Comment

The azide group is synthetically important due to its ability to be reduced under a variety of conditions, thus permitting the controlled introduction of an amine functionality (Scriven & Turnbull, 1988). Further reagents for the reduction of azides to form amines and amides continue to be discovered (Fazio & Wong, 2003); ruthenium(III) has been shown to be an efficient promoter for the formation of amides from azides and thioacids (Shangguan et al., 2003). Although catalytic hydrogenation is a particularly useful method of azide reduction, often providing excellent yields whilst leaving other sensitive functionalities intact, surprising complications are still discovered; thus catalytic reduction of a series of bicyclic azides (RN₃) resulted in the formation of a number of azoamines (RN=N-NH₂) arising from simple addition of hydrogen to the terminal nitrogen of the azide (Beacham et al., 1998). When the azido ester (1) was hydrogenated in the presence of palladium black in 1,4-dioxan, the majority of the products were derived from the amino ester (2) (Mayes, Simon et al., 2004; Mayes, Stetz, Watterson et al., 2004; Mayes, Stetz, Ansell & Fleet, 2004). However, significant amounts of the nitrile (3) were also formed during the reduction; this is unexpected, since the formation of the nitrile appears to be a formal oxidation occurring under reducing conditions. Although previous examples of the catalytic decomposition of primary azides to nitriles have been reported (Hayashi et al., 1976; Kappe, 1990; Kotsuki et al., 1997), this is the first example of the formation of a nitrile being formed under hydrogenation conditions. The structure of the unexpected product (3), including the relative configuration at C-5 (atom C13) bearing the nitrile, was firmly established by X-ray crystallographic analysis (Fig. 1); the absolute configuration arises from the use of D-galactose as the original starting material.

The crystal structure of (3) is unexceptional, consisting of layers of molecules lying parallel to the ab plane (Fig. 2). One face of the layer is relatively flat and consists of nitrile and methyl groups facing an identical face of the next layer. The other face of the layer is pleated, with the methyl carboxylate groups of one layer interleaving with the corresponding groups on the adjacent face. There are no unexpectedly short O-methyl or N-methyl contacts.

Figure 1 The title compound with displacement ellipsoids drawn at the 50% probability level. The H atoms are shown as spheres of arbitary radius.

Figure 2 Packing diagram of (3), viewed along the b axis.

Experimental

The azide ester (1) was hydrogenated in the presence of palladium black in 1,4-dioxan (Mayes, Simon et al., 2004) and the title material crystallized from ethyl acetate/hexane.

In the absence of significant anomalous scattering, Friedel pairs were merged, and the absolute configuration is arbitrarily assigned. The relatively large ratio of minimum to maximum corrections applied in the multiscan process (1:1.11) reflect changes in the illuminated volume of the crystal. The H atoms were all located in a difference map, but those attached to C atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H = 0.93-0.98 Å) and displacement parameters [U_iso(H) = 1.2-1.5U_eq(parent atom)], after which they 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.