ELUTION ORDERS IN THE SEPARATION OF ENANTIOMERS BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY WITH SOME CHIRAL STATIONARY PHASES

Enantioselectivity and elution order in the separation of various racemic compounds by high-performance liquid chromatography with some urea derivatives containing two asymmetric carbon atoms attached to two nitrogen atoms of the urea group derived from (S)- or (R)-valine (Val) and (S)- or (R)-1-(alpha-naphthyl)ethylamine (NEA) as chiral stationary phases (CSPs) were investigated in order to explain the mechanism of enantiomer separation. The chromatographic results showed that two kinds of diastereometric interactions are produced and each of the two chiral centres may contribute to the chiral recognition. In the separation of racemic amino acid methyl esters, the Val component may control the chiral recognition for N-acetyl derivatives and the NEA component for N-3,5-dinitrobenzoyl derivatives. In the direct separation of various racemic alcohols and esters, the Val component may control mainly the chiral recognition. The NEA component may efficiently increase the enantioselectivity as shown in allethrolone, etc., but it may also decrease the enantioselectivity as shown in terallethrin, etc. We can assume that the overall enantioselectivity and elution order on these CSPs are determined by the combination of the structure effects and the chiral recognition mechanisms on two chiral components.