3,5-Di-O-benzoyl-1,2-O-isopropylidene-α-d-ribo-hexos-3-ulo-1,4:3,6-difuranose

The title compound, C23H22O8, is a binary benzoyl ester whose nucleus consists of a fused system made up of a methylenedioxy ring and two tetrahydrofuran rings. One of the benzoyl ester groups is attached at the junction of the two tetrahydrofuran rings. The other is attached to the outer tetrafuran ring. Both the benzoyl ester groups are in an axial conformation with respect to the outer tetrhydrofuran ring. In the crystal, molecules are linked by two weak C—H⋯O hydrogen bonds, forming a chain running parallel to the a axis.

The title compound, C 23 H 22 O 8 , is a binary benzoyl ester whose nucleus consists of a fused system made up of a methylenedioxy ring and two tetrahydrofuran rings. One of the benzoyl ester groups is attached at the junction of the two tetrahydrofuran rings. The other is attached to the outer tetrafuran ring. Both the benzoyl ester groups are in an axial conformation with respect to the outer tetrhydrofuran ring. In the crystal, molecules are linked by two weak C-HÁ Á ÁO hydrogen bonds, forming a chain running parallel to the a axis.

Related literature
For details of the synthesis and absolute configuration of the nucleus, see: Tronchet & Bourgeois (1971). For applications of the nucleus, see: Xavier et al. (2009);Rajwanshi et al. (1999). For structure of a bicyclo-glycosyl compound, see: Zhang et al.  Table 1 Hydrogen-bond geometry (Å , ). The nucleus of molecule (I), Figure 1, consists of three fused rings, a methylenedioxy ring which is linked to two fused tetrahydrofuran rings. The methylenedioxy ring and a tetrahydrofuran ring are oriented in opposite directions with respect to the central tetrahydrofuran. One of the benzoyl ester groups is attached at the junction of the two tetrahydrofuran rings.
The other is attached to the outer tetrafuran ring. Both the benzoyl ester groups are in an axial conformation with respect to the outer tetrhydrofuran ring.
The molecules are linked by two weak C-H···O hydrogen bonds, Table 1, to form a chain which runs parallel to the a-axis, Figure 2.

Refinement
All H atoms were placed geometrically and treated as riding on their parent atoms with C-H are 0.93Å(aromatic), 0.96Å(methyl), 0.97Å(methylene) and 0.98Å(aliphatic) with U iso (H) =1.2U eq (C).

supplementary materials sup-2
In the absence of any significant anomalous scatterers in the molecule, attempts to confirm the absolute structure by refinement of the Flack parameter in the presence of 1675 sets of Friedel equivalents led to an inconclusive value of -0.10 (14). Therefore, the Friedel pairs were merged before the final refinement and the absolute configuration was assigned to correspond with that of the known chiral centres in a precursor molecule, which remained unchanged during the synthesis of the title compound.

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.
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 Rfactors(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.