Stereodirecting Effect of the Pyranosyl C-5 Substituent in Glycosylation Reactions

The stereodirecting effect of the glycosyl C-5 substituent has been investigated in a series of d-pyranosyl thioglycoside donors and related to their preferred positions in the intermediate (3)H(4) and (4)H(3) half-chair oxacarbenium ions. Computational studies showed that an axially positioned C-5 carboxylate ester can stabilize the (3)H(4) half-chair oxacarbenium ion conformer by donating electron density from its carbonyl function into the electron-poor oxacarbenium ion functionality. A similar stabilization can be achieved by a C-5 benzyloxymethyl group, but the magnitude of this stabilization is significantly smaller than for the C-5 carboxylate ester. As a result, the preference of the C-5 benzyloxymethyl to occupy an axial position in the half-chair oxacarbenium ions is much reduced compared to the C-5 carboxylate ester. To minimize steric interactions, a C-5 methyl group prefers to adopt an equatorial position and therefore favors the (4)H(3) half-chair oxacarbenium ion. When all pyranosyl substituents occupy their favored position in one of the two intermediate half-chair oxacarbenium ions, highly stereoselective glycosylations can be achieved as revealed by the excellent beta-selectivity of mannuronate esters and alpha-selectivity of 6-deoxygulosides.