dimethyl (5R,5'R,6R,6'R,10aS,10a'S)-1,1',5,5',9,9'-hexahydroxy-6,6'-dimethyl-8,8'-dioxo-5,5',6,6',7,7',8,8'-octahydro-10aH,10a'H-2,2'-bixanthene-10a,10a'-dicarboxylate