Multiple chiral centers

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

In principle, all forms of chirality have been used for liquid crystal synthesis. This involves compounds with single chiral centers, multiple chiral centers, chiral axes, chirality planes, etc. For example, chiral methylidencyclohex-anes can be found in [24], chiral aliens in [25], and binaphthyls in [26]. 

Answer (a) Cannot. Has no chiral center multiple-bonded atoms cannot be chiral centers. With 2 Cl s on the same double-bonded C, the compound cannot have a geometric isomer, (b) Can have an optical isomer. The C bonded to the Cl is a chiral center the four different groups are HgC, H, Cl, and CHzCH,. Note that we consider the entire group attached, e.g., H3C and CH3CH3. and not just the adjacent atom which happens to be a C in both of these groups, (c) Has geometric isomers. Each double-bonded carbon is attached to a pair of different groups or atoms H, CH3 are attached to one C and H, CHaCHj are attached to the other, (d) Has optical isomers. The C is a chiral center with 4 different atoms H and D are different. 

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. 

From among the variety of non-carbohydrate precursors, acetylenes and alkenes have found wide application as substrates for the synthesis of monosaccharides. Although introduction of more than three chiral centers having the desired, relative stereochemistry into acyclic compounds containing multiple bonds is usually difficult, the availability of such compounds, as well as the choice of methods accessible for their functionalization, make them convenient starting-substances for the synthesis. In this Section is given an outline of all of the synthetic methods that have been utilized for the conversion of acetylenic and olefinic precursors into carbohydrates. Only reactions leading from dialkenes to hexitols are omitted, as they have already been described in this Series.7... 

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