Dimethyl 7-methoxytetracyclo[6.4.0.02,4.03,7]dodeca-1(12),5,8,10-tetraene-3,4-dicarboxylate

The title compound, C17H16O5, is a previously unreported substituted semibulvalene cage compound (that is, a tricyclic hydrocarbon formed from one cyclopropane and two cyclopentene rings which also has one double bond fused to a benzene ring). It has one methoxy substituent attached to the bridgehead C atom that links only the two cyclopentene rings and two methyl carboxylate groups located on the C atom shared by all three non-benzene rings and that shared only between the cyclopropane and the cyclopentene rings. The stereochemistry of the two enantiomers (racemate) that assemble in each unit cell is RRRS and SSSR. In the crystal, molecules are linked via C—H⋯O hydrogen bonds and C—H⋯π interactions, forming double-layered sheets lying perpendicular to the a axis.

The title compound, C 17 H 16 O 5 , is a previously unreported substituted semibulvalene cage compound (that is, a tricyclic hydrocarbon formed from one cyclopropane and two cyclopentene rings which also has one double bond fused to a benzene ring). It has one methoxy substituent attached to the bridgehead C atom that links only the two cyclopentene rings and two methyl carboxylate groups located on the C atom shared by all three non-benzene rings and that shared only between the cyclopropane and the cyclopentene rings. The stereochemistry of the two enantiomers (racemate) that assemble in each unit cell is RRRS and SSSR. In the crystal, molecules are linked via C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions, forming double-layered sheets lying perpendicular to the a axis.

Experimental
The title compound (I) is the major product from the acetone sensitized irradiation of dimethyl 1-methoxy-4-hydro-1,4ethenonaphthalene-2,3-dicarboxylate (II). The melting range of (I) is 382-384 K. The barrelene (II) was of interest in connection with a study of polar substituents in pericyclic reactions (Bender et al., 1975) and was synthesized from the Diels-Alder reaction between 1-methoxynaphthalene and dimethyl acetylenedicarboxylate, as outlined in Figure 3.

Refinement
All hydrogen atoms were located on a difference map. Hydrogen atoms attached to carbon are treated as riding, with C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for methyl, C-H = 1.00 Å and U iso (H) = 1.2U eq (C) for methine and C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for aromatic and alkene H atoms. The highest residual peak is only a fraction of the electron density of a single H atom, 0.31 e.Å -3 , and is located between C1 and C5.

Figure 1
The molecular structure of (I) with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radius.

Figure 3
Scheme showing the photochemical preparation of (I) from the barrelene (II) as well as other preparative chemical steps. 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.