Synthesis of a trisaccharide repeating unit of the O-antigen from Burkholderia cenocepacia and its dimer.

The trisaccharide repeating unit of an O-antigen derived from Burkholderia cenocepacia and its dimer, i.e., α-L-Rhap-(1 → 3)-α-D-GalpNAc-(1 → 3)-β-D-GalpNAc-O(CH2 )3 N3 (1) and α-L-Rhap-(1 → 3)-α-D-GalpNAc-(1 → 3)-β-D-GalpNAc-(1 → 4)-α-L-Rhap-(1 → 3)-α-D-GalpNAc-(1 → 3)-β-D-GalpNAc-O(CH2 )3 N3 (2), respectively, were synthesized via a highly convergent strategy. Glycosylation of galactosaminyl acceptor 4 with galactosaminyl trichloroacetimidate donor 5 was followed by condensation of resulting disaccharide acceptor 12 with rhamnosyl imidate donor 6 to furnish stereoselectively trisaccharyl thioglycoside 3, which was used as a key and common glycosyl donor for the construction of both 1 and 2. Title molecule 1 was prepared by glycosylation of 3-azidopropanol with 3 and subsequently global deprotection, whereas coupling reaction of 3 with a trisaccharide acceptor 21 containing an 2,3-O-position acetonide-modified rhamnose residue, followed by global deprotection, generated the dimer 2 in a convergent [3 + 3] manner.