R-(-)-acetyl Chloromandelic Acid Preparation Via Enzymatic Dynamic Kinetic Resolution

Among the thousand pharmaceuticals currently in use all over the world, more than fivehundard are natural or semisynthetic, and most of them are chiral molecules in racemic form.The pharmacological differences between the two enantiomers render the preparation ofenantiopure chiral pharmaceutical a hot research topic. The R-(-)-acetyl chloromandelic acidprepared in this study is one of such important pharmaceutical intermmediates. R-(-)-acetylchloromandelic acid could be hydrolyzed into R-(-)-chloromandelic acid, which can be usedto synthesize the novel platelet aggregation inhibitor clopidogrel. With the expandingapplications of clopidogrel chloromandelic acid and its derivatives, the demand forR-(-)-chloromandelic Acid is increasing, but due to the limitations of traditional crafts, themarket demand is difficult to be met. Therefore, a new preparation approach is in urgent need.At the moment, kinetic resolution of racemic mixtures is one of the major methods forpreparing single enantiomers. However, in conventional kinetic resolution, a maximum ofonly50%of the racemic starting material could be converted to the desired product, while theother isomer remains untouched. To make the best use of all starting material, dynamic kineticresolution is a good solution, which utilizes the remaining isomer by racemization reaction,giving a maximum theoretical conversion of100%.In this study, racemic acetyl chloromandelic acid was used as the raw material to prepareR-(-)-acetyl chloromandelic acid through dynamic kinetic resolution, in which the resolutionproduct S-(+)-chloromandelic acid was racemized by mandelate racemase. Firstly, theconditions for enzymatic resolution, separation of resolution mixture, and enzymaticracemization were optimized. The optimal resolution conditions were:20mM substrate,pH=6.2(phosphate buffer),40oC,200r/min,4.25h, while the optimal conditions forracemization were:10mM resolution product of chloromandelic acid, pH=7.5(phosphatebuffer),25oC~30oC,200r/min,3h. Subsequently, to recycle chloromandelic acid, themixture of R-(-)-acetyl chloromandelic acid and chloromandelic acid resulted fromasymmetric hydrolysis of racemic acetyl chloromandelic acid should be efficiently separated.Separation experiments and extraction optimization were therefore conducted. The resultsshowed that two40-min extractions of the resolution mixure using equal volume ofchloroform at40oC,200r／min gave an extraction yield of96%for the unreactedR-(-)-acetyl chloromandelic acid; and then after two extractions of the residue solution usingequal volume of ethyl acetate, an extraction yield of98%was achieved for the productchloromandelic acid. Dynamic kinetic resolution of acetyl chloromandelic acid was thenconducted under the optimal conditions, achieving optically pure R-(-)-acetyl chloromandelicacid with a final yield of80%. In conclusion, the conditions for enzymatic resolution,chloromandelic acid recycling and racemization were optimized in this study, leading to thepreliminary establishment of the dynamic kinetic resolution process for R-(-)-acetylchloromandelic acid preparation, which has important guiding significance for futureindustrial applications.