9,10-Dihydroxy-4,4-dimethyl-5,8-dihydroanthracen-1(4H)-one

In the title molecule, C16H16O3, the ring system is planar and an intramolecular hydrogen bond is present. The molecular packing is dominated by an intermolecular hydrogen bond and by π-stacking interactions [interplanar separation 3.8012 Å].

In the title molecule, C 16 H 16 O 3 , the ring system is planar and an intramolecular hydrogen bond is present. The molecular packing is dominated by an intermolecular hydrogen bond and by -stacking interactions [interplanar separation 3.8012 Å ].

Related literature
For related literature, see: Allen (2002); Araya-Maturana et al. (2006, 2007; Desiraju (2002); Joshi et al. (1997); Valderrama et al. (1993).  Table 1 Hydrogen-bond geometry (Å , ). Data collection: SMART-NT (Bruker, 2001); cell refinement: SAINT-NT (Bruker, 1999); data reduction: SAINT-NT; program(s) used to solve structure: SHELXTL-NT (Sheldrick, 2008); program(s) used to refine structure: SHELXTL-NT; molecular graphics: SHELXTL-NT; software used to prepare material for publication: SHELXTL-NT. The molecule consists of three six-membered carbon rings fused trough atoms 4a, 9a in a side and trough carbons 8a, 10a in the other, to give rise an anthracene skeleton, substituted with an oxo, a gem-dimethyl and two hydroxyl groups at positions 1, 4, 9 and 10 respectively, as shown in Scheme 1. The central ring is aromatic, and double bonds are also found between carbons 2 -3 and 6 -7. This double bonds distribution leads to the core to be strictly planar. In fact, all the carbon atoms in the skeleton lies on the crystallographic mirror plane m from space group Pnma. The same happens with the oxo oxygen atom O1 and the hydroxyl groups O2 and O3. Interestingly, the hydroxyl hydrogen atoms H2 and H3 display a trans correlation. The planarity of the molecule together with the proximity of the oxo oxygen atom (O1) and the hydroxyl hydrogen atom (H2) leads to the presence of an intramolecular O-H···O hydrogen bond with O···O of 2.5172 (18) Å, suggesting a rather strong bond (Desiraju, 2002), which is present still in CDCl 3 solution, as indicated by NMR (Araya-Maturana et al, 2007). Few structures with this or some closely related pattern of substitution could be found in Cambridge Structural database (v 5.29, Allen, 2002), being 1,4-Dihydro-9,10-anthrahydroquinone the best, probably the one to the best of our knowledge, example (Joshi et al., 1997).

Experimental
The molecular packing is also dominated by the hydrogen bond, this time between vicinal molecules. As depicted in Figure 2, a planar chain is produced by means of the interacion of the "terminal" hydroxyl hydrogen atom H3 with the oxo oxygen O1 from the nearest molecule (x -1, y, z), in a "head to tail" arrangement in the [100] direction. The O···O distance, 2.8022 (18) Å, suggest a weaker interaction. Layers of molecules are defined in the packing by putting this chains one together the other, with no strong interaction between them. Any of the chain is contained in the x, 1/4, z plane. The next layer, x, 3/4, z is separated from the first in b/2, 3.8012 Å, a typical value for the aromatic π-stacking interaction.

Experimental
The molecule was synthesized by the Diels-Alder reaction between 8,8-dimethylnaphthalene-1,4,5(8H)-trione and butadiene). The cycloaddition takes place exclusively at external quinone doble bond affording the corresponding adduct I-a (See Scheme 2). Enolization of the adduct I-a with silicagel in toluene yield the hydroquinone I. (Valderrama et al., 1993). The 1 H-NMR spectrum in CDCl 3 of I exhibits a sharp singlet at 13.08 p.p.m. indicating that hydrogen bonding is also present in solution. This characteristic is important regarding antitumor and antioxidant properties.

Refinement
The hydrogen atoms positions were calculated after each cycle of refinement with SHELXL (Bruker,1999) using a riding model for each structure, with C-H distances in the range 0.95 to 0.99 Å and O-H equal to 0.84 Å. U iso (H) values were set equal to 1.5U eq of the parent carbon atom for methyl groups and hydroxyl hydrogen atoms, while 1.2U eq for the others. Fig. 1. Molecular structure diagramas for I showing atom numbering scheme. Displacement ellipsoids are at 33% probability level and H atoms are shown as spheres of arbitrary radii. Letter a corresponds to the names of fused carbon atoms according to the nomenclature rules. Letter B to symmcode x, -y + 1/2, z.   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (