2-[(1R,3S)-8-[(1R,3S)-3-(carboxymethyl)-5,10-dihydroxy-1-methyl-6,9-dioxo-3,4-dihydro-1H-benzo[g]isochromen-8-yl]-5,10-dihydroxy-1-methyl-6,9-dioxo-3,4-dihydro-1H-benzo[g]isochromen-3-yl]acetic acid