Approximate H(5) Ring Conformation of 2,3-O-Carbonate Protected α- and β-L-Rhamnopyranosides as Confirmed by X-Ray Crystallography.

2,3-O-Carbonate protected rhamnopyranosides with both the α- and β-anomeric configuration are shown crystallographically to have ring conformations that differ significantly from the chair and which approach the (o)H(5) half-chair. This distortion, which is greatest in the α-anomer, provides a basis for the α-selectivity of 2,3-O carbonate protected manno- and rhamnopyranosyl donors as well as the conformationally related 2,3-O-alkylidene derivatives, in homogeneous solution phase glycosylation reactions.