Hexamethyl 13,14-dioxapentacyclo[8.2.1.14,7.02,9.03,8]tetradeca-5,11-diene-1,4,5,6,11,12-hexacarboxylate

In the title compound, C24H24O14, the stereochemistry at the cyclobutane ring is cis-anti-cis and the –COOMe groups in the bicyclic rings are syn to each other. The molecule lies on a twofold rotation axis. In the crystal, weak C—H⋯O hydrogen bonds connect molecules into chains along [001], forming R 2 2(10) rings.

In the title compound, C 24 H 24 O 14 , the stereochemistry at the cyclobutane ring is cis-anti-cis and the -COOMe groups in the bicyclic rings are syn to each other. The molecule lies on a twofold rotation axis. In the crystal, weak C-HÁ Á ÁO hydrogen bonds connect molecules into chains along [001], forming R 2 2 (10) rings.

Experimental
The molecular structure of (I) is shown in Fig. 2. The molecule lies on a twofold rotation axis. In the crystal, weak C-H···O hydrogen bonds connect molecules into one-dimensional chains (Fig. 3) along [001] forming R 2 2 (10) rings (Bernstein et al., 1995).
For related structures, see the two preceding papers (Lough et al., 2012a,b).
Experimental 1,2,3-Trimethyl-7-oxabicyclo[2,2,1]hepta-2,5-diene-1,2,3-tricarboxylate (316 mg, 1.2 mmol) was weighed into an ovendried 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 (2 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 methanol to give the dimer (I). Colourless needles were grown by slow evaportation of a solution of (I) in methanol.

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 ).

Figure 2
The molecular structure of (I) showing 30% probability ellipsoids (symmetry code (a): -x + 2, y, -z + 1/2). 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.