(4R)-3-Hydroxy-7-isopropyl-4-methyl-5,6-dihydrobenzofuran-2(4H)-one

In the title compound, alternatively called α-hydroxy-γ-alkylidenebutenolide, C12H16O3, two independent molecules (A and B) crystallize in the asymmetric unit in each of which the 5,6-dihydrobenzo ring has an envelope conformation. The torsion angle along the butadiene chain in the γ-alkylidenebutenolide core is −177.9 (2)° for molecule A and 179.9 (2)° for molecule B. In the crystal, O—H⋯O hydrogen bonds between hydroxyl and carbonyl groups of adjacent independent molecules form dimers with R 2 2(10) loops.

In the title compound, alternatively called -hydroxy-alkylidenebutenolide, C 12 H 16 O 3 , two independent molecules (A and B) crystallize in the asymmetric unit in each of which the 5,6-dihydrobenzo ring has an envelope conformation. The torsion angle along the butadiene chain in the -alkylidenebutenolide core is À177.9 (2) for molecule A and 179.9 (2) for molecule B. In the crystal, O-HÁ Á ÁO hydrogen bonds between hydroxyl and carbonyl groups of adjacent independent molecules form dimers with R 2 2 (10) loops.
Supporting information for this paper is available from the IUCr electronic archives (Reference: JJ2187).

Introduction
Butenolides are an important class of organic compounds present in natural products that have been studied for over 50 years (Rao, 1964). Most of them exhibit interesting pharmacological activities, such as antibacterial, anticancer, antibiotic and phospholipase A2 inhibition activity (Ma et al, 1999). During the last decades γ-alkylidenebutenolides have been considered as attractive synthetic targets due to their structural diversity and biological properties. As a result, several synthetic procedures have been developed for the preparation of these substances Langer, et al., 2001;Xu, et al., 2007;Almeida, et al., 2010;Park, et al., 2012). Also, α-Hydroxy-γ-alkylidenebutenolides are particularly suitable building blocks for analogues of pharmacologically relevant natural products (Langer & Saleh, 2000). Herein, we report the crystal structure of a bicyclic α-hydroxy-γ-alkylidenebutenolide based on l-Menthone, an inexpensive and accessible reagent from the chiral pool, which is also an important structural motif found in natural products. To the best of our knowledge, there is only one report on the preparation of a similar γ-alkylidenebutenolide, but no structural data were presented (Schneider & Viljoen, 1997).

Synthesis and crystallization
Sodium hydride (60% dispersion in mineral oil, 2.44 g, 0.061 mmol) was stirred for 15 minutes in 200 mL of freshly distilled THF. Then a mixture of l-Menthone (7.71 g, 0.050 mmol) and di-ethyl oxalate (3.36 g, 0.023 mmol) in 100 mL of THF was added drop by drop. The resulting mixture was heated to reflux for 2 days. After this time, the solvent was removed by rotary evaporation. The crude reaction product was added to an ice-hydrochloric acid (1M) mixture and extracted with chloroform (3 x 50 mL). The organic layer was dried with MgSO 4 , filtered and the solvent was removed under vacuum to afford orange oil, which was purified by Kugelrohr distillation (413 °K, 5 x 10 -2 mbar, bulbs cooled with dry ice), to obtain the desired product as a yellow oil that solidifies (1.95 g, 41%). Suitable crystals for X-ray diffraction analysis were obtained by slow diffusion of hexane into a saturated solution of the compound in dichloromethane cooled at 263 °K for 3 days. Elemental analysis calculated for C 12 H 16 O 3 ×1/3H 2 O: C, 67.27 %, H 7.84 %. Found: C, 67.38 %, H 7.70 %.

Refinement
The positions of the two oxygen bound hydrogen atoms H3 and H6 were taken from a difference fourier synthesis and their positional parameters were refined. All other H atoms were included in calculated positions (C-H = 0.93 Å for aromatic H, C-H = 0.96 Å for methyl H, C-H = 0.98 Å for methylene H, and C-H = 1.00 Å for tertiary H), and refined using a riding model with U iso (H) = 1.2 Ueq or U iso (H) = 1.5 Ueq (for methyl groups) of the carrier atom.

Figure 1
View of independent molecules A and B of the title compound, C 12 H 16 O 3 , showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 50% probability level.    65, 20.23, 20.40, 21.95, 27.89, 28.25, 31.22, 128.25, 128.75, 135.33, 140.89, 167.80. [α] D 20 = +4.32 (c 0.018, CH 3 OH) 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.