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 Ulickýet al. Acta Cryst. (1991). C 47 , 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].