High-performance liquid chromatographic enantioseparation of fluorinated cyclic beta super(3)-amino acid derivatives on polysaccharide-based chiral stationary phases. Comparison with nonfluorinated counterparts

The stereoisomers of five fluorinated cyclic beta super(3)-amino acid derivatives and their nonfluorinated counterparts were separated on chiral stationary phases containing as chiral selectors cellulose tris-(3,5-dimethylphenyl carbamate), cellulose tris-(3-chloro-4-methylphenyl carbamate), cellulose tris-(4-methylbenzoate), cellulose tris-(4-chloro-3-methylphenyl carbamate), amylose tris-(3,5-dimethylphenyl carbamate) or amylose tris-(5-chloro-2-methylphenyl carbamate). The enantioseparations were carried out in normal-phase mode with n-hexane/alcohol/alkylamine mobile phases in the temperature range 5-40 degree C. The effects of the mobile phase composition, the nature and concentration of the alcohol and alkylamine additives, the structures of the analytes and temperature on the separations were investigated. Thermodynamic parameters were calculated from plots of ln alpha vs. 1/T. The Delta ( Delta H degree ) values ranged between -5.0 and +1.6kJ/mol, while Delta ( Delta S degree ) varied between -12.6 and +5.7J/mol/K. The enantioseparation was enthalpically controlled, the retention factor and the separation factor decreasing with increasing temperature, but entropically controlled separation was also observed. The elution sequence was determined for all of the investigated analytes.