Of phenols to quinones

Manganese dioxide on bentonite clay has also been used for oxidation of phenols to quinones (30-100%) [97] and Mn02 on silica effects the dehydrogenation of pyrrolidines (58-96%) [98]. 

Polyphenol oxidase occurs within certain mammalian tissues as well as both lower (46,47) and higher (48-55) plants. In mammalian systems, the enzyme as tyrosinase (56) plays a significant role in melanin synthesis. The PPO complex of higher plants consists of a cresolase, a cate-cholase and a laccase. These copper metalloproteins catalyze the one and two electron oxidations of phenols to quinones at the expense of 02. Polyphenol oxidase also occurs in certain fungi where it is involved in the metabolism of certain tree-synthesized phenolic compounds that have been implicated in disease resistance, wound healing, and anti-nutrative modification of plant proteins to discourage herbivory (53,55). This protocol presents the Triton X-114-mediated solubilization of Vida faba chloroplast polyphenol oxidase as performed by Hutcheson and Buchanan (57). 

The decisive step in this reaction is the transannular migration of the N-oxide oxygen atom (86+87). Such an intramolecular attack seems quite plausible, since model studies demonstrate that the oxygen atom of the N—0 group in 86 can approach within 2.6 A of the pseudo-geminal position in the other benzene ring. Analogous intermolecular reactions, such as oxidation of phenols to quinones by N-oxides, are known 87>. 

PPO catalyses the dependent oxidation of phenolics to quinones. The secondary reactions of quinones lead to the formation of polymeric brown or black pigments, which are responsible for significant post-harvest losses of fruits and vegetables [72]. Finally, induced PPO activity consists of both systemic and localized components. Systemic induction of PPO in tissues in response to all types of injuries may represent a broad, defensive role for PPO in protection of juvenile tissues from subsequent attack by a broad spectrum of pathogens and pests [71]. 

Butyl hypochlorite, 55 of phenols to quinones Benzoyl /-butyl nitroxide, 28 2,3-Dichloro-5,6-dicyano-l, 4-benzoqui-none, 104 Periodic acid, 238 of phosphorus compounds Dimethyldioxirane, 120 of selenium compounds Potassium permanganate, 258 of sulfides to sulfoxides and sulfones /-Butyl hydroperoxide-Dialkyl tar-trate-Titanium(IV) isopropoxide, 51 ra-Chloroperbenzoic acid, 76, 112 Dimethyldioxirane, 120 of thiols to sulfur compounds Trimethylsilyl chlorochromate, 327... 

Reaction LXXXIV. Addition of Phenols to Quinones. (A., 200, 251 215,134 B., 24,1341.)—Quinones readily react with 1 mol. of p-di-hydric phenols and 2 mols. of other phenols to form the highly coloured ether compounds, quinhydrones and phenoquinones respectively. 

The oxidation of diols by quinolinium dichromate (QDC) shows a first-order dependence on QDC and acid.5 The oxidation of phenols to quinones by quinolinium dichromate in aqueous acetic acid is acid catalysed rate-determining formation of a cationic intermediate is indicated by a p value of —3.79 and further analysis shows the rates to be influenced equally by both inductive and resonance effects of the substituents.6... 

Phenols — quinones. Oxidation of phenols to quinones can be effected with 1 eq. of H202 (60%) and excess iodine in methanol at room temperature. Yields are generally 70-98%. The actual oxidant is probably iodine and the function of... 

The browning offruitisa common example of the oxidation of phenols to quinones. Apples, pears, potatoes, etc. contain polyphenol oxidase (PPO), an enzyme that catalyzes the oxidation of naturally occurring derivatives of catechol (benzene-1,2-diol) by atmospheric oxygen. The products are ortho-quinones, which are unstable and quickly condense to give brown polymers. 

Lead dioxide (lead superoxide), PbOj, and red lead, Pb304, are used only to a limited extent. PbOj is used mainly for the oxidation of phenols to quinones [368, 430, 431] and of aromatic methyl groups to carboxyls... 

A single enzyme is sometimes capable of many various oxidations. In the presence of NADH (reduced nicotinamide adenine dinucleotide), cyclohexanone oxygenase from Acinetobacter NCIB9871 converts aldehydes into acids, formates of alcohols, and alcohols ketones into esters (Baeyer-Villiger reaction), phenylboronic acids into phenols sulfides into optically active sulfoxides and selenides into selenoxides [1034], Horse liver alcohol dehydrogenase oxidizes primary alcohols to acids (esters) [1035] and secondary alcohols to ketones [1036]. Horseradish peroxidase accomplishes the dehydrogenative coupling [1037] and oxidation of phenols to quinones [1038]. Mushroom polyphenol oxidase hydroxylates phenols and oxidizes them to quinones [1039]. 

The combination of Ti(OPr )4 and (BulO)2 or Bu 02I I has been utilized in a number of organic transformations including the Sharpless epoxidation,416 the conversion of alcohols to carbonyl compounds,417 the oxidation of phenols to quinones or ketols,418 and the oxidation of Ti enolates to o-hydroxyketones.419... 

Oxidation of phenols to quinones. Forrester and Thompson2 noted that potassium nitrosodisulfonate, an inorganic radical useful for the oxidation of phenols, amines,... 

Secondary alcohols have been oxidized to ketones with excess ferf-butylhydroperoxide in up to 93-99% yields using a zirconium catalyst.250 Zirconium catalysts have also been used with ferf-butylhydroperoxide in the oxidation of aromatic amines to nitro compounds and of phenols to quinones. Allylic oxidation of steroids in 75-84% yields has been performed with ferf-butylhydroperoxide and cop-... 

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