Synthesis of glycosylamides and 4-N-glycosyl-l-asparagine derivatives

Urea-catalyzed N-Glycosylation of Amides/Azacycles with Glycosyl Halides

The efficient synthesis of N-glycosides via direct N-glycosylation of amides/azacycles has been reported. The glycosylation of amides/azacycles with glycosyl halides in the presence of a catalytic amount of urea proceeded smoothly to provide the corresponding N-glycosylated amides or nucleosides in good to excellent yields with 1,2-trans-stereoselectivity. Moreover, by the addition of terpyridine, the 1,2-cis-stereoselectivity was achieved.

Palladium-catalyzed glycal imidate rearrangement: formation of alpha- and beta-N-glycosyl trichloroacetamides

A novel palladium(II)-catalyzed stereoselective synthesis of alpha- and beta-N-glycosyl trichloroacetamides has been developed. The alpha- and beta-selectivity at the anomeric carbon depends on the nature of the palladium-ligand catalyst. While the cationic palladium(II) promotes the alpha-selectivity, the neutral palladium(II) favors the beta-selectivity.

Synthesis of α- and β-Glycosyl Isothiocyanates via Oxazoline Intermediates

A practical synthesis of acylated glycosyl isothiocyanates from sugar oxazolines, by reaction with thiophosgene, is reported. In the absence of any additive, the reaction is governed by the reverse anomeric effect, leading to the equatorially oriented isothiocyanate. However, in the presence of copper(II) chloride, the reaction proceeds preferentially with retention of the configuration at the anomeric center, providing the axial anomer as the major product. Noteworthy, this strategy allows accessing per-O-acetylated glycopyranosyl isothiocyanates with 1,2-cis relative configuration (e.g., the alpha-anomer in the D-gluco and D-galacto series), a problem that was outside the scope of previous methodologies.

Glycopeptide synthesis and the effects of glycosylation on protein structure and activity

Despite the omnipresence of protein glycosylation in nature, little is known about how the attachment of carbohydrates affects peptide and protein activity. One reason is the lack of a straightforward method to access biologically relevant glycopeptides and glycoproteins. The isolation of homogeneous glycopeptides from natural sources is complicated by the heterogeneity of naturally occuring glycoproteins. It is chemical and chemoenzymatic synthesis that is meeting the challenge to solve this availability problem, thus playing a key role for the advancement of glycobiology. The current art of glycopeptide synthesis, albeit far from being routine, has reached a level of maturity that allows for the access to homogeneous and pure material for biological and medicinal research. Even the ambitious goal of the total synthesis of an entire glycoprotein is within reach. It is demonstrated that with the help of synthetic glycopeptides the effects of glycosylation on protein structure and function can be studied in molecular detail. For example, in immunology, synthetic (tumour-specific) glycopeptides can be used as immunogens to elicit a tumour-cell-specific immune response. Again, synthetic glycopeptides are an invaluable tool to determine the fine specificity of the immune response that can be mediated by both carbohydrate-specific B and T cells. Furthermore, selected examples for the use of synthetic glycopeptides as ligands of carbohydrate-binding proteins and as enzyme substrates or inhibitors are presented.

Detailed studies on substrate structure requirements of glycoamidases A and F

Glycoamidases (peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase, EC 3.5.1.52; also known as peptide: N-glycanases (PNGases) release N-linked oligosaccharides from glycopeptides and/or glycoproteins by hydrolyzing the glycosylated beta-amide bond of the asparagine side chain. The most widely used glycoamidases are those from Flavobacterium meningosepticum (glycoamidase F or PNGase F) and almond emulsin (glycoamidase A or PNGase A). To study the substrate structure requirement of these enzymes systematically, we synthesized >30 glycopeptides containing cellobiose, lactose, GlcNAc, and di-N,N'-acetylchitobiose (CTB). The length of the peptide was varied from one to five amino acids, and glycosylamines were linked to either Asn or Gln located at different positions in the peptide, including NH2 and COOH termini. Neither enzyme could cleave cellobiose and lactose glycopeptides, indicating that the 2-acetamido group on the Asn-linked GlcNAc is important in the recognition by both glycoamidases A and F. GlcNAc peptides could be cleaved by both enzymes, albeit not as effectively as CTB glycopeptides. Neither enzyme requires the Asn-Xaa-(Ser/Thr) sequence (required for N-glycosylation) for activity. Glycoamidase A could even hydrolyze a Gln-bound CTB glycopeptide, whereas the action of glycoamidase F on this substrate is minimal. While glycoamidase A could act on CTB dipeptides, glycoamidase F preferred a tripeptide or longer. The Km and Vmax values of glycoamidase A for t-butoxycarbonyl-(CTB)-Asn-Ala-Ser-OMe were 2.1 mM and 0.66 micromol/min/mg, respectively. A natural glycodipeptide, Man9-GlcNAc2-Asn-Phe, was also completely hydrolyzed by glycoamidase A.

Isothiocyanates and cyclic thiocarbamates of alpha,alpha'-trehalose, sucrose, and cyclomaltooligosaccharides

6,6'-Dideoxy-6,6'-diisothiocyanato-alpha,alpha'-trehalose (4), 6-deoxy-6-isothiocyanato-alpha-D-fructo-furanose beta-D-fructopyranose 1,2':2,1'-dianhydride (11), 6,6'-dideoxy-6,6'-diisothiocyanatosucrose (16), and per(6-deoxy-6-isothiocyanato)-cyclomaltohexaose (23), -cyclomaltoheptaose (27), and -cyclomaltooctaose (31) have been prepared in high yield by reaction of the corresponding amino sugars with thiophosgene. In the absence of base, all isothiocyanates were stable and could be stored and acetylated without decomposition. In the presence of triethylamine, 6,6'-dideoxy-6,6'-diisothiocyanato-alpha,alpha'-trehalose underwent intramolecular cyclisation involving HO-4 to give the corresponding bis(cyclic thiocarbamate). The product of cyclisation at a single glucopyranosyl unit was obtained in the treatment of the above diisothiocyanate with mixed (H+, HO-) ion-exchange resin. Under identical reaction conditions, 6,6'-dideoxy-6,6'-diisothiocyanatosucrose yielded exclusively the product of intramolecular cyclisation at the D-glucopyranosyl moiety, while derivatives of alpha-D-fructofuranose beta-D-fructopyranose 1,2':2,1'-dianhydride and cyclomaltooligosaccharides remained unchanged.

Chemical and enzymatic synthesis of glycopeptides

Progress recently made in the synthesis of biologically relevant N- and O-glycopeptides is illustrated by examples. In this context, developments in the preparation of complex saccharide side chains and in the subsequent coupling to peptide portions is described. Special emphasis is given to the synthesis of Lewis antigen-type structures. Furthermore, modern methods in solid phase peptide syntheses utilizing glycosylated building blocks are presented. Recent advances in glycopeptide syntheses employing enzymatic methods in deprotection steps as well as in peptide/saccharide chain elongation are reported.