3-Hydroxy-2-[(2-hydroxy-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(thiophen-3-yl)methyl]-5,5-dimethylcyclohex-2-en-1-one

The asymmetric unit of the title compound, C21H26O4S, consists of two independent molecules. In both molecules, intramolecular O—H⋯O hydrogen bonds stabilize the molecular structure. In the crystal, each molecule and its symmetry-related molecule by twofold rotation form a dimer through a pair of intermolecular C—H⋯O hydrogen bonds. In one of the molecules, the thiophene group is disordered over two sets of sites with occupancies of 0.735 (3) and 0.265 (3).

The asymmetric unit of the title compound, C 21 H 26 O 4 S, consists of two independent molecules. In both molecules, intramolecular O-HÁ Á ÁO hydrogen bonds stabilize the molecular structure. In the crystal, each molecule and its symmetry-related molecule by twofold rotation form a dimer through a pair of intermolecular C-HÁ Á ÁO hydrogen bonds. In one of the molecules, the thiophene group is disordered over two sets of sites with occupancies of 0.735 (3) and 0.265 (3).

Comment
We reported here the crystal structure of a bis compound which was synthesized from thiophene-3-carboxaldehyde and an active methylene compound, 5,5-dimethylcyclohexan-1,3-dione (dimedone) by using well-known Knoevenagel condensation reaction. This reaction was generally used in organic synthesis for C-C bond formation (Tietze & Beifuss, 1991) and active methylene compounds like dimedone was used as a precursor for many heterocyclic compounds (Suh et al., 2000;Choudhary et al., 2006).
There are two independent molecules in the the asymmetric unit ( Fig. 1) and all parameters are within normal ranges. In one of the molecules, the thiophene group was disordered over two positions with refined site occupancies  (Table 1) help to stabilize the molecular structure. In the crystal structure, the two sets of molecules are interconnected by two intermolecular C10A-H10A···O1A i and C16B-H16D···O4B ii hydrogen bonds to form dimers (Fig. 2).

Experimental
A mixture of thiophene-3-carboxaldehyde (4.46 mmol, 0.5 g) and dimedone (8.92 mmol, 1.25 g) in 20 ml of methanol was stirred overnight at room temperature. The completion of the reaction was monitored by TLC. After the reaction was completed, the crude product was separated, filtered, washed with methanol and dried. The isolated product was further purified by recrystallization from chloroform-methanol (1:1 v/v) to give the pure title compound in 85% yield.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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.
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 > 2sigma(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.