Purpurogallin Derivatives

Oxidation of fomentariol led to dehydrofomentariol (194) (322) which, together with the pyran derivatives anhydrofomentariol (195) (229) and anhydrodehydrofomentariol (196) (228), are minor constituents of F. fomentarius. The structures (193) to (196) are fully consistent with the H-n.m.r. spectra of these pigments and the position of the aldehyde side chain in dehydrofomentariol (194) was confirmed by X-ray analysis. The metabolite (194) has been converted to the pyran (196) with /7-toluenesulphonic acid in tetrahydrofuran (228). 

The occurrence of these purpurogallin derivatives seems to be restricted to Fomes fomentarius (38). 

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The oxidative ring cleavage undergone by purpurogallin quinone 14 is assumed to provide the excitation energy necessary to cause the tropolones to act as fluorescers. Perhaps dioxetane derivatives are key intermediates, as in the anthracene derivatives discussed in (IV. D.). 

Pyrogallol and some derivatives are oxidized to 3-hydroxy-o-benzoquinones which dimerize by 1,3-addition. The adducts react to purpurogallin by dehydrogenation and splitting off of CO2 (25, 26). By treating with alkali at 175°C. purpurogallin is transformed into 6,7,8-trihydroxynaphthalene-l-carboxylic acid (13, 22). Decarboxylation would be a further step in increasing aromatic character. 

Most pyrogallol derivatives did not precipitate either (Table III). A small precipitation occurred with pyrogallol (which may be considered as a hydroxyresorcinol) and it reacts with vanillin/HCl also (7). However, any additional substitution eliminates this reactivity, e.g., gallic acid (Table IV). Purpurogallin did precipitate with formaldehyde. It is not very soluble in the reaction mixture, and it is a tropolone. Structures of this type might interfere, but in tea and probably in wine the known tropolone derivatives are flavonoids (I, 25). 

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