Three dimeric anthracene derivatives from the fruits of Bulbine abyssinica

From the fruits of Bulbine abyssinica three new dimeric anthracene derivatives, (P)-8,9,1′,8′-tetrahydroxy-3,3′-dimethyl[10,7′-bianthracene]-1,4,9′,10′-tetraone (trivial name abyquinone A), (10R)-1,4,8,1′,8′-pentahydroxy-3,3′-dimethyl-[10,7′-bianthracene]-9,9′,10′(10H)-trione (trivial name abyquinone B), and (10R)-3′,4′-dihydro-1,4,8,3′,8′,9′-hexahydroxy-3,3′-dimethyl-[10,7′-bianthracene]-9,1′(10H,2′H)-dione (trivial name abyquinone C) were isolated. Despite their structural differences, these three compounds are connected to each other by the apparently biomimetic conversion of abyquinone C (a preanthraquinonylanthrone with two stereogenic centers) into B (an anthraquinonylanthrone with one stereogenic center) and finally into A (an axially chiral bianthraquinone) under mild conditions, involving a highly efficient center-to-axis chirality transfer. In addition, the known anthraquinones islandicin and chrysophanol were identified. The structures were determined on the basis of spectroscopical evidences, chemical transformations, and quantum chemical CD calculations.