Glycosylation Using Hemiacetal Sugar Derivatives: Synthesis of O-α-D-Rhamnosyl-(1→3)-O-α-D-rhamnosyl-(1→2)-d-rhamnose and O-α-D-Tyvelosyl-(1→3)-O-α-D-mannosyl-(1→4)-L-rhamnose

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-benzyl-α-D-rhamnopyranoside was prepared via the direct ditritylation of allyl α-D-mannopyranoside. 3-O-Acetyl-2,4-di-O-benzyl-D-rhamnopyranose was used as a precursor for the moiety of D-tyvelose (3,6-dideoxy-D-arabino-hexose, 3,6-dideoxy-D-mannopyranose, 3-deoxy-D-rhamnose) of the salmonella trisaccharide.