O-Quinones oxidation

Derivatives of phenol or aniline can be oxidized to quinones, the yield and ease of oxidation depending on the substituents. If an amino or hydroxyl group is in the para position, the reaction proceeds readily, as illustrated by the synthesis of quinone from hydroquinone by oxidation with a sodium chlorate-vanadium pentoxide mixture (5>6%) or with chromic-sulfuric acid mixture (92%). A para halogen atom usually has a favorable effect. Any group in the para position is eliminated or oxidized. o-Quinones are usually prepared from the corresponding catechols. A survey of procedures for the synthesis of benzoquinones by oxidation has been made. ... 

In addition to the enzymatic pathway of aroma formation, a thermal route also exists. At high temperatures, interactions of amino acids and sngars resnlt in the formation of various aldehydes. After thermal treatment, the tea becomes more tasty and pleasant, and has a better aroma. An essential source of secondary volatiles, formed during tea leaf processing, is oxidative. o-Quinone resulting from the oxidation of catechins can oxidize, besides amino acids and carotenes, unsaturated fatty acids as well. Linoleic and linolenic acids can be converted into hexenal and trans-hex-2-enal, respectively, and in addition, small amounts of other volatile compounds, especially hexanoic acid and trani-hex-2-enoic acid, can be formed from the same acids, respectively. Also the monoterpene alcohols, linalool and geraniol, play an important role in the formation of the aroma of black tea [38]. 

Copper(II)-quinone-bipyridine complexes have been structurally characterized [22,23]. Dinuclear catecholato complexes of copper(II), formed from strongly oxidizing o-quinones and copper(I) halides have also been described [24,25]. Some of these structures are strongly relevant to those assumed in Tsuruya et al. s scheme [19] ... 

The most elementary biosensors are fruit pulps or slices which have been combined with amperometric electrodes. A well-known example is the ba-nanatrode (Wang and tin 1988). This sensor, most useful for demonstration experiments, contains a paste mix of banana pulp, nujol and carbon powder which has been pressed into a glass tube with an electric contact (Fig. 7.39). The mass contains the enzyme polyphenolase, which catalyses the oxidation of polyphenols, among them important biological messengers like dopamine. The sensor can be tested by means of simple compounds like catechol, which can be detected in beer. As a result of air oxidation, o-quinone is formed. The latter is an electrochemicaUy active compound which can be detected e.g. by differential-pulse voltammetry. 

Unexpectedly, a completely different reaction took place in the oxidation of 2-(l-propenyl)phenol (111) with PdCh. Carpanone (112) was obtained in one step in 62% crude yield. This remarkable reaction is explained by the formation of o-quinone, followed by the radical coupling of the side-chain. Then the intramolecular cycloaddition takes place to form carpanone[131]. 

Adduct 100 is formed from the 1,4 cycloaddition of o-quinone (99) with the morpholine enamine of cyclohexanone (125). Treatment of styrene oxide with cyclic enamines at elevated temperatures (about 230°C) produces O.N-ketals possessing a furan nucleus (125a). 

Benzofuroxan may be obtained by oxidation of o-quinone dioxime. The first benzofuroxan derivative, 1,2-naphthofuroxan, was obtained by this method. Suitable oxidizing agents include alkaline ferri-cyanide, bromine water, chlorine, and nitric acid. The method is of practical value only when the o-quinone or its monooxime (o-nitrosophenol) is readily available, and since this is not generally the case, other routes, e.g., the oxidation of o-nitroanilines and the thermal decomposition of o-nitrophenyl azides/ are more commonly used. 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

The other common synthetic procedure for Bfx and Fx preparation is the oxidative cyclization of 1,2-dioximes. 1,2-Dioximes are excellent starting materials for the syntheses of the 1,2,5-oxadiazole N-oxide system in presence of oxidizing conditions to promote the cyclization. Its utiUty is restricted for Bfxs syntheses because the restriction of o-quinone dioximes availability, contrarily a-glyoximes, which are useful to prepare Fx, are more easily to prepare. In Table 1, products, conditions, and comments for the most recent Fx synthesis using 1,2-dioximes are shown. 

Despite the importance of the oxidative polymerization of 5,6-dihydroxyin-dole, in the biosynthesis of pigments, little experimental data are known on the oxidation chemistry of the oligomers of 1. For such reasons, three major dimers of 1, such as 2-4 (Scheme 2.9), have been computationally investigated at PBEO/ 6-31+G(d,p) level of theory both in gas and in aqueous solution (by PCM solvation model) to clarify the quinone methide/o-quinone tautomeric distribution. 

SCHEME 10.8 Hydroxychavicol oxidizes to an o-quinone that isomerizes to a o-hydroxy-p-QM. 

The effects of changing Jt-conjugation at the 4-position on both the rate of isomerization of the initially formed o-quinones to QMs and the reactivity of the quinoids formed from 4-propylcatechol, 2,3-dihydroxy-5,6,7,8-tetrahydronaphtha-lene (2-THNC), hydroxychavicol, and 4-cinnamylcatechol were studied (Fig. 10.6).9 These catechols were selectively oxidized to the corresponding o-quinones or QMs and trapped with GSH. Microsomal incubations with the parent catechols produced only o-quinone-GSH conjugates. However, if GSH was added after an initial incubation period both o-quinone- and QM-GSH conjugates were observed. The results indicate that the extended Jt-conjugation at the para position enhances the rate... 

SCHEME 10.11 Estrone and estradiol are oxidized to catechols and o-quinones, which isomerizes to /j-QMs. 

Quercetin is a naturally occurring flavonoid with both antioxidant and prooxidant activities (Scheme 10.12).90 It has been demonstrated in a variety of bacterial and mammalian mutagenicity experiments that quercetin has mutagenic properties that could be related to quinoid formation.91,92 Quercetin is initially oxidized to an o-quinone, which rapidly isomerizes to di-QMs that could also be called extended... 

Trans oxidative addition of CH3I to [Ir(acac)(cod)] affords the structurally determined complex [Ir(acac)(cod)(CH3)1].241 The reaction of [(Npet)2Ir]Cl, Npet = o-(diphenylphosphino)benzylide-nejethylamino, with tetrachloro-o-quinone yields the structurally characterized product (135).242 The synthesis and characterization of the water-soluble complex (136) have been described.243... 

Attempts of further nitration of dinitro derivative 83 under usual conditions failed. Using 100% nitric acid in fluorosulfonic acid or trifluoromethanesulfonic acid, reagents useful for nitration of deactivated aromatic systems led to the formation of moisture-sensitive nitration products, which undergo further oxidation to give o-quinone-like species 84 and 85. Using the latter conditions, compound 86 can be isolated in 20% yield and converted into the tetraoxo derivative 85 by heating at 220°C (Scheme 4) <1996JOC1898>. 

Lillie RD, Pizzolato P, Dessauer HC, et al. Histochemical reactions at tissue arginine sites with alkaline solutions of /J-naphthoquinone-4-sodium sulfonate and other o-quinones and oxidized o-diphenols. J. Histochem. Cytochem. 1971 19 487 197. 

The nitration of the polyheterocyclic compound 226 leads to the formation of moisture-sensitive nitration products, which undergo further oxidation to give o-quinone-like species (Scheme 55) <1996JOG1898>. 

The most synthetically useful methods for benzofuroxans are (1) oxidation of o-quinone dioximes (2) decomposition of o-nitroaryl azides and (3) oxidation of o-nitroanilines. Benzofuroxans can also be formed as a result of Boulton-Katritzky rearrangement (see Section 5.05.5.2.1). 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

As mentioned previously, PPO shows two catalytic activities the conversion of monophenols into o-diphenols (monophenolase activity) and the oxidation to the corresponding o-quinones (diphenolase activity) (Fig. 4.2) (Sanchez-Ferrer and others, 1995). 

The diphenolase activity involves the oxidation of two o-diphenols to two o-quinones... 

There is an irreversible enzymatic inactivation reaction, which occurs during the oxidation of the cyclizable and noncyclizable diphenols to oquinones. This inactivation process has been interpreted as being the result of a direct attack of an o-quinone on a nucleophilic residue (His) near the active enzyme center or of an attack of a copper-bound hydroxyl radical generated by the Cu(I)-peroxide complex. However, the latter hypothesis seems to be more probable, because inactivation also occurs in the presence of reducing agents that remove the o-quinones generated. 

A) In the presence of a reducing agent, such as ascorbic acid or NADH, o-quinones are reduced to o-diphenols that are stable compounds, and the reducing agent is oxidized to dehydroascorbic acid or NAD+. 

B) Several PPO substrates such as 3,4-dihydroxymandelate can be decarboxylated by PPO. The product of the enzyme action is an o-quinone, which, owing to its instability, evolves to the Anal product, 3,4-dihydroxybenzaldehyde, by a chemical reaction of oxidative decarboxylation. 

The oxidation of dihydroxy aromatic compounds under the conditions used by Goldschmidt ususually leads to the formation of quinones rather than diradicals. For example, >,/> -dihydroxydiphenyl gives >-diphenoquinone. Several attempts have been made to oxidize o.o -dihydroxydiphenyl, but without success. The product would be of special interest because of the possible equilibrium among diradical, quinone, and peroxide isomers ... 

In the oxidative bleaching processes, the decoloration of p- and o-quinones and of coniferaldehyde structures also seems to be involved. In the case of coniferaldehyde, the removal of the conjugated side chain is probably involved (Figure 3.20). 

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