High-performance liquid chromatographic enantioseparation of isoxazoline-fused 2-aminocyclopentanecarboxylic acids on a chiral ligand-exchange stationary phase

The application of a chiral ligand-exchange column for the direct high-performance liquid chromatographic enantioseparation of unusual -amino acids with a sodium N-((R)-2-hydroxy-1-phenylethyl)-N-undecylaminoacetate-Cu(II) complex as chiral selector is reported. The investigated amino acids were isoxazoline-fused 2-aminocyclopentanecarboxylic acid analogs. The chromatographic conditions were varied to achieve optimal separation. The effects of temperature were studied at constant mobile phase compositions in the temperature range 5-45 degrees C, and thermodynamic parameters were calculated from plots of lnk or ln versus 1/T. (H degrees) ranged from -2.3 to 2.2 kJ/mol, (S degrees) from -3.0 to 7.8 J mol-1 K-1 and -(G degrees) from 0.1 to 1.7 kJ/mol, and both enthalpy- and entropy-controlled enantioseparations were observed. The latter was advantageous with regard to the shorter retention and greater selectivity at high temperature. Some mechanistic aspects of the chiral recognition process are discussed with respect to the structures of the analytes. The sequence of elution of the enantiomers was determined in all cases.