(3R,3aR,6R,6aR)-Hexahydrofuro[3,2-b]furan-3,6-diyl dibenzoate

The title compound, C20H18O6, prepared from d-mannitol by a two-step procedure, is a functionalized fused bis-tetrahydrofuran. In the central fragment, consisting of two fused tetrahydrofuran rings, one O atom and its two adjacent C atoms, a methylene and a bridgehead C atom, are disordered over two sets of sites with an occupancy ratio of 0.735 (9):0.265 (9). In the major component, the ring containing the disordered O atom is a half-chair conformation with twisted methylene and benzoate-substituted C atoms, whereas the other ring has a half-chair or T-form conformation. In the minor component, the ring with the disordered O atom has an envelope conformation, with the O atom as the flap, and the other ring has a half-chair conformation, with the O atom and the other bridgehead CH atom being twisted. The two aromatic rings are inclined to one another by 20.00 (12)°. In the crystal, adjacent molecules are linked via C—H⋯π interactions, forming chains propagating along [010].

The title compound, C 20 H 18 O 6 , prepared from d-mannitol by a two-step procedure, is a functionalized fused bis-tetrahydrofuran. In the central fragment, consisting of two fused tetrahydrofuran rings, one O atom and its two adjacent C atoms, a methylene and a bridgehead C atom, are disordered over two sets of sites with an occupancy ratio of 0.735 (9):0.265 (9). In the major component, the ring containing the disordered O atom is a half-chair conformation with twisted methylene and benzoate-substituted C atoms, whereas the other ring has a half-chair or T-form conformation. In the minor component, the ring with the disordered O atom has an envelope conformation, with the O atom as the flap, and the other ring has a half-chair conformation, with the O atom and the other bridgehead CH atom being twisted. The two aromatic rings are inclined to one another by 20.00 (12) . In the crystal, adjacent molecules are linked via C-HÁ Á Á interactions, forming chains propagating along [010].
Cg is the centroid of the C15-C20 ring. Carbohydrate-based synthons have been used in a number of syntheses of complex natural chiral substances (Hanessian, 1993). Manipulation of the oxygenation pattern and stereochemistry of simple sugars proved to be a powerful mean to use nature-generated chirality. In this context D-mannitol plays an important role as a readily available chiral building block in organic synthesis (Babjak et al., 2002). In addition, mannitol and its derivatives are widely used as chiral reagents and chiral auxiliaries (Masaki et al., 1999) and can be transformed into biologically active and pharmaceutically important compounds (Lohray et al., 1999). As a continuation of our interest in oxidative processes mediated by transition metals oxo-species (Piccialli, Oliviero, Borbone et al., 2013;Piccialli, Tuzi, Oliviero et al., 2013;Piccialli, D′Errico, Borbone et al., 2013;Piccialli et al., 2012) we were interested in the synthesis and reactivity of functionalized fused bis-tetrahydrofurans.
In the title compound, molecule consists of two cis-fused tetrahydrofurane rings substituted at C2 and C5 positions ( Fig.1). A local C 2 symmetry of the molecule is observed at the junction. Tetrahydrofurane rings are disordered in two different positions (occupancy factor refined to 0.735 (9) for the A position and 0.265 (9) for the B position). Both disordered O1/C1/C2/C3A/C4 and O1/C1/C2/C3B/C4 rings adopt an envelope conformation with O1 at the flap. The disordered O2A/C3A/C4/C5/C6A and O2B/C3B/C4/C5/C6B rings are both in the envelope conformation that differ for the atom at the flap (C6A and O2B, respectively). The molecule of the title compound does not contain strong H-bonding donor and the crystal packing is stabilized by normal weak intermolecular interactions.

Experimental
The title compound was prepared according to a two-steps procedure starting from D-mannitol. Treatment of the latter with benzoyl chloride in pyridine for 16 h, followed by reaction with catalytic amounts of p-toluenesulfonic acid in 1,1,2,2-tetrachloroethane at reflux (Hockett et al., 1946) led to title compound as an amorphous white powder, after chromatography (CHCl 3 to CHCl 3 -MeOH, 95:5). The whole process consists of a double migration of the primary benzoates to the adjacent alcoholic functions followed by double water elimination and bis-cyclization (C1/C4 and C3/C6 ether ring closure). Recrystallization from methanol gave elongated white crystals suitable for X-ray analysis.

Refinement
All H atoms were generated stereochemically and refined by the riding model with U iso =1.2×U eq of the carrier atom.
Some constraints were introduced in the last stage of refinement to handle the disorder of one furane group (SAME and ISOR command of SHELXL program). In the absence of strong anomalous scatterer the Flack parameter is not meaningful. Data were merged using MERG 3 instruction and the absolute configuration was assigned on the assumption that the original mannitol configuration had been preserved during the acid-catalysed process.

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

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