Diethyl 9,10-endo-ethano-9,10-dihydro- anthracene-11,11-dicarboxylate

The title compound, (I) (Fig. 1), was created as an intermediate in the synthesis of 2-methylene malonic acid diethyl ester, (II). The alkene produced in the absence of anthracene is very unstable and polymerizes easily. The presence of the anthracene acts to trap the monomer in a Diels±Alder reaction and puri®cation of (I) prior to thermolysis allows the generation of (II) (by a retro-Diels±Alder reaction) in a much more stable form. The presence of excess maleic anhydride in this reaction ensures that the released anthracene is consumed by the formation of an anthracene-maleic anhydride adduct and is not free to regenerate (I). Thus, this type of reaction may be useful in the trapping of alkenes and allow for easier puri®cation.


Comment
The title compound, (I) (Fig. 1), was created as an intermediate in the synthesis of 2-methylene malonic acid diethyl ester, (II). The alkene produced in the absence of anthracene is very unstable and polymerizes easily. The presence of the anthracene acts to trap the monomer in a Diels±Alder reaction and puri®cation of (I) prior to thermolysis allows the generation of (II) (by a retro-Diels±Alder reaction) in a much more stable form. The presence of excess maleic anhydride in this reaction ensures that the released anthracene is consumed by the formation of an anthracene-maleic anhydride adduct and is not free to regenerate (I). Thus, this type of reaction may be useful in the trapping of alkenes and allow for easier puri®cation.

Experimental
A round-bottomed¯ask was ®tted with a still head and condenser and diethyl malonate (9.70 g, 9.2 ml, 61 mmol), anthracene (12.00 g, 67 mmol), paraformaldehyde (3.64 g, 0.12 mol), copper(II) acetate monohydrate (0.60 g, 3.0 mmol), acetic acid (50 ml) and xylene (50 ml) were quickly added. The reaction mixture was heated at 383 K for 15 h and a clear dark-green solution resulted. The temperature was increased in order to distil off the acetic acid, then the reaction mixture was cooled to room temperature and ®ltered under suction. The ®ltrate was retained and the xylene evaporated on a rotary evaporator to yield a green oil which was left to crystallize. Puri®cation was carried out by recrystallization from hot hexane. Filtration and washing with ice-cold hexane (25 ml) resulted in the pure anthracene adduct (I) ( 36.4, 43.9, 49.6, 60.0, 61.7, 123.3, 125.7, [De Keyser et al. (1988) give 125.68 and 125.74], 126.4, 139.8, 144.0 and 170.2.
All H atoms were geometrically placed in idealized locations and re®ned as riding on their carrier C atoms with CÐH distances set to 0.95, 0.98, 0.99 and 1.00 A Ê for aromatic, sp 2 , terminal sp 3 and bridgehead sp 3 hybrid C atoms, respectively. The constraint U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (methyl C) was applied as appropriate.
We thank the EPSRC UK National Crystallography Service (University of Southampton) for the data collection. Detail of a chain of molecules of (I) linked by CÐHÁ Á ÁO interactions.

Figure 1
View of (I) (50% displacement ellipsoids). H atoms are drawn as small spheres of arbitrary radius. 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.