Abstract :
A series of octyl glycosides di- to tetrasaccharides related to the GPI anchor of Trypanosoma brucei was prepared. Treatment of octyl 2-O-benzoyl-4,6-O-(1,1,3,3-tetraisopropyl-1,3-disiloxane-1,3-diyl)-α-d-mannopyranoside with ethyl 2,3,4,6-tetra-O-benzyl-1-thio-β-d-galactopyranoside under activation with bromine and silver trifluoromethanesulfonate afforded the α-linked disaccharide octyl 2-O-benzoyl-3-O-(2,3,4,6-tetra-O-benzyl-α-d-galactopyranosyl)-4,6-O-(1,1,3,3-tetraisopropyl-1,3-disiloxane-1,3-diyl)-α-d-mannospyranoside, the siloxane ring of which was regioselectively opened with a HF–pyridine complex to give the disaccharide acceptor octyl 3-O-(2,3,4,6-tetra-O-benzyl-α-d-galactopyranosyl)-2-O-benzoyl-4-O-(3-fluoro-1,1,3,3-tetraisopropyl-1,3-disiloxane-3-yl)-α-d-mannopyranoside (4). Mannosylation of 4 with benzobromomannose (7), followed by fluoride catalyzed desilylation gave the trisaccharide octyl 2-O-benzoyl-6-O-(2,3,4,6-tetra-O-benzoyl-α-d-mannopyranosyl)-3-O-(2,3,4,6-tetra-O-benzyl-α-d-galactopyranosyl)-α-d-mannospyranoside, which was deblocked via the deacylated intermediate octyl 3-O-(2,3,4,6-tetra-O-benzyl-α-d-galactopyranosyl)-6-O-(α-d-mannopyranosyl)-α-d-mannospyranoside to afford the octyl glycoside trisaccharide octyl 3-O-(α-d-galactopyranosyl)-6-O-(α-d-mannopyranosyl)-α-d-mannospyranoside. Glycosylation of 4 with 3,4,6-tri-O-acetyl-2-O-(2,3,4,6-tetra-O-benzoyl-α-d-mannopyranosyl)-α-d-mannopyranosyl trichloroacetimidate resulted in the tetrasaccharide octyl 2-O-benzoyl-4-O-(1-fluoro-1,1,3,3-tetraisopropyl-1,3-disiloxane-3-yl)-3-O-(2,3,4,6-tetra-O-benzyl-α-d-galactopyranosyl)-6-O-[2-O-(2,3,4,6-tetra-O-benzoyl-α-d-mannopyranosyl)-3,4,6-tri-O-acetyl-α-d-mannopyranosyl]-α-d-mannospyranoside, sequential desilylation, deacylation and debenzylation, respectively, of which via the intermediate octyl 2-O-benzoyl-3-O-(2,3,4,6-tetra-O-benzyl-α-d-galactopyranosyl)-6-O-[2-O-(2,3,4,6-tetra-O-benzoyl-α-d-mannopyranosyl)-3,4,6-tri-O-acetyl-α-d-mannopyranosyl]-α-d-mannospyranoside afforded the octyl glycoside tetrasaccharide octyl 3-O-(α-d-galactopyranosyl)-6-O-[2-O-(α-d-mannopyranosyl)-α-d-mannopyranosyl]-α-d-mannospyranoside.
