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The present invention relates to an industrially feasible and cost-efficient process for the preparation of isomerically pure S -amlodipine besylate hemipentahydrate ( 1 ), a useful calcium antagonist inhibitor. Previous workers reported that R -amlodipine-tartrate was crystallized out preferentially from the reaction mixture when naturally occurring l -tartaric acid and racemic amlodipine base in DMSO are mixed. In order to crystallize S -amlodipine-tartrate, the use of unnatural d -tartaric acid as a resolving agent in DMSO was required. However, the cost of d -tartaric acid was not conducive to overall cost efficiency in the resolution protocol. Subsequent to the above observations, we have developed a novel resolving system in which amlodipine base with natural l -tartaric acid in DMF as a solvent gave preferentially the S -form of amlodipine tartrate directly from the reaction. The optimization of this approach by adjusting the water percentage in DMF ensured consistent purity of S -amlodipine (+99%) and satisfactory resolution efficiency.
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Gotrane et al. (2010) studied this question.
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