Achiral Molecules with Two Chirality Centers

The situation is the same when the two chirality centers are present in a ring. There are four stereoisomeric l-bromo-2-chlorocyclopropanes a pair of enantiomers in which the halogens are trans and a pair in which they are cis. The cis compounds are diastereomers of the trans. 

A good thing to remember is that the cis and trans isomers of a particular compound are diastereomers of each other. 

In Section 7.5, the term relative configuration was used to describe the stereochemical relationship between a single chirality center in one molecule to a chirality center in a different molecule. Relative configuration is also used to describe the way multiple chirality centers within the same molecule are related. The two erythro stereoisomers of 2,3-dihydroxybutanoic acid possess the same relative configuration. The relationship of one chirality center to the other is the same in both, but different from that in the threo stereoisomer. 

Which stereoisomers of l-bromo-2-chlorocyclopropane possess the same relative configuration  

Now think about a molecule, such as 2,3-butanediol, which has two chirality centers that are equivalently substituted. 

Stereoisomeric 2,3-butanediols shown in their eciipsed conformations for convenience. Stereoisomers (a) and [b) are enantiomers. Structure (c) is a diastereomer of (a) and (h), and is achirai. it is caiied meso-2,3-butanedioi. 

Fischer projections can help us identify meso forms. Of the three stereoisomeric 2,3-butanediols, notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror-image halves. 

When using Fischer projections for this purpose, however, be sure to remember what three-dimensional objects they stand for. One should not, for example, test for superimposition of the two chiral stereoisomers by a procedure that involves moving any part of a Fischer projection out of the plane of the paper in any step. 

A meso stereoisomer is possible for one of the following compounds. Which one  

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Merrifield method See solid phase peptide synthesis Meso stereoisomer (Section 7 11) An achiral molecule that has chirality centers The most common kind of meso com pound IS a molecule with two chirality centers and a plane of symmetry... 

Biocatalysis is often used to prepare molecules with two or more chiral centers or to prepare intermediates from simple precursors when selective chemical transformations would be difficult [55]. Molecules with one chiral center are generally prepared by classical chemical resolution or by transferring chirality into achiral starting materials using chiral reagents. The launch of atorvastatin was initially carried out by classical conversion of 39 to the chiral diol 40 (Figure 3.27), and a biocatalyzed preparation of 40 was also developed [56]. The biocatalyzed route may now be the preferred route for preparing 40 [55]. 

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. 

In this exfflTipIe, addition to the double bond of an alkene converted an achiral molecule to a chiral one. The general term for a structural feature, the alteration of which introduces a chirality center in a molecule, is prochiral. A chirality center is introduced when the double bond of propene reacts with a peroxy acid. The double bond is a prochiral structural unit, and we speak of the top and bottom faces of the double bond as prochiral faces. Because attack at one prochual face gives the enantiomer of the compound formed by attack at the other face, we classify the relationship between the two faces as enantiotopic. 

There are two possible approaches for the preparation of optically active products by chemical transformation of optically inactive starting materials kinetic resolution and asymmetric synthesis [44,87], For both types of reactions there is one principle in order to make an optically active compound we need another optically active compound. A kinetic resolution depends on the fact that two enantiomers of a racemate react at different rates with a chiral reagent or catalyst. Accordingly, an asymmetric synthesis involves the creation of an asymmetric center that occurs by chiral discrimination of equivalent groups in an achiral starting material. This can be done either by enan-tioselective (which involves the reaction of a prochiral molecule with a chiral substance) or diastereoselective (which involves the preferential formation of a single diastereomer by the creation of a new asymmetric center in a chiral molecule) synthesis. 

It is now possible to see why, as mentioned on p. 95, enantiomers react at different rates with other chiral molecules but at the same rate with achiral molecules. In the latter case, the activated complex formed from the R enantiomer and the other molecule is the mirror image of the activated complex formed from the S enantiomer and the other molecule. Since the two activated complexes are enantiomeric, their energies are the same and the rates of the reactions in which they are formed must be the same (see Chapter 6). However, when an R enantiomer reacts with a chiral molecule that has, say, the R configuration, the activated complex has two chiral centers with configurations R and R, while the activated complex formed from the S enantiomer has the configurations S and R. The two activated complexes are diastereomeric, do not have the same energies, and consequently are formed at different rates. 

Diastereoisomerism is encountered in a number of cases such as achiral molecules without asymmetric atoms, chiral molecules with several centers of chirality, and achiral molecules with several centers of chirality (meso forms). Such cases can be encountered in acyclic and cyclic molecules alike, but for the sake of clarity these two classes of compounds will be considered separately. 

Interestingly, it is found that chiral exhibition is not simply limited to systems in which chiral molecules are adsorbed at achiral surfaces (i.e., adsorption of (R, R)-7A on the Ni or Cu surface) but it can also be displayed in systems where no initial chirality is present, i.e., from the adsorption of achiral molecules at achiral surfaces [203-211], Raval and coworkers [203] reported on the adsorption of succinic acid on Cu(l 1 0) and compared the results with those found for (R,R)-TA. Structurally, succinic acid is very similar to TA, with the only difference being that the two hydroxyl groups present in TA are replaced by hydrogen atoms, leading to a consequent loss of both chiral centers (Figure 14.7). 

Chirality is due to the fact that the stereogenic center, also called the chiral center, has four different substitutions. These molecules are called asymmetrical and have a Q symmetry. When a chiral compound is synthesized in an achiral environment, the compound is generated as a 50 50 equimolar mixture of the two enantiomers and is called racemic mixture. This is because, in an achiral environment, enantiomers are energetically degenerate and interact in an identical way with the environment. In a similar way, enantiomers can be differentiated from each other only in a chiral environment provided under... 

A chiral molecule has a nonsuperimposable mirror image. An achiral molecule has a superimposable mirror image. The feature that is most often die cause of chirality is an asymmetric carbon. An asymmetric carbon is a carbon bonded to four different atoms or groups. An asymmetric carbon is also known as a chirality center. Nitrogen and phosphorus atoms can also be chirality centers. Nonsuperimposable mirror-image molecules are called enantiomers. Diastereomers are stereoisomers that are not enantiomers. Enantiomers have identical physical and chemical properties diastereomers have different physical and chemical properties. An achiral reagent reacts identically with both enantiomers a chiral reagent reacts differently with each enantiomer. A mixture of equal amounts of two enantiomers is called a racemic mixture. 

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