Comparative Studies on Enantiomer Resolution of α-Amino Acids and Their Esters Using (18-Crown-6)-tetracarboxylic acid as a Chiral Crown Ether Selector by NMR Spectroscopy and High-Performance Liquid Chromatography

Abstract

In a previous study, our group and the Machida group have developed covalently bonded and/or dynamically coated chiral stationary phases (CSPs) derived from (+)-(18crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TA) for HPLC studies (Figure 1). These CSPs have been effectively applied in resolving several amino acids and various primary amino compounds. In addition, our group reported that 18-C-6-TA as an NMR chiral solvating agent is quite efficient to resolve some amino acid analytes in NMR spectroscopy. Therefore, we reported that the chiral crown ether selector 18-C-6-TA is useful for the enantiodiscrimination of phenylglycine, alanine, and diphenylalanine by NMR spectroscopy. Also other groups have reported some results using 18-C-6-TA as a chiral NMR solvating agent. Even though each analytical method by HPLC and NMR spectroscopy has been independently studied, the direct comparative study for enantiomer resolution of αamino acids and their esters using 18-C-6-TA as a chiral selector by two analytical methods has not been published. Especially, the enantiomer resolution results of two α-amino acids esters by NMR spectroscopy were only reported in our group. Therefore, in this study, we investigate the enantiomer resolution of various α-amino acids and their esters using (+)-18-C-6-TA as a chiral crown ether selector in NMR studies and compare these NMR results with liquid chromatographic resolution results obtained using 18-C-6TA derived CSP (CSP 1, Figure 1). Tables 1 and 2 show comparative results for the enantiomer resolution of several amino acids and their esters by NMR and HPLC. All investigated amino acids and esters were well resolved on CSP 1 derived from (+)-18-C-6-TA by HPLC (α = 1.18-2.74), except for leucine methyl ester and phenylalanine methyl ester (entries 2 and 3 in Table 2). However, all analytes including these two analytes showed chemical shift differences for the α-proton in the presence of 18-C-6-TA by NMR spectroscopy. It means that 18-C-6-TA successfully discriminated all tested amino acids and esters in this study by NMR. Typical H NMR spectra of valine (Val) and phenylalanine methyl ester (Phe-OMe) in the