Chiral molecules racemic forms

During classical asymmetric synthesis, the amplitude of these fluctuations are expected to decrease during the course of the reaction because more and more chiral molecules are formed and eeeXp declines. However, in the presence of chiral autocatalysis, the small ee caused by such fluctuations can be amplified. In such cases, the system is likely to be most sensitive in the initial stage of reaction when the concentration of chiral molecules is still small. If the autocatalytic species are concentrated they can be either in a racemic or optically active state but if they are highly diluted, as at the beginning of the reaction, statistical fluctuations can become significant so that the state... 

The presence of asymmetric C atoms in a molecule may, of course, be indicated by diastereotopic shifts and absolute configurations may, as already shown, be determined empirically by comparison of diastereotopic shifts However, enantiomers are not differentiated in the NMR spectrum. The spectrum gives no indication as to whether a chiral compound exists in a racemic form or as a pure enantiomer. 

This situation changed dramatically in 1996 with the discovery of strong electro-optic (EO) activity in smectics composed of bent-core, bowshaped, or banana-shaped achiral molecules.4 Since then, the banana-phases exhibited by such compounds have been shown to possess a rich supermolecular stereochemistry, with examples of both macroscopic racemates and conglomerates represented. Indeed, the chiral banana phases formed from achiral or racemic compounds represent the first known bulk fluid conglomerates, identified 150 years after the discovery of their organic crystalline counterparts by Pasteur. A brief introduction to LCs as supermolecular self-assemblies, and in particular SmC ferroelectric and SmCA antiferroelectric LCs, followed by a snapshot of the rapidly evolving banana-phase stereochemistry story, is presented here. 

The systems discussed up to now all showed chiral susceptibilities that were of the same order of magnitude or smaller than the achiral susceptibility components. The system that we discuss in this section has chiral susceptibilities that dominate the nonlinear optical response.53 The material is a chiral helicenebisquinone derivative shown in Figure 9.22. In bulk samples, the nonracemic, but not the racemic, form of the material spontaneously organizes into long fibers clearly visible under an optical microscope. These fibers comprise columnar stacks of helicene molecules.54,55 Similar columnar stacks self-assemble in appropriate solvents, such as n-dodecane, when the concentration exceeds 1 mM. This association can be observed by a large increase in the circular dichroism (CD) of the solutions. 

Finally, reference must be made to the important and interesting chiral crystal structures. There are two classes of symmetry elements those, such as inversion centers and mirror planes, that can interrelate. enantiomeric chiral molecules, and those, like rotation axes, that cannot. If the space group of the crystal is one that has only symmetry elements of the latter type, then the structure is a chiral one and all the constituent molecules are homochiral the dissymmetry of the molecules may be difficult to detect but, in principle, it is present. In general, if one enantiomer of a chiral compound is crystallized, it must form a chiral structure. A racemic mixture may crystallize as a racemic compound, or it may spontaneously resolve to give separate crystals of each enantiomer. The chemical consequences of an achiral substance crystallizing in a homochiral molecular assembly are perhaps the most intriguing of the stereochemical aspects of solid-state chemistry. 

As the cyclodextrins are chiral molecules, a racemic mixture of an optically active guest species has the possibility of forming two diaste-reomers on complexation with cyclodextrin, that is,... 

Thus, the anticholinergic activity of the alkaloid hyoscyamine is almost entirely confined to the (—)-isomer, and the (+)-isomer is almost devoid of activity. The racemic ( )-form, atropine, has approximately half the activity of the laevorotatory enantiomer. An anticholinergic drug blocks the action of the neurotransmitter acetylcholine, and thus occupies the same binding site as acetylcholine. The major interaction with the receptor involves that part of the molecule that mimics acetylcholine, namely the appropriately positioned ester and amine groups. The chiral centre is adjacent to the ester, and also influences binding to the receptor. 

Note 2 A polymerization in which, starting from the racemate of a chiral monomer, two types of polymer molecules, each containing monomeric units derived from one of the enantiomers, form in equal amounts is termed racemate-forming enantiomer-differentiating polymerization . The resulting polymer is optically inactive (see Note 4 of Definition 2). 

A collection containing only one enantiomeric form of a chiral molecule is referred to by several names including enantiopure, enantiomerically pure, or optically pure. A sample containing predominantly one enantiomer is called enantiomerically enriched or enantio-enriched. A collection containing equal amounts of two enantiomeric forms of a chiral molecule is called a racemic mixture or racemate. 

When this strategy was applied to the chiral azide 343, a severe loss of optical activity occurred, but the synthetic sequence was continued to afford the target molecule 347 in almost racemic form (66) (Scheme 9.66). 

In the meso-form 1, the absolute configurations are different, the molecules are identical, and there is at least one conformation with a symmetry plane, whereas the other conformations possess an enantiomeric equivalent. In contrast, molecules 2 and 3, both with groups of identical absolute configuration, are enantiomers, i.e., d,/-forms, often racemic mixtures. They are chiral and there are conformers with a twofold rotation axis. Thus, a differentiation between the meso- and the racemic form, simply by counting the number of 13C signals, is not possible. For each isomer the 13C-NMR spectrum contains a halved set of resonances, i.e., one signal for each pair of corresponding carbons in the two parts of the molecule. 

A chiral stereoisomer is not superimposable on its mirror image. It does not possess a plane or center of symmetry. The nonsuperimposable mirror images are called enantiomers. A mixture of equal numbers of molecules of each enantiomer is a racemic form (racemate). The conversion of an enantiomer into a racemic form is called racemization. Resolution is the separation of a racemic form into individual enantiomers. Stereomers which are not mirror images are called diastereomers. 

Addition of HBr to 1-butene yields a chiral molecule. The reaction is regioselective and a racemic mixture is formed. 

Chirality in Crystals. When chiral molecules form crystals the space group symmetry must conform with the chirality of the molecules. In the case of racemic mixtures there are two possibilities. By far the commonest is that the racemic mixture persists in each crystal, where there are then pairs of opposite enantiomorphs related by inversion centers or mirror planes. In rare cases, spontaneous resolution occurs and each crystal contains only R or only S molecules. In that event or, obviously, when a resolved optically active compound crystallizes, the space group must be one that has no rotoinversion axis. According to our earlier discussion (page 34) the chiral molecule cannot itself reside on such an axis. Neither can it reside elsewhere in the unit cell unless its enantiomorph is also present. 

Abstract It is well known that spontaneous deracemization or spontaneous chiral resolution occasionally occurs when racemic molecules are crystallized. However, it is not easy to believe such phenomenon will occur when forming liquid crystal phases. Spontaneous chiral domain formation is introduced, when molecules form particular liquid crystal phases. Such molecules possess no chiral carbon but may have axial chirality. However, the potential barrier between two chiral states is low enough to allow mutual transformation even at room temperature. Therefore the systems are essentially not racemic but nonchiral or achiral. First, enhanced chirality by doping chiral nematic liquid crystals with nonchiral molecules is described. Emphasis is made on ester molecules for their anomalous behavior. Second, spontaneous chiral resolution is discussed. Three examples with rod-, bent-, and diskshaped molecules are shown to give such phenomena. Particular attention will be paid to controlling enantiomeric excess (ee). Actually, almost 100% ee was obtained by applying some external chiral stimuli. This is very noteworthy in the sense that we can create chiral molecules (chiral field) without using any chiral species. 

Many pharmaceutical products are chiral molecules, either as single isomers or more commonly as racemic mixtures. In addition, many formulated products are mixtures of active compounds together with a number of additives such as excipients. For chiral molecules, the pressure to develop single isomer forms as... 

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