Liquid crystals, enantiomeric purity

A final word needs to be said about the supposedly unique features of enzymes, namely, their ability to produce enantiomerically pure products. This is not the place to speculate about the stereospecificity of enzymes, a problem that has been discussed elegantly by Comforth (15). It cannot be denied that the high (>99.9%) enantiomeric purity achieved by enzymes may be uniquely useful in the case of liquid products. However, when crystalline products are obtained in an asymmetric synthesis and the e.e. exceeds 80%, crystallization to enantiomeric purity without excessive loss of material is routinely achieved. 

Qualitative analysis of chiral compounds, including drugs, pesticides, carbohydrates, amino acids, liquid crystals, and other biochemicals Determination of enantiomeric purity of chiral compounds... 

There are many other examples of chiral synthesis that provide interesting intermediates of high enantiomeric purity and some of these are now being developed for inclusion in chiral liquid crystals e.g., Brown s chiral hydioboration procedure and Matesson s chiral homologation procedure). 

In racemate forming enantiomeric mixtures the enantiomeric excess with a higher enantiomeric purity than that the initial composition remains in the melt and the crystalline phase formed has a lower ee. For instance, at the crystallization of the enantiomeric mixture of frflws-chrysanthemic acid (CHRA) from melt the liquid phase contains a higher purity fraction than the eeo value of the initial mixture.21... 

When a chiral substance is dissolved in a nematic liquid crystal, a cholesteric mesophase is obtained(8). The cholesteric structure is characterized by its handedness (P-or M-helix) and pitch. Equal amounts of enantiomeric solutes of equal optical purity induce helical structures with identical pitch and opposite handedness... 

An important aspect of chiral liquid crystals is that they have the potential to be used to determine the enantiomeric purity of any substrate because they do not depend on any direct interaction between the liquid crystal and the substrate, but only depend on the two enantiomers aligning differently relative to the applied magnetic field. Chiral liquid crystals have been applied to a broad variety of compound classes including aliphatic hydrocarbons. " ... 

In such cases the enantiomeric excess is crystallized with a higher purity than that of in the initial composition, in a certain range of temperature. For example, the common intermediate of the synthesis of several prostaglandines is a lactone (PGL). Its enantiomeric mixtures (eeo) can be enriched by crystallization (eesoUd) from melt, while the racemic ratio (liquid residue, eeuquid) can be recovered for repeated resolution.20... 

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