Reactions of Ellagitannins Related to Their Metabolism in Higher Plants

Ellagitannins comprise a group of hydrolyzable tannins that contain hexahydroxydiphenoyl (HHDP), dehydrohexahydroxydiphenoyl (DHHDP), or their biogenetically related esters. Based on structural comparisons and the implicit assumption that polyphenols are susceptible to oxidation, it has been generally accepted that HHDP is produced via simple oxidative coupling of two galloyl (3,4,5-trihydroxybenzoyl) groups and further oxidation of HHDP to give DHHDP. However, the seasonal changes in ellagitannin composition of some plant leaves, and the non-enzymatic reactions of DHHDP esters isolated from plants, suggest that oxidative coupling of two galloyl groups first affords a DHHDP group, and subsequent, non-enzymatic redox disproportionation generates HHDP. The redox disproportionation of the DHHDP moiety and related quinone dimers was first suggested in the 1960s; however, the oxidation products that are produced simultaneously with reduction products are still unknown. This chapter describes various reactions involving DHHDP that are related to ellagitannin metabolism. In addition, the oxidative degradation of C -glycosidic ellagitannins is discussed.