Stereoisomers meso compounds

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. 

Meso stereoisomer (Section 7.11) An achiral molecule that has chirality centers. The most common kind of meso compound is a molecule with two chirality centers and a plane of symmetry. 

In open-chain compounds, the molecule can usually adopt that conformation in which H and X are anti periplanar. However, in cyclic systems this is not always the case. There are nine stereoisomers of 1,2,3,4,5,6-hexachlorocy-clohexane seven meso forms and a dl pair (see p. 161). Four of the meso compounds and the dl pair (all that were then known) were subjected to elimination of HCl. Only one of these (1) has no Cl trans to an H. Of the other isomers, the fastest elimination rate was about three times as fast as the... 

Up to this point, we have not had to address the unexpected photolysis behavior of the meso-compound, namely, that the stereochemical retention in the products resulting from the decomposition of this stereoisomer actually decreased at concentrations above the CMC. Intuitively, one would expect that the restricted environment created by micellization of the diazenes would enhance stereochemical retention. The (+)-diazene behaves as we would have predicted, but the meso-isomer does not. 

Since the two reactants cis and trans butenes are stereoisomers, being diastereomers, the product from cis is a meso compound and that from the trans give a pair of enantiomers, by definition both the reactions are stereospecific. 

Resistance to hydration was elucidated with tricyclic model compounds that lack the side chain and, hence, pharmacological activity. In this context, a useful comparison has been made between two meso compounds, namely 5W-dibenz.oja, dIcycloheplene 10,11-oxide (10.130, X = CH2) and d.v-slilbcnc oxide (10.7) [195]. The former compound proved to be a very poor substrate for rabbit liver microsomal EH, with a Km value comparable to that of cis-stilbene oxide, but Emax ca. 100-fold lower. This indicates that the two compounds have a comparable affinity for the enzyme, but that nucleophilic attack in the catalytic step is much less efficient for dibcnzo[ // cycloheplcnc 10,11-oxide than for d.v-slilbcnc oxide. This implies that the former compound acts better as an inhibitor than as a substrate of microsomal EH. Furthermore, there was also a fundamental steric difference in the reaction course of the two substrates, since the predominant stereoisomer formed from dibenzo //]cyclohep(ene 10,11-oxide had the (I OS, 11. -configuration,... 

This is essentially the same as the tartaric acid example, without the conformational complication. Thus, there are two chiral centres, and the groups around each centre are the same. Again, we get only three stereoisomers rather than four, since the cis compound is an optically inactive meso compound. There is a plane of symmetry in this molecule, and it is easy to see that one chiral centre is mirrored by the other, so that we lose optical activity. 

We can draw these three stereoisomers as Fischer projections, reversing the configurations at both centres to get the enantiomeric stereoisomers, whilst the Fischer projection for the third isomer, the meso compound, is characterized immediately by a plane of symmetry. For (-l-)-tartaric acid, the configuration is 2R, >R), and for (—)-tartaric acid it is (2S,3S). For both chiral centres, the group of lowest priority is hydrogen, which is on a horizontal line. In fact, this is the case in almost all Fischer projections, since, by convention, the vertical... 

A stereoisomer having more than one chiral center and having an internal plane of symmetry. Hence, meso compounds do not exhibit optical activity. 

A C2-symmetric ansa metallocene is a racemic mixture of an enantiomeric pair—an example is rac-(dimethylsilyl)bis(l-indenyl)zirconium dichloride (XXXIV), abbreviated as rac-(CH3)2SiInd2ZrCl2. The enantiomers are designated as (R, R) and (S, S) to describe the two coordination sites in each enantiomer. Actually, the synthesis of a C2 ansa metallocene usually produces a mixture of the racemic pair plus the meso compound (R, S). The meso compound, which is a diastereomer of the racemic pair, can be separated from the racemic mixture by physical techniques such as recrystallization. The meso stereoisomer possesses Cs symmetry, and its stereoselectivity is very different from that of the enantiomeric pair (Sec. 8-5a-3). 

Problem 5.23 Answer True or False to each of the following statements and explain your choice. ( ) There are two broad classes of stereoisomers, (b) Achiral molecules cannot possess chiral centers, (c) A reaction catalyzed by an enzyme always gives an optically active product, (d) Racemization of an enantiomer must result in the breaking of at least one bond to the chiral center, (e) An attempted resolution can distinguish a racemate from a meso compound. <... 

Problem 5.30 For the following compounds, draw projection formulas for all stereoisomers and point out their R,S specifications, optical activity (where present), and meso compounds (a) 1,2,3,4-tetrahydroxybutane, (b) l-chloro-2,3-dibromobutane, (c) 2,4-diiodopentane, (d) 2,3,4-tribromohexane, (e) 2,3,4-tribromopentane. 

C is correct. This question simply asks, "Which compound is the enantiomer to (.R,J )-2,3-dichlorobutane " Since wc have retention of configuration, only one enantiomer of 2,3-dichlorobutane is made. The one that is made will be a stereoisomer to the meso compound and will rotate plane-polarized light in the opposite direction of (R,Rf enantiomer. Enantiomers rotate polarized light, so compound A, B, or C must be this enantiomer. And since C and D are stereoisomers, one must be the meso compound of 2,3-dichlorobutane and one must be the ( ) enantiomer of (R,R )-2,3-dichlorobutane. 

C is correct. 2,3-dibromobutane has two enantiomers and a meso compound. Four stereoisomers are possible according to the 2" formula, but since there is a meso, only 3 exist. 

C is correct. Diastereomers - epimers, anomers, and geometric isomers - are stereoisomers that are not mirror images of each other. A meso compound is an achiral molecule, which is identical to its mirror image. 

It is possible for a compound to be its own mirror image. In such a case, the compound will not have a twin. It will be all by itself, and the total number of stereoisomers will be an odd number, rather than an even number. That one lonely compound is called a meso compound. If you try to draw the enantiomer (using either of the methods we have seen), you will find that your drawing will be the same compound as what you started with. 

You could have said that there are four stereoisomers but the following statement is much more helpful. There are three diastereoisomers, the syn,syn, the yn,anti, and the anti,anti. The syn,syn and the anti,anti are achiral (meso) compounds but the syn,anti is chiral and has two enantiomers,... 

There are four stereoisomers of 2,4-dibromo-3-chloropentane. C and D are enantiomers and are optically active. A and B are optically inactive meso compounds and are diastereomers. 

Commercial 2,4-pentanediol is a mixture of three stereoisomers (R,R), (S,S), and (R,S). The meso isomer shows three signals in its 13C NMR spectrum. Its diastereomers, the R,R and S,S enantiomeric pair, also show three signals, but two of these signals occur at different 8 values from the meso isomer. This is expected, because diastereomers differ in physical and chemical properties. One resonance from the meso compound accidentally overlaps with one signal from the enantiomeric pair. 

Of the nine stereoisomers of inositol, the scyllo-isomer has no axial hydroxyl, the myo-isomer has one, the epi-, chiro-, and neo-isomers have two, and the alio-, cis-, and mneo-isomers have three hydroxyl groups (Figure 1). Of these, six isomers scyllo-, myo-, epi-, neo-, cis-, and mr/co-i somers) have one or more planes of symmetry in the molecule (meso compounds) and are therefore not chiral. D-chiro- and L-e/u>o-i somers do not have a plane of symmetry and are chiral molecules moreover they are enantiomers of each other. The alio-isomer is unique - the conformational isomer of (10) (Figure 2) is (11) which is also its enantiomer Since interconversion between conformational isomers is rapid, a//o-inositol exists as a 50/50 mixture of the two enantiomers at room temperature. Therefore, although alio-inositol is chiral, the compound is optically inactive at room temperature because it is a racemic mixture a chiral reagent, such as an enzyme, would be expected to preferentially react with one enantiomer and not the other. 

Because one stereoisomer of 1,3-dibromocyclopentane is superimposable on its mirror image, there are only three stereoisomers, not four. A is an achiral meso compound and B and C are a pair of chiral enantiomers. A and B are diastereomers, as are A and C. 

Diastereomers are not miiror images of each other, and as such, their physical properties are different, including optical rotation. Figure 5.12 compares the physical properties of the three stereoisomers of tartaric acid, consisting of a meso compound that is a diastereomer of a pair of enantiomers. 

Draw all possible stereoisomers for each compound. Label pairs of enantiomers and diastereomers. Label any meso compound. 

Sometimes this reaction produces two stereoisomers, as in the case of c/s-2-butene, which forms an equal amount of two enantiomers. Sometimes it produces a single compound, as in the case of frans-2-butene, where a meso compound is formed... 

Problem 4.12 Draw stereochemical formulas for all the possible stereoisomers of the following compounds. Label pairs of enantiomers, and meso compounds. Tell which isomers, if separated from all other stereoisomers, will be optically active. Pick out several examples of diastereomers. 

The reactant, too, exists as diastereomers a pair of geometric isomers. If we start with, say, ci5-2-butene, which of the stereoisomeric products do we get A mixture of all of them No. cw-2-Butene yields only racemic 2,3-dibromobutane none of the meso compound is obtained. A reaction that yields predominantly one stereoisomer (or one pair of enantiomers) of several diastereomeric possibilities is called a stereoselective reaction. 

Problem 9.13 Draw structural formulas for all stereoisomers of the following. Label any meso compounds and indicate pairs of enantiomers. Do any (like c/s-1,2-dimcthylcyclohexane) exist as a non-resolvable racemic modification ... 

The dissimilarity of the two ends of an aldohexose molecule prevents the existence of meso compounds (Sec. 4.18), and hence we expect that there should be 2 or 16 stereoisomers—eight pairs of enantiomers. All 16 of these possible stereoisomers are now known, through either synthesis in the laboratory or isolation from natural sources only three—(+)-glucose, (+)-mannose, (+)-galactose— are found in abundance. 

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