Enantioenrichment by Crystallization

The separation of enantiomers is of great interest to the pharmaceutical industry since more than 50% of pharmaceutically active ingredients are chiral, and 9 of the top 10 drugs have chiral active ingredients. One particular enantiomer is usually preferred over the racemic mixture. In general, two methods are utilized for the production of a chiral active pharmaceutical ingredient (API) at a large scale: asymmetric synthesis and separation of enantiomers by crystallization. Neither process guarantees a product with an enantiomeric excess (ee) meeting regulatory requirements but instead generates a chiral mixture enriched with the desired enantiomer. In these cases, further chiral purification is required. This ee upgrade process remains largely an art that has not been systematically discussed. Development of a crystallization method for an ee enhancement should include three steps: (1) determine the thermodynamically stable phase of the racemate (conglomerate, racemic compound, or pseudoracemate) at the temperature of interest, (2) obtain the key solubility data, and (3) design the crystallization process. This review paper is intended to summarize recent publications and our own work concerning these areas and provide insight into how the process can be streamlined.