(3R,3R,4S,4aS)-2,2,3,3,4,4,9,9-Octahydro-1,1,4α,4,8,8-hexahydroxy-3β,3-dimethyl-9,9-dioxo-7,7-bi[4aH-xanthene]-4aα,4a-dicarboxylic acid dimethyl ester