Synthesis of an Asparagine-Linked Heptasaccharide − Basic Structure of N-Glycans

Galβ(1−4)GlcNAcβ(1−2)Manα(1−3)[Manα(1−6)]Manβ(1−4)GlcNAcβ(1−4)GlcNAcβ(1−N)Asn (1) was disconnected into building blocks 2−6. N-Dimethylmaleoyl (DMM) protected glucosamine 3 was readily obtained from glucosamine. Transformation of 3 into 4-O-unprotected glucosamine derivatives 9, 14, and 18 furnished the intermediates required for the incorporation of the three differently linked glucosamine residues. Thus, disaccharide 20 was obtained from acceptor 9 and glucosyl donor 19 and converted into protected Manβ(1−4)GlcNAc disaccharide donor 5 by inversion of the configuration at C-2 of the glucose residue. Glycosylation of acceptor 18 with known galactosyl donor 26 afforded protected lactosamine donor 6. The synthesis of asparagine building block 2 and of mannosyl donor 4 has already been reported. With building blocks 2−6 in hand, the synthesis of 1 was accomplished. Glycosylation of acceptor 14 with donor 5 gave trisaccharide 29 which was transformed into acceptor 30. Treatment of 30 with glycosyl donor 4 provided tetrasaccharide intermediate 31. Its transformation into 2d-O-unprotected acceptor 33 and then reaction with disaccharide donor 6 furnished hexasaccharide 34. Removal of the 4c,6c-O-benzylidene group gave 4c,6c-O-unprotected acceptor 35 which, on glycosylation with donor 4, led to heptasaccharide 36. Replacement of the N-DMM groups by N-acetyl groups and removal of all O-acyl groups, followed by transformation of the C-1a azido group into an amino group, and attachment of asparagine building block 2 led to the desired heptasaccharide β-linked to asparagine 39. Hydrogenolysis of all O-benzyl groups afforded target molecule 1.