N-ALKYLCARBAMOYL DERIVATIVES OF AMINO-ACIDS AS CHIRAL STATIONARY PHASES FOR HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY .1. AN EXAMPLE OF ENHANCING ENANTIOSELECTIVITY BY DELETING THE NON-ENANTIOSELECTIVE PI-PI INTERACTION SITE ON THE CHIRAL STATIONARY-PHASE

The role of the pi-pi interaction provided by the benzyl group of N-benzylcarbamoyl-derived chiral stationary phases (CSPs) was studied. Based on deleting the non-enantioselective pi-pi interaction site on an N-benzylcarbamoyl-(S)-phenylglycine-derived CSP, a series of seven CSPs were prepared by bonding N-alkylcarbamoyl derivatives of (S)-phenylglycine or (S)-phenylalanine either ionically or covalently on 3-amino-propyltriethoxysilane-modified silica gel. The chromatographic behaviour on the CSPs with regard to the resolution of dinitrobenzamide or dinitrobenzanilide derivatives of enantiomeric amino acids, amino alcohols, amines and carboxylic acid was studied. The best chromatographic results were found on ionic-type CSPs containing N-n-stearylcarbamoyl-(S)-phenylglycine or N-cyclohexylcarbamoyl-(S)-phenylglycine. It was also found that the chiral. recognition abilities of all of the four ionic-type N-alkylcarbamoyl-(S)-phenylglycine CSPs were better than that of the ionic-type CSP containing N-benzylcarbamoyl-(S)-phenylglycine for the discrimination of the same enantiomeric analytes. The results clearly confirm that enhancement of enantioselectivity is achieved by deleting the non-enantioselective pi-pi interaction site on the N-benzylcarbamoyl-(S)-phenylglycine-derived CSP.