Scope and limitation of the application of the (methoxydimethyl)methyl group in the synthesis of 2′-O-, 6′-O- and 2′,6′-di-O-(α-l-arabinofuranosyl)-β-d-galactopyranosyl-(1→6)-d-galactoses

For the characterisation of the anticipated epitope of an arabinogalactan, isolated from the extract of Echinacea purpurea, the trisaccharide α-l-Araf-(1→2)-β-d-Galp-(1→6)-d-Gal was synthesized. The easily available 3,4-O-isopropylidene-6-O-(methoxydimethyl)methyl-β-d-galactopyranosyl-(1→6)-1,2:3,4-di-O-isopropylidene-α-d-galactopyranose having the OH-2′ free served as aglycone upon direct arabinosylation at the 2′ position under Helferich conditions. The formed HgBr2 cleaved the acid sensitive protecting group at position 6′, but under basic conditions the desired, fully protected trisaccharide, or its symmetrical dimerization derivative linked 6′- to 6′-position via an isopropylidene bridge, could be obtained. An alternative route involved arabinosylation of a hepta-O-acetylated digalactose with free OH-2′. Two other oligosaccharides, namely, α-l-Araf-(1→6)-β-d-Galp-(1→6)-d-Gal and (α-l-Araf)2-(1→2,6)-β-d-Galp-(1→6)-d-Gal were also synthesized and characterised.