Enantiomeric resolution of N-methyl-α-amino acids and α-alkyl-α-amino acids by ligand-exchange chromatography

The enantiomeric resolution of N-methyl-DL-α-amino acids (NMe-AA) and DL-α-alkyl-α-amino acids (AAA) was achieved by ligand-exchange (LE) high performance liquid chromatography (HPLC) using silica bonded L-amino acids as the chiral selector (Chiral ProCu, Chiral HyproCu, Chiral ValCu, Nucleosil Chiral-1). Using aqueous solutions of copper sulfate or acetate adjusted to pH 4–6 and the addition of acetonitrile (10–20%) or methanol (10–40%), baseline resolution occurred in many cases. By comparison with enantiomers of known optical configuration, the elution order of a number of NMe-AA and AAA could be determined and on-line polarimeter detection enabled the assignment of the optical rotation, elution order and configuration of AAA enantiomers. Using a preparative column (300mm × 20mm i.d.) packed with Chiral ProCu, a baseline resolution of 60mg DL-α-methylphenylalanine was achieved. Further, LE-TLC, using a chiral stationary phase (“Chiralplate”), enabled the chiral separation of very hydrophobic AAA, characterized by increasingly large alkyl side chains,viz. the homologous series α-ethylalanine to α-nonylalanine and α-propyl-α-amino-n-butyric acid to α-hexyl-α-amino-n-butyric acid.