HIGH-PERFORMANCE CHIRAL DISPLACEMENT CHROMATOGRAPHIC SEPARATIONS IN THE NORMAL-PHASE MODE .1. RETENTION AND ADSORPTION STUDIES OF POTENTIAL DISPLACERS DEVELOPED FOR THE PIRKLE-TYPE NAPHTHYLALANINE SILICA STATIONARY-PHASE

Three families of potential displacers have been designed and synthesized for use with the Pirkle-type naphthylalanine silica stationary phases operated in the normal-phase mode. The displacers contain a pi-acid head group (3,5-dinitrobenzoate, 3,5-dinitrobenzamide or 3,5-dinitrophenylcarbamate groups), an H-bonding mid-section (C=O and/or NH groups) and a solubility adjusting tail section (n-alkyl groups). By varying the length of the alkyl chain between methyl and tetracosanyl, a large number of homologous compounds were synthesized in each family. The capacity factors of the homologues were measured in n-hexane-tetrahydrofuran eluents; the k' values vary regularly with both the tetrahydrofuran concentration of the eluent and the length of the alkyl chain. The individual excess adsorption isotherms of the homologues were also determined and the isotherm parameters were correlated with the length of the alkyl chain. Because the capacity factors and the adsorption strengths of these compounds cover a broad range, they can serve as displacers in a variety of normal-phase chiral displacement chromatographic separations.