- Efficient oxidative ipso-fluorination of para-substituted phenols using pyridinium polyhydrogen fluoride in combination with hypervalent iodine(III) reagents
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Diacetoxyiodobenzene (PIDA) and bis(trifluoroacetoxy)iodobenzene (PIFA) in the presence of pyridinium polyhydrogen fluoride (PPHF) are effective for the fluorination of para-substituted phenols to give a variety of 4-fluorocyclohexa-2,5-dienones in a good yield. (R,S)-1,1′-Bi-5,6,7,8- tetrahydro-2-naphthol (and its monoacetate) yields atropoisomeric fluorocyclohexadienones. The 4-substituted carbamate open-chain phenols were readily converted to fluorohydroindolenone and fluorohydroquinolenone derivatives by intramolecular conjugate addition.
- Karam, Omar,Martin-Mingot, Agnès,Jouannetaud, Marie-Paule,Jacquesy, Jean-Claude,Cousson, Alain
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p. 6629 - 6638
(2007/10/03)
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- Evidence of an electron-transfer mechanism in the peroxynitrite-mediated oxidation of 4-alkylphenols and tyrosine
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The mechanism of interaction of the peroxynitrite with some 4-alkylphenols and tyrosine was mainly studied by means of ESR spectroscopy and product analysis. The radical intermediates, detected as spin adducts to the 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO), were identified as carbon-centered radicals to the benzene ring. The reaction seems to proceed via an electron transfer process (ET), most likely mediated by a NOx derivative, leading to the intermediacy of a phenoxyl-type radical as proved by the detection of the corresponding Pummerer-type ketone. No evidence of the formation of hydroxyl radicals, due to the homolytic cleavage of the peroxynitrite at physiological pH was obtained, even though DEPMPO hydroxyl spin adducts were detected: the latter most likely arises from the direct attack of the spin trap by the oxidant species. The possible involvement of HCO3-/CO2, i.e., the formation of the nitrosoperoxycarbonate, ONOOCO2.-, was also investigated.
- Grossi, Loris
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p. 6349 - 6353
(2007/10/03)
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- The NO3 radical-mediated liquid phase nitration of phenols with nitrogen dioxide
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Analysis of Kyodai nitration, which refers to the mixture of NO2 and O3 in nonprotic solvents, may contribute to an understanding of the atmosphere because it can be used in the study of the nitration of aromatic compounds with NOsu
- Rindone,Barletta,Bolzacchini,Meinardi,Orlandi
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p. 2224 - 2230
(2007/10/03)
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- Isolation of cross-coupling products in model studies on the photochemical modification of proteins by tiaprofenic acid
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To gain insight into the chemical nature of drug-induced photoallergy, model studies have been carried out on the photochemical modification of proteins by tiaprofenic acid. Irradiation of decarboxylated tiaprofenic acid (DTPA) in the presence of p-cresol leads to C-C- and C-O-connected p-cresol 'dimers', together with DTPA hydrodimers. The p-cresol-DTPA cross-coupling product was not detected in this reaction. However, a product of this type is formed using a more hindered phenol, such as 2,6-di-tert-butylphenol. Similar results are obtained when tiaprofenic acid (TPA) or its methyl ester are used as photosensitizers. The observed formation of 'dimers' can be related to protein photo-crosslinking, through the coupling of two tyrosine units. On the other hand, phenol-(D)TPA cross-coupling may be relevant to the understanding of drug-protein photobinding.
- Miranda, Miguel Angel,Perez-Prieto, Julia,Lahoz, Agustin,Morera, Isabel M.,Sarabia, Zaideth,Martinez-Manez, Ramon,Castell, Jose V.
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p. 497 - 502
(2007/10/03)
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- Ingold-Fischer "Persistent Radical Effect", Solvent Effect, and Metal Salt Oxidation of Carbon-Centered Radicals in the Synthesis of Mixed Peroxides from tert-Butyl Hydroperoxide
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Mixed peroxides are formed from tert-butyl hydroperoxide (TBH), tert-butyl peroxalate (TBP), and a variety of substrates (p-cresol, cyclohexene, styrene, α-methylstyrene, acrylonitrile, 2-methylcyclohexanone). Also, the oxidation of THF in the presence of acrylonitrile under the same conditions gives the mixed peroxide, generated by addition of the tetrahydrofuranyl radical to the double bond and the cross-coupling of the radical adduct with the tert-butylperoxyl radical. Similarly, benzoyl peroxide, TBH, and acrylonitrile give the mixed peroxide by oxidative arylation of the double bond. Paradoxically, TBH acts as effective inhibitor of the polymerization of vinyl monomers (acrylonitrile, styrene). An overall kinetic evaluation suggests that the conditions for the Ingold-Fischer "persistent radical effect", characterized by the simultaneous formation of a persistent and a transient radical, are fulfilled in all cases. The reactions are strongly affected by solvents, which form hydrogen bonds with TBH. Catalytic amounts of Cu(II) and Fe(III) salts influence the selectivity; the possibility that the mixed peroxides can also be generated by metal salt oxidation of carbon-centered radicals is discussed.
- Bravo, Anna,Bjorsvik, Hans-Rene,Fontana, Francesca,Liguori, Lucia,Minisci, Francesco
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p. 3849 - 3857
(2007/10/03)
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- Redox Interactions of Cr(VI) and Substituted Phenols: Products and Mechanism
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The mechanisms of aqueous oxidation-reduction interactions between Cr(VI) and substituted phenols (RArOH) were characterized by kinetic analysis and determinations of reaction products and intermediates. A rapid, peroxidative equilibrium between HCrO4(-) and RArOH forms chromate ester intermediates, as verified by spectroscopy. The subsequent rate-limiting ester decomposition proceeds via innersphere electron transfer. The overall rate dependence on [H(+)] is well accounted for by three parallel redox pathways involving zero, one, and two protons. The two-proton pathway dominates at pH = 5. The parallel reaction rate expression was fitted to data for 4-methyl-, 4-methoxy-, 2,6-dimethoxy-, and 3,4-dimethoxyphenol for pH 1-6. Beside accurately predicting rates for the calibrated conditions, the model predicts a sharp decline in rates at pH >= 6. Rates subsequently measured at pH 7 agreed well with those calculated a priori. Such predictions suggest that the proposed mechanism is robust and accurate. Rate constants were correlated with Hammett-type substituent parameters. Reaction products indicated both one- and two-electron pathways.
- Elovitz, Michael S.,Fish, William
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p. 1933 - 1943
(2007/10/03)
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- Nucleophilic para-fluorination of 4-alkylphenols by hypervalent iodine reagent and pyridinium polyhydrogen fluoride (PPHF) a novel route to 4-fluorocyclohexa-2,5-dienones
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Ipso-fluorination of 4-alkylphenols with C6H5-I(OCOCF3)2-pyridinium polyhydrogen fluoride (PPHF) yields 4-fluorocyclohexa-2,5-dienones.
- Karam, Omar,Jacquesy, Jean-Claude,Jouannetaud, Marie-Paule
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p. 2541 - 2544
(2007/10/02)
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- Cycloaddition of Acylnitroso Compounds and Nitrosobenzene with the Pyrrolidine Dienamine of Pummerer's Ketone: X-Ray Crystal Structure of a Bridged 1,2-Oxazine Derivative
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When the pyrrolidine dienamine 2 of Pummerer's ketone 1 was treated with nitrosocarbonylmethane 4, generated in situ by periodate oxidation of acetohydroxamic acid, the major product was a bridged 1,2-oxazine, shown to have the structure 10 by X-ray cryst
- Freer, Andrew A.,Islam, Md. Azizul,Kirby, Gordon W.,Mahajan, Mohinder P.
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p. 1001 - 1007
(2007/10/02)
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- BIOGENESIS-LIKE TRANSFORMATION OF SALIDROSIDE TO RENGYOL AND ITS RELATED CYCLOHEXYLETANOIDS OF FORSYTHIA SUSPENSA
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Photooxygenation of salidroside (8) in methanol in presence of Rose Bengal afforded cornoside (9), which, on high pressure hydrogenation with 5percent palladium on activated carbon, yielded rengyoside B (6).Reduction of 6 with sodium borohydride gave rengyoside A(5) stereoselectively.By enzymatic hydrolysis, 9, 6 and 5 furnished rengyolone (4), rengyoxide (3) and rengyol (1), respectively. Similerly, p-hydroxyphenylethanol (10), the aglycone part of salidroside (8), was oxygenated photochemically to a dienone alcohol, which cyclized spontaneously to rengyolone (4).Hallerone (17) was obtained by the photooxygenation of p-hydroxyphenylethyl acetate (10b).Thus the plausible biosynthetic routes from salidroside (8) to rengyol (1) and the related natural cyclohexylethanoids were simulated chemically.
- Endo, Katsuya,Seya, Kazuhiko,Hikino, Hiroshi
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p. 3673 - 3682
(2007/10/02)
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- FORMATION OF PUMMER'S KETONE FROM 4-METHYLPHENOXENIUM ION. AN ACID-CATALYZED SOLVOLYSIS OF N-p-TOLUENESULFONYL-O-(4-TOLYL)HYDROXYLAMINE IN p-CRESOL
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N-p-ToluenesulfonylO-(4-tolyl)hydroxylamine (1) reacts with p-cresol in the presence of trifluoroacetic acid to yield biscresol (2) and Pummerer's ketone (3). KEYWORDS ---- phenoxenium ion; Pummerer's ketone; oxidation; oxidative coupling
- Endo, Yasuyuki,Shudo, Koichi,Okamoto, Toshihiko
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p. 3769 - 3770
(2007/10/02)
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