CRYSTAL AND MOLECULAR-STRUCTURE OF 2 2-DEOXY-2-HYDROXYIMINO DERIVATIVES OF BETA-D-ARABINO-HEXOPYRANOSE

The crystal structure of N-acetyl-N-(3,4,6-tri-O-acetyl-2-deoxy-2-hydroxyimino-beta-D-arabino-hexopyranosyl) amine (1) and 3,4,6-tri-O-acetyl-2-deoxy-2-hydroxyimino-beta-D-arabino-hexopyranosyl azide (2) were determined by direct methods and refined by full-matrix least squares to a final value of R = 0.034 for 3114 (1) and 2924 (2) reflections. The pyranoid rings of both compounds have almost identical conformation between the S-0(2) twist-boat and the B-3,B-0 boat. It was found using semi-empirical quantum mechanical calculations that the conformation observed in the solid state for both compounds, similar to that in solution, is more stable than the other possible, C-4(1) conformation. Similarity of geometries of 1 and 2 suggests that the pyranoid ring is less flexible than those with a chair conformation in other 2-hydroxyimino derivatives of pyranosides. On the other hand the observed conformations of 1 and 2 as well as the postulated way for acetyl group migration during hydrogenation of 2-acetoxyimino analog of 2 (-->1) confirm flexibility of the chair conformers. Results of conformation analysis of previous presented crystal structures of 2-oxyimino derivatives with a chair pyranoid ring and both title compounds suggest that the C-4(1) chair reversible arrow S-0(2) twist-boat/B-3,B-0 boat interconversion via the E(5) half-boat is possible.