Crystal structure and absolute configuration of (3aR,3′aR,7aS,7′aS)-2,2,2′,2′-tetramethyl-3a,6,7,7a,3′a,6′,7′,7′a-octahydro-4,4′-bi[1,3-benzodioxolyl], obtained from a Pd-catalyzed homocoupling reaction

The crystal structure of a homocoupled compound with absolute configuration 3aR,3′aR,7aS,7′aS was determined. The molecule contains two similar moieties composed of two fused rings. Its supramolecular structure is controlled mainly by C—H⋯O interactions.


Chemical context
Over the last few years, we have focused our efforts on the synthesis of vinylsulfimines as precursors in -lactamization reactions to generate asymmetric pyrrolidone derivatives which are of interest in medicinal chemistry (Silveira et al., 2012(Silveira et al., , 2014Silveira & Marino, 2013;Pereira et al., 2015). Encouraged by our previous experience in functionalizing halo-cyclohexadiendiols Labora et al., 2010;Heguaburu et al., 2010;Labora et al., 2008), we synthesized a vinylic sulfide (molecule 3 in Fig. 1) from protected iodo-cyclohexenediol (molecule 1 in Fig. 1). This latter compound was obtained firstly by regioselective reduction of iodocyclohexadienediol derived from the biotransformation of iodobenzene (Gonzá lez et al., 1997). The obtained compound was treated with lithium isopropylthiolate in the presence of 5% of Pd (PPh 3 ) 4 as catalyst to obtain the vinyl sulfide in 85% yield. Surprisingly, one of the attempts to perform this reaction proceeded to afford traces of the homocoupled product (molecule 2 in Fig. 1). Considering this finding, we decided to prepare this new compound via a palladium-catalyzed homocoupling reaction of the vinylic iodide (molecule 1 in Fig. 1), mediated by indium, according to the Lee protocol (Lee et al., 2005). Herein, we report this new synthetic method and the crystal structure of the title compound.

Structural commentary
The absolute configuration of the title compound ( Fig. 2) was determined to be 3aR,3 0 aR,7aS,7 0 aS by considering the synthetic pathway and confirmed by X-ray diffraction on the basis of the anomalous dispersion of light atoms only. The molecule is built up from two chemically identical moieties , respectively]). In fragment A, the flap of the envelope is oriented away from the five-membered ring while in fragment B, both C7 and the five-membered ring are on the same side of the plane of the envelope, making them conformationally different. The dihedral angle between the leastsquare planes through the six-membered rings is 43.15 (9) while the dihedral angles between the five and six-membered rings are 69.31 (10) and 76.95 (10) in A and B, respectively, leaving the two five-membered rings on opposite sides of the C4A-C4B bond and almost in the same plane, normal to the bisector plane of both six-membered rings.

Supramolecular features
In the crystal, weak C22A-H22FÁ Á ÁO3B i [symmetry code: (i) x, y, z À 1] interactions link the molecules in chains running along [001], see Fig. 3   The molecular structure of the title compound, showing anisotropic displacement ellipsoids drawn at the 50% probability level.

Synthesis and crystallization
A mixture of the vinylic iodide (molecule 1 in Fig. 1.) (140 mg, 0.5 mmol), Pd(PPh 3 ) 4 (10% wt., 14.4 mg, 0.025 mmol), indium (28.7 mg, 0.25 mmol), and lithium chloride (31.8 mg, 0.75 mmol) in dry THF (2 mL) was stirred at reflux for 4 h under a nitrogen atmosphere. The reaction mixture was quenched with NaHCO 3 (sat. aq.). The aqueous layer was extracted with ethyl acetate (3 Â 20 mL), and the combined organic phases were washed with brine, dried with Na 2 SO 4 , filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (hexanes/ ethyl acetate 95:5) to give the desired homocoupled product (43.5 mg, 57%). Crystals suitable for X-ray crystallographic analysis were obtained by dissolving the title compound in the minimum volume of ethyl acetate, adding hexanes until the solution became slightly turbid, and slowly evaporating the solvent at room temperature. 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to C were placed in calculated positions (C-H = 0.93-0.98 Å ) and included as riding contributions with isotropic displacement parameters set to 1.2-1.5 times the U eq of the parent atom. program(s) used to solve structure: SHELXS2014 (Sheldrick, 2008(Sheldrick, , 2015; program(s) used to refine structure: SHELXL2014 (Sheldrick, 2008(Sheldrick, , 2015; molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C7A 0.6166 (