Enantiomers configurational notation

Individual pairs of enantiomers are called threo or erythro. The threo form has substituents on opposite sides of the main backbone chain represented as Fischer projection, whereas the erythro form has them on the same side, respectively. These terms function correctly in case of linear sugar molecules, but in other cases there are often problems with determining what is actually the main chain [53]. In such cases, individual chiral carbon atoms can always be characterized using configurational notation (R and S). 

Chemical Properties. The notation used by Chemical Abstracts to reflect the configuration of tartaric acid is as follows (R-R, R )-tartaric acid [S7-69A-] (4) (S-R, R )-tartaric acid [147-71-7] (5) and y j O-tartaric acid [147-73-9] (6). Racemic acid is an equimolar mixture of the two optically active enantiomers and, hence, like the meso acid, is optically inactive. 

Serine hydroxymethyl transferase catalyzes the decarboxylation reaction of a-amino-a-methylmalonic acid to give (J )-a-aminopropionic acid with retention of configuration [1]. The reaction of methylmalonyl-CoA catalyzed by malonyl-coenzyme A decarboxylase also proceeds with perfect retention of configuration, but the notation of the absolute configuration is reversed in accordance with the CIP-priority rule [2]. Of course, water is a good proton source and, if it comes in contact with these reactants, the product of decarboxylation should be a one-to-one mixture of the two enantiomers. Thus, the stereoselectivity of the reaction indicates that the reaction environment is highly hydro-phobic, so that no free water molecule attacks the intermediate. Even if some water molecules are present in the active site of the enzyme, they are entirely under the control of the enzyme. If this type of reaction can be realized using synthetic substrates, a new method will be developed for the preparation of optically active carboxylic acids that have a chiral center at the a-position. 

Figure 2.11 Some plausible confoimations of (2/f, 4Y )-l-hydroxy-2,4-dimethylhex-5-ene. How many different torsional isomers might one need to examine, and how would you go about generating them [Note that the notation 2/f, 45 implies that the relative stereochemical configuration at the 2 and 4 centers is R,S - by convention, when the absolute configuration is not known the first center is always assigned to be R. However, the absolute conformations that are drawn here are S,R so as to preserve correspondence with the published illustrations of Stahl and coworkers. Since NMR in an achiral solvent does not distinguish between enantiomers, one can work with either absolute configuration in this instance.]...

The most significant handicaps associated with directionally specific locants are the inability to distinguish enantiomeric pairs of compounds directly from the notation and the arbitrary and often complicated hierarchical rules to determine what ligands are to be associated with which locants. Another complexity of locant notations that is sometimes overlooked is the need for additional symbols or other notation to indicate that there is less information meant than is expressed in the name with locants. Often the geometric configuration of a compound is known but not the absolute configuration. Thus the X in the third name is necessary in the locant notation because it is not possible to draw and number the cis chiral structure ambiguously. Similarly, vac is the recommended term to indicate a mixture of enantiomers. 

The +/— or djl notation is not a direct descriptor of the absolute configuration of an enantiomer (the arrangement of the substituents or ligands) for which the following prefixes are used. 

In all the listed amino acids, with the exception of glycine, the a-carbon is bound to four different substituents hence it is a stereogenic center. From this it follows that every amino acid can appear in the form of two enantiomers. In the following example, both the enantiomers of alanine are represented together with their absolute configurations. However, enantiomers of amino acids can also be represented by the traditional notation of chiral molecules that is called the relative configuration. This nomenclature for configuration was proposed Emil Fischer in the nineteenth century for the representation of the stereochemistry of carbohydrates. 

So far, the enantiomers have been named (+)-2-butanol and (—)-2-butanol, where the (+) and (-) refer to specific rotation. To describe the specific structure of the enantiomer, however, more information is required in part, a name. The designations (+) and (—) provide no structural information to reveal the absolute configuration of each enantiomer. One enantiomer of 2-butanol is drawn as 6, both in line notation and as a Fischer projection. The other enantiomer of... 

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