(−)-Benzyl 2,3-dideoxy-β-d-erythro-hex-2-enopyranoside

In the title compound, C13H16O4, the six-membered ring of the sugar moiety shows a half-chair conformation. In the crystal, molecules are connected via O—H⋯O hydrogen bonds, forming columns around twofold screw axes along the b-axis direction. There is a disorder of the benzyloxy group, which has two possible orientations with the phenyl group lying on a common plane [site-occupancy factors = 0.589 (9) and 0.411 (9)].

In the title compound, C 13 H 16 O 4 , the six-membered ring of the sugar moiety shows a half-chair conformation. In the crystal, molecules are connected via O-HÁ Á ÁO hydrogen bonds, forming columns around twofold screw axes along the b-axis direction. There is a disorder of the benzyloxy group, which has two possible orientations with the phenyl group lying on a common plane [site-occupancy factors = 0.589 (9) and 0.411 (9)].

Synthesis and crystallization
To a solution of 1c (669 mg, 2.17 mmol) in CH 3 CN (22 ml) were added Cs 2 CO 3 (1.41 g, 4.33 mmol) and BnOH (4.53 ml, 43.5 mmol) and the mixture was stirred for 8 h at room temperature. The mixture was diluted H 2 O and organic materials were extracted with CHCl 3 . The combined extract was washed with brine, dried over Na 2 SO 4 , and concentrated in vacuo.
The residue was purified by silica gel column chromatography (20 g). Elution with hexane-EtOAc (2:1) afforded the title compound (I) as a colorless solid (414 mg, 81%). The plate-like crystals of (I) were grown by slow cooling of a t-butylmethylether/hexane solution from ca 340 K to the room temperature. The specific rotation [α] D of (I) at 295 K is -107° (c 1.33, EtOH).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The absolute structure was assigned based on the known absolute configuration around the C6 atom, which originated from (+)-D-glucose. In the absence of significant anomalous scattering effects, Friedel pairs were averaged before the final refinement. There is an orientational disorder of the benzyloxy group, where the split phenyl group moieties lie on a common plane. The site occupation factor of the major part (O4A, C11A-C17A) was refined to 58.9 (9)%. All the H atoms were positioned geometrically, and refined as riding, with C-H = 0.93-0.98 Å and O-H = 0.82 Å, and with U iso (H) = 1.2U eq (C,O). The hydroxy groups were allowed to rotate but not to tip.

Comment
Glycosidic bond formation on glycals such as (1a in Fig. 3) accompanied with the migration of double bond from C 1 -C 2 to C 2 -C 3 had been recognized as Ferrier reaction to give 2-enopyranosides (2) (Ferrier & Prasad, 1969;Noshita et al., 1995). Generally, α-glycosides predominate by the well known oxygen anomeric stabilizing effect, and the anomeric stereochemistry was elucidated by X-ray structure analysis of 2a (Wingert et al., 1984). In contrast, Di Bussolo et al. (2002,2004) have demonstrated unique β-selective approach, starting from glycals such as 3 with leaving group at C 4 -OH. By neighboring group participation with free C 3 -OH, via an epoxide intermediate (4), Ferrier-like rearrangement occurred to give 5. Their proposed mechanism is shown in Fig. 3. In this case, a hydrogen bonding between epoxy ring in 4 and nucleophile dominates the stereochemistry to β rather than α. We submitted 3,6-di-O-acetyl-D-glucal (1b), which is very easily available by an enzymatic regioselective acylation of D-glucal (Calveras et al., 2010), to Crotti's protocol.
Introduction of methylsulfonyl group on free C 4 -OH (1c) and subsequent nuleophilic attack with benzyl alcohol provided the title compound, (I)(81%).
In the present study, the regio-and stereochemistry of (I) has been determined, although there is a complicated disorder. factors are 58.9 (9) and 41.1 (9)%, respectively. The C10-O4A and C10-O4B bond directions make an angle of 25.4 (7)°.

Figure 1
Molecular structure of the title compound with anisotropic displacement parameters drawn at the 30% probability level.

Refinement.
Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factor (gt).