Integration of Biocatalysis and Crystallization towards the Manufacture of Enantiomerically Pure Compounds

Crystallization of enantiomerically pure compounds from racemic mixtures that form racemic compounds requires an enantiomeric excess greater than the eutectic point between the compound and the pure enantiomer.  It is estimated that roughly 90% of racemic mixtures fall into this category; hence, development of a process that utilizes such an approach could be of significant value.

To achieve a necessary enantiomeric excess, we are investigating the chemoenzymatic reaction of racemic mixtures of amino acids. We have demonstrated the possibility of performing such a reaction at high concentrations, even at those that require substrate dissolution. Preliminary results show that even in supersaturated solutions, enantiomeric excesses greater than 99% can be achieved. The chemoenzymatic reaction of (RS)-phenylalanine to obtain (S)-phenylalanine has been performed in undersaturated, saturated, and supersaturated solutions. The reaction proceeds through the selective oxidation of the (R)-amino acid utilizing D-amino acid oxidase (D-AAO), which produces an achiral imino acid that is reduced into a racemic amino acid utilizing ammonia-borane. Moreover, extending such reaction to saturated or supersaturated solutions allows us to operate crystallization processes such that enantiomerically pure compounds are continuously recovered in a solid phase. Therefore, the implementation of this reactive separation process is beneficial because of the following factors: (1) this process allows higher productivity, (2) the rate of reaction is maximized, (3) the process has potential to be run continuously, and (4) organic solvents or low-MW chiral acid/bases (i.e., the formation of diastereomers) are not needed during separation.