Synthesis of blocked trisaccharide analogue related to the repeating unit of the O-antigen form E. coli: 025

Methyl 2-O-benzoyl-4, 6-O-benzylidene-O-alpha-D-glucopyranoside 1 was condensed with rhamnose-1-thio donor 2 in the presence of NIS/TMSOTf to yield the disaccharide methyl 2,3,4-tri-O-acetyl-O-alpha-L-rhamnopyranosyl-(1-3)-2-O-benzoyl-4,6-O-benzylidene-O-alpha-D-glucopyranoside 3. Acidic hydrolysis of compound 3 followed by tritylation, deacylation and benzylation yields the disaccharide methyl 2,3,4-tri-O-benzyl-O-alpha-L-rhamnopyranosyl-(1-3)-2,4-di-O-benzyl-6-O-trityl-O-alpha-D-glucopyranoside 6. Compound 6 on detritylation yields the acceptor methyl 2,3,4-tri-O-benzyl-O-alpha-L-rhamnopyranosyl-(1-3)-2,4-di-O-benzyl-O-alpha-D-glucopyranoside 7 which oil condensation with known glucose-I-thio donor 8 in the presence of NIS/TMSOTf yields the linear protected trisaccharide methyl 2,3,4,6-tetra-O-acetyl-O-beta-D-glucopyranosyl-(1-6)-2,3,4-tri-O-benzyl-O-alpha-L-rhamnopyranosyl-(1-3)-2,4-di-O-benzyl-O-alpha-D-glucopyranoside 9.