The experiments in Table were carried out to see if the carbamate would act as a catalyst rather than a self-promoter in the presence of an excess of competing nucleophile. We first confirmed that the competing nucleophile, isopropanol, does not react with the donor without addition of the carbamate "catalyst". This was confirmed by Entry 1, as only the starting material was recovered. When 0.2 equivalent of the carbamate is added the major product becomes the α-O-glycoside at both 0 • C and room temperature (Entry 2 and 3). The formation of the O-glycoside is highly α-selective suggesting that the carbamate is still a part of an associated ion pair and positioned so that either the carbamate or leaving group is blocking for attack from the upper side of the ring. However, the carbamate is not a good catalyst since N -glycoside product is also observed and thereby is consumed during the reaction. Such species formed by an irreversible reaction between a catalyst and a glycosyl donor have earlier been coined the rebound product by Miller. By comparing Entry 3 and Entry 4 it is seen that if the donor is the α-TCA the major product is the β-N -glycoside product with the β-O-glycoside product as a minor product. This is in line with the α-TCA carbamate ion pair being tighter than the β-TCA carbamate ion pair. Intrigued by the catalytic, albeit suboptimal, properties of the carbamate we hypothesized that if the formation of the rebound product (i.e. the N -glycoside) could be suppressed a new procedure for catalytic α-selective O-glycosylation could be developed. We tried several different carbamate and alcohol combinations but for all of them, the rebound product was still observed (SI Section 5). However, the insights from the mechanism of selfpromoted N -glycosylations have led us on the track of the discovery of new selective catalysts for glycosylation. We envision that if the formation of a nucleophilic counterion upon activation of the TCA is prevented either by steric hindrance or by decreasing the acidity of the catalyst so that the TCA is activated but without full proton transfer, stereospecific catalysis can be obtained. This is illustrated in Figure . Very acidic catalysts can form stable counterions, which give rise to unstable rebound products that are subject to anomerization, and the stereochemical information from the donor is often lost. This can, for instance, be observed with triflates 6, 8 or paratoluenesulfonic acid. If the acidity is decreased, the counterion becomes less stable and the rebound product becomes stable, making the reaction between the donor and counterion irreversible. In this way, the activator is no longer a catalyst but instead a self-promoter. By decreasing the acidity to the point where full protonation does not occur, but the TCA is still activated enough for reaction with another nucleophile might lead to transition back into a catalytic window with stereospecificity.