- Regioselectivity of Hydroxyl Radical Reactions with Arenes in Nonaqueous Solutions
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The regioselectivity of hydroxyl radical addition to arenes was studied using a novel analytical method capable of trapping radicals formed after the first elementary step of reaction, without alteration of the product distributions by secondary oxidation processes. Product analyses of these reactions indicate a preference for o- over p-substitution for electron donating groups, with both favored over m-addition. The observed distributions are qualitatively similar to those observed for the addition of other carbon-centered radicals, although the magnitude of the regioselectivity observed is greater for hydroxyl. The data, reproduced by high accuracy CBS-QB3 computational methods, indicate that both polar and radical stabilization effects play a role in the observed regioselectivities. The application and potential limitations of the analytical method used are discussed.
- Moores, Lee C.,Kaur, Devinder,Smith, Mathew D.,Poole, James S.
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p. 3260 - 3269
(2019/03/11)
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- Alkoxyl- and carbon-centered radicals as primary agents for degrading non-phenolic lignin-substructure model compounds
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Lignin degradation by white-rot fungi proceeds via free radical reaction catalyzed by oxidative enzymes and metabolites. Basidiomycetes called selective white-rot fungi degrade both phenolic and non-phenolic lignin substructures without penetration of extracellular enzymes into the cell wall. Extracellular lipid peroxidation has been proposed as a possible ligninolytic mechanism, and radical species degrading the recalcitrant non-phenolic lignin substructures have been discussed. Reactions between the non-phenolic lignin model compounds and radicals produced from azo compounds in air have previously been analysed, and peroxyl radical (PR) is postulated to be responsible for lignin degradation (Kapich et al., FEBS Lett., 1999, 461, 115-119). However, because the thermolysis of azo compounds in air generates both a carbon-centred radical (CR) and a peroxyl radical (PR), we re-examined the reactivity of the three radicals alkoxyl radical (AR), CR and PR towards non-phenolic monomeric and dimeric lignin model compounds. The dimeric lignin model compound is degraded by CR produced by reaction of 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH), which under N2 atmosphere cleaves the α-β bond in 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol to yield 4-ethoxy-3-methoxybenzaldehyde. However, it is not degraded by the PR produced by reaction of Ce4+/tert-BuOOH. In addition, it is degraded by AR produced by reaction of Ti3+/tert-BuOOH. PR and AR are generated in the presence and absence of veratryl alcohol, respectively. Rapid-flow ESR analysis of the radical species demonstrates that AR but not PR reacts with the lignin model compound. Thus, AR and CR are primary agents for the degradation of non-phenolic lignin substructures.
- Ohashi, Yasunori,Uno, Yukiko,Amirta, Rudianto,Watanebe, Takahito,Honda, Yoichi,Watanabe, Takashi
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experimental part
p. 2481 - 2491
(2011/05/14)
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- Enzymatic and nonenzymatic in vitro hydrolysis of 2-methyl-2-[2-(methoxy)phenoxy]-4H-1,3-benzodioxin-4-one and 2-methoxyphenyl O-acetylsalicylate
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This paper is concerned with the synthesis and physical properties as well as the enzymatic and nonenzymatic in vitro hydrolysis of the potential aspirin prodrug MR 693 (5) and the salicylic acid prodrug guacetisalum (6). The half-lives of both prodrugs and the amount of aspirin regenerated in each hydrolytic run for 5 have been estimated over a wide range of pH values.
- Hundewadt,Senning
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p. 545 - 547
(2007/10/02)
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- 29Si and 13C NMR Spectra of 4-Substituted 2-Methoxytrimethylsiloxybenzenes. Factors Determining the Chemical Shifts in Models of Lignin Constituents
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29Si and 13C NMR chemical shifts are reported for a series of twenty 4-substituted 2-methoxytrimethylsiloxybenzenes; the set of substituents incorporates a basic set of ten substituents differing in their relative polar and resonance effects and ten other
- Schraml, Jan,Kvicalova, Magdalena,Chvalovsky, Vaclav,Elder, Thomas,Brezny, Robert
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p. 973 - 978
(2007/10/02)
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- Electrophilic Hydroxylation with Bis(trimethylsilyl)peroxide. A Synthon for the Hydroxyl Cation
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The regiospecific introduction of an hydroxy group in aromatic and aliphatic compounds can be performed in good yields by electrophilic hydroxylation of their organometallic derivatives with bis(trimethylsilyl)peroxide.
- Taddei, Maurizio,Ricci, Alfredo
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p. 633 - 635
(2007/10/02)
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- Fragmentation of Trimethylsilyl Derivatives of 2-Alkoxyphenols: a Further Violation of the 'Even-electron Rule'
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The mass spectra of trimethylsilyl (TMS) ethers of 2-methoxyphenols show abundant +* ions originating from consecutive loss of two methyl radicals.This is illustrated by comparison of the accurate mass-measured and linked-scan spectra of the TMS derivatives of 2-methoxyphenol (guaiacol), 4-hydroxy-3-methoxybenzaldehyde (vanillin) and 3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid methyl ester (ferulic acid methyl ester) with those of the TMS derivatives of phenol, 4-hydroxybenzaldehyde, 3-(4-hydroxyphenyl)-2-propenoic acid methyl ester (p-coumaric acid methyl ester), 3-methoxyphenol and 4-methoxyphenol.This distinctive ortho effect is valuable in the identification of isomeric phenolic compounds.In the spectra of the TMS derivatives of 2-ethoxyphenol and 2-propoxyphenol the sequential loss of two radicals is less pronounced, because elimination of the side-chain and methyl group with rearrengement and hydrogen migration is competitive.
- Krauss, Dietlinde,Mainx, Hans Georg,Tauscher, Bernhard,Bischof, Peter
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p. 614 - 618
(2007/10/02)
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