Purity drop in optical resolution of DL-methionine by the diastereomer method

Abstract We carried out optical resolution of dl -methionine-ethyl-ester ( dl ) by the diastereomer method using acethyl- d -methionine ( d ′) as a resolving agent in order to elucidate the mechanism of purity drop in the course of optical resolution. We obtained about 90–95% of less soluble salt ( d ′  · d ) in the agitated system. However, the purity dropped to 85–90% in the non-agitated system. This purity drop in the non-agitated system was explained as follows with the help of calculated composition profiles of d ′  · d and d ′  · l in the stagnant film that developed near the d ′  · d crystal surface. d ′  · l is transported with the bulk flow induced by molecular diffusion of d ′  · d to the crystal–solution interface and accumulates on the crystal surface. This accumulation leads to nucleation of d ′  · l (impurity). Nucleation of d ′  · l causes the purity drop, especially in the non-agitated case. Although this type of nucleation can also occur in the agitated case, agitation enhances the deposition of required salt ( d ′  · d ), and hence the purity of agitated crystals is higher than that of non-agitated crystals.