Synthesis of lacto-N-neotetraose and lacto-N-tetraose using the dimethylmaleoyl group as amino protective group

The disaccharide donor O-[2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-(1→4)-3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-α,β-d-glucopyranosyl] trichloroacetimidate (7) was prepared by reacting O-(2,3,4,6-tetra-O-acetyl-α-d-galactopyranosyl) trichloroacetimidate with tert-butyldimethylsilyl 3,6-di-O-benzyl-2-deoxy-2-dimethylmaleoylamido-glucopyranoside to give the corresponding disaccharide 5. Deprotection of the anomeric center and then reaction with trichloroacetonitrile afforded 7. Reaction of 7 with 3′-O-unprotected benzyl (2,4,6-tri-O-benzyl-β-d-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-d-glucopyranoside (8) as acceptor afforded the desired tetrasaccharide benzyl (2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-(1→4)-(3,6-di-O-benzyl-2-deoxy-2-dimethylmaleimido-β-d-glucopyranosyl)-(1→3)-(2,4,6-tri-O-benzyl-β-d-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-d-glucopyranoside. Replacement of the N-dimethylmaleoyl group by the acetyl group, O-debenzylation and finally O-deacetylation gave lacto-N-neotetraose. Similarly, reaction of O-[(2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-(1→3)-4,6-O-benzylidene-2-deoxy-2-dimethylmaleimido-α,β-d-glycopyranosyl] trichloroacetimidate as donor with 8 as acceptor afforded the desired tetrasaccharide benzyl (2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-(1→3)-(4,6-benzylidene-2-deoxy-2-dimethylmaleimido-β-d-glucopyranosyl)-(1→3)-(2,4,6-tri-O-benzyl-β-d-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-d-glucopyranoside. Removal of the benzylidene group, replacement of the N-dimethylmaleoyl group by the acetyl group and then O-acetylation afforded tetrasaccharide intermediate 15, which carries only O-benzyl and O-acetyl protective groups. O-Debenzylation and O-deacetylation gave lacto-N-tetraose (1). Additionally, known tert-butyldimethylsilyl (2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-(1→3)-4,6-O-benzylidene-2-deoxy-2-dimethylmaleimido-β-d-glucopyranoside was transformed into O-[2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl)-(1→3)-4,6-di-O-acetyl-2-deoxy-2-dimethylmaleimido-α,β-d-glucopyranosyl] trichloroacetimidate as glycosyl donor, to afford with 8 as acceptor the corresponding tetrasaccharide 22, which is transformed into 15, thus giving an alternative approach to 1.