6-Azido-3-O-benzyl-6-deoxy-N,N-diethyl-1,2-O-isopropylidene-d-glycero-α-d-gluco-heptofuranuronamide

Reaction of 3-O-benzyl-1,2-O-isopropylidene-α-xylo-pentodialdo-1,4-furanose with N,N-diethyl-2-(dimethylsulfuranilidene)acetamide gave stereoselectively an epoxyamide, which was regioselectively opened by NaN3 in dimethyl formamide to give the title compound, C21H30N4O6. X-ray crystallography confirmed the relative stereochemistry of the title compound and the absolute configuration was determined by the use of d-glucose as the starting material. There are two molecules in the asymmetric unit (Z′ = 2). The crystal structure consists of two types of chains of O—H⋯O hydrogen-bonded molecules running parallel to the b axis, with each molecule acting as a donor and acceptor of one hydrogen bond.

Reaction of aldehyde 1, obtained from D-glucose, with the sulfur ylid gave epoxyamide 2 as the only product ( Fig.  1). The subsequent regioselective opening of the epoxide with sodium azide, with acetic acid as a catalyst, gave the title compound 3. The product was confirmed, by both X-ray crystallography and the use of D-glucose as the starting material, to have the D-glycero-D-gluco stereochemistry (Fig. 2) arising from initial attack of the ylid on the re face of the aldehyde. This configuration can be predicted from a Felkin-Ahn model as corroborated for a number of aldehyde sugars with either free aldehydes or in cyclohemiacetalic form (Valpuesta Fernández et al., 1990;López-Herrera et al., 1996). In the present compound 1, the influence of 3-O-benzyl group plays a crucial role to obtain complete selectivity.
There are two crystallographically distinct molecules in the asymmetric unit which are related by a pseudo 2-fold rotation axis (Fig 2). When the two molecules are mapped they show good overlap for the majority of the structure (Fig. 3) with RMS deviations of 1.0604 on the positions, 0.0140 for the bonds and 35.7787 for the torsion angles. The major difference between the two molecules is the orientation of the aromatic ring. The crystal exists as chains of hydrogen bonded molecules lying parallel to the b-axis, with each molecule acting as a donor and an acceptor of one hydrogen bond (Fig. 4). Only classical hydrogen bonding is considered. For one of the molecules in the asymmetric unit the ethylamine moiety exhibits a greater degree of thermal motion than in the other.

Refinement
In the absence of significant anomalous scattering, Friedel pairs were merged and the absolute configuration was assigned from the use of D-glucose as the starting material.
The H atoms were all located in a difference map, but those attached to carbon atoms were repositioned geometrically.
The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H