Cyclization reactions radical cations

The electrolysis of adamantylideneadamantane solutions affords the radical cation, which can add molecular (triplet) oxygen to give the peroxide radical anion, which can react with adamantylideneadamantane to give the 1,4-diradical and another molecule of adamantylideneadamantane radical cation. The latter reacts with oxygen, to continue the chain of the reaction, while the former cyclizes to the corresponding 1,2-dioxetane (Scheme 18) (81JA2098). 

PET reactions [2] can be considered as versatile methods for generating radical cations from electron-rich olefins and aromatic compounds [3], which then can undergo an intramolecular cationic cyclization. Niwa and coworkers [4] reported on a photochemical reaction of l,l-diphenyl-l, -alkadienes in the presence of phenanthrene (Phen) and 1,4-dicyanobenzene (DCNB) as sensitizer and electron acceptor to construct 5/6/6- and 6/6/6-fused ring systems with high stereoselectivity. 

Other radical cyclization approaches to the synthesis of piperidines include a CAN-mediated stereoselective cyclization of epoxypropyl cinnamyl amines <06TL705> and a cyclization of (-trimethylsilylmethylamine radical cation, generated via a photoinduced electron transfer reaction to a tethered -functionality <06JOC8481>. 

Anodic oxidation reactions have been utilized to reverse the polarity of enol ethers and to initiate radical cation cyclizations. As shown below, the ketene acetal 97 is oxidized on a... 

In the oxygen-independent Type III reactions the excited/sensi-tized psoralen donates its excitation energy directly to, or reacts with, the target compound. This occurs if the substrate and the target compound (e.g., DNA) are already in close proximity or intercalated. The reactions will proceed very rapidly via the excited singlet state, and are, typically, cyclization reactions or electron-transfer between the sensitizer and the target. In addition, the psoralen can be ionized, either directly or via the excited state, and react with the target compound in the form of a radical cation. Furocoumarins are also employed in treatment of cutaneous T-cell lymphoma and some infections connected with AIDS, by so-called photopheresis processes [71, 74-76]. In this case, peripheral blood is exposed to, e.g., photoactivated (sensitized) 8-methoxypsoralen (8-MOP) in an extracorporeal flow system. This... 

Diels-Alder catalysis.1 This radical cation can increase the endo-selectivity of Diels-Alder reactions when the dienophile is a styrene or electron-rich alkene. This endo-selectivity obtains even in intramolecular Diels-Alder reactions. Thus the triene 2, a mixture of (Z)- and (E)-isomers, cyclizes in the presence of 1 to 0° to the hydroindanes 3 and 4 in the ratio 97 3. Similar cyclization of (E)-2 results in 3 and 4 in the ratio 98 2 therefore, the catalyst can effect isomerization of (Z)-2 to (E)-2. Even higher stereoselectivity is observed when the styrene group of 2 is replaced by a vinyl sulfide group (SC6H5 in place of QHtOCT ). 

Radiative cooling, 23 13-14 Radiative heating/cooling, 23 25-26 Radical catalysts, 14 274 Radical cations, 12 249 Radical chain reactions, 14 274 Radical cyclization approach, 21 147 Radical decomposition reaction, 10 600 Radical generating systems, alternative, 14 299... 

Relatively few kinetic data are available for the carbon-carbon bond forming reactions of alkene radical cations. Nevertheless, rate constants for the cyclization illustrated in Scheme 9, with generation of the alkene radical cation by the fragmentation method, have been measured. These cyclization rate constants are significantly faster than those of the corresponding neutral radicals [89]. 

In a rare study of a radical cyclization of fragmentation-derived alkene radical cations, it was discovered that the stereochemistry of the precursor can have significant consequences on the outcome of the reaction. Thus, a gluco-... 

In the gluco case (Scheme 13) the radical cyclization, with its requirement for the formation of a czs-fused ring junction [129,130], takes place uneventfully on the opposite face of the alkene radical cation to the one shielded by the phosphate anion, whereas in the manno series cyclization is severely retarded by the presence of the phosphate group above the face of the radical cation on which cyclization must occur. This steric retardation of the cyclization step results in a breakdown of chain propagation and results in the longer reaction times observed. Furthermore, the retardation of the radical cyclization step in the manno case enables the alkene radical cation to take... 

An example of a 6-endo cyclization of an alcohol onto an alkene radical cation/phosphate anion pair has also been described (Scheme 22). In order to bring about fragmentation of the primary alkyl phosphate bond in this reaction it was necessary to work in a 1 1 mixture of benzene and acetonitrile [139,140],... 

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