Stereocenters meso compounds

A meso compound has stereocenters, but the compound also has symmetry that allows it to be the mirror image of itself. Consider cis-1,2-dimethylcyclohexane as an example. This molecule has a plane of symmetry cutting the molecule in half. Everything on the left side of the plane is mirrored by everything on the right side ... 

The Still-Gennari olefination in Figure 11.21 is recommended to anyone who wants to enjoy a third stereochemical dehcacy. The substrate is a dialdehyde that contains oxygenated stereocenters in both a-positions. Nevertheless, this aldehyde is achiral because it has a mirror plane and thus represents a mew-compound. Meso-compounds can sometimes be con-... 

One source defines a meso compound as an achiral compound with stereocenters. Why is this a poor definition ... 

Chiral diols have also been prepared starting from meso-compounds [68-71]. Since meso-compounds are, in essence, symmetric molecules, the same applies as for the other symmetric starting materials. Indeed, this is exactly what was found Even though the stereocenters of the protected heptane tetrol are far away from the ester groups that are to be hydrolysed stereoselectively, this is what happens [69, 70]. The high selectivity is partly due to the fact that the secondary alcohol groups are protected as a cyclic acetal, giving additional structural information to the enzyme (Scheme 6.20 A). A cyclic acetal also provides additional structural information in the enantioselective hydrolysis of a pentane tetrol derivative (Scheme 6.20 B) [71]. In both cases Pseudomonas fluorescens lipase (PFL) proved to be the enzyme of choice. 

In the products C retains the R configuration since none of its bonds were broken and there was no change in priority. The configuration at C, the newly created chiral center, can be either R or 5. As a result, two diastereomers are formed, the optically active RR enantiomer and the optically inactive RS meso compound. b) No. The numbers of molecules with S and R configurations at C are not equal. This is so because the presence of the C -stereocenter causes an unequal likelihood of attack at the faces of C. Faces which give rise to diastereomers when attacked by a fourth ligand are diastereotopic faces. 

When a molecule has more than one source of asymmetry, two optical isomers may be neither perfect reflections of each other nor superimposeable some but not all stereocenters are inverted. These molecules are an example of diastereomers They are not enantiomers. Diastereomers seldom have the same physical properties. Sometimes, the stereocentres are themselves symmetrical. This causes the counterintuitive situation where two chiral centres may be present but no isomers result. Such compuonds are called meso compounds. 

There are two stereocenters for 2,3-dichlorobutane but there are only three stereoisomers. There is a pair of enantiomers (I and II). Unlike the situation with 2,3-dichloropentane, structures III and IV are not a pair of enantiomers. Structures III and IV are superimposable by rotating either one 180° in the plane of the paper. This occurs because the two stereocenters (C-2 and C-3) have the same set of four different substituents (H,C1,CH3, CHCl — CH3). A correct answer for this problem must show either the equal sign between structures III and IV or only one structure from III and IV. The lone compound, whether shown as III or IV, is referred to as a rrteso compound. The meso compound has a plane of symmetry (symbolized by a dotted line in the structure shown below). The meso compound is achiral and optically inactive even though it possesses two stereocenters. 

Ans. There are three stereoisomers since the two stereocenters have the same four different substituents. There is a trans pair of enantiomers and a cis meso compound. Each of the trans enantiomers is a diastereomer of the meso and vice versa. 

Merrifield solid-state peptide synthesis Automated synthesis using polystyrene to anchor a peptide chain. Meso compound A compound with at least two stereocenters yet superimposable on its mirror image. Meta- (m-) Common-name prefix for 1.3-disubstituted benzenes. 

In a meso compound the stereocenters must be identically substituted and opposite in configuration. 

In addition to molecules containing prochiral sites, racemic molecules can participate in catalytic asymmetric transformations. In some transformations, the stereocenter is destroyed in the course of the reaction, and equilibrating prochiral intermediates are formed. An example of such a process is the asymmetric arylation of ketones (Figure 14.8C). In other cases, one of the enantiomers of the substrate reacts with the asymmetric catalyst significantly faster than the other enantiomer. In this case, an enantioenriched product is formed, and the opposite enantiomer of the reactant remains. This last process is called a kinetic resolution and is illustrated for the conjugate reduction of enones via hydrosilylation (Figure 14.8D). In this case, the top and bottom faces of the C-C double bond are diastereotopic because reaction at each face of the enone generates diastereomeric products. This section of the chapter first presents the principles that relate to reactions at prochiral centers of achiral substrates and then presents the principles that relate to reactions of racemic or meso compounds. 

Another way to verify that (c) is achiral is to see that it has a plane of symmetry that bisects the molecule in such a way that the top half is the reflection of the bottom half. Thus, even though (c) has two stereocenters, it is achiral. The stereoisomer of tartaric acid represented by (c) or (d) is called a meso compound, defined as an achiral compound that contains two or more stereocenters. 

Enantiomers are nonsuperposable mirror images. A meso compound is an achiral compound with two or more stereocenters, that is, a compound with two or more stereocenters that has a superposable mirror image. 

Locate all stereocenters and use the 2" rule to determine the maximum number of stereoisomers possible. Determine which, if any, of the possible stereoisomers are meso compounds. 

Cyclohexanediol has two stereocenters, and, according to the 2" rule, a maximum of 2 = 4 stereoisomers is possible. The trans isomer of this compound exists as a pair of enantiomers. The cis isomer has a plane of symmetry and is a meso compound. Therefore, although the 2" rule predicts a maximum of four stereoisomers for 1,3-cyclohexanediol, only three exist—one pair of enantiomers and one meso compound ... 

A meso compound contains two or more stereocenters assembled in such a way that its molecules are achiral. 

Meso compound An achiral compound possessing two or more stereocenters. 

Tartaric acid (systematic name 2,3-dihydroxy-butanedioic acid) is a naturally occurring dicarboxylic acid containing two stereocenters with identical substitution patterns. Therefore it exists as a pair of enantiomers (which have identical physical properties but which rotate plane-polarized light in opposite directions) and an achiral meso compound (with different physical and chemical properties from those of the chiral diastereomers). 

A compound that contains two (or, as we shall see, even more than two) stereocenters but is superimposable with its mirror image is a meso compound (mesos, Greek, middle). A characteristic feature of a meso compound is the presence of an internal mirror plane, which divides the molecule such that one half is the mirror image of the other half. For example, in 2,3-dibromobutane, the 2R center is the reflection of the 35 center. This arrangement is best seen in an eclipsed hashed-wedged line structure (Figure 5-11). The presence... 

Figure 5-11 meso-2,3-Dibromobutane contains an internal min or plane when rotated into the eclipsed conformation shown. A molecule with more than one stereocenter is meso and achiral as long as it contains a mirror plane in any readily accessible conformation. Meso compounds possess identically substituted stereocenters. 

In Summary Meso compounds are diastereomers containing a molecular plane of symmetry. They are therefore superimposable on their mirror images and achiral. Molecules with two or more identically substituted stereocenters may exist as meso stereoisomers. 

Two stereocenters in a molecule result in as many as four stereoisomers—two diastereomerically related pairs of enantiomers. The maximum number of stereoisomers that a compound with n stereocenters can have is 2". This number is reduced when equivalently substituted stereocenters give rise to a plane of symmetry. A molecule containing stereocenters and a mirror plane is identical with its mirror image (achiral) and is called a meso compound. The presence of a mirror plane in any energetically accessible conformation of a molecule is sufficient to make it achiral. 

A meso form with any one of the enantiomers of tartaric acid represents a pair of diastereomers. Although it may not be true for this compound because of the meso form, in general, if you have n stereocenters, there are 2n stereoisomers possible (see Post-Lab question no. 3). 

Mechanistic studies with stereoisomeric 2,5-hexanediols showed that an intramolecular Sn2 type mechanism is operative high stereoselectivity with configurational inversion at one stereocenter has been reported [90,91,93]. meso-l.S-Hex-anediol (7), consequently, undergoes dehydration to yield tran5-2,5-dimethyloxo-lane (Scheme 6), whereas the racemic compound is converted to dy-2,5-dimethy-loxolane. 

Interligand asymmetric induction. Group-selective reactions are ones in which heterotopic ligands (as opposed to heterotopic faces) are distinguished. Recall from the discussion at the beginning of this chapter that secondary amines form complexes with lithium enolates (pp 76-77) and that lithium amides form complexes with carbonyl compounds (Section 3.1.1). So if the ligands on a carbonyl are enantiotopic, they become diastereotopic on complexation with chiral lithium amides. Thus, deprotonation of certain ketones can be rendered enantioselective by using a chiral lithium amide base [122], as shown in Scheme 3.23 for the deprotonation of cyclohexanones [123-128]. 2,6-Dimethyl cyclohexanone (Scheme 3.23a) is meso, whereas 4-tertbutylcyclohexanone (Scheme 3.23b) has no stereocenters. Nevertheless, the enolates of these ketones are chiral. Alkylation of the enolates affords nonracemic products and O-silylation affords a chiral enol ether which can... 

Note The presence of both R and 5 stereocenters in a structure does not mean that the compound is automatically meso. For it to be meso, its stereocenters must have identical sets of substituents, and the molecule as a whole must have an internal mirror plane of symmetry. 

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