Application of Crystallization Inhibitors to Chiral Separations. 1. Design of Additives to Discriminate between the Racemic Compound and the Pure Enantiomer of Mandelic Acid

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Abstract

In the context of the potential use of crystallization for spontaneous separation of racemates, it is well-known that the frequent appearance of racemic solid forms (racemic compounds) is a serious obstacle. Given previous studies on the selective inhibition of centric versus noncentric polymorphic structures, we report here the application of a similar rationale for the selective crystallization inhibition of a racemic compound compared to a pure enantiomer. We have chosen the system R,S-mandelic acid (MA) in water which, from racemic solutions, crystallizes as two polymorphs of a racemic compound. The selection of potential additives is described and from the effects of additive concentration on the crystallization of (R,S)-MA compared to (S)-MA, at various crystallization temperatures, it is shown that while none of the additives dramatically inhibit the crystallization of (S)-MA even at 3% they significantly inhibit the crystallization of the racemate. With 10% additive, the crystallization of (R,S)-MA is totally inhibited for over 4 months. This leads the way to a new potential crystallization process for chiral enrichment discussed in the accompanying paper in this issue.

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