Dual-mode of Reversed and Normal-phase Liquid Chromatographic Enantiomer Separation on Coated Cellulose Derived Chiral Stationary Phases

Abstract

For their broad applicability of enantiomer separation of a number of chiral compounds, many chiral stationary phases (CSPs) based on polysaccharide derivatives have been developed and extensively used. Among these CSPs, cellulose tris(3,5-dimethylphenylcarbamate) derived CSP (Chiralcel OD or Chiralcel OD-H) and cellulose tris(4methylbenzoate) derived CSP (Chiralcel OJ or Chiralcel OJH) have proven to be highly useful CSPs. These type CSPs are usually prepared with coating or adsorbing the polysaccharide derivatives of the chiral selectors on a silica matrix. It has been used with a limited range of solvents and, therefore, it has been mainly employed with normal phase eluents, such as hexane/2-propanol mixtures. Compared to normal-phase enantiomer separation, however, reversedphase enantiomer separation using aqueous mobile phases may often provide advantages. Especially, it is important for samples that have problems for analysis under normal phase conditions because of their solubility and intrinsic aqueous biological samples or synthetic analytes with aqueous solvents. Since these coated cellulose derived CSPs like Chiralcel OD and Chiralcel OJ have been applied with normal phase eluents, only a few of results for enantiomer separation using aqueous mobile phases on these CSPs have been reported. The first attempt of reversed-phase enantiomer separation on the polysaccharide derived CSP has been reported by Ikeda et al. Four chiral drug analytes have been enantioseparated with good separation factors (α = 1.101.61) on Chiralcel OD using aqueous buffer-acetonitrile solutions. The second report on enantiomer separation of one hydantoin biological analyte of a major metabolite of phenytoin on Chiralcel OJ using aqueous ethanolic solution (Rs = 2.2) has been published by Eto et al. Also, the enantiomer resolution of four drug analytes on Chiralcel OJ using aqueous acetonitrile solution has been performed by Ishikawa et al. A series of quaternary tropane alkaloids have been resolved with good separation factors on Chiralcel OD using aqueous ion pair mobile phases and normal phases by Hempe et al., respectively. Weinz and his co-workers have reported on enantiomer resolution of only two biological samples of glutethimide and its analog analytes on Chiralcel OD using aqueous acetonitrile solution (α = 1.21, 1.14). In this study, we present enantiomer resolution of Nfluorenylmethoxycarbonyl (FMOC) α-amino acids on Chiralcel OJ-H and Chiralcel OD-H under the dual-mode of reversed as well as normal phase conditions. This is the first report on both reversed and normal phase liquid chromatographic enantiomer separation using Chiralcel OJ-H and Chiralcel OD-H, coated cellulose based CSPs. Table 1 and 2 show liquid chromatographic results for the separation of the enantiomers of N-FMOC α-amino acids on Chiralcel OJ-H and Chiralcel OD-H under reversed and normal phase conditions. In general, the enantioselectivities on these two CSPs for the resolution of N-FMOC α-amino acids under reversed phase condition of 40% acetonitrile in 50 mM phosphate buffer are lower than those under normal