Tetramethyl 1,4-dimethyl-13,14-dioxapentacyclo[8.2.1.14,7.02,9.03,8]tetradeca-5,11-diene-5,6,11,12-tetracarboxylate

In the title compound, C22H24O14, the relative stereochemistry at the cyclobutane ring is cis-anti-cis and the methyl groups in the bicyclic rings are syn to each other. The two carboxylate groups attached to the same —C=C— bond are disordered over two sets of sites in a 0.603 (2):0.397 (2) ratio. In the crystal, weak C—H⋯O hydrogen bonds connect molecules into C(12) chains along [001] incorporating R 2 2 2(10) rings.

In the title compound, C 22 H 24 O 14 , the relative stereochemistry at the cyclobutane ring is cis-anti-cis and the methyl groups in the bicyclic rings are syn to each other. The two carboxylate groups attached to the same -C C-bond are disordered over two sets of sites in a 0.603 (2):0.397 (2) ratio. In the crystal, weak C-HÁ Á ÁO hydrogen bonds connect molecules into C(12) chains along [001] incorporating R 2 2 2 (10) rings.   Table 1 Hydrogen-bond geometry (Å , ). We have recently investigated the Ru-catalyzed isomerization and dimerization reaction of oxanorbornadiene compounds (Ballantine et al., 2009). When dissolved in 1,2-dichloroethane in the presence of Cp*Ru(COD)Cl, 1-methyl-2,3-dimethyl-7-oxabicyclo[2,2,1]hepta-2,5-diene-2,3-dicarboxylate will dimerize into (I) (Fig. 1). The desired product was resolved using fractional crystallization in methanol. The stereochemistry and regioschemistry of the product was determined by this single-crystal X-ray analysis. The only dimer product obtained was found to have a cis-anti-cis stereochemistry at the cyclobutane ring of the dimer, and the two Me groups in the bicyclic rings were found to be syn to each other.

Related literature
The molecular structure of (I) is shown in Fig. 2. Two of the carboxylate groups attached to the same -C═C-bond were refined as disordered over two sets of sites with refined occupancies 0.603 (2) and 0.397 (2). In the crystal, weak C-H···O hydrogen bonds connect molecules into C(12) chains (Bernstein et al., 1995) Lough et al., 2012a,b) are reported in the two following papers.

Experimental
1-Methyl-2,3-dimethyl-7-oxabicyclo[2,2,1]hepta-2,5-diene-2,3-dicarboxylate (45 mg, 0.20 mmol) was weighed into an oven-dried vial, purged with nitrogen and transferred into a Dry Box. In the Dry Box, Cp*Ru(COD)Cl (10 mol%) was added to another oven dried vial and dissolved in 1,2-dichloroethane (0.3 ml). The Ru-catalyst was then transferred into the vial containing the 7-oxanorbornadiene. The vial was sealed with a screw cap and removed from the Dry Box. The reaction was heated at 333 K with stirring for 18 h. The crude product was purified by column chromatography (EtOAc:hexanes=2:3) followed by recrystallization in hexanes to give the dimer (I). Slow evapotation of a solution of (I) in hexanes gave colourless plates.

Refinement
Hydrogen atoms were placed in calculated positions with C-H distances of 0.98 and 1.00 Å. They were included in the refinement in a riding-model approximation with U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ). Constraints were applied to both the geometry and displacement parameters of the atoms in the disorder components using the FLAT, SADI and EADP instructions in SHELXL (Sheldrick, 2008).

Figure 1
Reaction scheme

Figure 2
The molecular structure of (I) showing 30% probability ellipsoids.  Part of the crystal structure showing weak hydrogen bonds as dashed lines. The disorder is not shown.  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.