Cyclo-reversion processes

Radical Anions.—Mention has already been made of the reverse cycloaddition which occurs on reduction of (66) to (67) and which might be a concerted allowed process in the radical anion. Another example is reported. The radical anion (426) is made by reduction of the hydrocarbon with Na-K alloy at —110 °C. On warming to — 20°C it rearranges to the radical anion (427) of pleiadene—perhaps a [ 2 -I- 2J cyclo-reversion allowed because of the extra electron. ... 

Kinetic parameters for the gas-phase cyclo-reversion ( 2 +J1 + 2) processes of cis-tris-a-homobenzene (221), diademane (222), and 1,6-homodiademane (223)... 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

Ethenolysis of cycloalkenes is a convenient route for production of certain polyunsaturated compounds. Shell have developed a process for the manufacture of a,w-diolefins via ethenolysis of cyclic olefins (the reverse of Eq. 4) [21,22]. Thus, 1,5-hexadiene and 1,9-decadiene were produced via ethenolysis of cyclo-octadiene and cyclooctene, respectively (Eqs. 7a,b). 

The application of thermolytic reactions (e.g. Diels-Alder and reverse Diels-Alder processes) for the synthesis of fluoro-organic compounds has been reviewed. The cycloaddition of the meso-ionic dithiolone or thiazolone compounds (209 X = S or NPh) with the Diels-Alder adduct of cyclo-octatetraene and dimethyl acetylenedicarboxylate occurs at the cyclobutene ring to give the exo-adducts (210). Pyrolysis of compounds (210) yields dimethyl phthalate and 7,9-diheteratricyclo[4,2, 1,0 ]non-3-en-8-ones (211 X = S or NPh) by retro-[4-t- 2] addition. The similar additions of (209) and related compounds to dimethyl 7-oxabicyclo[2,2,l]-hepta-2,5-diene-2,3-dicarboxylate afforded [4 -1-2] adducts which on pyrolysis suffered double thermal fragmentation e.g. loss of XCO and 3,4-dimethoxycarbonyl-furan) to afford five-membered aromatic heterocyclics. ... 

The class of enzymes called transaminases use the coenzyme pyridoxamine phosphate 23 to convert a substrate a -keto acid to an o -amino acid, while the pyridoxamine itself is converted to pyridoxal phosphate 24 (Fig. 1.12). Later the pyridoxal phosphate coenzyme is converted back to the pyridoxamine form by transaminations in the reverse direction from a sacrificial amino acid. We set out to mimic this interesting process by attaching pyridoxamine to a cyclodextrin, so there would be a preference for transaminating keto adds that could bind to the cyclo dextrin in water. 

Only one or two [6 + 2] reactions have yet been discovered. The transformation (Equation 6.70) possibly involves a [ 65 + 2s) interaction in the first step followed by a reverse cyclo-addition of the [ 2 + 2s + 2 ] type in the second step. Chlorosulphonyl isocyanate reacts with cycloheptatriene (Equation 6.71) to yield three cyclo-adducts, apparently by formal [ 6s +w2al and [jr2s + Tr2al processes. However, it seems more likely that the 1,4-dipoiar species (S3) is an intermediate in the change. 

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