- Reversible formation of aryloxenium ions from the corresponding quinols under acidic conditions
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Quinols, 1, are products of the hydration of O-aryloxenium ions, 2, and N-arylnitrenium ions, 3, and they are being investigated for medical uses. Under acidic conditions (pH 1-3) kinetics and products of Br- trapping demonstrate that 1a, 4-phenyl-4-hydroxy-2,5-cyclohexadienone, and 1b, 4-p-tolyl-4-hydroxy-2,5-cyclohexadienone, generate the corresponding oxenium ions 2a and 2b, respectively, as steady-state intermediates. Formation and trapping of the oxenium ions occurs in competition with the acid catalyzed dienone-phenol rearrangement. Because oxenium ion formation is reversible, the ion can only be detected by trapping with a nucleophile. Br- is an efficient trap under acidic conditions because, unlike N3 -, it is not protonated under those conditions. Attempts to detect the oxenium ions 2a and 2b at pH 4.6 and 7.1 with N3- were unsuccessful indicating that oxenium ion formation only occurs under acidic conditions. The oxenium ion 2c could not be detected under acidic conditions from the quinol 1c, 4-(benzothiazol-2-yl)-4-hydroxy-2,5-cyclohexadienone, by Br- trapping methods, even though this ion can be detected during hydrolysis of the corresponding ester, 4c. Although the benzothiazol-2-yl group is a resonance electron donor that is capable of stabilizing an O-aryloxenium ion, it is also a strong inductive electron withdrawing group that hinders the formation of 2c from 1c by decreasing the extent of protonation of 1c to generate 1cH+ and by destabilizing the transition state for ionization of 1cH+. Generation of an oxenium ion from the corresponding quinol is feasible under acidic conditions as long as the 4-substituent of the quinol is both a resonance and inductive electron donor. Copyright
- Chakraborty, Mrinal,Brzozowski, Christopher F.,Novak, Michael
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- Novel access to cyclohexane-1,4-diones and 1,4-hydroquinones via radical 1,2-acyl rearrangement on 2-(halomethyl)cyclopentane-1,3-diones using cobaloxime-mediated electroreduction or tributyltin hydride
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A new preparative access to synthetically useful cyclohexane-1,4-diones 2 and their oxidized analogues, hydroquinones 3, with the option of introducing alkyl and aryl substituents, was developed by radical 1,2-acyl rearrangement on 2-(halomethyl)cyclopentane-1,3-diones 1, accessible from 1,2-bis(trimethylsiloxy)cyclobutene and α-bromo ketone dimethyl acetals. The electroreduction of monoacetals of 1 in the presence of cobaloxime as a catalyst afforded the cyclohexane-1,4-dione monoacetals in good yields. The Bu3SnH-reduction of 2-aryl 1 under refluxing in benzene effected the rearrangement, affording 2, and when the reaction was prolonged, aromatization to 3 proceeded in moderate yields.
- Kawafuchi, Hiroyuki,Inokuchi, Tsutomu
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p. 2051 - 2054
(2007/10/03)
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- Acid-catalyzed rearrangement of O-(2-arylphenyl)hydroxylamines to aryldihydroazepinones
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Acid-catalyzed rearrangement of O-(2-arylpheynl) hydroxylamines followed by ring enlargement afford 7-aryl-2, 3-dihydro-1H-azepin-2-ones. 2-Amino-2-phenyl-3, 5-cyclohexadienone, an intermediate of the reaction, was trapped as the N-trifluoroacetamide.
- Endo, Yasuyuki,Kataoka, Ken-Ichiro,Haga, Naoki,Shudo, Koichi
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p. 3339 - 3342
(2007/10/02)
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- 1,3-Dipolar Cycloaddition Reactions of 1,4-Benzoquinones with Nitrilimines
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The cycloaddition reactions of some aryl conjugated p-benzoquinones with nitrilimines were studied.Depending upon the reaction conditions mono- and/or bis- adducts were isolated only from carbon-carbon double bond.The structure determination was unequivocally established by an X-ray analysis carried out on a bis-adduct.The observed regio-, as well as site-selectivity was qualitatively correlated with frontier molecular orbitals of the reacting species.
- Argyropoulos, N. G.,Mentzafos, D.,Terzis, A.
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p. 1983 - 1988
(2007/10/02)
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- Stabilized polyglycol for high temperatures
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A composition comprising a poly-alkyleneglycol is stablized against molecular degradation and viscosity loss at elevated temperatures by about 0.1 to about 10% by weight of an arylhydroquinone such as p-acetylphenylhydroquinone.
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