In the optical resolution process of Dl-serine m-xylene-4-sulfonate dihydrate by the preferential crystallization procedure, the stability of the supersaturation state of the unseeded isomer was investigated in connection with the cooling conditions. The experiment was initiated with the model solution and was then extended to the actual resolution process. The metastable region of the unseeded isomer on a two-dimensional diagram with the supersaturation degree and cooling time could be divided into the first and the second metastable regions. The limits of the first and the second metastable regions were expressed by a supersaturation degree and by an area on the diagram respectively. Both limiting values were constant, independent of cooling speed and the lowest arrival temperature, if the conditions for the crystallizer, the initial concentration of the Dl-form, the amount of the seed crystals, and the degree of stirring were fixed.
DL-Proline was found to form new diastereoisomeric solid complexes with L-tartaric acid in a molar ratio of 1: 1 from an aqueous ethanol. The complex of L-proline was less soluble than that of n-proline. On the basis of these properties, DL-proline was easily resolved by the usual chemical resolution technique without the necessity of converting to the derivative. The optically impure D-proline was racemized by heating at 170•Ž for 4hr in water con taining an equimolar amount of sodium hydroxide and was reused for resolution step.
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%).
In the optical resolution of Dl-serine m-xylene-4-sulfonate dihydrate by the preferential crystallization procedure, the prediction of the resolution process was studied. The solution saturated with the Dl-form at 20 °C was cooled under a programmed cooling pattern and was seeded with the l-isomer. The decrease in the concentration of the l-isomer during the process was predicted by a conventional calculation method. Then, the waiting time for the spontaneous crystallization of the d-isomer was predicted by using the limiting values of the first and the second metastable regions, which were discussed in a previous report. As a result, a good agreement was obtained between the predicted results and the experimental results.
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