Formation of Dehydrohexahydroxydiphenoyl Esters by Oxidative Coupling of Galloyl Esters in an Aqueous Medium Involved in Ellagitannin Biosynthesis

Abstract

Hexahydroxydiphenoyl (HHDP) and dehydrohexahydroxydiphenoyl (DHHDP) groups are the major acyl components of ellagitannins, which are polyphenols whose biosynthesis have attracted considerable attention; however, the mechanisms of the production of HHDP and DHHDP in the ellagitannin biosynthesis have not been clarified. With the aim of elucidating such a mechanism, this study investigates the CuCl2‐mediated oxidation of simple galloyl derivatives in an aqueous medium. It is shown that the oxidation of methyl gallate affords a DHHDP‐type dimer, whose reduction with Na2S2O4 yields an HHDP‐type dimer. However, the oxidation of the HHDP‐type product over CuCl2 does not afford the parent DHHDP ester. The oxidation of 1,4‐butanediol digallate under the same conditions produces a DHHDP‐type product via the intramolecular coupling of galloyl groups. These results strongly suggest that the DHHDP group is the initial product of the oxidative coupling of two galloyl groups in the ellagitannin biosynthesis, and subsequent reductive metabolism affords HHDP esters.

Enzymatic Oxidation of Tea Catechins and Its Mechanism

“…The redox potential of the galloyl group is higher than that of B rings and its reactivity with o-quinones is comparatively low. A total of 60-80% of the total tea catechins possess galloyl esters located at the C-3 hydroxy group and oxidation of galloyl groups may be important to the formation of black tea pigment [26]; only limited examples of oxidative coupling of galloyl groups have been reported [17,[27][28][29]. Besides, enzymes preferentially oxidize the catechol B-rings, and the resulting quinone subsequently oxidizes the pyrogallol rings for the redox potential of which is lower than that of the catechol rings [30][31][32][33].…”

Conformationally Flexible Ellagitannins: Conformational Analysis of Davidiin and Punicafolin via DFT Calculation of 1H NMR Coupling Constants

“…4 The DHHDP group is also produced by CuCl2-mediated oxidation of galloyl ester derivatives in aqueous media. 5 These results strongly indicate that the DHHDP group is the initial product of the oxidative coupling of two galloyl groups in ellagitannin biosynthesis and the subsequent reductive metabolism yields HHDP esters (Figure 1a). 4,5 Glucose derivatives can adopt various conformations, such as chair (C), boat (B), skew-or twist-boat (S), envelope (E), and half-chair (H).…”

“…5 These results strongly indicate that the DHHDP group is the initial product of the oxidative coupling of two galloyl groups in ellagitannin biosynthesis and the subsequent reductive metabolism yields HHDP esters (Figure 1a). 4,5 Glucose derivatives can adopt various conformations, such as chair (C), boat (B), skew-or twist-boat (S), envelope (E), and half-chair (H). 6 Many ellagitannins are likely biosynthesized from a common precursor, 1,2,3,4,6-penta-O-galloyl-β-D-glucose (1) with a 4 C1 conformation (Figure 1b).…”

“…Corilagin (5), a desgalloyl analogue of 4, is an ellagitannin with a flexible conformation; it was reported to be present as an equilibrium mixture of B1,4 and O,3 B (or 1 C4) conformational states in DMSO-d6 and a slightly perturbed O,3 B (or 1 C4) one in acetone-d6. 10a,b Moreover, its JH,H values change in DMSO-d6 depending on the temperature.…”

Phytochemistry and pharmacology of sea buckthorn (Elaeagnus rhamnoides; syn. Hippophae rhamnoides): progress from 2010 to 2021

“…In this way tellimagrandins II and casuarictin are formed. The addition of more galloyl units, combined with the oxidative coupling reactions (giving HHDP and dehydrohexahydroxydiphenoyl (DHHDP) groups), the removal of gallic acid residues, other modifications, as well as oligomerization lead to formation of plethora of compounds (Ascacio-Valdes et al 2011; Salminen 2014 ; Yamashita et al 2021 ).…”