Isopropyl 2,5-anhydro-6-azido-3,6-dideoxy-d-xylo-hexonate

Sugar amino acids (SAAs) are carbohydrates which contain amine and acid groups; SAAs have been the focus of much interest as dipeptide isosteres, foldamers and library scaffolds (Hill et al., 2001; Gruner et al., 2002; Schweizer, 2002; Chakraborty, Srinivasu, Tapadar & Mohan, 2004; Trabocchi et al., 2005). The preference for SAA oligomers (carbopeptoids) to adopt compact conformations as relatively short homooligomers will provide insight into the paradigm of protein folding. SAAs (2,5-O-cis configuration) structurally related to SAA (3) have a high propensity to adopt repeating -turn conformations (Hungerford et al., 2000; Smith et al., 2003; Chakraborty, Srinivasu, Sakunthala et al., 2004). In contrast, some 2,5-O-trans SAAs have been shown to adopt helical conformations (Claridge et al., 1999; Claridge et al., 2005). The conformational complexity of these dipeptide isosteres is being further explored by preparation of structurally related analogues of the original SAA systems i.e. SAA (3) and corresponding diastereoisomers (Watterson et al., 2003).

The X-ray crystal structure ( Fig. 1) firmly established the relative stereochemistry of the stereogenic centres in the title compound, (2). The absolute configuration of (2) (see scheme) is determined by the use of d-gulono-1,4-lactone as starting material.
The crystal packing consists of chains of molecules linked by hydrogen bonds and lying parallel to the a axis (Fig. 2). There are no unusual intermolecular contacts.
Attempted refinement of the Flack (1983) parameter gave an inconclusive result, in the absence of Friedel pairs and the presence of only weak anomalous scattering effects. The absolute configuration was assigned from the known configuration of the starting material. The H atoms were all located in a difference map, but those attached to C 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 = 0.93-0.98 Å and O-H = 0.82 Å ) and U iso (H) values (in the range 1.2-1.5 times U eq of the parent atom), after which they were refined with riding constraints.
Financial support from the EPSRC to AAE and MPW is gratefully acknowledged.   The title compound, with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radius.
The X-ray crystal structure (Fig. 1) firmly established the relative stereochemistry of the stereogenic centres in (2). The absolute configuration of (2) (see scheme) is determined by the use of D-gulono-1,4-lactone.
The crystal packing consists of chains of molecules linked by hydrogen bonds and lying parallel to the a axis (Fig. 2).
There are no unusual intermolecular contacts.