The Role of Diastereomer Impurity in Oiling-Out during the Resolution of trans-4-Methyl-2-piperidine Carboxylic Ethyl Ester Enantiomers by Crystallization

Isolating enantiomers by crystallization via diastereomer salt formation is widely used in the pharmaceutical industry. Unfortunately, we found that, when some diastereomer impurity exists, oiling-out will take place and significantly decrease the resolution efficiency. To identify the role of impurities in oiling-out, three series of chiral resolution of trans -4-methyl-2-piperidine carboxylic ethyl ester ( trans -4MPE) with different levels of diastereomer impurity were performed. Using process analytical technologies (PAT) technologies, the demixing region was identified by measuring T L–L curves and supersolubility curves for different series. It was found that a series with a higher level of impurity has a higher T L–L point and a wider demixing region. Besides, it seems that the resolution efficiency through oiling-out, as well as the crystallization kinetics, was significantly decreased. Here we initially put forward this particular diastereomer purification problem, where the impurity has a positive effect on oiling-out. To reduce or eliminate oiling-out, an efficient prepurification strategy must be ensured.