Catechols, quinone synthesis

Recently it has been shown, that iodine(v) reagents can also be used for such transformations. An efficient regioselective method for the oxidation of phenols to ortho-quinones 40 can be achieved using 2-iodoxybenzoic acid (IBX) 7. With a subsequent reduction this proves to be a useful procedure for the synthesis of a variety of catechols 41, Scheme 20 [94]. 

Due to some special structural and magnetic peculiarities, in particular, free-radical properties, the quinone ligands and their metal complexes are apart from the other kinds of coordination compounds [125a, 138a], as will be shown below. Here we present an overview of the main methods for the synthesis of complexes containing benzoquinone, semiquinone, and catecholate ligands, and peculiarities of the products. 

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). 

Catechol oxidation catalyzed by peroxidases can be used not only for the synthesis of sulfur-substituted catechols but also for the preparation of synthetic compounds related to pheomelanins, which contain benzothiazine units. In fact, the quinone undergoes an extremely easy nucleophilic addition by thiols. For example, treating the neurotransmitter dopamine with cysteine, in the presence of HRP/H2O2, gives rise to 2-S- and 5-5-cysteinyl-catecholamine and a smaller amount of the 2-S,5-S,-di-cysteinyl-catecholamme conjugate [48, 49] (Fig. 6.3e). 

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. ... 

Simple four-step synthesis of unsaturated seven-membered carbo- and heterocycles from catechols and quinones via an entropy/strain-reduction strategy 04SL933. 

Coumestans represented by 321 are an oxygenated class of aromatic natural products, which have phytoalexin and estrogenic activities. From the biogenetic point of view, 321 will be formed from two units, 4-hydroxycoumarin (322) and catechol. Thus, the first synthesis of 321 was carried out by an electrochemical method. Catechol was initially oxidized to o-quinone, which was attacked by 322 to afford 321 in 95% yield (Scheme 69). ... 

As mentioned above, quinones can be formed from derivatives of resorcinol or catechol as well as from the parent hydroxyl compounds. When dimethoxy compound 144 was treated with ceric ammonium nitrate (CAN), for example, a 92% yield of quinone 145 was obtained as part of De Kimpe s synthesis of isagarin.222... 

Dwivedy, I., Devanesan, P., Cremonesi, P., Rogan, E., and Cavalieri, E., Synthesis and characterization of estrogen 2,3- and 3,4-quinones. Comparison of DNA adducts formed by the quinones versus horseradish peroxidase-activated catechol estrogens. Chem. Res. Toxicol. 5, 828-833 (1992). 

Catechols, hydroquinones, and their methyl ethers readily afford quinones on Ce oxidation.Partial demethylative oxidation is feasible, as shown in the preparation of several intramolecular quinhydrones (eq 5) and a precursor of daunomy-cinone. Sometimes the dual oxidant system of CAN-NaBrOs is useful. In a synthesis of methoxatin (eq 6) the o-quinone moiety was generated from an aryl methyl ether. 

The first step of the domino process again was the laccase-catalyzed oxidation of catechol 25 to o-quinone 26, which reacted through Michael addition with N,N-dimethyl barbituric add 31 to yield the intermediate 32. This compound underwent a further oxidation followed by cyclization through two Michael additions. Finally, laccase-catalyzed oxidation provided polycyclic dispiropyrimidinone 35 in excellent yield of 90% (Scheme 9.7). Recently, the same authors reported the synthesis of... 

Illustrations can be chosen among the vast amount of data, and applications to organic synthesis were reviewed.i As an example, when submitted to mechanical treatment, solid o,o -di-t-butyl catechol and the corresponding quinone react to give the semiquinone radical anion. Polymers, synthetic or natural (lignin), undergo cleavage, but polymerization can also be induced. ... 

Diphenyl selenoxide has been advocated for the oxidation of catechols to ortho-quinones in the synthesis of benzylisoquinoline alkaloids. ... 

Although the data above established that NAcDEE could undergo an a,p-desaturation on the side chain following oxidation to the quinone, we felt it desirable to see whether further information could be gleaned about the mechanism and rate of synthesis, since we were interested in this reaction as a model for the reactivity of peptidyl DOPA. The chemistry of catechols and their quinones is notoriously complex, and it was not obvious that a realistic extrapolation from this low molecular... 

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