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-Ara f-(1→2)-β- d-Gal p-(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 HgBr 2 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-Ara f-(1→6)-β- d-Gal p-(1→6)- d-Gal and (α- l-Ara f) 2-(1→2,6)-β- d-Gal p-(1→6)- d-Gal were also synthesized and characterised.