Racemic Structures and Optical Resolutions by Preferential Crystallization of (±)-1-Amino-2-propanol Salts of Organic Acids

Abstract

The optical resolutions by preferential crystallization of (±)-1-amino-2-propanol salts of organic acids have been studied. The salts employed were 2-aminobenzoate (2-AB salt), 4-hydroxybenzoate (4-HB salt), 4-methoxybenzoate (4-MTB salt), 4-methylbenzoate (4-MB salt), benzoate (BA salt), 4-chlorobenzoate (4-CB salt), 3-nitrobenzoate (3-NB salt), 4nitrobenzoate (4-NB salt), 5-sulfosalicylate (SS salt), and methanesulfonate (MS salt). The racemic structures of these (±)-salts at the melting points were determined on the basis of the energies of formation of racemate and the binary phase diagrams of melting point. The results indicated that (±)-3-NB and MS salts were conglomerate, and that (+)-4MTB salt was racemic solid solution. (±)-2-AB, 4-HB, 4-MB, BA, 4-CB, 4-NB, and SS salts formed racemic compounds. However, (±)-BA and 4-CB salts were suggested to be unstable racemic compounds at the melting points. The comparison of the infrared spectra and solubilities of (±)-BA and 4-CB salts with those of the corresponding (+)-salts, or the ternary phase diagrams of solubility at 10 and 30°C of 4-CB salt indicated that these (+)salts existed as conglomerate in the vicinity of room temperature. The optical resolutions by preferential crystallization of (±)-BA, 4-CB, 3-NB, and MS salts have been carried out at 10°C. Though (+)-BA and 4-CB salts with optical purity higher than 90% were obtained, the degrees of resolution were low. However, in the optical resolution of (±)-MS and 3NB salts, the (+)-MS salt with optical purity of 80.6% was obtained in degree of resolution of 54.8%, and (+)-3-NB salt with optical purity of 81∼93% in 43∼65%.