Crystallization of enantiomers

Crystallization is one of the most evident manifestations of chiral recognition. It is well known that crystallization from a racemic solution can yield a range of scenarios, from a conglomerate, in which resolution of isomers is complete through solid solutions, in which it may be partial, to a racemic compound in which no resolution occurs. This underlying behaviour, discovered largely as a result of Pasteur’s seminal works more than 150 years ago, has given rise to the development of different crystallization modes. In a conglomerate forming system mirror image homochiral crystal packings are spontaneously formed and preferential crystallization may be used to selectively and alternatively crystallize one enantiomer out of a close to 50-50 composition. In a compound forming system the so called pasteurian resolution -and its numerous variants – must be used. Here a specific resolving agent gives two different crystal lattices with the two enantiomers and the difference in stability- i.e. in solubility- gives the reason for the success of this method which relies on a simple selective crystallization based on stable thermodynamic equilibria.
However, recently for compound-forming systems the first successful preferential crystallization separations have been performed starting from partially resolved, i.e.
enantiomerically enriched solutions. Studying and applying this option within the integrative concept of the project significantly widens the applicability of crystallization processes for enantioseparation. Further, a supramolecular, solid state approach has recently been explored which tries to engulf, selectively a specific guest chiral molecule in a specific chiral host molecule. This approach is promising but, for the moment, less developed at the industrial scale.

This work is part of European Union Framework Program 7 project INTENANT

INTegrated synthesis and purification of single ENANTiomers (INTENANT)

Several industries have an increasing interest in improving their access to pure single
enantiomers of biologically active substances. The latter often form the basis for valued added products relevant for, among others, the pharmaceutical, cosmetic, and food industries.
Up to now there exist two generally rivalling concepts for producing pure enantiomers. One approach is based on developing and applying enantioselective synthesis techniques, which often consist of a large number of reaction steps. Alternatively, simple non-selective synthesis can be applied in combination with advanced downstream separation techniques and recycling strategies. These two competing approaches have been up to now hardly combined and considered in a holistic way.
It is the goal of the proposed project to combine the available chemical and physical methods to effectively and efficiently produce single enantiomers at high purity. For this a strong consortium of academic and industrial experts in various fields of synthesis and separation technologies will work together on several well selected model systems as well as industrially relevant components. Based on case studies performed, main outcome of the project will be innovative, shorter, cheaper and widely applicable pathways for the production of pure
enantiomers, which will be brought into industrial practice and enhance the competitiveness of European industry.

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