ABSTRACT
The chemical structures of the hydrolyzable tannins are basically composed of a central sugar core, typically a glucose unit, to which gallic acid moieties are esteri‚ed. β-Glucogallin is the simplest glucosyl gallate known and serves as a galloyl unit donor in the biosynthesis of the fully galloylated β-d-glucopyranose (β-PGG), which is itself considered to be the immediate precursor of the two subclasses of hydrolyzable tannins, that is, gallotannins and ellagitannins (ETs) (Figure 20.1) (Gross 1999, 2008; Niemetz and Gross 2005; Quideau et al. 2011). Gallotannins are the result of further galloylations of β-PGG and are characterized by the presence of one or more meta-depsidic digalloyl moieties, as exempli‚ed with the hexagalloylglucose 3-O-digalloyl-1,2,4,6-tetra-O-galloyl-β-dglucopyranose (1a in Figure 20.1). Alternatively, β-PGG can be subjected to intra-and intermolecular
20.6.2.4 About the C-Glucosidic Ellagitannins ..............................................................453 20.6.2.5 About the Flavano-Ellagitannins ..................................................................... 454
Acknowledgments ...................................................................................................................................455 References .............................................................................................................................................. 456
phenolic coupling processes that create connections between spatially adjacent galloyl residues by forming C-C biaryl and C-O diaryl ether bonds. The so-called hexahydroxydiphenoyl (HHDP) biaryl unit generated by intramolecular coupling is the structural characteristic that de‚nes hydrolyzable tannins as ETs. The nature of the atropisomeric form of these chiral biaryl motifs, such as the (S)-HHDP unit of the tellimagrandins or the (R)-HHDP unit of geraniin, is determined by the position of the galloyl motifs on the glucopyranose core in either its 4C1-or its 1C4-conformation. Besides, the HHDP motif is susceptible to many additional transformations, among which its oxidation leads to the dehydrohexahydroxydiphenoyl (DHHDP) unit characteristic of the dehydroellagitannin natural products, such as geraniin (Quideau and Feldman 1996; Khanbabaee and van Ree 2001a; Feldman 2005; Pouységu et al. 2011).
