Diastereomeric molecules

Diastereomers include all stereoisomers that are not related as an object and its mirror image. Consider the four structures in Fig. 2.3. These structures represent fee four stereoisomers of 2,3,4-trihydroxybutanal. The configurations of C-2 and C-3 are indicated. Each stereogenic center is designated J or 5 by application of the sequence rule. Each of the four structures is stereoisomeric wife respect to any of fee others. The 2R R and 25,35 isomers are enantiomeric, as are fee 2R, iS and 25,3J pair. The 21 ,35 isomer is diastereomeric wife fee 25,35 and 2R,3R isomers because they are stereoisomers but not enantiomers. Any given structure can have only one enantiomer. All other stereoisomers of feat molecule are diastereomeric. The relative configuration of diastereomeric molecules is fiequently specified using fee terms syn and anti. The molecules are represented as extended chains. Diastereomers wife substituents on the same side of the extended chain are syn stereoisomers, whereas those wife substituents on opposite sides are anti stereoisomers. 

As a unichiral group which reacts with the racemate to form diastereomeric molecules which can be separated by achiral adsorption processes. 

Due to the vast number of literature papers containing NMR spectral data of cyclic diastereomeric molecules, only the basic features, according to ring sizes, will be discussed. Pertinent material is also available in other reviews13,25 77. 

Properties of Diastereomers In contrast to enantiomeric pairs, the correpond-ing spatial distances in diastereomeric pairs are not all identical. For example, errand trans-1,2-difluoroethene (Figure 1.2.4), differ in their F-F and H-H distances. This results into different energy contents and different properties between diastereomeric molecules. The difference in properties of diastereomers is illustrated with cis- and trans-1-phenyl-1,3-butadiene, which show markedly different physicochemical properties [12] (Figure 1.2.5). Further investigation of stereochemical isomers is beyond the scope of this book, and discussion in subsequent chapters is limited to constitutional isomers. 

Partition methodologies have been classically used in which charge densities are concerned however, their application to other properties such as energy and volume, is less common. In the paper [33], we describe how the differences in these atomic contributions shed light in the relative properties of diastereomeric molecules. 

The most common diastereomeric molecule is one that contains two asymmetric carbons. This situation is illustrated by the compounds... 

Figure 5 The structures of the diastereomeric molecules ephedrine and pseudoephedrine (only one enantiomer of each substance is presented).

Molecules with a single stereocenter as the only source of configurational stereoisomerism can exist as one of two enantiomers no configurational diastereomers are possible. Conformational stereoisomers, on the other hand, are most commonly diastereomeric. Molecules with two dissimilar stereocenters as the source of stereoisomerism can exist as one of... 

Students are familiar with the terms applied to relationships between stereoisomeric molecules homomeric molecules (superposable molecules), enantiomeric molecules (nonsuperposable mirror images), and diastereomeric molecules (stereoisomers that are not mirror images of one another). These familiar terms are parallel to the terms that we have introduced above homotopic, enantiotopic, and diastereotopic, which are applied to nuclei or groups within the molecule. 

PROBLEM 4.14 We have seen diastereomers twice before. Show two different kinds of diastereomeric molecules. Hint Start with a good definition of diastereomer. 

The resolutions described above are based on the formation of diaste-reomeric complexes with a column stationary phase or an enzyme. The more common alternative is to bond the enantiomers covalently to a chiral resolving agent to make stable diastereomeric molecules, separate those diastereomers by chromatography or recrystallization, and then disassemble each purified diastereomer to obtain the resolved enantiomers. 

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