Optical Resolution, Characterization, and X-Ray Crystal Structures of Diastereomeric Salts of Chiral Amino Acids with (S)-(−)-1-Phenylethanesulfonic Acid

Abstract

Abstract Ten Dl-Amino acids (AA), including neutral, and basic amino acids, and an imino acid, were optically resolved, without derivatization into their covalent compounds, by means of fractional crystallization of their diastereomeric salts with (−)-1-phenylethanesulfonic acid (PES) in various solvents. Several pairs of the diastereomeric crystalline salts formed during the resolutions were analyzed by DSC and spectroscopy, which showed that the successful resolutions were attributable to differences in various physicochemical properties between the more-soluble d-AA·(−)-PES and less-soluble l-AA·(−)-PES. Chiral recognition of the most successfully resolved species, Dl-p-hydroxyphenylglycine (HPG) salt, was explored by comparing the X-ray crystal structures of d- and l-HPG·(−)-PES. The two crystal structures differed obviously in their hydrogen-bonding networks: the less-soluble l-HPG·(−)-PES only had strong hydrogen-bonded infinite chains of HPG in a “head-to-tail” arrangement through the p-hydroxyl group, the structure of which was more geometrically stable than that of the more-soluble d-HPG·(−)-PES. The differences in the two crystal structures related to striking differences in their solubilities and thermal properties.