Asymmetric centers meso compounds with

When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is more complex than in acyclic compounds. A cyclic compound which has two different asymmetric carbons with different sets of substituent groups attached has a total of 2 = 4 optical isomers an enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso compound and only the trans isomer is chiral. (See Fig. 1.15.)... 

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

The term meso (Greek, middle ) was used to describe an achiral member of a set of diastereomers, some of which are chiral. The optically inactive isomer seemed to be in the middle between the dextrorotatory and levorotatory isomers. The definition just given ( an achiral compound with chirality centers ) is nearly as complete, and more easily applied, especially when you remember that chirality centers are usually asymmetric carbon atoms. 

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. 

The two glyceraldehyde isomers of 4-13 are identical in all physical properties except that they rotate the plane of polarized light in opposite directions and form enantiomorphous crystals. When more than one asymmetric center is present in a low-molecular-weight species, however, stereoisomers are formed which are not mirror images of each other and which may differ in many physical properties. An example of a compound with two asymmetric carbons (a diastereomer) is tartaric acid, 4-16, which can exist in two optically active forms (d and L, mp 170 C), an optically inactive form (meso, mp 140 C), and as an optically inactive mixture (dl racemic, mp 206°C). 

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. 

It is easy to recognize when a compound with two asymmetric centers has a stereoisomer that is a meso compound because the four atoms or groups bonded to one asymmetric center are identical to the four atoms or groups bonded to the other asymmetric center. For example, both of the asymmetric centers in the following compound are bonded to an H, OH, CH3, and CH(OH)CH3. 

A compound with the same four groups bonded to two different asymmetric centers will have three stereoisomers— namely, a meso compound and a pair of enantiomers. 

A diastereoisomer with two or more asymmetric centers and a plane of symmetry within the molecule reducing the number of possible enantiomers. A meso compound is not optically active. 

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