CONFORMATIONAL EQUILIBRIA OF DERIVATIVES OF (2S)-1-PHENYL-2-PHTHALIMIDOPROPANE FROM HETEROCHROMO-PHORIC EXCITON CIRCULAR-DICHROISM STUDIES

CD spectra of C(1) and C(3)-substituted N-phthaloyl derivatives of (2S )-2-amino-1-phenylpropane, PhCH(X)CH(NPht)R', belonging to either syn (threo) or anti (erythro) series were measured and the Cotton effects in the 250-200 nm region were analyzed in terms of exciton-type intera ction between the electric dipole moments of the phenyl 1L(a) and the phthalimide pi-pi transitions. The sign and amplitudes of the Cotton effects point to a contribution of two conformers, trans and gauche, i n the syn series and to a dominant contribution of the trans conformer in the anti series. This is supported by the exciton Cotton effects d ue to benzoate - phthalimide coupling which were obtained for the corr esponding benzoates of parent 1-phenyl-2-phthalimidopropanols from the differential CD curves.