Chiral properties

In chemoinformatics, chirality is taken into account by many structural representation schemes, in order that a specific enantiomer can be imambiguously specified. A challenging task is the automatic detection of chirality in a molecular structure, which was solved for the case of chiral atoms, but not for chirality arising from other stereogenic units. Beyond labeling, quantitative descriptors of molecular chirahty are required for the prediction of chiral properties such as biological activity or enantioselectivity in chemical reactions) from the molecular structure. These descriptors, and how chemoinformatics can be used to automatically detect, specify, and represent molecular chirality, are described in more detail in Chapter 8. 

Optical properties of cyanines can be usefiil for both chiral substituents/environments and also third-order nonlinear optical properties in polymer films. Methine-chain substituted die arbo cyanines have been prepared from a chiral dialdehyde (S)-(+)-2-j -butylmalonaldehyde [127473-57-8] (79), where the chiral properties are introduced via the chiral j -butyl group on the central methine carbon of the pentamethine (die arbo cyanine) chromophore. For a nonchiral oxadicarbocyanine, the dimeric aggregate form of the dye shows circular dichroism when trapped in y-cyclodextrin (80). Attempts to prepare polymers with carbocyanine repeat units (linked by flexible chains) gave oligomers with only two or three repeat units (81). However, these materials... 

The oldest method of resolving enantiomers by TLC takes advantage of the natural chiral properties of cellulose and triacetylcellulose resulting from the helical structure of the polymers (98). Amino acid derivatives have been resolved on silica gel layers impregnated with chiral acids or bases, for example. 

Preparation of chiral mesoporous materials has become a great interest for material scientists. Normally chiral property is introduced into chiral mesoporous material via an organic chiral templating component. But, by using a sonochemical method, Gabashvili et al. [36] have prepared mesoporous chiral titania using a chiral inorganic precursor and a non-chiral dodecylamine as a template. Size of the pores was 5.5 nm. 

Chirality Properties conferred by the presence of one or more chiral centres. 

Several 4-(3-alkyl-2-isoxazolin-5-yl)phenol derivatives that possess liquid crystal properties have also been obtained (533-535). In particular, target compounds such as 463 (R = pentyl, nonyl) have been prepared by the reaction of 4-acetoxystyrene with the nitrile oxide derived from hexanal oxime, followed by alkaline hydrolysis of the acetate and esterification (535). A homologous series of 3-[4-alkyloxyphenyl]-5-[3,4-methylenedioxybenzyl]-2-isoxazolines, having chiral properties has been synthesized by the reaction of nitrile oxides, from the dehydrogenation of 4-alkyloxybenzaldoximes. These compounds exhibit cholesteric phase or chiral nematic phase (N ), smectic A (S4), and chiral smectic phases (Sc ), some at or just above room temperature (536). 

It is easy to prove that the function 0o > has the same chiral properties as the zero-mode function To > - Then... 

Chirality is an essential property of life, which can be found throughout all biological self-assembled and self-organized architectures. Over many millennia nature has, through trial and error, learned how to utilize the chiral properties of the small building blocks, for example, amino acids and nucleic acids and how to express this structural property in a hierarchical process at the quaternary level. This expression of chirality at the quaternary level in turn... 

Nonlinear optical techniques are extremely useful in characterizing the chiral properties of materials, as is pointed out by Verbiest and Persoons in Chapter 9. These authors give an in-depth discussion of this tool, both from an experimental and theoretical point of view, paying special attention to the characterization of chiral surfaces and thin films. In the second part of their contribution they highlight the role chiral materials can play in the field of nonlinear optics and photonics, which opens the way for a variety of applications. 

If any two of the quantities a, b, c, d in Eq. (1) are equal, then 0=0. If the values of any two of them are interchanged, 0 changes sign. If a, b, c, d are identified with some property of the ligands, therefore, then is a chiral property of the molecule, and is thus a candidate for an approximate representation of the rotatory power. Crum Brown and Guye identified the quantities a, b, c, d with the masses of the ligands. 

The function 0 in (1) may or may not accurately describe an experimentally interesting chiral property it does, however, possess the necessary symmetry properties for doing so, and therefore is an example of what is called a chirality function (to be defined precisely in Section III). Its form was arrived at basically through symmetry considerations, but not in a way that would systematically yield analogous formulas for other skeletons. The question arises of just how much can be said about chirality functions in a systematic way based on symmetry considerations alone. It is to be expected that group theory should play a major role in any study of this question. 

As it turns out, however, neither of these functions can be used to describe a chiral property of a wide class of molecules without encountering a fundamental difficulty. For example, consider an isomer mixture in which the molecules I, II, III of Fig. 9 appear in equal concentrations. Xi for this mixture would be, apart from a multiplicative constant, simply... 

Either of these functions by itself, then, is incapable of giving a sufficiently general description of a chiral property, as each vanishes identically in situations where there is no reason of symmetry why it should, i.e., no reason inherent in the definition of a chirality function. The sum of the two, %i + %2, appears to offer better prospects than either one by itself but it is not clear at this point whether this function is sufficiently general, or in general, how one decides whether a chirality function will encounter difficulties of this kind or not. 

For a given skeleton, the set of chiral Young diagrams (representations) permits us to define a set of numbers which are characteristic of the chiral properties of the skeleton. In this subsection, we consider the properties of these numbers for the case of achiral ligands, <5 =<3 . 

Miiller-Schwarze, D., Ravid, U., Claesson, A., et al. (1978a). The deer lactone source, chiral properties, and responses by black-tailed deer. JoarnaZo/C/jcm/ca/Eco/cgy4,247-256. 

In this section, we consider systems in which chiral properties have been the focus of attention. Examples " have already been mentioned, and earlier sections should also be consulted. The synthesis and photophysical properties of dinuclear complexes with... 

What happens for a nonracemic mixture of enantiomers Is it possible to calculate the values of the chiral properties of the solution from knowledge of the properties of the enantiopure compound In principle, yes, on the condition that there is no autoassociation or aggregation in solution. Then, the observed properties will be simply the weighted combination of the properties of two enantiomers. A nice example of where this normal law may be broken was discovered by Horeau in 1967 it is the nonequivalence between enantiomeric excess (ee) and optical purity (op, with op = [a]exi/[ ]max) for 2,2-methylethyl-succinic acid. In chloroform op is inferior to ee, while in methanol op = ee. This was explained by the formation of diastereomeric aggregates in chloroform, while the solvation by methanol suppresses the autoassociation. 

Li, X.-C. et al.. Absolute configuration, conformation, and chiral properties of fiavanone-(3 8")-flavone biflavonoids from Rheedia acuminata. Tetrahedron, 58, 8709, 2002. 

Use of the optically resolved complex leads to the optically active polymer, but this property, which arises from the helical chain structure, is found only in the swollen polymer and is easily lost in toluene or dichloroacetic acid solution 144). The polymerization occurs with a high degree of enantioface selection, and the model for the product backbone is indeed chiral. However, because of the presence of a mirror, plane in the polymer chain (effects of chain termini neglected), the product does not have chiral properties in solution. 

---