Finally, we probed the pathway for the formation of the anomeric β-triflate (Rβ) from its α-anomer. This in situ anomerization scheme has been forwarded to account for the formation of the α-glucosyl products, while it has been excluded for the mannosyl case because of the presumed instability of the β-mannosyl triflate. Indeed, Asensio and co-workers have previously been able to detect the β-glucosyl triflate by NMR spectroscopy of an C-labeled glycosyl triflate. We have also been able to detect the elusive β-mannosyl triflate by C CEST as described in the preceding report, urging us to revisit the anomerization reaction in both cases. As shown in the reaction profiles for the glucosyl and mannosyl triflates (Figure , left pathways), the activation energy (TSIV) for the formation of the anomeric β-triflate (Rβ) is slightly higher for the mannoside (+16.5 kcal mol -1 ) than for the glucoside (+15.8 kcal mol ). When the entire potential energy surface is regarded, the key distinction between the mannoside and the glucoside reaction profiles becomes apparent: for the glucosyl triflate it is energetically more favorable to form the β-triflate (+15.8 kcal mol -1 ) than the dioxanium ion from the α-triflate (+19.9 kcal mol -1 ), while for the mannosyl triflate the reverse is true (TSIV and TSIII are respectively +16.5 kcal mol -1 and +14.0 kcal mol -1 ). The formation of the two different reactive intermediates accounts well for the dichotomy in the stereochemical outcome for the 3-O-benzoyl benzylidene mannose and glucose systems. The mannosyl dioxanium ion, found in the gas-phase and in solution by C CEST NMR, is responsible for the stereoselective formation of the α-mannosyl glycosylation products. In contrast, the glucosyl 1,3-dioxanium ion could be formed and characterized in the gas phase, but no evidence was found in solution. In this case, the generation of the mixtures of anomeric products can be explained using the anomeric triflates as product forming intermediates, with reactive nucleophiles being capable of displacing the more prevalent α-glucosyl triflate and the weaker nucleophiles reacting with the more reactive β-triflates. As the pathway towards the glucosyl 1,3-dioxanium ion requires significantly more energy than the formation of the reactive β-triflate, as revealed by the computed potential energy surfaces, these ions do not partake in the reaction explaining the analogous behavior of the 3-O-benzyl and 3-O-benzoyl benzylidene glucosyl donors. Since the glucosylation reactions with the 2,3-di-O-benzyl benzylidene donors proceeded with identical stereoselectivity, it is likely they proceed through a similar SN2-like mechanism. for all data of the stationary points of the reaction profiles.