Pentafluorophenyl (3R,4R,5S)-5-{[(3R,4R,5S)-5-azidomethyl-3,4-dimethoxy-2,3,4,5-tetrahydrofuran-3-carboxamido]methyl}-3,4-dimethoxy-2,3,4,5-tetrahydrofuran-3-carboxylate

The title compound, C22H25F5N4O9, is a stable pentafluorophenyl ester intermediate in the synthesis of novel homo-oligomeric structures containing branched carbon chains. The structure is epimeric to the previously characterized dimeric pentafluorophenyl ester with stereochemistry (3R,4R,5R), which was synthesized using d-ribose as starting material. The crystal structure of the title molecule removes any ambiguities arising from the relative stereochemistries of the six chiral centres. Two hydrogen bonds, bifurcating from the NH group, stabilize the crystal: one intramolecular and one intermolecular, both involving O atoms of the methoxy groups. The asymmetric unit contains two independent molecules not related by any pseudo-symmetry operators. The major conformational differences are localized, leading to one molecule being extended compared to the other. The collected crystal was twinned (twin ratio is 0.939:0.061), and the azide group is positionally disordered over two positions in one molecule [occupancy ratio 0.511 (18):0.489 (18)].

The title compound, C 22 H 25 F 5 N 4 O 9 , is a stable pentafluorophenyl ester intermediate in the synthesis of novel homooligomeric structures containing branched carbon chains. The structure is epimeric to the previously characterized dimeric pentafluorophenyl ester with stereochemistry (3R,4R,5R), which was synthesized using d-ribose as starting material. The crystal structure of the title molecule removes any ambiguities arising from the relative stereochemistries of the six chiral centres. Two hydrogen bonds, bifurcating from the NH group, stabilize the crystal: one intramolecular and one intermolecular, both involving O atoms of the methoxy groups. The asymmetric unit contains two independent molecules not related by any pseudo-symmetry operators. The major conformational differences are localized, leading to one molecule being extended compared to the other. The collected crystal was twinned (twin ratio is 0.939:0.061), and the azide group is positionally disordered over two positions in one molecule [occupancy ratio 0.511 (18):0.489 (18)].
Financial support to MIS provided through the European Community's Human Potential Programme under contract HPRN-CT-2002-00173 is gratefully acknowledged.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BH2308).
Foldamers provide increased understanding of the factors which induce secondary structures in proteins . Pentafluorophenyl esters have been shown to be particularly useful in the synthesis of homo-oligomers of SAAs (Mayes, Cowley et al., 2004;Mayes, Simon et al., 2004). Hitherto, all SAAs (Risseeuw et al., 2007) used as peptidomimetics contain linear carbon chains. Efficient syntheses of branched sugars by the Ho-crossed aldol condensation (Ho & Wong, 1985;Simone et al., 2005) allows access to carbon-branched SAA scaffolds, such as (1), which may provide monomers for new classes of foldamers.
The title material (2) crystallizes with two molecules in the asymmetric unit (Z'= 2, Fig. 2 Table 2). The especially large deviations for C7-C11-N12-C13 and N12-C13-C14-O25 leads to molecule A being partially folded back on itself so that it is less extended than molecule B (Fig. 4). Molecule B has disorder in the azide that can be modelled as two distinct sites. In both A and B, some of the atoms in the 5-membered rings and adjacent methyl groups show large adp's. This is consistent with the ring fluxion commonly seen in this class of compounds, and cannot really be modelled as split atoms. The non-linearity of the azide group [N22-N23-N24: 170.6 (8)°] and slight alternation in the N adp's are common in this class of structures (Humphreys et al., 2005).
The crystal structure consists of infinite chains with an equivalent hydrogen bond linking molecule A to B as that linking B to the next A (Fig. 5). These chains are stacked side-by-side to form layers (Fig. 6). One face of this layer consists of pentafluorophenyl groups, the other face contains the terminal azide groups. The aromatic face is essentially flat, and opposes the aromatic face of the adjacent layer. The azide face is pleated (as a result of the differing over-all length of the molecules), with the ridges in one layer fitting into the hollows of the next.

Refinement
In the absence of significant anomalous scattering, Friedel pairs were merged and the absolute configuration assigned from the known starting materials.
The relatively large ratio of minimum to maximum corrections applied in the multiscan process (1:1.30) reflects changes in the illuminated volume of the crystal. These were kept to a minimum, and were taken into account by the multi-scan inter-frame scaling (DENZO/SCALEPACK, Otwinowski & Minor, 1997).
The H atoms were all located in a difference map, but those attached to C atoms were repositioned geometrically. All H atoms except that on C118 (at the start of the disordered azide group) were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H in the range 0.93-0.98, N-H to 0.86 Å) and U iso (H) (in the range 1.2-1.5 times U eq of the parent atom), after which the positions were refined with riding constraints (Cooper et al., 2010).