1,2,3,4-Tetramethylcyclopent-2-ene-1,4-diol

The title compound, C9H16O2, crystallizes with two molecules in the asymmetric unit. The structure displays intermolecular O—H⋯O hydrogen bonding.

The title compound, C 9 H 16 O 2 , crystallizes with two molecules in the asymmetric unit. The structure displays intermolecular O-HÁ Á ÁO hydrogen bonding.
Financial support by the Otto-von-Guericke-Universitä t Magdeburg is gratefully acknowledged.

S1. Comment
In the solid state, alcohols generally form hydrogen-bonded networks resulting in a variety of ring, chain, or helix structures (Brock & Duncan, 1994). The hitherto unknown title compound, 1,2,3,4-tetramethylcyclopen-2-ene-1,4-diol, was obtained in minor quantities (less than 5% isolated yield) in the form of colorless crystals during a preparation of 1,2,3,4-tetramethylcyclopentadiene according to the literature (Fendrick et al., 1988). The structure of the title compound is shown in Figure 1. Dimensions are available in the archived CIF. Especially notable is the hydrogen-bond network in the crystal structure. As depicted in Figure 2, four molecules of 1,2,3,4-tetramethylcyclopen-2-ene-1,4-diol are connected via hydrogen-bonds to give cyclic tetramers. Further hydrogen-bonding between adjacent tetrameric units results in an extended hydrogen-bond network.

S2. Experimental
Crystals of the title compound were obtained as a minor by-product during the synthesis of 1,2,3,4-tetramethylcyclopentadiene according to the literature preparatio (Fendrick et al., 1988).

S3. Refinement
H atoms bonded to C were refined with fixed individual displacement parameters [U(H) = 1.2 U eq (C) or U(H) = 1.5 U eq (C methyl )] using a riding model with C-H(methylen) = 0.99 Å or C-H(methyl) = 0.98Å, respectively. The H atoms bonded to O were refined isotropically.

Figure 1
The molecule of the title compound in the crystal. Displacement ellipsoids represent 50% probability levels. H-Atom radii are arbitrary.

Figure 2
The hydrogen-bond network.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.41 e Å −3 Δρ min = −0.23 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 R-factors(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.