Comparative Liquid Chromatographic Enantiomer Resolution on Two Chiral Stationary Phases Derived from Amylose Tris(3,5-dimethylphenylcarbamate)

Abstract

Polysaccharide-derived chiral stationary phases (CSPs) are known to show high chiral recognition ability in HPLC and have been extensively used to separate a broad range of racemic compounds. These type CSPs are usually prepared coating or adsorbing the polysaccharide derivatives on silica gel. Therefore, the solvents such as chloroform, methylene chloride, and tetrahydrofuran which dissolve or swell the chiral selectors of the polysaccharide derivatives cannot be used as mobile phases. For example, in case of Chiralpak AD prepared by coating amylose tris(3,5dimethylphenylcarbamate) derivative, one of the widely used polysaccharide type CSPs, suitable mobile phases like hexane, 2-propanol and ethanol should be used for the column safety. It may be damaged in case of even the use of only a little amount of inappropriate solvents used as mobile phases and/or sample solvents. These limitations represent a disadvantage for new applications using these CSPs and, especially, preparative separation due to solubilization of analytes. To overcome these problems, therefore, the development of polysaccharide-derived covalently bonded CSPs has been of great interest and various results of different attempts have been reported. Recently, Chiralpak IA prepared by chemically bonding amylose tris(3,5-dimethylphenylcarbamate) derivative on silica gel, which is used as the same chiral selector of coated type Chiralpak AD has been introduced. In this study, we present the comparative liquid chromatographic enantiomer resolution of N-fluorenylmethoxycarbonyl (FMOC) protected αamino acids ethyl ester derivatives on two polysaccharidederived CSPs, covalently bonded type Chiralpak IA and coated type Chiralpak AD. This is the first report concerning the comparison of the chiral separations on Chiralpak IA and Chiralpak AD using normal mobile phases. For enantiomer separation of N-FMOC α-amino acids ester derivatives, very a few results on CSPs have been reported. Rizzi has separated three FMOC α-amino acids methyl esters enantiomers on cellulose triacetate type column. Miyazawa et al. have reported on the resolution of only one analyte of FMOC 2-aminobutanoic acid methyl ester enantiomers among many different N-protected amino acid derivatives. Kusters et al. have reported on the resolution of fifteen N-FMOC α-amino acids methyl and isopropyl esters enantiomers on a polysaccharide-derived CSP. To our knowledge, our results are the first reported for the enantiomer resolution of N-FMOC α-amino acids ethyl ester derivatives. Table 1 shows the effect of mobile phase on the enantiomer separation of some N-FMOC α-amino acids ethyl esters on Chiralpak IA. The enantioselectivities and retention times are greatly influenced by the nature of mobile phase. As shown in Table 1, in general, 5% 2propanol in hexane as a mobile phase afforded the greatest enantioselectivity with the highest resolution factor, whereas 20% chloroform in hexane afforded the lowest enantioselectivity. It is notable that the elution orders of three analytes using 10% tetrahydrofuran or 10% ethyl acetate or 20% chloroform in hexane are different from those using 5% 2-propanol in hexane. Table 2 shows the comparative data of enantiomer separation of N-FMOC α-amino acids ethyl esters on Chiralpak IA and Chiralpak AD using 2-propanol in hexane as a mobile phase. In general, Chiralpak IA showed slightly lower enantioseparation than Chiralpak AD for enantioresolution of N-FMOC α-amino acids ethyl esters. Several results have reported that a certain decrease in the enantio-