3-Acetyl-4-hydroxy-6,7-dimethyl-2H-chromen-2-one

In the title coumarin derivative, C13H12O4, the 2H-chromene ring system is essentially planar [maximum deviation = 0.047 (1) Å]. An intramolecular hydrogen bond is observed between the hydroxy and the ketonic O atoms. In the crystal, pairs of intermolecular C—H⋯O hydrogen bonds link inversion-related molecules into dimers. Additional intermolecular C—H⋯O hydrogen bonds further interconnect these dimers into two-dimensional arrays incorporating R 2 2(9) ring motifs.

In the title coumarin derivative, C 13 H 12 O 4 , the 2H-chromene ring system is essentially planar [maximum deviation = 0.047 (1) Å ]. An intramolecular hydrogen bond is observed between the hydroxy and the ketonic O atoms. In the crystal, pairs of intermolecular C-HÁ Á ÁO hydrogen bonds link inversion-related molecules into dimers. Additional intermolecular C-HÁ Á ÁO hydrogen bonds further interconnect these dimers into two-dimensional arrays incorporating R 2 2 (9) ring motifs.

Experimental
Acetyl chloride (1 ml) was added to a solution of 4-hydroxy-6,7-dimethylcoumarin (5.2 mmol, 1.0 g) in pyridine (10 ml) which contains piperidine (one drop) on ice bath. The reaction mixture was kept at room temperature for 7 days. The solution was then poured into ice-cold water and hydrochloric acid was added to afford the precipitate, which was washed with water, dried and recrystallized from ethanol to get the pure title compound in 70% yield.

Refinement
Atom H1O2 was located in a difference Fourier map and allowed to refine freely. The remaining H atoms were placed in their calculated positions, with C-H = 0.93-0.96 Å, and refined using a riding model, with U iso = 1.2 or 1.5 U eq (C). The rotating group model was applied to the methyl groups.  3-Acetyl-4-hydroxy-6,7-dimethyl-2H-chromen-2-one

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.