crenata) (Okuda and Ito 2011). This is a flavo-galloyl unit bound glucosidically to
C-1, with an additional three hydrolysable ester bonds to a D-glucose-derived
open-chain polyol. More example of complex tannin is epigallocatechin gallate
(28), which is widely present in tea leafs and responsible for variety of medicinal
activities or camelliatannin A (29) (Fig. 21.4) obtained from Camellia japonica.
21.4 Biosynthetic Pathways of Tannins
The biosynthesis of tannins follow two different synthetic pathways for hydrolysable tannins and condensed tannins. The biosynthesis of hydrolysable tannins
comprises via the key intermediate formation of 1,2,3,4,6-pentagalloyl glucose
(Hagenah and Gross 1993). The starting material or the precursor is quinic acid.
Gallotannins or depsidic metabolites can be formed by the galloylation of the
O
HO
HO
OH HO
OH
OH
O
O
O
O
O
O
O
O
O
O
HO
O
OH
OH
O
O
OH
OH
OH
HO
HO
HO
OH
HO
HO
O
O
HO
9
10
O
OH
HO
O
O
OH
HO
OH
O
HO
HO
OH
O
O
HO
OH
OH
11
O
O
OH
OH
OH
O
O
O
HO
OH
O
O
OH
OH
O
HO
HO
OH HO
OH
OH
O
O
12
O
HO
O
O
O
O
O
O
O
O
OH
OH
HO
HO
HO
HO
HO
HO
HO
O O
OH
OH
OH
OH
OH
OH
13
O
O
O
OH
OH
OH
O
O
O
O
OH
OH
OH
HO
OH
OH
O
O
O
O
HO
HO
HO
HO
HO
HO
15
OH
OH
HO
OH
HO
HO
HO
O
O
O
O
HO
HO
OH
O
O
O
O
OH
OH
OH
HO
O
O
OH
HO
14
Fig. 21.2 Structures of some ellagitannins
726
S. Mal and D. Pal