Efficient synthesis of building blocks for branched rhamnogalacturonan I fragments

Starting from allyl 2-O-acetyl-3-O-benzyl-α-l-rhamnopyranoside (3), allyl (2,3,4,6-tetra-O-benzoyl-β-d-galactopyranosyl)-(1→4)-2-O-acetyl-3-O-benzyl-α-l-rhamnopyranoside (5) was synthesized under Helferich conditions. Module 5 was converted to 2,3,4,6-tetra-O-benzoyl-β-d-galactopyranosyl-(1→4)-2-O-acetyl-3-O-benzyl-α-l-rhamnopyranosyl bromide (8) which was then coupled with methyl (allyl 2,3-di-O-benzyl-β-d-galactopyranosid)uronate (11) to provide methyl (2,3,4,6-tetra-O-benzoyl-β-d-galactopyranosyl)-(1→4)-(2-O-acetyl-3-O-benzyl-α-l-rhamnopyranosyl)-(1→4)-(allyl 2,3-di-O-benzyl-β-d-galactopyranosid)uronate (14). Alternatively, module 5 was transformed into allyl 2,3,4,6-tetra-O-benzoyl-β-d-galactopyranosyl-(1→4)-3-O-benzyl-α-l-rhamnopyranoside (9) suitable as an acceptor for the glycosylation with methyl 4-O-acetyl-2,3-di-O-benzyl-α/β-d-galactopyranosyluronate N-phenyl trifluoroacetimidate (13) to yield allyl (methyl 4-O-acetyl-2,3-di-O-benzyl-α-d-galactopyranosyluronate)-(1→2)-[2,3,4,6-tetra-O-benzoyl-β-d-galactopyranosyl]-(1→4)-3-O-benzyl-α-l-rhamnopyranoside (15). Both trisaccharides modules are suitable for the synthesis of branched pectin fragments.