Catechol oxidative coupling

Benzodioxanes (6, 516). The earlier synthesis of benzodioxanes by oxidative coupling of catechol derivatives with methoxypropenylphenols has been extended to the first synthesis of the complex benzodioxane silybin (3) shown in equation (I).2 The starting materials are (2R,3R)-dihydroqucrcctin (I) and coniferyl alcohol (2). In this case, the reaction is not regioselective, 3 and the isomeric 4 being obtained in nearly equal amounts. 

Pal et al. (1994) compared the catalysis of oxidative coupling reactions of various phenolic compounds by the enzymes, laccase and tyrosinase, and mineral catalyst, birnessite. Birnessite acts as a heterogeneous catalyst whereas laccase and tyrosinase function as homogeneous catalysts. Laccase and tyrosinase continue to oxidize catechol after repeated additions of the chemical, while birnessite lost its oxidizing activity after the first addition of catechol (Figure 2.20). In the case of birnessite,... 

Figure 2.20. Transformation of catechol by laccase (0.4 units mT1), tyrosinase (0.4 units ml-1) and birnessite (600ugml 1) after repeated addition of substrate. Reprinted from Pal, S., Bollag, J.-M., and Huang, P. M. (1994). Role of abiotic and biotic catalysts in the transformation of phenolic compounds through oxidative coupling reactions. Soil Biol. Biochem. 26, 813-820, with permission from Elsevier.

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

Scheme 22. Triphenylene synthesis by VOC -mediated oxidative coupling of catechol ethers with biphenyls.

In the presence of bulky groups, such as in 3,5-di-ferf-butylquinone, the quinone is stable and can be isolated, but in most cases, it easily undergoes other reactions, such as Michael addition by nucleophiles present in solution, including the catechol itself. On the other hand, in some cases, the substrate oxidation can be driven to selected products [44], as in the synthesis of neurotrophic americanol A and isoamericanol by HRP-catalyzed oxidative coupling of caffeic acid [45] (Fig. 6.3b). 

Several other groups have observed oxidative coupling reactions of phenols (see Equation 5.10) in the presence of ozone. Chrostowski et al. (1983) exposed several polyphenolic compounds to ozone at various pHs and observed changes in color for example, catechol (86) was observed to develop first a red and then a brown color. 

Oxidative coupling of catechol-ethers with MoCf Preparation of 5,5 -bis(2-chloroethoxy)-4,4 -dimethoxy-2,2 -dimethylbiphenyl (409) [80]... 

The oxidative coupling of catechol proceeded in the presence of a peroxidase catalyst [11], during which an unstable o-quinone intermediate may be formed, followed by complicated reaction pathways to give poly(catechol). 

Polypyrrole shows catalytic activity for the oxidation of ascorbic acid,221,222 catechols,221 and the quinone-hydroquinone couple 223 Polyaniline is active for the quinone-hydroquinone and Fe3+/Fe2+ couples,224,225 oxidation of hydrazine226 and formic acid,227 and reduction of nitric acid228 Poly(p-phenylene) is active for the oxidation of reduced nicotinamide adenine dinucleotide (NADH), catechol, ascorbic acid, acetaminophen, and p-aminophenol.229 Poly(3-methylthiophene) catalyzes the electrochemistry of a large number of neurotransmitters.230... 

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