Trans isomer specific isomers

Huonne specifically weakens the carbon-carbon bond opposite the carbon atom beanng fluonne by about 4-5 kcal/mol per fluonne atom [ 25] It has been shown expenmentally that isomenzation of cir-l, l-difluoro-2,3-dimethylcyclopropane to the trans isomer has an activation energy )of 49 7 kcal/mol [126], which is about 10 kcal/mol lower than that of the parent hydrocarbon [127] (equation 26)... 

The P-C isomer selectivity seems to be tissue-specific a preferential uptake of the all-trans isomer was shown in hepatic stellate HSC-T6 cells and in cell-free system from rat liver microsomes, but not in endothelial EAHY cells or U937 monocyte-macrophages (During et al., 2002). When Caco-2 cells were incubated with only 9-cis P-C, all -trans P-C did not increase in cells or in the basolateral medium, indicating that there is no cis-trans isomerization occurring in intestinal cells. Thus, the isomerization of 9-cis P-C observed in vivo (You et al., 1996) could take place in the... 

The chloroacetoxylation reaction is synthetically useful since the chloride can be substituted with either retention [Pd(0)-catalyzed reaction] or inversion (Sjv2 reaction) by a number of nucleophiles. In this way both the cis and trans isomers are accessible and have been prepared from a number of allylic acetates (Schemes 5 and 6). In a subsequent reaction the allylic acetate can be substituted by employing a copper- or palladium-catalyzed reaction. The latter reactions are stereo specific. 

While Baldwin and Barden119 found that triphenyltin deuteride added to the carbon-carbon triple bond of phenylacetylene in a stereo specific reaction, several workers120-124 have found that the addition of a trialkyltin hydride to a carbon-carbon triple bond gives a mixture of the cis and trans isomers (equation 89). The more stable trans isomer is produced in the highest yield. 

Sigma lone pair nonbonded interaction in 1,2-difluorocyclopropane presents a similar situation to that observed in 1,2-difluoroethane (see Pattern b Scheme 1). Specifically, sigma nonbonded attraction may favor the trans isomer. This can be seen in the results of CNDO/2 calculations shown above where it appears that the sigma nonbonded interaction is repulsive in the cis isomer. 

The above predictions are partially confirmed by the available electrochemical data. Specifically, the half wave reduction potentials for cis and trans 1,2-disubsti-tuted ethylenes have been measured and it is found that it is easier to reduce the trans isomer, i.e. E, > E°fa243 248). 

The addition of alkenes to A -methylenium amide cations is stereo-specific in the sense that it obeys the cis principle of Alder and Stein, cis-2-butene giving a cis product (105, R = R = Me) and the trans isomer giving trans product (105, R = R = Me). The cycloaddition of unsymmetrical alkenes is highly regiospecific. Knowing how the alkene will react with a large cation allows prediction of the product (105). Thus styrene, butadiene, and vinyl acetate all react so that its substituent appears at position 6 of the dihydrooxazinium salt (105, R = Ph, vinyl, or OAc). 

Competitive inhibitors bind to specific groups in the enzyme active site to form an enzyme-inhibitor complex. The inhibitor and substrate compete for the same site, so that the substrate is prevented from binding. This is usually because the substrate and inhibitor share considerable stmctural similarity. Catalysis is diminished because a lower proportion of molecules have a bound substrate. Inhibition can be relieved by increasing the concentration of substrate. Some simple examples are shown below. Thus, sulfanilamide is an inhibitor of the enzyme that incorporates j9-aminobenzoic acid into folic acid, and has antibacterial properties by restricting folic acid biosynthesis in the bacterium (see Box 11.13). Some phenylethylamine derivatives, e.g. phenelzine, provide useful antidepressant drags by inhibiting the enzyme monoamine oxidase. The cA-isomer maleic acid is a powerful inhibitor of the enzyme that utilizes the trans-isomer fumaric acid in the Krebs cycle. 

In 1986, Kesselmayer and Sheridan reported the matrix isolation and IR spectrum of chloro-methoxymethylene. They were able to show that two isomeric forms were present, trans (12) and cis (13). This is in line with Schaefer s " finding that hydroxymethylene should exist in cis- and frons-forms. While Kesselmayer and Sheridan were able to assign several of the bands in the matrix mixture to the cis- and trans-forms, they were unable to assign the majority of the bands in the IR spectrum they obtained to the specific isomer. In order to further confirm their isolation of these alkoxy carbenes, as well as to help in the interpretation of the observed IR spectrum, we undertook the calculation of the IR spectra of 12 and 13. ... 

Dichloroethylene is a mixture of two geometric isomers, cis and trans-, the proportion of the cis isomer to the trans isomer varies from mixture to mixture, depending on the manufacturer s specifications. The properties of the mixture are expected to be similar to those of the individual isomers. 

Unlike desferrioxamine analogs designed for specific therapeutic purposes described above, chiral DFO analogs that form conformationally unique complexes with iron(lll) were designed to serve as chemical probes of microbial iron(lll) uptake processes. As mentioned above, ferrioxamine B can form a total of five isomers when binding trivalent metal ions, each as a racemic mixture. Muller and Raymond studied three separate, kinetically inert chromium complexes of desferrioxamine B (N-cis,cis, C-cis,cis and trans isomers), which showed the same inhibition of Fe-ferrioxamine B uptake by Streptomyces pilosus. This result may indicate either that (i) ferrioxamine B receptor in this microorganism does not discriminate between geometrical isomers, or that (ii) ferrioxamine B complexes are conformationally poorly defined and are not optimal to serve as probes. 

The protein phosphatase calcineurin was of particular interest since it mediates the immunosuppressive effect of the pharmaceuticals cyclosporin and FK506, often used in organ and tissue transplantations. The biochemical point of application of both pharmaceuticals was unclear for a long time. In initial experiments, it was found that cyclosporin and FK506 bind specifically to two proteins known as cyclophilin and FK506 binding protein, respectively. Both proteins function as peptidyl prolyl cis/trans isome-rases (review Fischer, 1994). 

This polymer is a hard plastic that occurs naturally as gutta-percha or balata. Since the trans isomer packs better than the cis isomer, it has a higher specific gravity, a higher degree of crystallinity, and a higher melting point (67 C) than the ds isomer of polyisoprene. The chemical and the solvent resistance of the trans polymer are simitar to those of the ds polymer. 

These reactions, called electrocydic rearrangements,365 take place by pericyclic mechanisms. The evidence comes from stereochemical studies, which show a remarkable stereo-specificity whose direction depends on whether the reaction is induced by heat or light. For example, it was found for the thermal reaction that cw-3,4-dimethylcyclobutene gave only cis,trans-2,4-hexadiene, while the trans isomer gave only the trans-trans diene 366... 

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