1,3,4-Tri-O-acetyl-2-N-(trifluoroacetyl)-β-l-fucose

The title compound, C14H18F3NO8, was produced through conjugation of 1,3,4-tri-O-acetyl-2-azidodeoxy-α,β-l-fucose with trifluoroacetyl chloride in the presence of bis(diphenylphosphino)ethane in tetrahydrofuran at room temperature. The X-ray crystal structure reveals that the β-anomer of the product mixture crystallizes from ethyl acetate/hexanes. The compound exists in a typical chair conformation with the maximum possible number of substituents, four out of five, located in the sterically preferred equatorial positions. The major directional force facilitating packing of the molecules are N—H⋯O hydrogen bonds involving the amide moieties of neighboring molecules, which connect molecules stacked along the a-axis direction into infinite strands with a C 1 1(4) graph-set motif. Formation of the strands is assisted by a number of weaker C—H⋯O interactions involving the methine and methyl H atoms. These strands are connected through further C—H⋯O and C—H⋯F interactions into a three dimensional network

The title compound, C 14 H 18 F 3 NO 8 , was produced through conjugation of 1,3,4-tri-O-acetyl-2-azidodeoxy-,-l-fucose with trifluoroacetyl chloride in the presence of bis(diphenylphosphino)ethane in tetrahydrofuran at room temperature. The X-ray crystal structure reveals that the -anomer of the product mixture crystallizes from ethyl acetate/hexanes. The compound exists in a typical chair conformation with the maximum possible number of substituents, four out of five, located in the sterically preferred equatorial positions. The major directional force facilitating packing of the molecules are N-HÁ Á ÁO hydrogen bonds involving the amide moieties of neighboring molecules, which connect molecules stacked along the a-axis direction into infinite strands with a C 1 1 (4) graph-set motif. Formation of the strands is assisted by a number of weaker C-HÁ Á ÁO interactions involving the methine and methyl H atoms. These strands are connected through further C-HÁ Á ÁO and C-HÁ Á ÁF interactions into a three dimensional network
Beginning with an anomeric mixture of 1,3,4-tri-O-acetyl-2-azidodeoxy-α,β-L-fucoses, treatment with trifluoroacetyl chloride in the presence of bis(diphenylphosphino)ethane in THF at room temperature afforded a mixture of 1,3,4-tri-Oacetyl-2-N-(trifluoro)acetyl-α,β-L-fucoses as a colorless syrup. The mixture of anomers was purified by column chromatography, and one of the two anomers was selectively isolated by vapor diffusion crystallization using ethyl acetate and hexanes, with the other anomer remaining in solution. The crystals (m.p. 157-159 °C) were initially identified as the β-anomer by 1 H NMR spectroscopy. Single-crystal diffraction was then employed in order to unambiguously confirm the configuration of the anomer isolated by crystallization. Figure 1 shows a depiction of one molecule of 1,3,4-tri-O-acetyl-2-N-(trifluoro)acetyl-β-L-fucose as present in the solid state. From the crystal structure, it was confirmed that the isolated and crystallized fraction is indeed the β anomer, as had been already suspected based on 1 H NMR shifts and coupling constants. The molecule crystallizes in a chair conformation as typical for pyranose sugar derivatives (Rao et al., 1998) with the choice of the chair conformation -from the two that are possible -being apparently the result of sterical interactions. The maximum possible number of substituents, four out of five, are located in the sterically preferred equatorial positions. Only the O-acetyl group at carbon atom C4 is, out of necessity to be able to maintain the overall chair conformation, forced into an axial position.
This conformation is in agreement with the observed solution conformation as determined from the 1 H NMR coupling constants (see experimental section). The acetyl and amide groups are, as expected, nearly perfectly planar -the r.m.s. deviation from planarity for the five atoms of the amide group is only 0.0052. Those for the four atoms of each acetate group at C1, C3 and C4 are 0.0196, 0.0087 and 0.0475, respectively -and they are tilted to variable degrees against the average plane of the pyranose moiety (48.17 (9)° at C1, 57.98 (9)° at C3, 79.04 (7)° at C4, and 71.97 (8)° for the amide group).
This conformation allows for dense packing of the molecules in the solid state, with no significant distortions of the molecules or voids present in the solid state. The major directional force facilitating packing of the molecules are N-H···O hydrogen bonds involving the amide moieties of neighboring molecules. Through these interactions, molecules stacked along the direction of the a-axis (created through translation from each other) are connected into infinite strands, with a C 1 1 (4) graph set motif for the N-H···O hydrogen bonds. Formation of the strands is assisted by a number of weaker but nevertheless still attractive C-H···O interactions involving the methine and methyl hydrogen atoms, with three of these interactions featuring H···O separations of 2.5 Å or less (2.38, 2.41 and 2.45 Å, involving H14A, H8B and H3, respectively. See Table 1 for details and symmetry operators). These strands, Figure 2, are in turn connected with each other through further C-H···O and C-H···F interactions into a three dimensional network, Figure 3 and Table 1.
The stabilizing and directional effect of the C-H···F interactions might also have contributed to the ordered nature of the supplementary materials sup-2 Acta Cryst. (2014). E70, o134-o135 trifluoro methyl group. No signs of rotational disorder, as often observed for CF 3 groups, is evident for this structure.

Refinement
H atoms attached to carbon and nitrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with carbon hydrogen bond distances of 1.00 and 0.98 Å for C-H and CH 3 and 0.88 Å for N-H moieties, respectively. Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. U iso (H) values were set to a multiple of U eq (C/N) with 1.5 for CH 3 and 1.2 for C-H and N-H units, respectively. Reflection 0 1 1 was affected by the beam stop and was omitted from the refinement.  Thermal ellipsoid plot, with atom labels for non-hydrogen atoms and 50 percent probability ellipsoids.  View along the a axis direction, showing C-H···O and C-H···F interactions connecting parallel strands with each other.

Computing details
For atoms involved and symmetry operators, see Table 1.