A new polymorph of physcion

The structure of the title compound, 7-meth­oxy-2-methyl-4,5-dihy­droxy­anthracene-9,10-dione, C₁₆H₁₂O₅, was originally reported by Ulický et al. [Acta Cryst. (1991). C47, 1879-1881] in the space group P2₁2₁2₁ [polymorph (Io)]. The new polymorph, (Im), crystallizes in the space group P2₁/c. The mol­ecular structures are closely similar, with both -OH groups forming intra­molecular hydrogen bonds to one of the neighbouring quinone O atoms, thus slightly lengthening this C=O bond; the pattern of C-C bond lengths in the ring system is consistent with some contribution from a resonance form with a negative charge at the hydrogen-bonded quinone O atom and an aromatic region around its neighbouring C atoms. The packing of (Im) is simpler than the extensively crosslinked pattern of (Io), with mol­ecular tapes connected by classical (but three-centre) and `weak' hydrogen bonds, parallel to [20].