O-α-D-Rhamnopyranosyl-(1→3)-O-α-D-rhamnopyranosyl-(1→2)-D-rhamnopyranose, a repeating trisaccharide of the O-specific polysaccharides (OPSs) of Pseudomonades, and O-α-D-tyvelopyranosyl-(1→3)-O-α-D-mannopyranosyl-(1→4)-L-rhamnopyranose, a trisaccharide composing the OPSs of Salmonella typhi, were synthesized by in-situ activating glycosylation reactions using hemiacetal sugar derivatives. Allyl 2,4-di-O-