1,2:1′,2′-Di-O-isopropylidenedifuranose-C12 higher carbon sugar

In the title compound, C18H28O8, the five-membered ring with one O atom attached to the ethyl substituent has a twisted conformation about the C—O bond. The adjacent cis-fused ring with two O atoms also has a twisted conformation about one of the C—O bonds. The dihedral angle between these rings (all atoms) is 59.05 (12)°. The five-membered ring linked to the ethynyl susbtituent is twisted about a C—C bond; the cis-fused adjacent ring is twisted about a C—O bond [dihedral angle between the rings (all atoms) = 71.78 (12)°]. Two intramolecular O—H⋯O hydrogen bonds occur. In the crystal, molecules are linked by O—H⋯O hydrogen bonds, generating [001] chains.

In the title compound, C 18 H 28 O 8 , the five-membered ring with one O atom attached to the ethyl substituent has a twisted conformation about the C-O bond. The adjacent cis-fused ring with two O atoms also has a twisted conformation about one of the C-O bonds. The dihedral angle between these rings (all atoms) is 59.05 (12) . The five-membered ring linked to the ethynyl susbtituent is twisted about a C-C bond; the cis-fused adjacent ring is twisted about a C-O bond [dihedral angle between the rings (all atoms) = 71.78 (12)

Comment
The term higher carbon sugars is customarily employed with monosaccharides containing seven or more consecutive carbon atoms in the chain. Higher carbon sugars have been attracting the increasing attention of organic chemists in the past decades due to the fact that they can be used as non-metabolized analogues of di-and oligosaccharides and are components of some antibiotics (Iwasa et al., 1978;Harada et al., 1981) and also that they are carbohydrate precursors for higher carbon amino sugars (Liu et al., 2006). C 18 H 28 O 8 ,the title compound (I), is a free C12 higher carbon sugar,whose structure consists of a fused system made up of two methylenedioxy ring and two tetrahydrofuran rings. Both of them, one methylenedioxy ring connects parallelly to tetrahydrofuran, give two fragments with V-shaped models. In the crystal, O-H···O hydrogen bonds (Table 1), link the molecules into [001] chains.
The crystal packing is shown in Figure 2.

Refinement
All H atoms were placed geometrically and treated as riding on their parent atoms with C-H are 0.96 Å (methylene) or 0.93 Å (aromatic), 0.82 Å (hydroxyl)and U iso (H) =1.2U eq (C). Attempts to confirm the absolute structure by refinement of the Flack parameter in the presence of 1412 sets of Friedel equivalents led to an inconclusive value of 0.3 (2). Therefore, supplementary materials sup-2 Acta Cryst. (2013). E69, o1399 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.

Figure 1
The molecular structure of (I) showing 30% probability displacement ellipsoids.  Packing diagram. 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.