVE CYANOHYDRIN FORMATION CATALYZED BY CYCLO-(S)-PHE-(S)-HIS

A model was proposed for the stereochemical course of the highly enant ioselective hydrocyanation of aromatic aldehydes catalysed by cyclo-(S )-Phe-(S)-His (1). The conformation populations of 1 were determined b y substituting the NMR coupling constants of the relevant protons in t he Karplus equations. In addition the relative stabilities of these co nformations were calculated using the semi-empirical AM1 and PM3 metho ds. The nature of the interaction between HCN and the imidazole moiety of 1 was calculated using a model system. No experimental evidence co uld be obtained for the interaction of benzaldehyde with 1 using NMR o r IR.