Synthesis of an octasaccharide fragment of high-mannose-type glycans of glycoproteins

Multisite Partial Glycosylation Approach for Preparation of Biologically Relevant Oligomannosyl Branches Contribute to Lectin Affinity Analysis

High-mannose-type glycans play essential biological roles, e.g., immune response and glycoprotein quality control, and preparing a series of oligomannosyl branches of high-mannose-type glycans is critical for biological studies. However, obtaining sufficient amounts of the various oligomannosyl branches is challenging. In this study, we demonstrated a partial glycosylation strategy for the single-step synthesis of various biologically relevant oligomannosyl-branched structures. First, Manα1-6(Manα1-3)Man-type oligomannosyl branch was synthesized via double glycosylation from a 3,6-di-OH mannosyl acceptor and fluorinated mannosyl donor with perfect α-selectivity. Subsequent partial glycosylation by reducing the equivalent of the mannosyl donor enabled to obtain biologically relevant Manα1-2Manα1-6(Manα1-2Manα1-3)Man, Manα1-6(Manα1-2Manα1-3)Man, Manα1-2Manα1-6(Manα1-3)Man, and Manα1-6(Manα1-3)Man in one-pot. Each oligomannosyl branch could be easily purified by liquid chromatography. The resulting structural isomers were identified by 2D-HMBC NMR. A systematic lectin affinity assay using the prepared oligomannosyl branches showed different specificities for the Galanthus nivalis lectin between structural isomers of the oligomannosyl branches with the same number of mannose residues..

Convergent synthesis of oligomannose-type glycans via step-economical construction of branch structures

Oligomannose-type glycans on glycoproteins are important signaling molecules in the glycoprotein quality control system in the endoplasmic reticulum. Recently, free oligomannose-type glycans generated by the hydrolysis of glycoproteins or dolichol pyrophosphate-linked oligosaccharides were recognized as important signals for immunogenicity. Hence, there is a high demand for pure oligomannose-type glycans for biochemical experiments; however, the chemical synthesis of glycans to achieve high-concentration products is laborious. In this study, we demonstrate a simple and efficient synthetic strategy for oligomannose-type glycans. Sequential regioselective α-mannosylation at the C-3 and C-6 positions of 2,3,4,6-unprotected galactose residues in galactosylchitobiose derivatives was demonstrated. Subsequently, the inversion of the configuration of the two hydroxy groups at the C-2 and C-4 positions of the galactose moiety was successfully carried out. This synthetic route reduces the number of the protection-deprotection reactions and is suitable for constructing different branching patterns of oligomannose-type glycans, such as M9, M5A, and M5B.

Chemical synthesis of homogeneous glycopeptides and glycoproteins

Oligosaccharides linked to proteins are known to play important roles in several biological events. However, oligosaccharides are heterogeneous, which has hindered detailed elucidation of oligosaccharide functions. In order to solve this problem, glycoproteins having homogeneous oligosaccharides have long been required. For this purpose, an efficient preparative method of complex-type oligosaccharides has been investigated from a natural source and this method was found to afford over 24 kinds of diverse complex-type oligosaccharides by use of chemical methods and branch-specific sequential glycosidase digestion. The sufficient amount of homogeneous complex type oligosaccharides obtained enabled us to examine the synthesis of homogeneous glycopeptides as well as glycoproteins by use of solid phase glycopeptide synthetic method and native chemical ligation. This review describes recent progress related to the efficient method of oligosaccharide preparation and synthesis of glycoproteins including bioactive erythropoietin.

Improved synthesis of 5-hydroxylysine (Hyl) derivatives*

The synthesis of 5-hydroxylysine (Hyl) derivatives for incorporation by solid-phase methodologies presents numerous challenges. Hyl readily undergoes intramolecular lactone formation, and protected intermediates often have poor solubilities. The goals of this work were twofold: first, develop a convenient method for the synthesis of O-protected Fmoc-Hyl; secondly, evaluate the efficiency of methods for the synthesis of O-glycosylated Fmoc-Hyl. The 5-O-tert-butyldimethylsilyl (TBDMS) fluoren-9-ylmethoxycarbonyl-Hyl (Fmoc-Hyl) derivative was conveniently prepared by the addition of tert-butyldimethylsilyl trifluoromethanesulfonate to copper-complexed Hyl[epsilon-tert-butyloxycarbonyl (Boc)]. The complex was decomposed with Na+ Chelex resin and the Fmoc group added to the alpha-amino group. Fmoc-Hyl(epsilon-Boc, O-TBDMS) was obtained in 67% overall yield and successfully used for the solid-phase syntheses of 3 Hyl-containing peptides. The preparation of Fmoc-Hyl[epsilon-Boc, O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)] was compared for the thioglycoside, trichloroacetimidate and Koenigs-Knorr methods. The most efficient approach was found to be Koenigs-Knorr under inverse conditions, where Fmoc-Hyl(epsilon-Boc)-OBzl and peracetylated galactosyl bromide were added to silver trifluoromethanesulfonate in 1,2-dichloroethane, resulting in a 45% isolated yield. Side-reactions that occurred during previously described preparations of glycosylated Hyl derivatives, such as lactone formation, loss of side-chain protecting groups, orthoester formation, or production of anomeric mixtures, were avoided here. Research on the enzymology of Lys hydroxylation and subsequent glycosylation, as well as the role of glycosylated Hyl in receptor recognition, will be greatly aided by the convenient and efficient synthetic methods developed here.

An approach for a synthesis of asparagine-linked sialylglycopeptides having intact and homogeneous complex-type undecadisialyloligosaccharides

This paper describes synthesis of asparagine-linked sialylglycopeptides. The typical feature of our strategy for the synthesis of a sialylglycopeptide is to employ undecadisialyloligosaccharyl Fmoc-asparagine (Fmoc-Asn(CHO)-OH) 1 without protecting groups on its hydroxyl groups except for the benzyl ester of the NeuAc residues. Our synthetic methodology solved the problem of esterification toward sugar hydroxyl groups by activated amino acids during the elongation of a peptide chain. When employing high concentrations of the Fmoc-amino acid, esterification markedly occurred, but the esterification scarcely occurred when employing low concentrations of reactants. Taking advantage of these findings, we examined the synthesis of a high molecular sialylglycopeptide, CTLA-4 fragment (113-150) 13 having two complex-type sialyloligosaccharides by use of native chemical ligation (NCL). As a result, we succeeded in the synthesis of a sialylglycopeptide having a cysteine residue at the N-terminus (CTLA-4: 129-150 fragment) 11 and a sialylglycopeptide-thioester (CTLA-4: 113-128 fragment) 12. Finally, the sialylglycopeptides synthesized were applied to NCL reactions. The reaction successfully afforded the desired product, CTLA-4 (113-150) 13 containing mature and pure complex-type sialyloligosaccharides in excellent purity.

Rationally designed syntheses of high-mannose and complex type undecasaccharides

Synthetic routes are described to a high-mannose type triantenary undecasaccharide 1 and a completely protected form 39 of the complex type biantenary undecasaccharide 2 carrying alpha-(2-->3)-linked sialic acid residues. Starting from a previously reported trisaccharide 4, the core pentasaccharides 15 and 37 were synthesized through regio- and/or stereo-selective mannosylation with a suitably protected mannosyl donor. Chain elongation of 15 by stepwise addition of a mannose residue afforded an undecasaccharide 20 that was eventually deprotected to give 1. On the other hand, coupling of 38 and a trisaccharide glycosyl donor 36 afforded 39.