Dissymmetry conformational

These considerations, thus, lay the groundwork for tests among several semi-empirical approaches to the estimation of optical rotation of bond systems regarded as helices. Should it be necessary to use Eq. (lb) rather than (la), then a sweeping reassessment of the use of the helical conductor model will be required. However that test turns out, a test between that model and the simple conformational dissymmetry model becomes possible on the basis of the material shown in Table 1. At this point it should be said that our calculations on twistane 16> support the helical conductor model but that the results obtained by Pino and his co-workers 17 18> on the chiroptical properties of isotactic polymers prepared from chiral a-olefins support the conformational dissymmetry model. [We are not able, at present anyhow, to account for their results with the helical conductor model]. 

TABLE 7. Analysis of (S)-3-methylhexane (Simple Conformational Dissymmetry) ... 

Rotivity contributions for some common conformational units were calculated by the use of equation 12 and are compared with those used with the empirical Conformational Dissymmetry Model (Table 13). 

In the original Conformational Dissymmetry Model (above), the digauche conformations 20c and 21 were considered to be forbidden all others were equally allowed . It would be more realistic to consider that conformers are neither forbidden nor allowed. 

To apply the Conformational Dissymmetry Model to 31, the bond angle and torsion angle corrections of the Helix Model are required ... 

TABLE 24. Conformational dissymmetry analysis of steroid side chains at C, ... 

Optical activity in high-symmetry chiral molecules has been discussed, and the importance of the role of conformational dissymmetry emphasized by comparison of allenes and spiro[3,3]heptanes. Baboulene and Sturtz have discussed the relationship between stereochemistry and pharmacological activity in l-aminomethyl-2-benzoylcyclopropanes. ... 

Optical isomerism is the result of a dissymmetry in molecular suhstitution. The basic aspects of optical isomerism are discussed in various textbooks of organic chemistry. Optical isomers (enantiomers) may have different physiological activities from each other provided that their interaction with a receptor or some other effector structure involves the asymmetric carbon atom of the enantiomeric molecule and that the three different substituents on this carbon atom interact with the receptor. The Easson-Stedman hypothesis assumes that a three-point interaction ensures stereospecificity, since only one of the enantiomers will fit the other one is capable of a two-point attachment only, as shown in figure 1.13 for the reaction with a hypothetical planar receptor. However, it is reasonable to assume that receptor stereospecificity can also undergo a change when the receptor conformation is altered by a receptor-drug interaction. 

Again it was surprising that enantioseparation by HPLC was successful because the conformational flexibility of the wheel and the thread entail reduced structural dissymmetry of the enantiomers compared with more rigid molecules. For the rotaxanes 80m and 100 separation factors a were found to be 1.48 and 1.69, respectively. The circular dichrogram of the cycloenantiomeric [ljrotaxane 100 is given in Figure 50. 

In addition to the dissymmetry generated by the trisfchclate) structure of octahedral complexes, it is possible to have dissymmetry in the ligand as well. For example, Ihe gauche conformation of ethylenediamine is dissymetric (Fig. 12.27) and could be resolved were it not for the almost complete absence of an energy barrier preventing... 

A. L. Sweeting, L. M. unpublished study). Disorder is not common in the 9-substituted anthracenes, but is observed for anthryl esters 7 and 9. We conclude that disorder may provide local dissymmetry, deforming the lattice and allowing charge accumulation. Unfortunately crystallography cannot determine how the sites of different conformation are distributed in the crystal. We examined the local environment by testing some of the materials for the generation of a second harmonic on laser irradiation no correlation with triboluminescence was found. 

Extrinsic Cotton effects are due to the inherent dissymmetry of the enzyme-bound chromophore (an inherent effect) and/or to the interactions of the chromophore with the encompassing dissymmetric environment (interactive effects). The inherent effects are those which the free chromophore would exhibit if its conformation were identical with that of the enzyme-bound form. The interactive effects result from protein-ligand interactions or ligand-ligand interactions. The main problem in interpretation of die CD of enzyme-bound chromophores is distinguishing between the inherent and the interactive effects. 

Molecular mechanics calculations and temperature-dependent CD spectra may reveal details of the conformational map not attainable by DNMR. Thus, double minima within each of the anti and syn forms were inferred from molecular mechanics calculations and were also supported by the temperature dependence of the CD spectra. Rotational strength calculations for the stable conformations predicted by the molecular mechanics calculations (or for any other arrangement) are in fair agreement with respect to both the sign and the dissymmetry factor for many of the studied compounds and may also allow for tentative assignments of absolute configuration in cases where this is not known (87JA492). 

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