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