Introduction to Absolute Configuration of Polysaccharide

  • The absolute configuration represents the true arrangement of the individual groups in the chiral molecule in space, ie the absolute spatial relationship. A compound, when its structural formula is expressed as a stereostructure, conforms to the true stereostructure of the compound molecule, and the configuration of the stereostructure is an absolute configuration.

     

    The absolute configuration is a description of the chiral molecular entities (or groups) of the atomic arrangement of the space and their stereochemistry.

     

    When a configuration expresses a stereoisomer as specified, if the true configuration of the determined stereoisomer is the same as the configuration expressed by the configuration, then the configuration represented by the configuration is called For absolute configuration. The labeling of the absolute configuration is a conformational labeling method established by Ingol and Kane, characterized by the arrangement of groups of chiral carbon atoms in different directions in space, namely the R and S configuration marking methods.

     

     

     

    Determination of absolute configuration

    1. If the four different groups a, b, c, and d of the chiral carbon atom are arranged in the order of a→b→c→d, the order is determined according to the Ingol-Kane rule;
    2. The last d of the sequence is taken as the top end of the chiral carbon atom tetrahedron, a, b, c are the triangles at the bottom of the tetrahedron, and then the order of a, b, c is viewed from the bottom of the tetrahedron to the top direction;
    3. If it is arranged clockwise, it is R type; if it is arranged counterclockwise, it is S type.

     

    Method for determining absolute configuration

    The determination of the absolute configuration of the chiral center requires the actual determination of the spatial structure of the molecule. The determination of the absolute configuration is very important for studying the physicochemical properties of chiral compounds and the mechanism of asymmetric catalytic reactions. The commonly used methods are as follows:

     

    1. X-ray diffraction method

    In single crystal X-ray analysis of a chiral center containing heavy atoms or known absolute configurations, the interference image of the diffracted beam depends not only on the distance between the atoms but also on their spatial orientation, thereby determining the absolute nature of the compound to be tested. structure.

    1. Chiral optical method

    It has been found that in the same series of compounds, the same chemical reaction changes the optical rotation in the same direction without changing the direction of the optical rotation. It is thus possible to derive its absolute configuration from the substrate used to prepare it after a series of reactions known as stereochemistry. In recent years, the ORD and CD curves of chiral compounds were determined by optical dispersion and circular dichroism, and the Cotton effect was compared with the corresponding map of structural analogs of known absolute configuration to determine the absolute structure of the sample to be tested.

    1. Chemical correlation method

    A chiral compound of unknown absolute configuration, after a number of stereochemically known reactions, can be inferred to the absolute configuration of a compound if it can be associated with a compound of known absolute configuration.

    1. NMR method

    The absolute configuration of the chiral compound can be determined by an NMR method such as an Overhauser nuclear effect (NOE effect), a chiral Mosher reagent derivatization method, or a chiral anisotropic reagent.

     

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