2- C -Azidomethyl-2-deoxy-3,4- O -isopropylidene-D -ribono-1,5-lactone

X-ray crystallographic analysis firmly establishes the ribo stereochemistry and the unusual boat conformation of the title branched carbon chain lactone, C9H13N3O4, arising from an unexpected rearrangement in the nucleophilic substitution of a trifluoro­methane­sulfonate. There are two molecules in the asymmetric unit.

X-ray crystallographic analysis firmly establishes the ribo stereochemistry and the unusual boat conformation of the title branched carbon chain lactone, C 9 H 13 N 3 O 4 , arising from an unexpected rearrangement in the nucleophilic substitution of a trifluoromethanesulfonate. There are two molecules in the asymmetric unit.

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-dribulose allowed the isolation of the isopropylidene derivative of arabinono-1,5-lactone, (1) (Hotchkiss et al., 2006), shown to crystallize in a boat conformation .
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 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.
Financial support (to FPC) provided by the Fundacao para a Ciencia e a Tecnologia, Portugal, is gratefully acknowledged.

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.

S1. 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 .
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 Xray 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 analysis of (4) firmly establishes that the relative configuration of the azidomethyl branch at C2 is in a bow-sprit 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.
From Fig. 2, it seems evident that a pseudo-translational operator of the form (0.48 + x, 0.48 + y, +z) is clearly detectable. The multi-scan technique was used to correct for changes in the illuminated volume.

S3. Refinement
In the absence of significant anomalous dispersion effects, Friedel pairs were merged before refinement. H atoms were seen in a 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 adjacent supporting information sup-2 Acta Cryst. (2006). E62, o321-o323 atom], after which their positions were refined with riding constraints.

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