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

Abstract

A study was made of the role of the phenylalanine phenyl ring in the enantioselectivity of several chiral stationary phases (CSPs) whose chiral selectors consist of several N-(3,5-disubstituted)benzoyl derivatives of this amino acid covalently bonded to silica gel. Racemic compounds with π-acceptor, π-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)-cyclohexylalanine. In all instances the best enantioselectivities were obtained with CSPs derived from ( S)-cyclohexylalanine. The results show that the phenyl ring in the phenylalanine moiety does not have an electronic role in the recognition of the racemic compound by the chiral selector on the CSP, a non-classical π-π interaction between the 3,5-dinitrobenzoyl group in the racemic compound and in the CSP acts in the resolution of N-(3,5-dinitro)benzoyl derivatives of amino acids on CSPs with the same group and the change in the arrangement of solutes in the diastereomeric solute-stationary phase complexes can take place without an inversion of the elution order of enantiomers.