Tetra‐O‐acetyl‐β‐D‐glucopyranosyl chloride: Occurrence of gg and gt rotamers in the crystal and correlation between conformation at C‐6 and anomeric stabilization

Abstract

AbstractAn X‐ray structural analysis of 2,3,4,6‐tetra‐O‐acetyl‐β‐D‐glucopyranosyl chloride (5β) shows the crystal lattice to contain consecutive layers of two forms, each in the 4C1 conformation, but differing in the orientation of the 6‐acetoxy group relative to the pyranoid ring: the gauche‐gauche (gg) rotamer, exhibiting standard bond lengths and torsional angles, and the gauche‐trans (gt) form, in which an unusually long C‐5—O‐5 bond distance of 1.536 Å is observed, though without major distortion of the chair conformation. In solution, only the gg form prevails. The similarity of conformational features of 5β in its gg form and of its β‐methoxy analog 7β is so close that the anomeric effects for equatorial Cl and OCH3 on a pyranoid ring must be grossly identical. — Force‐field calculations on the anomeric 1‐chloro‐ and 1‐methoxy derivatives of tetra‐acetyl‐D‐glucopyranose allow a realistic estimation of the magnitude of the anomeric effect and a surprisingly accurate assessment of the degree of anomeric stabilization in the individual C‐6 rotameric forms, the gg conformer invariably being the energetically most favored.