Donor Catalyst T ( From Figure it is seen that the donor and acceptor are very close in space after INT2 is formed. This leads to a favorable trajectory for attack by the deprotonated acceptor on the opposite side of the leaving group, which results in inversion of the stereochemistry. This is also in line with a loss of inversion when the glycosyl donor has an axial C-2. Thus to try to explain the loss of selectivity and the observed side products in glycosylations with the β-TCA we decided to search for similar ion pairs between the β-TCA and the acceptor. Two ion pairs were located, and these are shown in Figure . Due to the β-configuration, the protonated imidate cannot hydrogen bond to the O-2 oxygen as in INT2. Instead, the protonated imidate is hydrogen bonded to the endocyclic oxygen in both INTβa and INTβb, thus the carbamate is further away in space than for the corresponding ion pair with the α-TCA (INT2). As a consequence, the carbamate can be orientated, so that an attack would lead to retention of stereochemistry (INTβa). Additionally, even when the carbamate is positioned in a way that would lead to inversion (INTβb) the carbamate nitrogen is further away from the anomeric carbon (5 Å) compared to the distance in INT2 (3.4 Å) which might leave room for competing nucleophiles and explain the increased amount of side products. To investigate this hypothesis, we decided to conduct a competition experiment.