Ethyl (4aR*,7S*,8S*,8aS*)-1-oxo-7-phenyl-3,4,4a,7,8,8a-hexahydro-1H-isochromene-8-carboxylate

In the title compound, C18H20O4, both the tetrahydropyranone ring and the cyclohexene ring adopt envelope conformations. The crystal packing is stabilized by weak intermolecular C—H⋯O hydrogen bonding.

In the title compound, C 18 H 20 O 4 , both the tetrahydropyranone ring and the cyclohexene ring adopt envelope conformations. The crystal packing is stabilized by weak intermolecular C-HÁ Á ÁO hydrogen bonding.

Related literature
The title compound is a derivative of 1-oxo-hexahydro-1Hisochromene, which has been reported as a key intermediate towards the total syntheses of natural products such as eleutherobin and tetronothiodin, see: Kim et al. (2000); Jung et al. (2000); Page et al. (2003). For microwave-assisted intramolecular Diels-Alder cycloaddition, see: Wu et al. (2006Wu et al. ( , 2007; Wang et al. (2009

Comment
The title compound, C 18 H 20 O 4 , refers to the derivative of 1-oxo-hexahydro-1H-isochromenes, which has been reported as a key intermediate towards total syntheses of natural products such as eleutherobin (Kim et al., 2000;Jung et al., 2000) and tetronothiodin (Page et al., 2003). The title compound has recently been obtained during microwave-assisted intramolecular Diels-Alder cycloaddition along with a minor diastereomer with a 74:26 diastereomeric ratio (Wu et al., 2006(Wu et al., , 2007Wang et al., 2009). The compound has four stereogenic centers but crystallizes as a racemate as indicated by the centrosymmetric space group. We report here its crystal structure.
In the molecular structure of the title compound ( Fig. 1), there are one pyranone ring and one cyclohexene ring. Both of the two rings C1-C2/C6-C9 and C2-C3/O2/C4-C6 adopt envelope conformation. The crystal packing is stabilized by weak non-classical intermolecular C-H···O hydrogen bonds (Table 1).

Experimental
A 10 mL pressured process vial was charged ethyl (3E,5E)-6-phenylhexa-3,5-dien-1-yl fumarate (172.0 mg, 0.57 mmol) followed by adding MeCN (4 mL). The loaded vial was then sealed with a cap containing a silicon septum and put into the cavity of a technical microwave reactor with the temperature measured by an IR sensor. After heating at 453 K for 1 h, the reaction mixture was concentrated under reduced pressure. The residue was purified by flash column chromatography (silica gel, 20% EtOAc in petroleum ether) to furnish the title compound (95.0 mg, 52%), along with a minor diastereomer (32.0 mg, 19%), as colorless needles. mp 392-394 K (EtOAc-hexane). Single crystals, as a racemate, suitable for X-ray diffraction of the title compound were grown at ambient temperature in the mixed solvent of ethyl acetate and hexane (v:v = 10:1).

Refinement
H atoms were placed in calculated positions with C-H = 0.93-0.98 Å, and refined in riding model with U iso (H) = 1.5U eq (C) for methyl and 1.2U eq (C) for the others.  Fig. 1. The molecular structure of the title compound with the atom numbering scheme. Displacement ellipsoids are drawn at 40% probability level. H atoms are presented as a small spheres of arbitrary radius.

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.
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 Rfactors(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.