Gallotannins are hydrolysable tannins with a polyol core (referring to a compound with multiple hydroxyl groups) substituted with 10-12 gallic acid residues. Gallotannins contain the characteristic meta-depside bonds (1.91) between gallic acid residues. This bond is more labile than an aliphatic ester bond, and can be methanolyzed with a weak acid in methanol. In contrast, methanolysis of an aliphatic ester bond requires methanol with a strong mineral acid and heat.
The most commonly found polyol is D-glucose, although some gallotannins contain catechin and triterpenoid units as the core polyol. Gallotannins with a D-glucose core are synthesized from 1,2,3,4, 6-pentagalloylglucose (1,2,3,4,6-penta-0-galloyl-(-D-glucopyranose; 1.92; see also Chapter 3). An example of a gallotannin is the hexagalloylated compound 2-O-digalloyl-1,3,4,6-tetra-O-galloyl-( -D-glucopyranose (1.93), where the additional gallic acid residue is located on C2 of the glucopyranose ring.
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