Meso-isomers

FIGURE 1.15 Isomers of cyclopropane-1,2-dicarboxyUc acid, (a) Trans isomer b) meso isomer. 

After recovery of L-lysine, the residual dl-(49) is epimerized to a mixture of the DL and meso isomers, and the latter is subjected to the same decarboxylation step. This reaction is a part of a microbial process in which glucose is fermented by a lysine auxotroph of E. coli to meso- which accumulates in the medium. Meso-(49) is quantitatively decarboxylated to L-lysine by cell suspensions oi erobacteraerogenes (93). However, L-lysine and some... 

The steric bulk of the three iodine atoms in the 2,4,6-triiodoben2ene system and the amide nature of the 1,3,5-substituents yield rotational isomers of the 5-A/-acyl-substituted 2,4,6-triiodoisophthalamides. Rotational motion in the bonds connecting the side chains and the aromatic ring is restricted. These compounds also exhibit stereoisomerism when chiral carbon atoms are present on side chains. (R,5)-3-Amino-l,2-propanediol is incorporated in the synthesis of iohexol (11) and ioversol (12) and an (3)-2-hydroxypropanoyl group is used in the synthesis of iopamidol (10). Consequendy, the resulting products contain a mixture of stereoisomers, ie, meso-isomers, or an optical isomer. 

Self-condensation of the substituted propiophenone, 15, by the pinacol reaction proceeds to give the glycol, 16, as the meso isomer. (If it is assumed that the transition state for this reaction resembles product, this stereoselectivity can be rationalized on the grounds of steric interaction compare A, which leads to the observed product, with B.) Dehydration under very specialized conditions (acetyl chloride, acetic anhydride) affords the bisstyrene-type diene (17). Removal of the acyl groups by means of base affords the synthetic estrogen, dien-... 

Some physical properties of the three stereoisomers are listed in Table 9.3. The (+)- and (-j-tartaric acids have identical melting points, solubilities, and densities but differ in the sign of their rotation of plane-polarized light. The meso isomer, by contrast, is diastereomeric with the (+) and (-) forms. As such, it has no mirror-image relationship to (+)- and (-)-tartaric acids, is a different compound altogether, and has different physical properties. 

These terms are best illustrated by examples. Thus, if maleic acid treated with bromine gives the dl pair of 2,3-dibromosuccinic acid while fumaric acid gives the meso isomer (this is the case), the reaction is stereospecific as well as stereoselective because two opposite isomers give two opposite isomers ... 

The first important evidence for the existence of this mechanism was the demonstration that retention of configuration can occur if the substrate is suitable. It was shown that the threo dl pair of 3-bromo-2-butanol when treated with HBr gave (f/-2,3-dibromobutane, while the erythro pair gave the meso isomer ... 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

The solution to this problem has been to isolate the lactide and to polymerize this directly using a tin(ii) 2-(ethyl)hexanoate catalyst at temperatures between 140 and 160 °C. By controlling the amounts of water and lactic acid in the polymerization reactor the molecular weight of the polymer can be controlled. Since lactic acid exists as d and L-optical isomers, three lactides are produced, d, l and meso (Scheme 6.11). The properties of the final polymer do not depend simply on the molecular weight but vary significantly with the optical ratios of the lactides used. In order to get specific polymers for medical use the crude lactide mix is extensively recrystallized, to remove the meso isomer leaving the required D, L mix. This recrystallization process results in considerable waste, with only a small fraction of the lactide produced being used in the final polymerization step. Hence PLA has been too costly to use as a commodity polymer. 

Scheme 6.13 gives some examples of Cope and oxy-Cope rearrangements. Entry 1 shows a reaction that was done to compare the energy of chair and boat TSs. The chiral diastereomer shown can react through a chair TS and has a AG about 8 kcal/mol lower than the meso isomer, which must react through a boat TS. The equilibrium is biased toward product by the fact that the double bonds in the product are more highly substituted, and therefore more stable, than those in the reactant. 

Inclusion complexation of racemic 63 with the meso isomer of 2 was also examined. The host compound included racemic 63 as did 2a, and photoreaction of the complex in the solid state gave racemic 65 again 23). 

Open-chain vicinal dihalides are apparently reduced conceitedly via a conformation in which the two halogen atoms are anti to each other. This conclusion is based upon the fact that meso isomers of structures 47 ait generally more easily reduced than the dl isomers, presumably because steric repulsions are less for meso than for dl in the transition state for electron transfer. The stereochemistry of the products of reduction of simple open chain vicinal di-... 

Up to this point, we have not had to address the unexpected photolysis behavior of the meso-compound, namely, that the stereochemical retention in the products resulting from the decomposition of this stereoisomer actually decreased at concentrations above the CMC. Intuitively, one would expect that the restricted environment created by micellization of the diazenes would enhance stereochemical retention. The (+)-diazene behaves as we would have predicted, but the meso-isomer does not. 

These differences reflect the conformations of (+)- and meso-isomers as they sit at the air-water interface. What is much harder to elucidate is the effect of stereochemistry on intermolecular interactions. How does changing the stereochemistry at one chiral center affect interactions between diastereomers Ab initio molecular orbital calculations have been used to address the problem of separating stereochemically dependent inter- and intra-molecular interactions in diastereomeric compounds (Craig et al., 1971). For example, diastereomeric compounds such as 2,3-dicyanobutane exhibit significant energetic dependence on intramolecular configuration about their chiral centers. So far, however, little experimental attention has been focused on this problem. 

Figures 46 and 47 show the effect of successive incorporation of meso-diastereomer on the phase transition characteristic of the ( )-isomer as the concentration of meso-isomer increases, the phase transition surface pressure IT occurs at higher n. The same result was found (Arnett et al., 1988b) for mixtures of ( )- and meso-C-15 6,6 and C-15 9,9, which are not shown here. According to the surface phase rule, this is indicative of...

Maleic acid when treated with bromine gives dl pair of 2, 3 dibromosuccinic acid while fumaric acid gives the meso isomer. 

Use of the kinetic advantage method thus points clearly to the occurrence of chemical catalysis with the low-valent metalloporphyrins. This is confirmed by repeating, with iron(I) octaethylporphyrin and cobalt (I) etioporphyrin, the stereochemical experiments carried out earlier with the anion radical of 1,4-diacetylbenzene. Complete stereospecificity is observed in both cases The meso isomer of 4,5-dibromooctane is converted totally into the c/.v-olcfin the d,l isomer is converted totally into the trans-olefin. The reaction again exhibits a clear antiperiplanar preference. 

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