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

That the ring conformation of glycopyranosyl oxacarbenium ions can influence the stereochemical outcome of glycosylation reactions has been postulated for some time. Some new ionization calculations show that the ultimate conformation 4H3 or 5S1 of d-glucopyranosyl oxacarbenium ions depends on the initial image (CH-1-C-1–S+-SCH3) conformation of anomeric thiosulfonium ions. Evidence is also presented that nucleophile:electrophile hydrogen bonded complexes, 1,6-anhydro-carbenium ions and electron rich carbon nucleophile:oxacarbenium ion complexes are all probably artifacts of neglecting counter ions or nucleophiles in the DFT calculation. All three cationic species are likely important for glycosylation reaction side reactions but not as productive species.