Ellagitannins are also hydrolysable tannins derived from pentagalloylglucose (1.92), but unlike gallotannins, they contain additional C-C bonds between adjacent galloyl moieties in the pentagalloylglucose molecule. This C-C linkage is formed through oxidative coupling between the two adjacent galloyl residues, and results in the formation of a hexahydroxydiphenoyl (HHDP) unit, which can have either the S- (1.94) or the ^-configuration (1.95).
The chirality is the result of the limited free rotation around the axis of the C-C bond due to the two ester bonds between the galloyl residues and the polyol (indicated by the wavy bonds in 1.94 and 1.95), combined with the presence of the ortho-substituents that create steric hindrance.
The name ellagitannins is derived from ellagic acid (1.96), which is formed spontaneously from hexahydroxydiphenic acid (1.94/1.95) in aqueous solution via an intra-molecular esterification reaction.
With the glucopyranose molecule in the 4Ci conformation (a chair conformation with C4 above the plan, as shown in (1.92)), the most common linkages are between galloyl residues at the 2- and 3-positions of the glucopyranose ring, and/or between those at the 4- and 6-positions. These are referred to as Group A ellagitannins. In addition, ellagitannins with the less common 3,4-linkage, such as identified in the compound cercidinin A from the bark of Cercidiphyllum japonicum are included in this group (Tanaka et al., 2001). Group B ellagitannins have a glucopyranose molecule in the energetically less favorable 1C4 chair or boat conformations. In this case 1,6-, 1,3-, 2,4- or 3,6-C-C linkages can be formed between galloyl residues. Figure 1-4 displays the possible different configurations. It should be noted that it is possible to have combinations of different linkages, such as, for example, 3,6- and 2,4-linkages.
The configuration of the HHDP esters (R or S; as shown in Figure 1-4) varies depending on the position of the HHDP unit, and is in the energetically most favorable configuration. This is dictated by the stereochemistry of the sugar molecule (Haslam and Cai, 1993). Aside from the glucopyranose ring, there are also ellagitannins consisting of an open-chain polyol. The open chain ellagitannins identified so far all contain a 2,3-linked HHDP unit (Haslam and Cai, 1993).
Further modification of the HHDP unit is possible. Adjacent galloyl residues can undergo further oxidative coupling with participation of the aromatic hydroxyl groups. The valoneoyl unit (1.97), for example, arises from the linkage between a galloyl residue with an HHDP-unit. Furthermore, the meta- and para-hydroxyl groups on one of the galloyl-moieties can be oxidized, resulting in a dehydro-HHDP unit (1.98).
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