Weso-tartaric acid

Fischer projections are especially useful in the case of compounds with more than one chirality center. For example, it is easy to see the plane of symmetry in meso-tartaric acid. As was the case with regular structures, interchanging any two groups in a Fischer projection results in inversion of configuration at the chirality center. Thus, interchanging the H and OH on the lower chirality center of weso-tartaric acid inverts the configuration at that chirality center, resulting in the (27 ,3R)-stereoisomer, (-i-)-tartaric acid. It is also easy to see that this stereoisomer does not have a plane of symmetry. 

Careful examination of these mirror images shows that they are, in fact, identical. If the (25,37 )-stereoisomer is rotated 180° in the plane of the paper, the structure on the right is produced and it is identical to the (27 ,350-stereoisomer on the left. Because the compound is superimposable on its mirror image, it is not chiral and does not rotate plane-polarized light. Another way to determine that this compound is not chiral is to note that it has a plane of symmetry that bisects the C-2—C-3 bond. Compounds such as this one, which contain chirality centers but are not chiral are called weso-stereoisomers. me.vo-Tartaric acid is human-made and does not occur in nature. Overall, then, tartaric acid has only three stereoisomers the two enantiomers of the chiral diastereomer (often called the d,/-diastereomer) and the weso-diastereomer. 

Typically, condensations to polyamide (II), are carried out between an activated, i.e., esterified (diester, ester/lactone, dilactone) form of the aldaric acid (I), and a primary diamine in a polar protic solvent such as methanol at room temperature. Under those conditions any starting single ester form of a five or six carbon aldaric acid is rapidly converted to an equilibrium mixture of acyclic diester and ester/lactone forms. Dilactone, if formed at all, is present in only small amounts. The starting monomer esterified form(s) of the aldaric acid differ from one acid to an other. For example, the monomers of choice for L-tartaric acid (1) and weso-galactaric acid (4) are simple diesters, while for D-glucaric acid suitable monomers are ester/lactones, dilactone, or even the entire alcohol esterified mixture (10). Xylaric acid is also conveniently polymerized as its esterification mixture while the D-mannaric acid monomer is the 1,4 6,3-dilactone (13a). 

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