Synthesis of β-C-galacto-Pyranosides with Fluorine on the Pseudoanomeric Substituent

β-C-galacto-Pyranosides with CHF and CF₂ substitutes for the glycosidic oxygen were prepared through a four-step sequence starting from a central 1-thio-1,2-O-isopropylidene acetal alcohol and different α-fluoro- and α,α-difluoro acids. The key step in the synthesis is the oxocarbenium cyclization of an intermediate enol ether-thioacetal to a C1-substituted glycal.