Chiral Recognition of Amino Acid Derivatives: An NMR Investigation of the Selector and the Diastereomeric Complexes

Article abstract of DOI:10.1021/jo00264a033

A tetraamidic selector containing two chiral synthons ((S)-phenylalanine) spaced by a 3,6,9-oxadecanoyl bridge (Phe-3-O-TA) (1) was used as stationary phase in capillary GC to perform chiral resolution of N-TFA-amino esters.In this paper we report an investigation on the mechanism of chiral recognition by NMR spectroscopy (1D and 2D, COSY, NOESY, and J-resolved experiments and 13C relaxation times).First the conformation of the selector in CDCl3 and CD3OD was studied to evaluate the structural features that might justify the enantiomeric discrimination ability.Thenthe self-associations of the selector and of the enantiomers (S)- and (R)-methyl (2a,2b) and (S)- and (R)-n-butyl N-TFA-phenylalaninate (3a, 3b) were investigated at variable temperature and concentration in CDCl3.Finally, titration experiments were carried out to detect the sites of the binding interactions between the selector and the enantiomers.A simple recognition mode is proposed to account for both the enantioselectivity of the phase and the elution order of the enantiomers observed in GC (t'_RS).This is the first spectroscopic study on the mechanism of chiral recognition in GC concerning the real phase and not a model system.