REACTION OF N-ACETYLDEHYDROPHENYLALANYL-S-PROLINE WITH CA(II) AND NI(II) IONS AND ITS ROLE IN ASYMMETRIC HYDROGENATION

N-Ac-DELTAPhe-S-Pro forms strong coordination complexes with the CO2H group of the proline residue and weak coordination complexes through t he -C(= O)-N groups with Ca(II) and Ni(II) ions in 95% aqueous methano l as indicated by the pK(a) values, PMR spectroscopy, and paramagnetic broadening of the C-13 NMR signals in the nickel complex. Complex for mation enhances the rigidity of the dehydrodipeptide conformation, lea ding to a strong increase in the optical yield upon hydrogenation over achiral catalysts.