Joachim Seifert

Chemoenzymatic synthesis of deca and dodecasaccharide N-glycans of the ‘bisecting’ type†

Abstract The synthesis of N -glycans carrying the ‘bisecting’ GlcNAc modification is difficult due to steric hindrance at the central β-mannoside. A facile introduction of a ‘bisecting’ GlcNAc residue was accomplished by using an excess of a glucosaminyl fluoride. After deprotection of the resulting octasaccharide a 6-aminohexanoyl spacer was attached. Enzymatic elongation of the carbohydrate chains using three different glycosyltransferases gave full length ‘bisecting’ N -glycans of the complex type terminating with galactose, α-2,6 or α-2,3 linked sialic acid.

Synthesis of a core-fucosylated, biantennary octasaccharide as a precursor for glycopeptides of complex N-glycans

Abstract A convergent synthesis of the biantennary and core-fucosylated octasaccharide 20 (a protected form of A ) is described. Octasaccharide 20 is designed to serve as a precursor for dodecasaccharide 1 , a complex N -glycan frequently found in glycoproteins of the serum and the cell surface.

Synthesis of Multiantennary Complex Type N‐Glycans by Use of Modular Building Blocks

A modular set of oligosaccharide building blocks was developed for the synthesis of multiantennary N-glycans of the complex type, which are commonly found on glycoproteins. The donor building blocks were laid out for the elongation of a core trisaccharide acceptor (beta-mannosyl chitobiose) conveniently protected with a single benzylidene moiety at the beta-mannoside. Through two consecutive regio- and stereoselective couplings the donors gave N-glycans with three to five antennae in high yields. Due to the consistent protection group pattern of the donors the deprotection of the final products can be performed by using a general reaction sequence.

Synthesis of three biantennary N-glycans containing the α-1,6 core-fucosyl motif

Combined chemical and enzymatic synthesis gave access to the biantennary corefucosylated N-glycans A,B and C terminating with galactose, α-2,6 or α-2,3 linked sialic acid. The oligosaccharides were functionalized at the anomeric center with a 6-aminohexanoyl spacer to facilitate their incorporation into neoglycoproteins. As a versatile precursor served the chemically synthesized core-fucosylated octasaccacharide 2.

Application of novel, N-DTPM protected d-glucosamine building blocks in oligosaccharide synthesis

Abstract A study of glycosylation reactions was performed, using novel N -DTPM protected glucosamine donors 1 – 3 and acceptors 4 – 9 ranging from achiral primary alcohols to various alcohols within the d -GlcNAc-, d -Gal- and d -Glu-series. The results show high yielding reactions with good β-selectivities.

Syntheses of α-dystroglycan derived sialylated glycosyl amino acids carrying a novel mannosyl serine/threonine linkage

Abstract An efficient access to the Fmoc protected glycosyl amino acids 20a-d which correspond to the unique sequence NeuNAc -α-(2,3)- Gal -β-(1,4)- GlcNAc -β-(1,2)- Man -α- Ser Thr recently found in laminin-binding protein α-dystroglycan, is described. Tetrasaccharide donor 18 was constructed from monosaccharide blocks 2, 3, 8 and 9 and coupled with Ser Thr derivatives 19a-d. Resultant glycosyl amino acid 20d was completely deprotected into 1a.

Syntheses of α-dystroglycan derived glycosyl amino acids carrying a novel mannosyl serine/threonine linkage

Abstractα-Dystroglycan (α-DG) is a membrane-associated, extracellular glycoprotein. It is anchored to the cell-membrane by binding to the transmembrane glycoprotein β-dystroglycan (β-DG) to form an α/β-DG-complex. It was discovered that the bovine peripheral nerve α-DG possesses the Ser/Thr linked tetrasaccharide as the major constituent of the O-linked carbohydrates, which was proposed to contribute laminin binding activity of this glycoprotein.This structure has a striking feature in terms of the mode of linkage between oligosaccharide and the core protein. It has a mannose residue linked to the core protein through Ser/Thr residue. A similar structure was proposed to exist in brain derived HNK-1 immunoreactive O-glycans. Being interested in the structural novelty and potential biological significance of this type of glycan chains, the chemical synthesis of Ser/Thr linked mannose containing tetrasaccharide was investigated. Tetrasaccharide donor was constructed from monosaccharide blocks and coupled with Ser/Thr derivatives. Subsequent deprotection afforded target tetraosyl serine. Furthermore, synthetic routes to lower homologues, namely Gal-β-(1,4)-GlcNAc-β-(1,2)-Man-α-Ser and GlcNAc-β-(1,2)-Man-α-Ser were also provided.