ET’s complexity and diversity are directly linked to their biosynthetic variability, and there are limitless possible structures as a result thereof. In fact, more than 1000 ETs have been identified to date. Some ET structures, characteristic groups and their precursor, β-pentagalloyl glucose, are depicted in
Figure 2. ETs are formed via oxidative C-C coupling of at least two galloyl units of the β-pentagalloyl glucose (
Figure 2), leading to an axially chiral HHDP unit
[1]. Further steps can lead to the formation of a second HHDP group (e.g., Casuarictin,
Figure 2) or to the cleavage of the formed HHDP or galloyl groups (e.g., Corilagin,
Figure 2). Trimer and tetramer forms of the galloyl group can result from a further oxidative coupling. Such is the case of Castalagin and Vescalagin (
Figure 2), which have a nonahydroxytriphenoyl (NHTP) group, also known as flavogallonyl. HHDP groups can also suffer further oxidation to form other units, such as dehydrohexahydroxydiphenoyl (DHHDP) (e.g., Mallotusinic acid in
Figure 2) or chebuloyl (e.g., Chebulagic acid in
Figure 2). C-O bonding of HHDP groups is another possibility, resulting in sanguisorboyl, tergalloyl and valoneoyl groups (
Figure 2), among others
[25]. Thus, via the oxidative C-O coupling between galloyl and hexahydroxydiphenoyl moieties, ET monomers can form dimers, trimers and tetramers with molecular weights up to several thousands of Da (e.g., Sanguiin H-6, a Casuarictin dimer, in
Figure 2). The nature of the bonds between monomers, either biphenyl or diarylether, sets up a method for their classification
[25]. Lastly, ETs can give rise to hybrid structures by joining with other classes of molecules: e.g., Epiacutissimin B (
Figure 2), a flavano-ellagitannin, has epicatechin at the C-1 center of the open-chain glucose core
[26].
It is certain that the pentagalloyl glucose oxidation pathway plays a central role in ellagitannins biosynthesis, but differing structural principles have been recognized for this class, which still leave many gaps, not only in the identification of enzymes catalyzing the synthesis of different linkage types, but also regarding some physiological aspects, such as seasonal variation of metabolite concentrations and enzyme activities
[27]. Detailed postulations on ET’s biosynthesis fall outside of the scope of this review and can be found elsewhere
[1][27]. Additionally, a detailed discussion on structural revisions of some ETs can be found in a recent review
[3], reinforcing once again the complexity and structural diversity of this tannin class.