Enantiomeric separation of DNS-amino acids and DBS-amino acids by ligand exchange chromatography with S- and R-phenylalaninamide modified silica gel

(S)- and (R)-phenylalaninamide [(S)- and (R)-pheA, respectively] were covalently bonded to silica, and the novel chiral stationary phases (CSps) obtained were used for ligand-exchange chromatographic (LEC) separations with Cu(II) as complexing metal ion. These CSps perform good separations of dansyl-I (Dns)- and dabsyl-I (Dbs) amino acids. The results [enantioselectivity factor (α), retention factor (k') and elution order] were compared with the data obtained previously using one of the selectors as an additive to the mobile phase. Moreover, these CSPs were successfully employed in the direct resolution of a five-membered ring cyclic sulphonamide (γ-sultam) without any prederivatization procedure. The chiral recognition process was found to be consistent with a ligand-exchange mechanism, the geometry of the mixed diastereomeric complexes being strongly dependent on the structural features of the selector covalently bonded to the phase and on the steric bulk of the Dns- and Dbs-amino acid protecting groups. In order to investigate the nature of the interactions involved in the discrimination process, the influence of temperature on the chromatographic parameters (k', α) was also evaluated