Methyl 3-O-α-l-fucopyranosyl β-d-glucopyranoside tetrahydrate

The title compound, C13H24O10·4H2O, is the methyl glycoside of a disaccharide structural element present in the backbone of the capsular polysaccharide from Klebsiella K1, which contains only three sugars and a substituent in the polysaccharide repeating unit. The conformation of the title disaccharide is described by the glycosidic torsion angles ϕH = 51.1 (1)° and ψH = 25.8 (1)°. In the crystal, a number of O—H⋯O hydrogen bonds link the methyl glycoside and water molecules, forming a three-dimensional network. One water molecule is disordered over two positions with occupancies of 0.748 (4) and 0.252 (4).

The title compound, C 13 H 24 O 10 Á4H 2 O, is the methyl glycoside of a disaccharide structural element present in the backbone of the capsular polysaccharide from Klebsiella K1, which contains only three sugars and a substituent in the polysaccharide repeating unit. The conformation of the title disaccharide is described by the glycosidic torsion angles ' H = 51.1 (1) and H = 25.8 (1) . In the crystal, a number of O-HÁ Á ÁO hydrogen bonds link the methyl glycoside and water molecules, forming a three-dimensional network. One water molecule is disordered over two positions with occupancies of 0.748 (4) and 0.252 (4).
Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: PLATON (Spek, 2009). disaccharide structural element is also present in the CPS produced by the thermophilic cynaobacterium Mastigocladus laminosus (Gloaguen et al., 1999) and the exopolysaccharide from Enterobacter amnigenus (Cescutti et al., 2005). The three-dimensional structure of the title compound, as a methyl glycoside model compound, represents an important part in these polysaccharides and may be used as a suitable starting point in modeling of the polymeric structures.
The major degrees of freedom in an oligosaccharide are the torsion angles φ H , ψ H , and ω. For the title compound (I) the two former are present at the glycosidic α-(1 → 3)-linkage. Furthermore, for the glucose residue the φ H torsion angle is also important. The ω torsion angle describes the conformation of the hydroxymethyl group in the glucose residue. In the title compound both of the φ H torsion angles in the structure are described by the exo-anomeric conformation with φ H = 51.1 (1)° for the fucose residue and φ H = 45.3 (1)° for the glucose residue (Fig. 1). The torsion angle conformation of ψ H = 25.8 (1)°.
The conformation of the hydroxymethyl group is described by one of the three rotamers, gauche-trans, gauche-gauche, or trans-gauche with respect to the conformation of C6-O6 to C5-O5 and to C5-C4, respectively. In the present case the glucose residue has the gt conformation with ω = 77.02 (9)°, i.e., shifted away somewhat from a canonical gauche conformation. Extensive water-water hydrogen bonding was observed (Table 1) for the four water molecules present in the crystal. Partial occupancy was observed for one of the water molecules, in a 3:1 relative ratio between OW4A and OW4B, with a distance of 0.90 Å in between the disordered water molecules.
The crystal structure of methyl 3-O-α-L-fucopyranosyl α-D-galactopyranoside was recently determined (Eriksson & Widmalm, 2012), but in contrast to the title compound it did not crystallize as a hydrate. Two stereochemical differences are present between the compounds: firstly at the reducing end where the O-methyl group is located equatorially (βconfiguration) in the title compound but axially (α-configuration) in methyl 3-O-α-L-fucopyranosyl α-D-galactopyranoside; secondly, and more interesting, the configuration at the C4 atom of the hexopyranose residue is different, equatorial in the title compound but axial in the previously investigated disaccharide, i.e., from gluco-to galactoconfiguration. For the φ H dihedral angle the conformation at the glycosidic linkage is almost identical with φ H = 51° supplementary materials sup-2 . E68, o3180-o3181 herein and φ H = 55° in methyl 3-O-α-L-fucopyranosyl α-D-galactopyranoside. However, the conformation at the ψ H dihedral angle differs significantly with ψ H = 26° herein, compared to ψ H = -24° in the previously determined compound, i.e., a difference of 50°. Whether the difference of ca 25° from an eclipsed conformation at the ψ H dihedral angle represents an intrinsic difference at the glycosidic linkage between 3-O-substituted glucose and galactose residues, or is just due to packing/hydration effects, remains to be elucidated. We note that in water solution the 13 C NMR glycosylation shifts (Δδ C ), i.e., differences in chemical shifts between the disaccharide and its constituent monosaccharides, for both the C1 and C3 atoms at the glycosidic linkage are lower by ca 1 p.p.m. in the title compound (Δδ C ~7.2 p.p.m.) compared to et al., 1988). These 13 C NMR chemical shift differences may be related to different conformational preferences at the ψ H dihedral angle.

Experimental
The synthesis of the title compound was described by Baumann et al. (1988) in which the fucose and glucose residues have the L and D absolute configurations, respectively. The compound was crystallized by slow evaporation of a mixture of water and ethanol (1:1) at ambient temperature.

Refinement
All hydrogen atoms, except those on the water molecules, were geometrically placed and constrained to ride on the parent atom. The C-H bond distances were set to 0.98 Å for CH 3 , 0.99 Å for CH 2 , 1.00 Å for CH. The O-H bond distance was set to 0.84 Å for OH groups. The U iso (H) = 1.5 U eq (C, O) for the CH 3 and OH, while it was set to 1.2 U eq (C) for all other H atoms. One of the water positions, OW4 was disordered over two positions with the occupancy 0.748 (4) for OW4A and 0.252 (4) for OW4B. Using the non-merged dataset for refinement, the Flack parameter refined to x = 0.0 but the s.u. was estimated to 0.3. This low accuracy of x is a result of the absence of significant anomalous scattering effects, thus the value of the Flack parameter was not considered as meaningful, and the 2677 Friedel equivalents were  A view of the molecule with atom numbering scheme. The displacement ellipsoids are drawn at the 50% probability level for non-H atoms.

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.

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