Structure and synthesis of phlobatannins related to the (4β,6:4β,8)-bis-fisetinidol-catechin profisetinidin triflavanoid☆

Abstract Additional members of the class of natural phlobatannins, resulting from stereoselective C-ring isomerization of the 2,3- trans -3,4- cis -flavan-3-ol moieties in the (4β,6:4β,8)-bis-fisetinidol-catechin triflavanoid have been identified from natural sources. These comprise three functionalized hexahydrodipyrano[2,3- f :2′,3′- h ]-chromenes and a fisetinidol-(4β,10)-tetrahydropyrano[2,3- f ]chromene. The complex structures of these novel natural condensed tannins were confirmed by synthesis via base-catalysed pyran rearrangement of the 4- O (E)-methyl ether of their postulated biogenetic triflavanoid precursor.