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. The compounds were characterized by 1H and 13C NMR and the structures determined using single-crystal X-ray diffraction.

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 ÁCH 3 CN. The anthraquinone core of the molecule is essentially planar and both hydroxy groups participate in intramolecular O-HÁ Á ÁO (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-HÁ Á ÁO(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.

Structural commentary
The asymmetric unit of 1,4-dihydroxy-2-propoxy anthraquinone (1), crystallized from acetonitrile solvent, contains a single anthraquinone molecule and one acetonitrile solvate molecule as shown in Fig. 1. The two intramolecular hydrogen bonds (Table 1) are typical for the 1,4-dihydroxy anthraquinones and 1-hydroxyanthraquinones. These hydrogen bonds are maintained in chloroform solution, as shown by the chemical shift of 13.47 and 13.56 ppm for the two hydroxyl protons. The anthraquinone moiety is planar, with an average root mean square (r.m.s.) deviation of atoms C1 to C14 of 0.021 Å , in which the maximum deviation from the plane defined by atoms C1 to C14 is 0.044 (1) Å for C9. The propyl chain is angled slightly above the plane of the anthraquinone moiety, with deviations of 0.043 (2), 0.143 (2) and 0.247 (2) Å for atoms C15, C16 and C17, respectively, from the plane defined by atoms C1-C14. The acetonitrile is angled towards H10 with a N1Á Á ÁC10 distance of 3.401 (2) Å . The final difference map shows several peaks of 0.2 to 0.7 e Å À3 in the anthraquinone plane that suggest the presence of minor whole-molecule disorder in which the anthraquinone is translated in the plane and/or flipped over.

Figure 1
Molecular structure of (1) with the included acetonitrile. Displacement ellipsoids of non-H atoms are drawn at the 50% probability level and H atoms are shown as circles of arbitrary size.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms potentially involved in hydrogen-bonding interactions were located by difference methods and initially restrained in the refinement with O-H = 0.84 (2) Å and with U iso (H) = 1.2U eq (O). Other H atoms were included in the refinement at calculated positions, C-H = 0.95 Å for aromatic, C-H = 0.99 Å for methylene and C-H = 0.98 Å for methyl hydrogens with U iso (H) = 1.2U eq (C) for aromatic and methylene H atoms and 1.5U eq (C) for methyl H atoms.  For both structures, data collection: SMART (Bruker, 2014); cell refinement: SMART (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015b); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: X-SEED (Barbour, 2001).

1,4-Dihydroxy-2-propoxyanthraquinone acetonitrile monosolvate (1)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.