How do Various Reaction Parameters Influence Anomeric Selectivity in Chemical Glycosylation with Thioglycosides and NIS/TfOH Activation?

The reaction of glycosyl donor phenyl 2,3,4,6-tetra-O-benzyl-1-thio-β-D-glucopyranoside with NIS/TfOH(cat.) was systematically studied under various reaction conditions. Neither the molecular sieve pore size nor amount of NIS activator was found to have an effect on the α/β-ratio in the reaction with L-menthol as glycosyl acceptor. Increasing concentration and the amount of triflic acid catalyst, however was found to increase the β-selectivity in certain cases. Moreover, lowering temperature was found to have a strong effect on the glycosylation outcome.