O-Dihydroxy compounds

The photoaddition of aldehydes to o-quinones is a general reaction leading to the formation of products which are to be regarded as derivatives of methylene ethers of o-dihydroxy compounds (A).198 194 247 Such compounds exhibit ring-chain tautomerism and are generally assigned open-chain structures (B).188 242... 

Fluorescence on long-wave UV (only for o-dihydroxy compounds) ... 

Antimony salts, in acid solution, react with aromatic o-dihydroxy compounds (e.g., p3n ocatechol, pyrogBllol) to produce slightly soluble, white, or light yellow crystalline precipitates which can be formulated as cyclic esters of antimonous acid. The yellow dyestuff 9-methyl-2,3,7-trihydroxy-6-fluorone (I), which can be viewed as a derivative of pyrocatechol, forms a red antimonous compound (II). 

Linear polyethers derived from three or more molecules of aliphatic dihydroxy compounds, particularly when the chain length exceeds ten units, are most conveniently named by open-chain replacement nomenclature. For example, CH3CH2—O—CH2CH2—O—CH2CH3 could be 3,6-dioxaoctane or (2-ethoxy)ethoxyethane. 

Potential 2,5-dihydroxy compounds (185) exist in the dicarbonyl forms (186). Succinic anhydride (186 Z = O) on silylation is converted into 2,5-bis(trimethylsilyloxy)furan (187) the latter compound readily participates in Diels-Alder addition reactions (80TL3423). Reaction of thiosuccinic anhydride (186 Z = S) with the triphenylphosphorane Et02CH=PPh3 gives a product which exists in the aromatic form (188) (75LA1967). 

Catechol and other o-dihydroxy aromatic systems may be protected by the formation of the methylenedioxy compound, most conveniently achieved by the following PTC procedure.44 A mixture of water (20 ml), dibromomethane (0.15 mol) and Adogen 464 (1 mmol), is vigorously stirred and heated to reflux. The air in the system is displaced by nitrogen. A solution of the appropriate o-dihydroxybenzene (0.1 mol) and sodium hydroxide (0.25 mol) in water (50 ml) is added at such a rate that addition is complete after 2 hours. After addition is complete, the reaction mixture is stirred and refluxed for a further hour. The product is isolated by standard work-up procedures. 

Furthermore, some fused tropolones give typical reactions of o-dihydroxy or o-diketo (389) tautomers they form o-dimethoxy compounds (e.g., 418a) or derivatives of dicarbonyl compounds (dioximes and quinoxalines like 513), respectively (65MI3). Another diketone exhibits remarkable stability against enolization to form indolotropolone 324 (Scheme 83), due to steric interaction (78BCJ3579). 

All these scavenging capacities are based on one fact The polyphenol compounds are attacked by superoxide or hydroxyl radicals predominantly at the o-dihydroxy site. The semiquinone radicals formed from the reaction of the o-dihydroxy structures with the radicals are quite stable, probably because of the presence of hydrogen bonding. However, if the PPG has a methoxy radical at the ortho position, the aroxyl radical derived from its reaction with superoxide or hydroxyl radical is less stable owing to the absence of intramolecular hydrogen bonding [114]. 

Reaction of the ArC,o unit with 1-pentynylmagnesium bromide occurred smoothly in tetrahyrofuran containing cuprous chloride without formation of alienic impurities. An analogous type of route was used for the synthesis of 1,3-dimethoxy-5-[(ZZ)-pentadec-8,11-dienyl]benzene, (15 2)-cardol dimethyl ether (ref. 105). For the synthesis of (15 2)-urushiol, 1,2-dihydroxy-3-[(ZZ)-penta-dec-8.11-dienyl]benzene and its dimethyl ether, 7-(2,3-dimethoxyphenyl)heptyl bromide and the corresponding 2,3-dihydroxy compound, obtained by demethylation with boron tribromide, have been used (ref. 149). 

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