Chiral ligand-exchange chromatography of amino acid derivatives

Abstract

The chiral resolution of various N-acyl-DL-amino acids [acyl = formyl, acetyl, monochloroacetyl, trifluoroacetyl, benzyloxycarbonyl (Z), tert. butyloxycarbonyl (Boc)] as well as dansyl-DL-amino acids and side chain protected DL-serines was investigated by ligand-exchange high-performance liquid chromatography (LE-HPLC) using a commercially available stationary phase of silica bonded L-proline (‘Chiral ProCu’) and aqueous eluents containing Cu²⁺ as complex forming ion. In the instances investigated the D-enantiomers eluted before the L-enantiomers. Since in most cases baseline resolution, or almost baseline resolution, of the diastereomeric amino acid/Cu²⁺ complexes was achieved LE-HPLC is considered as a suitable method for the determination of the optical purity of the respective amino acid derivatives. The investigation of Boc- and Z-protected amino acids is of particular interest because these derivatives are among the most frequently used in peptide synthesis. However, 9-fluorenylmethyloxycarbonyl(Fmoc)-DL-amino acids were not, or unsatisfactorily, resolved by LE-HPLC under the conditions used.