Phenol/quinone addition

The synthesis of conocurvone (la) also makes use of the well established principle of phenol/quinone addition, even though the term was inspired by the reaction of quinones with hydroxy-2//-l-benzopyran-2-ones, a reaction that was discovered later but proceeds in a similar manner. Brief warming of... 

Phenols can be mononitrated in a two-phase system (ether/water) by sodium nitrate and aqueous hydrochloric acid in the presence of a catalytic amount of lanthanum(III) nitrate in yields genei ly above 80%. Phenol itself gives 2 1 ortho.para nitration, while phenols substituted in the para position give predominantly ortho nitration with some meta nitration (equation 21). As shown recently nitration of phenols in the two-phase system occurs even without lanthanum(III) nitrate. By use of sodium nitrate, aqueous sulfuric acid and a catalytic amount of sodium nitrite in the two-phase system mononitration of phenols can be achieved in high yields. Under these conditions starting from 3-substituted phenols in addition to the nitro compounds formation of quinones has been observed. Polyhydroxy aromatics are typically difficult to nitrate directly. Thus, 4,6-dinitroresorcinol (45) has been synthesized from resorcinol diacetate (44) with nitric acid or mixed solutions of nitric acid/sulfuric acid containing urea as a nitrous trap (equation 22). ... 

The polycyclic aromatic hydrocarbon carcinogens, which are very ubiquitous, are metabolized by the microsomal mixed-function oxidase system of target tissues to a variety of metabolites such as phenols, quinones, epoxides, dihydrodiols and dihydrodiol-epoxides ( ). The mqjor pathway of activation of benzo(a)pyrene (BP) leads to the formation of dihydrodiol-epoxide of BP which interacts predominantly with the 2-amino of guanine of DNA. The dihydrodiol-epoxide of BP appears to be the major ultimate electrophilic, mutagenic, and carcinogenic metabolite of BP ( ). Nevertheless, other metabolites such as certain phenols, epoxides and quinones may contribute to the overall carcinogenic activity of BP. In addition, a free radical mechanism may also be partly involved in its carcinogenic activity. 

Natural fats possess a certain degree of resistance to oxidation, owing to the presence of compounds termed antioxidants. These prevent the oxidation of unsaturated fats until they themselves have been transformed into inert products. A number of compounds have this antioxidant property, including phenols, quinones, tocopherols, gallic acid and gallates. In the European Union, propyl, octyl or dodecyl-gallate, butylated hydroxyanisole, butylated hydroxytoluene and ethoxyquin may be added to edible oils as antioxidants in amounts specified in the EC Community Register of Feed Additives 2009. Other substances such as synthetic a-, 7- and 8-tocopherols and various derivatives of ascorbic acid may be used without limit. 

In addition, toluene extraction reveals traces of organic materials, essentially poly-aromatic hydrocarbons (below 0.5 wt%). A number of oxygenated chemical groups have been found on carbon black surface, such as carbonyls, carboxyls, pyrones, phenols, quinone, lactol, etc., but in minute quantities and all are removed by heating at 950°C in an oxygen free atmosphere. 

Above 160°C it is believed that additional cross-linking reactions take place involving the formation and reaction of quinone methides by condensation of the ether linkages with the phenolic hydroxyl groups (Figure 23.14). 

Coelenterazine (A) is oxidized into dehydrocoelenterazine (D) by MnC>2 in a mixed solvent of ethanol and ether (Inoue et al., 1977b). Dehydrocoelenterazine (C26H19O3N3) can be obtained as dark red crystals. It does not have the capability of chemiluminescence. The ultraviolet absorption spectrum (Fig. 5.6) shows its absorption maxima at 425 nm (e 24,400) and 536 nm (g 12,600) in ethanol. An addition of NaOH significantly increases the 536 nm peak at the expense of the 425 nm peak. Dehydrocoelenterazine can take a tautomeric structure of quinone type (not shown), in which the phenolic proton on the 2-substituent is shifted onto the N(7) of the imida-zopyrazinone ring. Dehydrocoelenterazine can be readily reduced to... 

The quinone methide can also be generated in situ, at least in aqueous NaOH, directly from the peracetate, as hydrolysis of the phenolic acetate is faster than the benzylic acetate (see an example in Section 12.5.3). This method was used to demonstrate the addition of anthrahydroquinone (AHQ) and anthranol to (actual polymeric) lignin quinone methides in studies elucidating the anthraquinone (AQ)-catalyzed 8-0-4-aryl ether cleavage mechanisms in alkaline pulping.64-66... 

It is seen that the substitution of the part of amine or phenol by quinone prolongs the induction period by two or three times. The mechanism of synergistic action of quinone is the same as in the case of nitroxyl radicals. Quinone reacts with InH with production of semiquinone radicals. The latter rapidly reacts with peroxyl radicals and provokes the additional rapid chain termination [47],... 

In addition to this reaction, quinones and other alkyl radical acceptors retard polymer oxidation by the reaction with alkyl radicals (see earlier). As a result, effectiveness of these inhibitors increases with the formation of hydroperoxide groups in PP. In addition, the inhibiting capacity of these antioxidants grows with hydroperoxide accumulation. The results illustrating the efficiency of the antioxidants with cyclic chain termination mechanisms in PP containing hydroperoxide groups is presented in Table 19.12. The polyatomic phenols producing quinones also possess the ability to terminate several chains. 

Consequently, in an inert atmosphere/= 2(1 + k(lls/krcc) > 2. When phenoxyl radicals react only with peroxyl radicals, /= 2 and there is no regeneration. At low dioxygen pressures, phenoxyl radicals react with both peroxyl and alkyl radicals / ranges between 2 and 2(1 +kdis/krec) and increases with decreasing p02- In addition to this, the product of phenol oxidation, quinone, becomes the efficient alkyl radical acceptor at low dioxygen pressure (see earlier). 

Michael addition of di- and tri-hydric phenols to /V-cinnamoylimidazoles followed by a lactonisation offers a route to 4-aryI-3,4-dihydrocoumarins and their [/]-benzologues <00S123>. The lactonisation of the naphthoquinone derivative 66 is sensitive to the acidic cyclising medium and it is possible to obtain the thermodynamically less stable o-quinone derivative exclusively (Scheme 44) <00TL3007>. Some related quinones have been obtained from 1-benzylisoquinolines via an arylnaphthoquinone <00T6O23>. 

Chemically, conversion into a vat consists in a 1 6-addition of hydrogen and recalls exactly the conversion of quinone into quinol. Like quinol, indigo white , also a dihydric phenol , is a weak add, the alkali salts of which are coloured intensely yellow. 

The anodic coupling of aryl ethers is reviewed in Ref. [180]. Aryl ethers are more selectively coupled than phenols for the following reasons The carbon-oxygen coupling is made impossible and the ortho-coupling and the oxidation to quinones become more difficult. A mixture of triflu-oroacetic acid (TFA) and dichloromethane proved to be the most suitable electrolyte [181]. TFA enhances the radical cation stability and suppresses the nucle-ophilicity of water. Of further advantage is the addition of alumina or trifluo-roacetic anhydride [182]. Table 12 compiles representative examples of the aryl ether coupling. 

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