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

A trisaccharide repeating unit of the O-antigen from Burkholderia anthina, α-L-Rha-(1→2)-α-L-Rha-(1→2)-β-D-Gal-O(CH2)3NH2 (1), and its dimer, α-L-Rha-(1→2)-α-L-Rha-(1→2)-α-D-Gal-(1→3)-α-L-Rha-(1→2)-α-L-Rha-(1→2)-β-D-Gal-O(CH2)3NH2 (2), were synthesized via a highly convergent and efficient assembly strategy. Sequential glycosylation of galactosyl acceptor 6 with rhamnosyl thioglycoside 7, followed by condensation of the resulting disaccharide acceptor 9 with rhamnosyl imidate donor 10, gave the title molecule 1 after global deprotection. The title hexasaccharide 2 was assembled in a convergent [2+2+2] manner, in which α-1,2-linked disaccharide 12 was initially obtained by the coupling reaction of disarmed thiorhamnoside acceptor 15 with armed thiogalactoside donor 14. Sequential glycosylation of disaccharide acceptor 9 with thioglycoside donors 12 and 13 afforded the target compound 2 after global deprotection.