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-tri­hydroxy­anthra­quinone (purpurin) using sodium hydride followed by reaction with either 1-bromo­propane or 1-bromo­butane. 1,4-Dihy­droxy-2-propoxyanthra­quinone crystallizes as a 1:1 solvate from aceto­nitrile, C(17)H(14)O(5)·CH(3)CN. The anthra­quinone core of the mol­ecule is essentially planar and both hy­droxy groups participate in intra­molecular O—H⋯O (carbon­yl) hydrogen bonds. The propyl chain is angled slightly above the plane of the anthra­quinone moiety with a maximum deviation of 0.247 (2) Å above the plane for the terminal carbon atom. In contrast, 2-but­oxy-1,4-di­hydroxy­anthra­quinone, C(18)H(16)O(5), crystallizes from nitro­methane with two independent mol­ecules in the asymmetric unit. The anthra­quinone core of each independent mol­ecule is essentially planar and both hy­droxy groups on both mol­ecules participate in intra­molecular O—H⋯O(carbon­yl) hydrogen bonds. The butyl chain in one mol­ecule is also angled slightly above the plane of the anthra­quinone 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 mol­ecule is twisted out of the plane of the anthra­quinone 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.