Comment

The Kiliani reaction of ketoses with cyanide (Hotchkiss et al., 2004; Soengas et al., 2005) provides access to a novel class of carbohydrate scaffold which contains a branched carbon chain. Such sugar building blocks have hitherto been rare and difficult to prepare in large quantities (Bols, 1996; Lichtenthaler & Peters, 2004). However, naturally occurring ketoses restrict the branched carbon chain to a hydroxymethyl group. A further class of branched carbohydrates is available from the Kiliani ascension on 1-deoxyketoses, themselves prepared by addition of organometallic reagents to sugar lactones. Thus, reaction of cyanide with a protected 1-deoxy-D-ribulose allowed the isolation of the isopropylidene derivative of arabinono-1,5-lactone, (1) (Hotchkiss et al., 2006), shown to crystallize in a boat conformation (Punzo, Watkin, Jenkinson & Fleet, 2005).

The value of protected sugar lactones such as (1) depends on being able to modify the tertiary alcohol functionality to other groups. Thus, the esterification of the free alcohol (1) with triflic anhydride in pyridine afforded the trifluoromethanesulfonate, (2), which on further reaction with sodium azide in dimethylformamide gave the ribo-azide, (3), as the major product in good yield, even though the overall reaction is a nucleophilic displacement at a very hindered position. It was possible that neighbouring group participation by an O atom might have been involved in the reaction but the X-ray crystal structure (Punzo, Watkin, Jenkinson, Cruz & Fleet, 2005) showed that the reaction proceeded with inversion of configuration to give the ribonolactone (3) in a boat conformation, with the C2-methyl group in a hindered flag-pole position. A small quantity of a second crystalline azide, the title compound, (4), was also isolated.

X-ray crystal-structure analysis of (4) firmly establishes that the relative configuration of the azidomethyl branch at C2 is in a bowsprit conformation. The absolute configuration of (4) was determined by the use of D-erythronolactone as the starting material for the synthesis. Azides (3) and (4) are likely to be useful building blocks for the synthesis of novel branched prolines and pipecolic acids, respectively.

In Fig. 2, a pseudo-translational operator of the form (0.48 + x, 0.48 + y, +z) is clearly detectable.

Figure 1 The asymmetric unit of (4), containing two molecules, with displacement ellipsoids drawn at the 50% probability level. H-atom radii are arbitrary.

Figure 2 A packing diagram of (4), viewed down the a axis.

Experimental

The title lactone, (4) {m.p. 365-367 K; []_D²³ -168.2% (c 1.0 in MeCN)}, was crystallized by dissolving it in ethyl acetate, adding cyclohexane and allowing slow competitive evaporation of the two solvents until clear colourless crystals formed. The multi-scan technique was used to correct for changes in the illuminated volume.

In the absence of significant anomalous dispersion effects, Friedel pairs were merged before refinement. H atoms were seen in difference Fourier maps. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry [C-H in the range 0.93-98 Å and U_iso(H) in the range 1.2-1.5 times U_eq of the parent atom], after which their positions were refined with riding constraints.

Data collection: COLLECT (Nonius, 2001); cell refinement: DENZO/SCALEPACK (Otwinowski & Minor, 1997)'; data reduction: DENZO/SCALEPACK; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS.