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.