Glycosyl Phosphites as Glycosylation Reagents: Scope and Mechanism

The glycosylation reactions with glycosyl phosphites in the presence of catalytic amounts of TMSOTf at low temperature have been studied with different donors and acceptors for the synthesis of several glycosides, including O-glycosides, S-glycosides, C-glycosides, and glycopeptides. Mechanistic investigations of the reactions indicate that the glycosyl phosphite is activated by either TfOH or TMSOTf, depending on how the substrates are mixed. When the acceptor is treated with TMSOTf first, the glycosyl phosphite is activated by the resulting TfOH. The glycosyl phosphite can also be activated by TMSOTf directly. The best result is, however, to mix the acceptor and TMSOTf first, followed by addition of the glycosyl phosphite.