Hydroxylation, free-radical

Free-radical hydroxylation of heterocyclic compounds has almost always been studied in the context of the relation of chemical hy-... 

Rather scanty evidence exists for the participation of free radicals in Alzheimer s disease and Down s syndrome. However, more recendy, reports have appeared that suggest possible free-radical involvement in the pathogenesis of these two conditions. Zemlan et al. (1989) repotted that the activity of the free-radical scavenging enzyme, SOD, was significantly increased in fibroblast cell lines derived from familial Alzheimer s and Down s patients. They hypothesized that the elevation in SOD activity observed in the Alzheimer patients supports the theory that paired helical filaments are formed by free-radical hydroxylation of proline residues. They further su ested that SOD levels might also be increased in the brains of Alzheimer s and Down s patients, and that the increase in SOD may reflect an enhanced generation of free radicals. 

Irradiation of the pyrimidines (241a,b) using triphenylene-sensitization in tetrahydrofuran results in their conversion to the spirotriketones (242). The path by which this occurs involves fission of bond a in (241) affording a biradical which cyclizes to the observed products (242). The evidence for a biradical intermediate is supported by the formation of (243) from the irradiation of pyrimidine (241c). This product arises by hydrogen abstraction within the biradical formed by the fission of a followed by a free radical hydroxyl at ion. The source of the hydroxyl radical is thought to be the peroxide of tetrahydrofuran produced in the reaction mixture. ... 

Benzene reacts with the titanous-peroxide system to give the adduct (41) in which the unpaired spin is deloealized over the residual aromatic system it is inferred that this is an intermediate in the formation of phenol, which is isolable from the reaction, so that the mechanism of free-radical hydroxylation is analogous to that of phenylation (Dixon and Norman, 1963a, 1964b). 

PC Bisphenol A, stilbenes, water, some organic pigments Chain scissions, crosslinks, free radicals, hydroxyl groups, ethers, unsaturations Chain scissions, hydroperoxides, free radicals, carbonyl groups... 

The extracts of several oilseeds, especially borageandeveningprimrose, were evaluated for their free radicals, hydroxyl radical, superoxide and hydrogen peroxide scavenging activities. In this, 2,2-diphenyl-l-piperylhydrazyl radical PPH ) was used to monitor the efficacy of the extracts in terms of tiieir scavenging capacity. Generalized reduction reactions between DPPH and additives (AH) may be written as ... 

Analogous to the free-radical hydroxylation using tris(phenylthio)antimony, the trapping of radicals derived from O-acyl-thiohydroxamates with tris(phenylthio)phosphorous proceeds via a free-radical chain process (Scheme 15). The formation of the thiophosphonate end product is the result of a consecutive rearrangement of a pentavalent intermediate under the reaction condition and hydrolysis under aqueous work-up. 

Nitrations are highly exothermic, ie, ca 126 kj/mol (30 kcal/mol). However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and the operating conditions. The reactions usually are either ionic or free-radical. Ionic nitrations are commonly used for aromatics many heterocycHcs hydroxyl compounds, eg, simple alcohols, glycols, glycerol, and cellulose and amines. Nitration of paraffins, cycloparaffins, and olefins frequentiy involves a free-radical reaction. Aromatic compounds and other hydrocarbons sometimes can be nitrated by free-radical reactions, but generally such reactions are less successful. 

Etherification and esterification of hydroxyl groups produce derivatives, some of which are produced commercially. Derivatives may also be obtained by graft polymerization wherein free radicals, initiated on the starch backbone by ceric ion or irradiation, react with monomers such as vinyl or acrylyl derivatives. A number of such copolymers have been prepared and evaluated in extmsion processing (49). A starch—acrylonitrile graft copolymer has been patented (50) which rapidly absorbs many hundred times its weight in water and has potential appHcations in disposable diapers and medical suppHes. 

Heavily pigmented wools such as karakul require a more stringent approach, known as mordant bleaching, in which a metal salt is first appHed. The metal cations ate preferentially absorbed by the melanin pigment, where they subsequendy decompose hydrogen peroxide to produce highly aggressive hydroxyl free radicals, which then attack and bleach the melanin (114). 

Electron spin resonance (esr) (6,44) has had more limited use in coal studies. A rough estimate of the free-radical concentration or unsatisfied chemical bonds in the coal stmcture has been obtained as a function of coal rank and heat treatment. For example, the concentration increases from 2 X 10 radicals/g at 80 wt % carbon to a sharp peak of about 50 x 10 radicals/g at 95 wt % carbon content and drops almost to zero at 97 wt % carbon. The concentration of these radicals is less than that of the common functional groups such as hydroxyl. However, radical existence seems to be intrinsic to the coal molecule and may affect the reactivity of the coal as well as its absorption of ultraviolet radiation. Measurements from room... 

Beyer synthesis, 2, 474 electrolytic oxidation, 2, 325 7r-electron density calculations, 2, 316 1-electron reduction, 2, 282, 283 electrophilic halogenation, 2, 49 electrophilic substitution, 2, 49 Emmert reaction, 2, 276 food preservative, 1,411 free radical acylation, 2, 298 free radical alkylation, 2, 45, 295 free radical amidation, 2, 299 free radical arylation, 2, 295 Friedel-Crafts reactions, 2, 208 Friedlander synthesis, 2, 70, 443 fluorination, 2, 199 halogenation, 2, 40 hydrogenation, 2, 45, 284-285, 327 hydrogen-deuterium exchange, 2, 196, 286 hydroxylation, 2, 325 iodination, 2, 202, 320 ionization constants, 2, 172 IR spectra, 2, 18 lithiation, 2, 267... 

A large proportion (30-90% in tropical waters) is absorbed by bacteria and oxidized to FfjS in order to allow the sulfur to be used by these organisms. Once in the atmosphere, DMS is oxidized by various free radicals such as hydroxyl and nitrate ions. In the presence of low concentrations of NO the hydroxyl reaction... 

The flask should have creased sides and a conical indentation in the bottom and should be equipped with a high-speed, propeller-type stirrer rotated to force the liquid downwards. The stirrer should be constructed of glass because metals may interfere with the generation and utilization of the hydroxyl free radicals. 

One of the important consequences of neuronal stimulation is increased neuronal aerobic metabolism which produces reactive oxygen species (ROS). ROS can oxidize several biomoiecules (carbohydrates, DNA, lipids, and proteins). Thus, even oxygen, which is essential for aerobic life, may be potentially toxic to cells. Addition of one electron to molecular oxygen (O,) generates a free radical [O2)) the superoxide anion. This is converted through activation of an enzyme, superoxide dismurase, to hydrogen peroxide (H-iO,), which is, in turn, the source of the hydroxyl radical (OH). Usually catalase... 

The hydroxyl hydrogen in phenol is particularly susceptible to abstraction by a free radical. 

Free-radical substitutions of heterocyclic compounds have been carried out with alkyl, aryl, and hydroxyl radicals in solution and with halogen atoms in the gas phase. Of these, arylations have been the most extensively investigated. 

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