Crystal structure of bis[3-methoxy-17β-estra-1,3,5(10)-trien-17-yl] oxalate

The title symmetrical steroid oxalate diester is substantially twisted about the central O2C—CO2 bond, leading to an overall shallow V-shape for the molecule, which may correlate with its reactivity under flash vacuum pyrolysis. C—H⋯O hydrogen bonds help to establish the packing.


Chemical context
The pyrolysis of esters possessing aliphatic -hydrogen atoms is a known route to alkenes via radical mediated -elimination (Brown, 1980). As part of our studies in this area , we now describe the crystal structure of the title compound, (I), an oxalate diester of 17--estradiol 3-methyl ether (Reck et al., 1986;Schö nnecker et al., 2000). Flashvacuum pyrolysis (FVP) of (I) at 873 K and 0.2 torr led to estratetraene 3-methyl ether in 47% yield.

Structural commentary
The atom labelling scheme (Fig. 1) for (I) relates equivalent atoms in the two halves of the molecule by adding 50, e.g. C1 and C51. The C19-C69 bond length of 1.513 (6) Å for the oxalate unit is exactly as expected for an sp 2 -sp 2 carboncarbon single bond but significantly shorter than the typical C-C bond length of about 1.57 Å in isolated oxalate ions (Dinnebier et al., 2003). The mean C-O C bond length is 1.324 Å and the mean C O bond length is 1.197 Å . The dihedral angle between the C19/O1/O2 and C69/O51/O52 planes of 61.5 (5) indicates a substantial twist. This leads to an overall shallow V-shaped conformation for the molecule, with the C18 and C68 methyl groups facing each other [C18Á Á ÁC68 = 4.64 Å ]. This could be significant in terms of the radicalreactivity of this molecule under FVP .

Supramolecular features
In the crystal, molecules are linked by weak C-HÁ Á ÁO interactions (Table 1). Interestingly, these three bonds all arise from one 'end' of the molecule. Two of these bonds are accepted by the same oxalate O atom and a three-dimensional network arises.

Database survey
In the closely related dehydroepiandrosterone oxalate diester (Cox et al., 2007), the dihedral angles between the CO 2 planes of the oxalate linkers in the two asymmetric molecules are 24.2 (3) and 51.46 (11) .
A search of the Cambridge Structural Database (Version 5.31; Allen & Motherwell, 2002) revealed four other structures containing an oxalate diester bridge between two fragments connected to the bridge by a secondary carbon atom. In C 22 H 34 O 4 polymorph-I (Barnes & Weakley, 2004a) the dihedral angle between the CO 2 groups in the oxalate fragment is 12.5 (9) and the bornyl substituents adopt a syn orientation. C 22 H 34 O 4 polymorph-II (Barnes & Weakley, 2004b) contains one-and-a-half molecules in the asymmetric unit, with the half-molecule completed by inversion symmetry, hence the oxalate bridge is planar by symmetry; in the complete molecule, the oxalate dihedral angle is 12.2 (5) . In both molecules, the bornyl substituents are in an anti orientation.

Synthesis and crystallization
The title compound was prepared by the method of Lotowski & Guzmanski (2005)   A view of the molecular structure of the title molecule, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. All the H atoms except those bonded to the chiral C atoms have been omitted for clarity.

Refinement
The crystal quality was only fair, which may correlate with the rather high R int value. The H atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and refined as riding atoms with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C). The methyl groups were allowed to rotate, but not to tip, to best fit the electron density. Experimental details are given in Table 2 (Farrugia, 2012); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

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.