Tartaric acids, configuration

Chemical Properties. Because of its chiral center, malic acid is optically active. In 1896, when tartaric acid was first reduced to malic acid, the levorotatory enantiomer, S(—), was confirmed as having the spatial configuration (1) (5,6). The other enantiomer (2) has the R configuration. A detailed discussion of configuration assignment by the sequence rule or the R and S system is available (7). 

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. 

Let s return for a last look at Pasteur s pioneering work. Pasteur took an optically inactive tartaric acid salt and found that he could crystallize from it two optically active forms having what we would now call the 2R,3R and 2S,3S configurations. But what was the optically inactive form he started with It couldn t have been meso-tartaric acid, because meso-tartaric acid is a different chemical compound and can t interconvert with the two chiral enantiomers without breaking and re-forming chemical bonds. 

Table sugar, sec Sucrose Tagatose, structure of, 975 Talose. configuration of, 982 Tamiflu, molecular model of, 130 Tamoxifen, synthesis of, 744 Till] DNA polymerase, PCR and, 1117 Tartaric acid, stereoisomers of, 305-306... 

Meso compound A symmetrical compound containing two chiral centres configured so that the chirality of one of the centres is equal and opposite to the other. Such internal compensation means that these compounds have no overall effect on polarised light (e.g., meso tartaric acid). 

The configuration of (-)-glyceraldehyde was related through reactions of known stereochemistry to (+)-tartaric acid. 

The distinction between the various configurations of monocentric, tetrahedral molecules is dependent on the ability to differentiate between all four ligands. In systems with more than one asymmetric C-atom these can be regarded as the chiral subunits of the total configuration. If there is sufficient flexibility one can expect 2n configurations with n different asymmetric C-atoms. If there are asymmetric C-atoms of the same kind, as in the isomers of tartaric acid, a lower number of distinguishable configurations is encountered. 

The equilibrium ensemble of the configurations of meso-tartaric acid 10 a—c has umin = 0. From the definition of umm follows immediately that for all achiral molecules min = 0 holds. This applies also to those cases where asymmetric C-atoms are present but cancel each other because of overall molecular symmetry. 

In principle, separation of resonances of diastereomeric compounds (such as dl and meso isomers) may be increased simply through use of an appropriate achiral solvent. Chiral solvents may in some cases be especially effective in producing a separation, particularly if the diastereomers differ in configuration about a center that is amenable to analysis by the CSA method. Kaehler and Rehse (89) give a detailed account of conditions necessary for measurement of the ratio of meso- and dZ-tartaric acid employing A,N-dimethyl PEA. Bomyl acetate used as solvent for l,2-difluoro-l,2-dichloroethane (90) allows measurement of the diastereomeric composition. Paquette and co-workers (91,92), using TFAE, were able to determine the diastereomeric purity of the recrystallized adducts 47 of... 

Now for a rather unexpected twist. We have seen that if there are n chiral centres there should be 2" configurational isomers, and we have considered each of these for n = 2 (e.g. ephedrine, pseudoephedrine). It transpires that if the groups around chiral centres are the same, then the number of stereoisomers is less than 2". Thus, when n = 2, there are only three stereoisomers, not four. As one of the simplest examples, let us consider in detail tartaric acid, a component of grape juice and many other fruits. This fits the requirement, since each of the two chiral centres has the same substituents. 

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... 

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