Chiral references

Although main-chain chirality refers to both polymers with stereogenic centers in the main chain (configurational main-chain chirality) and polymers with main chains consisting of helical stereogenic bonds induced by chiral side groups or end termini (conformational main-chain chirality), in this chapter we mainly focus on polysilanes exhibiting the latter type of chirality. 

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. 

Compounds of unknown chirality to which a chiral reference is added in order to form a covalent derivative, a salt, or even a solvate. In the latter case, a compound of known chirality is incorporated by way of cocrystallization in the crystal structure of a compound of unknown absolute configuration, or vice versa. 

Determinations of absolute configuration by internal comparison are becoming rather frequent (see Table 5) but they may be unreliable since occasionally authors seem to only assume the absolute configuration of the chiral reference component in the structure. 

Table 5. Determinalion of Absolute Configuration by Internal Comparison to a Chiral Reference...

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

S) can be determined. The imprinted polymers show differences in their sensitivities towards the enantiomers of the analytes (Fig. 12, left), whereas the non-chiral reference polymer shows no difference between the two enantiomers (Fig. 12, right). Therefore, a chiral discrimination by the imprinted polymers is proved this can be described by a separation factor a (Table 1). The separation factor is defined here as the ratio of the sensitivity of the template to the sensitivity of the antipode. 

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... 

Chiral refers to the property of chirality. As applied to a molecule, the term has been used differently by different workers. Some apply it exclusively to the whole molecule, whereas others apply it to parts of a molecule. For example, according to the latter view, a meso compound is considered to be composed of two chiral parts of opposite chirality sense this usage is to be discouraged. See enantiomorph... 

D.V. Augusti et al., Rapid quantitative chiral analysis of sugars by electrospray ionization tandem mass spectrometry using modified amino acids as chiral reference compounds. Anal. Chem. 74, 3458-3462 (2002)... 

By means of sophisticated 2D NMR experiments, using D-mycosamine as an internal chiral reference, and by the preparation of some hydrogenated derivatives, Lancelin and Beau [2] succeeded in the elucidation of the complete stereostructure of natamycin. 

Chirality then is the property of handedness, and the adjective chiral refers to the molecule as a whole, rather than to a particular atom. Molecules such as CC14 and CH2C12 that can be superimposed on their mirror images are said to be achiral . 

Mie, A., Jomten-Karlsson, M., Axelsson, B.O., Ray, A., Reimann, C.T., Enantiomer separation of amino acids by complexation with chiral reference compounds and high-field asymmetric waveform ion mobility spectrometry preliminary results and possible limitations. Anal Chem. 2007, 79, 2850. 

FIGURE 9.2 General schematic for studying enantioselective discrimination using the KM. A metal (M) is mixed in solution with a chiral reference compound (CR) and an analyte of interest (A ). A ternary complex is isolated in the gas phase and induced to fragment. The logarithm of the ratio of product ion channels is measured and can be correlated with the enantiomeric purity of A. ... 

Schug, K.A., Lindner, W. (2005) Stereoselective discrimination and quantification of arginine and N-blocked arginine enantiomers by formation and dissociation of calcium-mediated diastereomeric trimer complexes with a chiral reference compound using electrospray ionization-ion trap tandem mass spectrometry. 7. Am. Soc. Mass Spectrom., 16,825-834. 

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. 

Chiral refers to a molecule with a structure that is not sup>erimposable on its minor image. (See also enantiomers.)... 

Enantiomeric composition of ibuprofen was determined using a kinetic method based on the formation of trimeric complexes of this compound with chiral reference compounds (o-glucose, o-mannose, D-tartaric acid, D-galactose, and D-ribose) in electrosprayed solutions in the presence of divalent metal cations [32]. The described method can be applied to investigate interconversion of ibuprofen enantiomers under various conditions of the drug administration. This is of importance in view of ibuprofen standing in the pharmaceutical industry. 

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