Catechol-containing flavonoids

Hendrickson HP, Kaufman AD, Lunte CE (1994) Electrochemistry of catechol-containing flavonoids. J Pharmac Biomed Anal 12 325-334. 

The first chapter by Reihmann and Ritter reviews the recent developments of peroxidase-catalyzed oxidative polymerization of phenol and derivatives with a phenolic OH group. The importance of enzymatic polymerization in general is emphasized. Properties of product polyphenols, characteristics of the enzyme catalysis, and significance of the process and the product are discussed. The second chapter by Uyama and Kobayashi is concerned with the oxidative polymerization of polyphenols, which are compounds containing more than two phenolic OH groups. These compounds include catechols and flavonoids... 

Swain and Goldstein (6, 7) noted a rather large difference in the molar color yield from different phenols with the Folin-Denis reagent. They attributed this to differences in relative oxidation-reduction potentials of the different phenols, but under their conditions pyrogallol gave about half the color of catechol and more than resorcinol. However, they also reported that the molar absorptivity produced by a flavonoid was approximately equal to the sum of the values for the separate phenolic moieties which it contained. 

As indicated by the structures of these molecules, the A ring contains resorcinol phenolic hydroxyls, while the B ring contains the catechol or adjacent phenolic hydroxy groups, both of which would be expected to be highly reactive in resin formation. This high reactivity would also hold for the condensed tannins present in the bark extract, since they are polymeric flavonoids (14). 

Flavonoids are widely distributed in fruits and vegetables and are very common nutritional supplements as antioxidants. The results on antioxidant activities of simple catechols provide a useful basis for evaluating results for the many, more complex natural compounds containing the catechol structure, such as the flavonoids, steroidal catechols and hormonal catecholamines. There are several reviews on the antioxidant properties of flavonoids and several reports on experimental " and theoretical evidence linking their antioxidant properties to the catechol moiety usually found in their structure. The basic flavonoid structure (29) is shown in Chart 1, with a few selected examples (30-36) from different groups to illustrate some of the relationships between their detailed structures and related antioxidant properties. Efforts to elucidate these relationships are hampered by their very low solubility in non-polar solvents, and the tendency of some researchers to employ metal ions as initiators of oxidation in aqueous media so that one cannot distinguish between their action as chain-breaking... 

The chemistry is characterised by a number of phenolic constituents, including flavonoids (Hyperoside, spireoside, kaempferol glycoside), salicylates (salicylalde-hyde, gaultherin, methylsalicylate, salicin, salicylic acid), tannins (water-soluble, hydrolysable types, also some catechols). The volatile oil contains salicylates, benzaldehyde, ethylbenzoate, heliotropin, phenylacetate and vanillin. 

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