Highly Oxidized Ellagitannins of Carpinus japonica and Their Oxidation–Reduction Disproportionation

• Published in: Journal of natural products (Washington, D.C.) Vol. 83; no. 11; pp. 3424 - 3434
• Main Authors: Kojima, Daisetsu, Shimizu, Kengo, Aritake, Kosuke, Era, Manami, Matsuo, Yosuke, Saito, Yoshinori, Tanaka, Takashi, Nonaka, Gen-ichiro
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society and American Society of Pharmacognosy 25.11.2020; Amer Chemical Soc
• Subjects: Chemistry, Medicinal; Life Sciences & Biomedicine; Pharmacology & Pharmacy; Plant Sciences; Science & Technology; HYDROLYZABLE TANNINS; DEHYDROHEXAHYDROXYDIPHENIC ACID; HEXAHYDROXYDIPHENIC ACID; GALLIC ACID; BLACK TEA; ESTERS; PLANTS; EPIGALLOCATECHIN; ENZYMATIC OXIDATION; GERANIUM-THUNBERGII SIEB
• ISSN: 0163-3864; 1520-6025
• DOI: 10.1021/acs.jnatprod.0c00893

In the research on ellagitannin metabolism, two unique dehydroellagitannins, carpinins E (1) and F (2), bearing dehydrohexa­hydroxy­diphenoyl (DHHDP) and hydrated biscyclo­hexenetrione dicarboxyl ester (HBCHT) groups, were isolated from young leaves of Carpinus japonica. Upon heating in H2O or treatment with pH 6 buffer at room temperature, 1 and 2 afforded the reduction product 3, isocarpinin A, with an (R)-hexahydroxy­diphenoyl (HHDP) group, suggesting the occurrence of redox disproportionation of the (S)-DHHDP group. This was supported by the increase in production of 3 in the pH 6 buffer solution by coexistence of epigallocatechin-3-O-gallate (15), accompanied by oxidation of 15. In contrast, treatment of 1 and 2 with ascorbic acid yielded 4, carpinin A, with an (S)-HHDP group. Upon heating with ascorbic acid, the HBCHT group was also reduced to an (S)-HHDP group, and 2 was converted to 2,3;4,6-bis­(S)-HHDP glucose. In leaves of C. japonica, the tannins 1 and 2 are dominant in young spring leaves, but compounds 3 and 4 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.