Crystal and molecular structure of 2,3,4-tri-O-acetyl-β-d-arabinopyranosyl azide

✍ Scribed by Peter Luger; Zoltán Györgydeák

• Publisher: Elsevier Science
• Year: 1993
• Tongue: English
• Weight: 232 KB
• Volume: 247
• Category: Article
• ISSN: 0008-6215
• DOI: 10.1016/0008-6215(93)84263-6

✦ Synopsis

The known tendency of halogen or alkoxyl groups on C-l in a pyranose system to adopt an axial conformation is generally designated as the anomeric effect. This effect plays an important and frequently discussed role in the field of conformational analysis of carbohydrates and related acetals '. The essential consequence of this phenomenon manifests itself not only in a relative stability but also in the reactivity of the anomeric centre and in such structural characteristics as bond lengths and bond angles. The origin of the anomeric effect has been discussed in terms of electrostatic interactions, n-n (and n-u *) interactions, double bond-no bond resonance, and polar-polar bond interactions'-3.

In our previous study4 of glycopyranosyl compounds with nitrogen-containing moieties on C-l, we proposed a sequence of groups with decreasing anomeric effect where the azido group falls between the OAc and NHCOCF, groups. An X-ray structure analysis of 2,3,4-tri-O-acetyl-o-p-arabinopyranosyl azide (1) showed5 that the equatorial azido group was oriented gauche with respect to the ring oxygen with a torsion angle O-5-C-l-N-l-N-2 of 75.6". This value is comparable with a bending of the 0-CH, bond in methyl P-pyranosides (ca. 70-769, which demonstrates that the exq-anomeric effect of the azido group is of the same magnitude as the effect of a methoxyl group. These findings were supported by a further X-ray analysis of 2,3,4-tri-0-acetyl-/3-D-xylopyranosyl azide (21, where a gauche arrangement with respect to the ring oxygen was also found for the equatorial azido group6.