Study of chromatographic enantioseparation of the esters of N-dinitrobenzoyl (N-DNB) and N-benzoyl (N-B) alpha-amino acids on novel chiral stationary phases containing structurally matching N-DNB and N-B-alpha-AA amides in the chiral selector

Enantioseparation ability and enantiopreference of chiral stationary phases CSP 1-CSP 3, containing a terminal N-3,5-dinitrobenzoyl (N-DNB) unit, and CSP 4, containing a terminal N-benzoyl (N-B) unit, are studied. Separation factors (a) for the two sets of test racemates (TR) that structurally match the chiral selector of these CSPs have been determined. The first set consists of seven N-DNB α-amino acid isopropylesters (TR 1A-TR 7A), and the second one of their N-B analogues (TR 1-TR 7). The best enantioseparation (α a v e r a g e 1.27) is obtained when n-acceptor DNB unit is present in both TR and CSP. One π-acceptor unit, either in the analyte or in CSP, suffices for efficient enantioseparation (α a v e r a g e 1.19). Interaction between π-neutral units in the CSP and test racemate does not afford effective enantioseparation (α a v e r a g e 1.03). Using (S)-en-antiomers of all TRs as standards, CD detection has revealed regular preference of the CSPs for the enantiomers containing amino acid amide of the same absolute configuration. The possible origin of such enantiopreference is discussed.