Enantiomers order

Enantiomer Ordering and Separation During Molecular Inclusion... 

Even greater enantiomer ordering occurs if 14 is crystallised from dimethyl sulfoxide (DMSO), because the compound (14)2 (DMSO) is produced. This material, in space group P2i2j2, is a conglomerate where each crystal contains... 

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. 

In other approaches, chirahty descriptors were developed with the intention not of measuring chirality but of describing chirality in a way that correlations could be established with observable properties. These descriptors have different values for opposite enantiomers, in order that chirality-dependent properties can be predicted from them. They are usually multidimensional. 

When the lowest ranked substituent (the methyl group) is away from us the order of decreasing precedence of the remaining groups must appear in a clockwise sense in the R enantiomer... 

The minor images of bromochlorofluoromethane have the sane constitution. That is, the atoms are connected in the sane order. But they differ in the anangement of then-atoms in space they are stereoisomers. Stereoisomers that are related as an object and its nonsuperimposable minor image are classified as enantiomers. The word enantiomer describes a paiticulai- relationship between two objects. One cannot look at a single molecule in isolation and ask if it is an enantiomer any more than one can look at an individual human being and ask, Is that person a cousin Fuithennore, just as an object has one, and only one, minor image, a chiral molecule can have one, and only one, enantiomer. 

When chiral, drugs and other molecules obtained from natural sources or by semisynthesis usually contain one of the possible enantiomeric forms. However, those obtained by total synthesis often consist of mixtures of both enantiomers. In order to develop commercially the isolated enantiomers, two alternative approaches can be considered (i) enantioselective synthesis of the desired enantiomer or (ii) separation of both isomers from a racemic mixture. The separation can be performed on the target molecule or on one of its chemical precursors obtained from conventional synthetic procedures. Both strategies have their advantages and drawbacks. 

Among the existing separation techniques, some - due to their intrinsic characteristics - are more adapted than others to processing large amounts of material. Such processes, which already exist at industrial level, can be considered in order to perform an enantioselective separation. This is the case for techniques such as distillation and foam flotation, both of which constitute well-known techniques that can be adapted to the separation of enantiomers. The involvement of a chiral selector can be the clue which changes a nonstereoselective process into an enantioselective one. Clearly, this selector must be adapted to the characteristics and limitations of the process itself. 

Each glycopeptide CSP has unique selectivity as well as complementary characteristics, and a considerable number of racemates have been resolved on all three of them. Interestingly, most of the resolved enantiomers have the same retention order on these macrocyclic CSPs. When they are mixed or coupled with each other, the selectivity on one CSP will not be canceled by another. Even if some compounds may not have the same retention order, the complementary effects will result in an identifiable selectivity. Therefore, the coupled chiral columns can be used as a screening tool and save chromatographers substantial time in method development. 

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