Racemization of Optically Active Amino Acid Salts and an Approach to Asymmetric Transformation of Dl-Amino Acids

Abstract

In connection with optical resolution of DL-amino acids, the racemization process of optically active amino acid salts was studied. A wide variety of salts of optically active amino acids with sulfonic acids or mineral acids could be racemized by heating in a medium of acetic acid at 80–100 °C for 1 h in the presence of 0.1 molar equivalents of an aldehyde and free DL-amino acid. From the reaction mixture, the racemized DL-amino acid salts were recovered in high yield (86–92%) and in high purity. An asymmetric transformation between two enantiomers of amino acid salt was attempted by the combination of preferential crystallization of the desired enantiomer and racemization of the antipode in the liquid phase. DL-Alanine p-chlorobenzenesulfonate was partially converted to L-isomer (yield 16%, optically purity 97%). Another approach was carried out by transformation between two diastereoisomeric salts. The combination of selective precipitation of less soluble diastereoisomeric salt and epimerization of soluble diastereoisomeric salt allowed DL-phenylglycine d-camphor-10-sulfonate to be partially converted to D-phenylglycine d-camphor-10-sulfonate (yield 68%, optical purity 96%).