COMPARISON BETWEEN SILICA-BONDED CHIRAL STATIONARY PHASES DERIVED FROM 3,5-DISUBSTITUTED N-BENZOYL-(S)-PHENYLALANINE AND (S)-CYCLOHEXYLALANINE IN THE RESOLUTION OF RACEMIC COMPOUNDS BY LIQUID-CHROMATOGRAPHY

A study was made of the role of the phenylalanine phenyl ring in the e nantioselectivity of several chiral stationary phases (CSPs) whose chi ral selectors consist of several N-(3,5-disubstituted)benzoyl derivati ves of this amino acid covalently bonded to silica gel. Racemic compou nds with pi-acceptor, pi-donor or both characters were resolved on two series of CSPs derived from N-(3,5-dimethyl)benzoyl, N-(3,5-dimethoxy )benzoyl and N-(3,5-dinitro)benzoyl-(S)-phenylalanine and (S)-cyclohex ylalanine. In all instances the best enantioselectivities were obtaine d with CSPs derived from (S)-cyclohexylalanine. The results show that the phenyl ring in the phenylalanine moiety does not have an electroni c role in the recognition of the racemic compound by the chiral select or on the CSP, a non-classical pi-pi interaction between the 3,5-dinit robenzoyl group in the racemic compound and in the CSP acts in the res olution of N-(3,5-dinitro)benzoyl derivatives of amino acids on CSPs w ith the same group and the change in the arrangement of solutes in the diastereomeric solute-stationary phase complexes can take place witho ut an inversion of the elution order of enantiomers.