Pyranoside‐into‐Furanoside Rearrangement of 4‐Pentenyl Glycosides in the Synthesis of a Tetrasaccharide‐Related to Galactan I of Klebsiella pneumoniae

An efficient strategy for synthesis of a spacer‐armed tetrasaccharide related to galactan I of Klebsiella pneumoniae is described, which uses newly developed acid‐free conditions for the pyranoside‐into‐furanoside (PIF) rearrangement of a digalactoside bearing a 4‐pentenyl group at the anomeric position. The 4‐pentenyl aglycon was successfully used both as a leaving group in the glycosylation of 3‐(trifluoroacetamido)propanol, and as a temporary anomeric protecting group, allowing conversion into an imidate donor. Regioselective coupling of the disaccharide blocks gave the desired tetrasaccharide sequence required for investigation of the interaction of galactan I with immune‐system proteins.