4-Nitrophenyl α-l-rhamnopyranoside hemihydrate1

The absolute configuration of the title compound, C12H15NO7·0.5H2O, was assigned from the synthesis. There are two rhamnoside molecules and one water molecule in the asymmetric unit, displaying O—H⋯O hydrogen bonding. One of the nitro groups does not conjugate efficiently with the benzene ring.

The absolute configuration of the title compound, C 12 H 15 NO 7 Á0.5H 2 O, was assigned from the synthesis. There are two rhamnoside molecules and one water molecule in the asymmetric unit, displaying O-HÁ Á ÁO hydrogen bonding. One of the nitro groups does not conjugate efficiently with the benzene ring.

Comment
Para-nitrophenyl-α-L-rhamnoside is an important substrate in the studies on α-L-rhamnosidase, for its chromogenic property of the released para-nitrophenol (Garegg et al., 1978). It also serves as synthetic intermediate for glycosidic compounds (Martearena et al., 2003).
In order to develop a greener synthetic method, a series of approaches have been carried out in this lab. A fairly convenient route was found finally, in which the title compound was synthesized in only two steps. First, L-rhamnose (1) was acetylated and chlorinated to yield 2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl chloride (2) in the presence of acetyl chloride; then it was converted to the target molecule (3) in the condition of phase transfer catalyst (Scheme 1). The synthetic route was more concise compared with published methods (Garegg & Norberg, 1983). Additionally, the bioactivity of the synthetic compound was confirmed by enzymatic assay (Nishio et al., 2004) Suitable crystals of target product were obtained by slow crystallization from 95% ethanol. The crystal structure was determined in order to ascertain its stereochemistry and solid-state conformation. These data are consistent with the proton and carbon NMR studies. Due to the absence of heavy atoms, refinement of the Flack parameter was not possible, and the absolute configurations could not be determined directly. Instead, they were assigned based on the knowledge of stereochemistry of the synthetic precursors and the mechanisms of synthesis. The crystal of rhamnoside has two molecules and one water molecule in the independent part of the unit cell. The configuration, conformation and atom numbering are shown in Fig. 1.
Similar to the known structures of the nitrophenyl glycopyranosides, the analyzed rhamnopyranoside (3) crystallizes in the P 2 1 space group. Besides, one of the nitro groups is slightly rotated with respect to the phenyl fragments. The angles between the best planes of the phenyl ring and the nitro groups are 13.3° and 0.5°, respectively. This finding partly supports the earlier opinion that the nitro group does not conjugate effectively with the benzene ring (Temeriusz et al., 2005). The sugar moieties adopt 4 C 1 conformations. Fig. 2 shows the intermolecular interactions in the crystal lattice. The crystal structure of (3) consists of molecular sheets lying perpendicular to the b axis (Fig. 2), in which the molecules are linked by short hydrogen bonds (Table 1).
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Refinement
In the absence of any significant anomalous scattering, the Flack (1983) parameter was indeterminable (Flack & Bernardinelli, 2000). Hence, the Friedel equivalents were merged prior to the final refinements, and the absolute structure was set by reference to the known chirality of the enantiopure starting sugar employed. Fig. 1. The molecular structure of (3), with displacement ellipsoids drawn at the 50% probability level. H-atom radii are arbitrary.