Complications of modeling glycosylation reactions: can the anomeric conformation of a donor determine the glycopyranosyl oxacarbenium ring conformation?

► Donor H-bonding, 1,6-anhydro formation, and O-2 electron rich C nucleophile complexes identified. ► Anomeric dihedral determines the ring conformation of a pyranosyl oxacarbenium ion. ► A hypothesis that by suitable choice of thioglycoside and sulfenyl promoter, specific oxacarbenium ion ring conformations could be accessed.