Structural revisions of macrocyclic ellagitannin dimers bridged by two hexahydroxydiphenoyl groups

Ellagitannin dimers, 1,1′;2,2′-diHHDP-diglucose and 3,3′;4,4′-diHHDP-6,6′-digalloyl-diglucose, were recently isolated from the wood of the sweet chestnut (Castanea sativa Mill.). These compounds were originally reported to possess unprecedented macrocyclic dimeric structures bridged by two hexahydroxydiphenoyl (HHDP) groups. However, a reinvestigation of their spectroscopic data revealed that these compounds correspond to known ellagitannin monomers, namely 2,3-O-(S_a)-HHDP-d-glucopyranose and gemin D (3-O-galloyl-4,6-O-(S_a)-HHDP-d-glucopyranose), respectively. A comparison of the originally reported NMR data for 1,1′;2,2′-diHHDP-diglucose and 3,3′;4,4′-diHHDP-6,6′-digalloyl-diglucose with those of authentic 2,3-O-(S_a)-HHDP-d-glucopyranose and gemin D, combined with DFT-based NMR chemical shift calculations, unequivocally confirmed these structural revisions.