Glutathione-mediated conversion of the ellagitannin geraniin into chebulagic acid

Geranlin (1), a widely distributed ellagitannin having a dehydrohexahydroxydiphenoyl (DHHDP) ester moiety, was converted into chebulagic acid (2), an ellagitannin having a chebuloyl ester moiety, which was reported to be a potent inhibitor of DNA topoisomerases. This was achieved by addition of the thiol group of glutathione to the six-membered acetal ring form of the DNHDP moiety (1a) with concomitant hydrolytic ring cleavage and subsequent reductive desulfurization with Raney nickel. The concurrent addition of the thiol to the five-membered acetal ring form of the DHHDP group (1b) generated the product 14 and its precursor 13, which are structurally related to naturally occurring ellagitannins isolated from Euphorhiaceous plants, Thus, the rearrangements observed in these reactions may be relevant to the metabolism of DHHDP esters in plants.