Synthesis and crystal structures of two purpurin derivatives: 1,4-dihy­droxy-2-propoxyanthra­quinone and 2-but­oxy-1,4-di­hydroxy­anthra­quinone

The title compounds were obtained by deprotonation of 1,2,4-tri-hydroxy-anthra-quinone (purpurin) using sodium hydride followed by reaction with either 1-bromo-propane or 1-bromo-butane. 1,4-Dihy-droxy-2-propoxyanthra-quinone crystallizes as a 1:1 solvate from aceto-nitrile, C17H14O5·CH3CN. The anthra-quinone core of the mol-ecule is essentially planar and both hy-droxy groups participate in intra-molecular O-H⋯O (carbon-yl) hydrogen bonds. The propyl chain is angled slightly above the plane of the anthra-quinone moiety with a maximum deviation of 0.247 (2) Å above the plane for the terminal carbon atom. In contrast, 2-but-oxy-1,4-di-hydroxy-anthra-quinone, C18H16O5, crystallizes from nitro-methane with two independent mol-ecules in the asymmetric unit. The anthra-quinone core of each independent mol-ecule is essentially planar and both hy-droxy groups on both mol-ecules participate in intra-molecular O-H⋯O(carbon-yl) hydrogen bonds. The butyl chain in one mol-ecule is also angled slightly above the plane of the anthra-quinone moiety, with a maximum deviation of 0.833 (5) Å above the plane for the terminal carbon atom. In contrast, the butyl group on the second mol-ecule is twisted out of the plane of the anthra-quinone core with a torsion angle of 65.1 (3)°, resulting in a maximum deviation of 1.631 (5) Å above the plane for the terminal carbon atom.