Crystal structure of 3,6-dihydroxy-4,5-dimethylbenzene-1,2-dicarbaldehyde

The planar achiral title compound has C 2v symmetry and crystallizes in the chiral space group P21.

The title compound, C 10 H 10 O 4 , was synthesized from tetramethyl-1,4-benzoquinone. In the crystal, the almost planar molecule (r.m.s. deviation = 0.024 Å ) forms intramolecular hydrogen bonds between the aldehyde and hydroxy groups and exhibits C 2v symmetry. This achiral molecule crystallizes in the chiral space group P2 1 with intermolecular O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonding and C-HÁ Á Á and C OÁ Á Á interactions stabilizing the crystal packing.

Chemical context
A number of benzo-and naphthoquinone derivatives with one or two side chains being capable of alkylation after reduction were found to exhibit inhibitory activity against the growth of transplantable tumours in mice. Furthermore, inhibition of nucleic acid biosynthesis and of the activities of coenzyme Q mediated enzyme systems are also known for related compounds composed of 3,6-dihydroxy-4,5-dimethylbenzene-1,2-dicarbaldehyde (Lin & Loo, 1977;Lin et al., 1978). According to the literature, these compounds are synthesized from tetramethyl-1,4-benzoquinone (Lin & Loo, 1977;Lin et al., 1978). Here we report the molecular and crystal structure of an achiral derviative crystallizing in a chiral space group.

Structural commentary
The molecular structure of the title compound consists of a benzene ring substituted by two methyl groups, two hydroxy groups and two aldehyde groups (Fig. 1). The molecular point group symmetry is C 2v (H atoms excluded). The C-C bond lengths of the methyl substituents are 1.511 (2) and 1.508 (2) Å , the C-O bond lengths of the hydroxy substituents are 1.354 (2) and 1.350 (2) Å , and the C-C bond lengths of the aldehyde substituents are 1.464 (2) and 1.462 (2) Å . Two intramolecular O-HÁ Á ÁH hydrogen bonds between the hydroxy and aldehyde functions are observed (Table 1 and Fig. 1). The molecule is essentially planar (r.m.s. deviation = 0.024 Å ), with the largest deviation found for atom O2 [0.047 (1) Å ].

Supramolecular features
In the crystal, molecules are connected along the b axis by O-HÁ Á ÁO hydrogen bonds and along the c axis by C-HÁ Á ÁO hydrogen bonds (Table 1 and Fig. 2). As a result, chiral crystals composed of achiral molecules are formed. Many examples of such chiral crystals forming from achiral molecules have been reported for decades, but the prediction of chiral crystallization is still impossible (Koshima & Matsuura, 1998;Matsuura & Koshima, 2005).
The C8 O2 carbonyl group is stacked on top of the aromatic ring, with the O2Á Á ÁCg1 distance being 3.4846 (19) Å (Cg1 is the centroid of ring C1-C6).

Figure 2
A view of the O-HÁ Á ÁO hydrogen bonds (dashed lines) present in the crystal lattice of the title compound.

Figure 3
Part of the crystal packing showing the C-HÁ Á Á stacking interactions.

Figure 1
The molecular structure of the title compound, showing the atomnumbering scheme and displacement ellipsoids drawn at the 50% probability level. other and complexed with cobalt, nor with the title compound where the two aldehyde O atoms are involved in intramolecular hydrogen bonds and the two aldehyde H atoms are facing each other.

Synthesis and crystallization
A mixture of tetramethyl-1,4-benzoquinone (2.0406 g, 12.4 mmol) and concentrated piperidine (98.0%, 35 ml) was stirred at room temperature for 35 h. The mixture was evaporated and a white intermediate product was obtained. To a solution of the obtained intermediate product dissolved in acetic acid (18 ml), a mixture of CrO 3 (1.77 g) and 50% acetic acid (35 ml) was added dropwise at 353 K. After 10 min, the reaction mixture was poured onto crushed ice (100 g). The solution was filtered by vacuum filtration and a crude compound was obtained. The crude compound was dissolved in toluene and purified by silica column chromatography to afford 0.567 g (yield 23.5%) of the title compound as a yellow solid (single crystals served for X-ray analysis). IR (KBr, cm À1 ): 1633 (s), 3436 (m).

Refinement
Crystal data, data collection and structure refinement details are summarized in   Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008), WinGX (Farrugia, 2012), Mercury (Macrae et al., 2006) and publCIF (Westrip, 2010). 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.