Melanin. I. Kinetics of the oxidative cyclization of dopa to dopaquinone

Young, Thomas E.; Griswold, John; Hulbert, Matthew H.

doi:10.1021/jo00927a057

Cited by 159 publications

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“…In addition, Adams (4,5) has shown that p-benzoquinones with electronwithdrawing substituents can be nucleophilically attacked by water to yield a trihydroxy compound, which can be further oxidized to the hydroxyquinone. Several other workers (6)(7)(8)(9) have investigated the reactions of o-quinones from Dopa, catechol, and 4-methyl catechol in aqueous solutions, but the emphasis has been mostly on acidic solutions and at low concentrations, which significantly suppress the coupling reaction. Outside of the work by Stum and Suslov (10), there has been little electrochemical work done on the coupling reactions of quinones.…”

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confidence: 99%

The Electrochemical Oxidation of Substituted Catechols

Ryan

Yueh

Chen

1980

J. Electrochem. Soc.

204

115

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The oxidation of substituted catechols was studied by cyclic voltammetry, chronoamperometry, rotating ring‐disk electrode, and coulometry. The results showed that the quinones that were formed from the oxidation of substituted catechols reacted with the basic forms of the starting material to yield the dimeric product. These products were generally unstable and rapidly polymerized or underwent some other irreversible reaction to form an electroinactive product. For 3,4‐dihydroxyacetophenone and propriophenone, the intermediate was stable long enough to be observed in cyclic voltammetry. The rate of the coupling reaction was found to correlate well with the Hammett ρ‐σ parameters and indicated that there was substantial negative charge in the transition state. Finally, an analysis of the coulometric n‐values along with the inormalat1/2/C values indicated that the initial coupling product was a diphenyl ether. Analysis of the coulometry products showed extensive polymerization.

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confidence: 99%

The Electrochemical Oxidation of Substituted Catechols

Ryan

Yueh

Chen

1980

J. Electrochem. Soc.

204

115

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“…A peak current ratio (I p C1 /I p A1 ) of nearly unity, particularly during the repetitive cycling of potentials may be considered as a criterion for the stability of p-quinone produced at the surface of the electrode under experimental condition. In other words, any hydroxylation [17][18][19][20][21][22] or dimerization [23][24][25] reaction is too slow to be observed on the cyclic voltammetry time-scale.…”

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confidence: 99%

A New Way for Synthesis of Phenoxazine and Diphenoxazine Derivatives via Electrochemical Method

Davarani

Bojdi

Mehdinia

2011

CHEMICAL & PHARMACEUTICAL BULLETIN

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Electrochemical oxidation of hydroquinone (1a) and 2,3-dimethylhydroquinone (1b) have been studied in the presence of 2-aminophenol (3a) and 2-amino-4-chlorophenol (3b), as nucleophiles in phosphate buffer solution (pH 7.2) using cyclic voltammetry and controlled potential coulometry. We proposed different mechanisms for the electrode process. The products were derived with good yield and purity using controlled-potential electrochemical oxidation of 1a, b in the presence of 3a and 3b at the graphite electrode in an undivided cell.

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“…15,16,17 These studies provided the first order rate constants for the cyclisation of the free amines (k 2 , Scheme 3) as well as the overall rate constants for aminochrome formation (k f , Table 2 Scheme 3) is the rate limiting process. Subsequently, a combined theoretical and kinetic study of the oxidation of L-dopa using mushroom tyrosinase under acidic conditions (pH 5.0-6.0) was able to derive values of the rate constants for both the first order deprotonation step (k 1 ) and the cyclisation step (k 2 )(Scheme 3).…”

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confidence: 99%

ortho-Quinone amines and derivatives: the influence of structure on the rates and modes of intramolecular reaction

Land¹,

Ramsden²,

Riley³

2006

Arkivoc

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The influence of N-substituents and chain length on the mode and rate of intramolecular reaction of ortho-quinone amines is reviewed. These reactions have been the subject of a series of studies over the last decade using a combination of pulse radiolysis, enzyme oximetry and preparative chemistry. Alternative reactions are cyclisation, spirocyclisation and para-quinomethane formation. The observed mode of reaction is determined by both the length of the ortho-quinone side chain and the nature of the N-substituents. The rates of the competing reactions are discussed and compared with rates determined by other workers using cyclic voltammetry. Formation of a para-quinomethane is relatively slow and only occurs when cyclisation is unfavourable due to steric, entropy or electronic effects. Spirocyclisation is rapid and usually reversible but in one case a stable spirocyclic product has been isolated.

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Melanin. I. Kinetics of the oxidative cyclization of dopa to dopaquinone

Abstract: 96, 1256 (1974).(2) (a) For a recent review of alkaloids based on these ring systems, see J. E. Saxton in the "Alkaloids," Vol. 2, Specialist Periodical Reports of the Chemical Society, London, 1972. (b) For recent synthetic methods in the pyrrolizidine series, see N.

Cited by 159 publications

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The Electrochemical Oxidation of Substituted Catechols

The Electrochemical Oxidation of Substituted Catechols

A New Way for Synthesis of Phenoxazine and Diphenoxazine Derivatives via Electrochemical Method

ortho-Quinone amines and derivatives: the influence of structure on the rates and modes of intramolecular reaction

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