Catechol B-rings

Figure 3. Polyphenolic patterns occurring in tannins. Key XVII, resorcinol A-pyrogallol B XVIII, resorcinol A-catechol B XIX, phtoroglu-cinol A-pyrogallol B and XX, phloroglucinol A-catechol B ring systems.

As expected, the catechol B-ring is more oxidizable than the resorcinol A-ring, and these results are in good agreement with the acid-base properties discussed previously on each ring, the more basic site is the most oxidable phenolic function. 

Coumaranones such as (31) have been suggested as intermediates in the synthesis of the yellow aurone pigments and it is apposite that a related structure (32) has recently been found in the heartwood of Acacia crombei. ° This represents the first report of a spirocoumaranone in nature. At one time, all known plant aurones had a catechol B-ring [e.g. (33)] and their synthesis seemed to be in some way related to this fact. However, an exception to this regularity has now appeared. Asen and Plimmer have discovered an aurone with a single hydroxy-group in its B-ring, namely (34), in yellow flowers of a Limonium cultivar, where it occurs in conjunction with (33) and the chalcone (26). 

Fig. 16.6 The 1,3-diphenyl propane skeleton of flavonoids and the numbering system for flavonoids. Three structural features optimise the radical scavenging properties of a flavonoid (i) an orto-dihydroxy structure of the B-ring (catechol) (ii) 2,3 double bond in conjugation with a 4-oxo group (iii) 3- and 5-hydroxy groups (Bors and Saran, 1987).

Oxidation of catechols in the presence of a protein may lead to extensive catechol-protein covalent coupling (Figure 9.4) as demonstrated in the case of the chlorogenic acid-BSA couple. Autoxidation of EGCG at pH 4.9 in the presence of Zn(II) cations was shown to generate semiquinone radicals (stabilized by Zn(II) binding) mainly on the B ring moiety. 

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). 

CCA preservatives interfere with the curing of any phenolic adhesive, but particularly so with tannin cold-sets. This is due to the higher capability of complexation Cu and Cr with ortho-diphenols (such as the catecholic and pyro-gallolic B rings of flavonoids). The problem is not grave under normal ambient gluing conditions, but it becomes more evident at ambient temperatures of 30 °C or higher. The problem, of course, lies with CCA as a wood preservative rather... 

When the B-ring possesses a catechol type structure, this nucleus acts mostly as a reducing agent. Once oxidized, it can undergo either coupled oxidation or Michael addition as a powerful electrophile, both mechanisms restore the catechol structure. The main grape anthocyanins, as their B-ring lacks the catechol hydroxylation pattern. 

An ortho-dihydroxy structure (catechol moiety) in the B-ring, which is the radical target site for flavonoids with a saturated C-2 and C-3 double bond, confers a high stability to the flavonoid aroxyl radical and participates in electron delocalization. 

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