Achiral materials

To describe the propagation of light in an anisotropic medium, we write tire wave equation of (5.8) in the form  

Coordinate system to describe light propagation in a uniaxial system. 

The transformation matrix from the primed to the unprimed system, and the inverse matrices read as  

The dielectric tensor and its inverse are diagonal in the primed system and are expressed as  

Transforming the dielectric tensor and its inverse to the laboratory system we get  

---

As was the case for kinetic resolution of enantiomers, enzymes typically exhibit a high degree of selectivity toward enantiotopic reaction sites. Selective reactions of enaiitiotopic groups provide enantiomerically enriched products. Thus, the treatment of an achiral material containing two enantiotopic functional groups is a means of obtaining enantiomerically enriched material. Most successful examples reported to date have involved hydrolysis. Several examples are outlined in Scheme 2.11. 

The above results are valuable in that an optically active compound is produced in bulk from achiral material. Only a few successful examples of photochemical conversion of achiral into chiral material in the absence of a chiral source have been reported hitherto 49, and in these cases the conversion was carried out on a fragment of a chiral crystal. In our case, chiral crystals are available in bulk, and mass production of the chiral compound is possible. 

Obviously, in a relatively small work such as this it is not possible to be comprehensive. Preparations of bulk, achiral materials (e.g. simple oxiranes such as ethylene oxide) involving key catalytic processes will not be featured. Only a handful of representative examples of preparations of optically inactive compounds will be given, since the emphasis in the main body of this book, i.e. the experimental section, is on the preparation of chiral compounds. The focus on the preparation of compounds in single enantiomer form reflects the much increased importance of these compounds in the fine chemical industry (e.g. for pharmaceuticals, agrichemicals, fragrances, flavours and the suppliers of intermediates for these products). 

SPONTANEOUS CHIRAL CRYSTALLIZATION OF ACHIRAL MATERIALS AND ABSOLUTE ASYMMETRIC TRANSFORMATION IN THE CHIRAL CRYSTALLINE ENVIRONMENT... 

Kondepudi and McBride indicate that stirred crystallization is effective to accelerate the enantiomeric excess (ee) of crystals in the recrystallization step. [7] Kondepudi reported that the ee of the crystal greater than 95% can easily be obtained in stirred crystallization of achiral materials that crystallize in chiral form, such as sodium... 

The chiral crystallization of achiral materials and the asymmetric transformation in the chiral crystal environment are described. Many successful examples are presented however, it is still rare to find materials which show this behavior. Recently, new asymmetric reactions using chiral crystals in homogeneous conditions have been developed. [39],[40] These reactions used the frozen chirality generated by chiral... 

Another interesting approach in asymmetric ring-closing metathesis is the catalytic enantiose-lective desymmetrization of achiral starting materials [21 ] (61 62). Starting with achiral material... 

Starting from achiral materials, two stereoisomeric phosphonylated dihydroxy pyrrolidines (275) and (276), containing four stereogenic centers, have been synthesized enantioselectively, employing a combination of enzymatic and transition-metal-mediated methods. Both compounds contain features of the transition state of the enzyme-catalysed fucosyl transfer reaction and represent building blocks of potential inhibitors against this class of enzymes. The synthesis of new sugar-derived phosphonic acids e.g. (277) from protected... 

Abstract Enantioselective heterogeneous catalysis requires surfaces with structures that are chiral at the atomic level. It is possible to obtain naturally chiral surfaces from crystalline inorganic materials with chiral bulk structures. It is also possible to create naturally chiral surfaces from achiral materials by exposing surfaces that have atomic stractures with no mirror symmetry planes oriented perpendicular to the surface. Over the past decade there have been a number of experimental and theoretical demonstrations of the enantiospecific physical phenomena and surface chemistry that arise from the adsorption of chiral organic compounds on the naturally chiral, high Miller index places of metals. 

Like ketone 2, ketone 3 can give two possible products and the nucleophile could attack the ketone from one face or from the other. However, this time, the two lines of attack lie on either side of the mirror plane that runs through ketone 3a. The two products are enantiomers and the two faces of the molecule are thus enantiotopic. This reaction was the only reaction that started with an achiral material but gave a chiral product. Because a reaction of ketone 3 leads to chiral products we can also describe ketone 3 as prochiral. 

We end with a stereochemically involved synthesis of a prostaglandin 155. The racemic synthesis is summarised below - each compound is a single diastereoisomer and all from 146 to 155 are chiral but all are racemic as the synthesis starts with achiral materials. There are 14 steps in the synthesis and the final product 155 contains four chiral centres.37 If we want a single enantiomer, where should we resolve ... 

When the nematic phase is composed of optically active materials (either a single component or a multicomponent mixture made up of chiral compounds or chiral compounds mixed with achiral materials), the phase itself becomes chiral and has reduced environmental space symmetry. The structure of the chiral nematic (or cholesteric) modification is one where the local molecular ordering is identical to that of the nematic phase, but in the direction normal to the director the molecules pack to form a helical macrostructure, see Fig. 5. As in the nematic phase the molecules have no long-range positional order, and no layering exists. The pitch of the helix can vary from about 0.1 x 10 m to almost infinity, and is dependent on optical purity and the degree of molecular... 

This achiral material is termed a meso compound. This situation had been predicted by Le Bel. He considered that two identical chiral groups could compensate for each other and so cancel out the effect of rotation of plane polarized light. 

---