Gallic acid structure

The identification of non-peptidic lead structures remains a challenge. Screening of natural product extracts led to the identification of two po-lyhydroxylated biphenyls ((10a) and (10b), Figure 2.13) that show submicromolar inhibition of the viral protease [156]. A recent report discloses polyesters of glucose (11) and gallic acid (12) as micromolar inhibitors of the NS3 protease [157]. 

The remaining shlklmates in Table III also are relatively simple, well known compounds. The phenolic structures of vanillin (125) and gallic acid (127) and the prephenolic structures of the common quinic acid (128) and chlorogenic acid (129) make them candidates for physiological activity. Gallic acid is the monomer for tannins, biological polymers found in the cotton plant (15, 37). 

FIGURE 11.4 Hydrolyzable tannin. In the structure shown, five gallic acid units surround a sugar (hexose). 

Figure 11. Structures of Two Hydrolysis Products of Geraniin Brevifolin and Gallic Acid...

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). 

The phenolics include anthocyanins, anthraquinones, benzofurans, chromones, chromenes, coumarins, flavonoids, isoflavonoids, lignans, phenolic acids, phenylpropanoids, quinones, stilbenes and xanthones. Some phenolics can be very complex in structure through additional substitution or polymerization of simpler entities. Thus xanthones can be prenylated and flavonoids, lignans and other phenolics can be glycosylated. Condensed tannins involve the polymerization of procyaninidin or prodelphinidin monomers and hydrolysable tannins involve gallic acid residues esterified with monosaccharides. As detailed in this review, representatives of some major classes of plant-derived phenolics are potent protein kinase inhibitors. 

C.I. Natural Brown 6 (constitution not available), basic structure gallic acid (gallotannin 1), is based on a large variety of vegetable products having a high content of hydrolyzable tannin, which mostly form gallic acid when hydrolyzed. 

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