Multiple chirality centers

The best examples of substances with multiple chirality centers are the carbohy drates (Chapter 25) One class of carbohydrates called hexoses has the constitution... 

Steroids are another class of natural products with multiple chirality centers One such compound is cholic acid which can be obtained from bile Its structural formula IS given m Figure 7 12 Cholic acid has 11 chirality centers and so a total (including cholic acid) of 2" or 2048 stereoisomers have this constitution Of these 2048 stereoiso mers how many are diastereomers of cholic acid s Remember Diastereomers are stereoisomers that are not enantiomers and any object can have only one mirror image Therefore of the 2048 stereoisomers one is cholic acid one is its enantiomer and the other 2046 are diastereomers of cholic acid Only a small fraction of these compounds are known and (+) cholic acid is the only one ever isolated from natural sources... 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Because the configuration at C-2 is lost on enolization, the enediol intermediate can revert either to D-glucose or to D-mannose. Two stereoisomers that have multiple chirality centers but differ in configuration at only one of them are refened to as... 

Highly enantioselective atom transfer radical cydization reactions catalyzed by chiral Lewis acids have been reported by Yang et al. [80]. Two main advantages of these enantioselective cyclizations include installing multiple chiral centers and retaining a halogen atom in the product, which allows for further functionalization. 

Monosaccharides generally have multiple chiral centers. 

Some important organic compounds have more than one chiral center. Multiple chiral centers indicate the presence of multiple stereoisomers. 

For samples containing multiple chiral centers, the use of a diastereo-mer, if available, for method development is a good option. Diastereomers are often difficult to separate so their use in method development may lead to a more highly resolving method that, therefore, may have a higher probability of being stability-indicating. 

The synthesis of new chiral organic compounds and the improved synthesis of known substances will always be a major task for the professional chemist. When constructing target molecules with multiple chirality centers, a scientist must consider either total synthesis step by step or assembly from smaller chiral blocks as an alternative approach. 

The feature of 2-chlorobutane that makes it chiral is the presence of a carbon attached to four different groups. Such carbons are another type of stereocenter. The currently accepted term to describe such a carbon, or any other tetrahedral atom attached to four different groups, is chirality center. (Some older terms that you may encounter are chiral carbon atom or asymmetric carbon atom.) Any molecule with one chirality center as its only stereocenter is chiral. (As we shall see shortly, many, but not all, molecules with multiple chirality centers are also chiral.) So, another way to identify a chiral molecule is to look for a single chirality center, which requires some practice. It helps to remember that any carbon that is attached to two identical groups (this includes all doubly and triply bonded carbons) is not a chirality center. Consider these examples ... 

Many of these compounds have quite complex structures, often with multiple chirality centers. In addition, many of them are pharmacologically active that is, they have dramatic physiological effects on any organism that ingests them. For both of these reasons they have always interested organic chemists, and the determination of their structures and their syntheses continue to play an important role in the development of organic chemistry. 

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