Highly Oxidized Ellagitannins of and Their Oxidation-Reduction Disproportionation.

In the research on ellagitannin metabolism, two unique dehydroellagitannins, carpinins E () and F (), bearing dehydrohexahydroxydiphenoyl (DHHDP) and hydrated biscyclohexenetrione dicarboxyl ester (HBCHT) groups, were isolated from young leaves of . Upon heating in HO or treatment with pH 6 buffer at room temperature, and afforded the reduction product , isocarpinin A, with an ()-hexahydroxydiphenoyl (HHDP) group, suggesting the occurrence of redox disproportionation of the ()-DHHDP group. This was supported by the increase in production of in the pH 6 buffer solution by coexistence of epigallocatechin-3--gallate (), accompanied by oxidation of . In contrast, treatment of and with ascorbic acid yielded , carpinin A, with an ()-HHDP group. Upon heating with ascorbic acid, the HBCHT group was also reduced to an ()-HHDP group, and was converted to 2,3;4,6-bis()-HHDP glucose. In leaves of , the tannins and are dominant in young spring leaves, but compounds and become the major components of tannins in mature leaves. These results suggest that, in ellagitannin biosynthesis, oxidative coupling of the two galloyl groups first generates a DHHDP group, and subsequent reduction of DHHDP esters produces HHDP esters.