Highly efficient stereospecific preparation of Tn and TF building blocks using thioglycosyl donors and the Ph₂SO/Tf₂O promotor system

The activation of 2-azido-2-deoxy Tn and TF thioglycosyl donors by the powerful thiophilic promoter system Ph₂SO/Tf₂O has been investigated. Glycosylation of an Fmoc-protected threonine derivative gave 1,2-cis glycosides in high yields and excellent stereoselectivities. The galactosylation of phenyl 2-azido-4,6-O-benzylidene-2-deoxy-1-thio-β-D- galactopyranoside was achieved in high yield and without orthoester formation using a trichloroacetimidate donor carrying a 2-O-(2,5-difluorobenzoyl) group. The anomeric thiophenyl group of the constructed TF disaccharide could directly be activated by the van Boom promotor for the glycosylation of a threonine derivative.