Chirality reference systems

Chirality element enumeration is essential for the classification of stereoselective reactions 27>. For instance, in order to distinguish an asymmetrically induced synthesis from other reactions whose stereoselectivity is also due to a chiral reference system, one must compare the number of chirality elements in the starting materials and the products. 

An additional chiral reference system based on chiral pairs of oriented lines has recently been proposed.140... 

In an asymmetric synthesis the effect of a chiral reference system is due to the fact that it interacts with a chiral molecular system and the mirror image of the latter in a different manner, just like a right hand interacts differently with right and left-handed gloves. These differences in interaction are chemically observable as stereoselectivity. Complex schemes of chemical reactions may contain asymmetric syntheses as partial reactions which can be conceptually separated from the other parts of the scheme. In some of these cases the whole reaction scheme may even not qualify as an asymmetric synthesis, because it leads to dissipation of chirality due to the presence of chirality dissipating components whose contribution to the overall result is predominating... 

Another possibility of constructing a chiral membrane system is to prepare a solution of the chiral selector which is retained between two porous membranes, acting as an enantioselective liquid carrier for the transport of one of the enantiomers from the feed solution of the racemate to the receiving side (Fig. 1-5). This system is often referred to as membrane-assisted separation. The selector should not be soluble in the solvent used for the elution of the enantiomers, whose transport is driven by a gradient in concentration or pH between the feed and receiving phases. As a drawback common to all these systems, it should be mentioned that the transport of one enantiomer usually decreases when the enantiomer ratio in the permeate diminishes. Nevertheless, this can be overcome by designing a system where two opposite selectors are used to transport the two enantiomers of a racemic solution simultaneously, as it was already applied in W-tube experiments [171]. 

Central Chirality. The system Cxyzw (5) has no symmetry when x, y, z, and w are different groups, and this system is referred to as a central chiral system. 

S) atoms. In both cases, the reference system does not change the three-dimensional arrangement of the groups around the chiral center. For example, if the sense of rotation had been chosen by convention to go from the small to the large atom (Schemes 11c and d), the corresponding symbols would have been reversed, but the absolute configuration of the molecule would have remained the same. 

We have briefly surveyed the chirality in photochromism. As there are a variety of strategies to incorporate chirality in photochromic systems, it is difficult to describe them in a systematic manner. In this chapter, we categorized the chiral photochromic systems in terms of the reaction modes and compounds in order to show the latest trends of the relevant photochromic compounds. Those who wish to study this field should refer to general reviews and books on photochromism [1,93,94] and become familiar with the compounds first. The book by Feringa [95] should be consulted also. 

Chirality refers to molecules that have nonsuperimpos-able mirror images. Most chiral molecules contain one or more asymmetric carbon atoms. Chiral molecules are widespread in biological systems and are important in drug design. 

Many chiral alicyclic rings have been used for liquid crystals. Often, a chiral ring system is only documented by a few examples. Figure 4.12 shows some of them. The references can be found in [3]. 

In this section we briefly discuss the most important aspects of the phenomenological description of nematic and chiral nematic systems, needed for the imderstanding of our particular examples in Sections 12.4 and 12.5. More details on this subject can be found in the general references [25], [26], [27], [28], [29], [30], [39], and references therein. 

The hydrosilylation of ketones is referred to above when a chiral phosphine-platinum (ii) complex is used as catalyst, chiral alcohols are obtained, as their silyl ethers, in optical yields of 5-18%. Chiral alcohols are also obtained in 15-20 % optical yield by condensation of ketones with magnesium alcoholates derived from N-methylephedrine. A study of the chiral reducing system, a-phenylethylamine-borane, has been referred to previously, as has Corey s refinement of prostaglandin reduction. 

If a tetrahedral center in a molecule has two identical substituents, it is referred to as prochiral since, if either of the like substituents is converted to a different group, the tetrahedral center then becomes chiral. Consider glycerol the central carbon of glycerol is prochiral since replacing either of the —CH9OH groups would make the central carbon chiral. Nomenclature for prochiral centers is based on the (R,S) system (in Chapter 3). To name the otherwise identical substituents of a prochiral center, imagine... 

Figure 8.3 Examples of different biological effects of enantiomers. S and R refer to a particular system of nomenclature used to describe chiral carbon, (see Appendix A8.1)...

Chiral dendrimers are a class of compounds which offer the possibility to investigate the impact of chirality in macromolecular systems. Their specific properties are based on their well defined highly ordered structures with nano-scopic dimension (in this report we refer to dendrimers if the molecule has a core with at least three branches attached and a defined structure otherwise we will use the term dendritic compound). 

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