A stereocontrolled construction of 2-azido-2-deoxy-1,2-cis-α-galactosidic linkages utilizing 2-azido-4,6-O-benzylidene-2-deoxygalactopyranosyl diphenyl phosphates: stereoselective synthesis of mucin core 5 and core 7 structures

TMSOTf-promoted glycosidation of 2-azido-4,6-O-benzylidene-2-deoxygalactosyl diphenyl phosphates with fluorenylmethoxycarbonyl (Fmoc)-protected serine and threonine derivatives in THF/Et₂O (1:1) gave glycosyl amino acids in high yields and with excellent levels of α-selectivity (α/β=94:6–95:5). The synthetic utility of the present glycosidation method was demonstrated by a stereoselective synthesis of mucin-type glycopeptide core 5 and core 7 building blocks, which are suitable for Fmoc-based solid-phase synthesis of O-glycopeptides.     

A Bacterial Chitinase Acts as Catalyst for Synthesis of the N‐Linked Oligosaccharide Core Trisaccharide by Employing a Sugar Oxazoline Substrate

A chitinolytic enzyme, chitinase A1 from Bacillus circulans WL‐12, was found to catalyze a glycosyl‐transferring reaction to form the N‐linked oligosaccharide core structure, Man(β1‐4)‐GlcNAc(β1‐4)‐GlcNAc, by employing Man(β1‐4)‐GlcNAc‐oxazoline as glycosyl donor. When the reaction was carried out in the presence of 20 v/v% acetone, the trisaccharide was obtained in 32% yield. It has been shown for the first time that a chitinase behaves like an endo‐β‐N‐acetylglucosaminidase in spite of low structural similarity between them.     

Propargyl mediated intramolecular aglycon delivery (IAD): applications to the synthesis of core N-glycan oligosaccharides

The use of propargyl mediated intramolecular aglycon delivery (IAD) for the synthesis of the key Manβ(1→4)GlcNAc linkage of N-glycan oligosaccharides, including the core N-glycan pentasaccharide, is investigated. Isomerisation of a 2-O-progargyl group of manno thioglycoside donors to an allene is followed by iodonium ion mediated mixed acetal formation with the 4-OH of protected GlcNAc acceptors, and subsequent intramolecular glycosylation occurs with complete control of anomeric stereochemistry to form the Manβ(1→4)GlcNAc linkage. A variety of linear and convergent approaches (1+2, 3+1, 3+2) to the core pentasaccharide are investigated as means of probing the generality and limitations of this type of intramolecular aglycon delivery for the formation of β-mannoside linkages in complex oligosaccharides.     

Oligosaccharide synthesis with glycosyl phosphate and dithiophosphate triesters as glycosylating agents

Described is an efficient one-pot synthesis of alpha- and beta-glycosyl phosphate and dithiophosphate triesters from glycals via 1,2-anhydrosugars. Glycosyl phosphates function as versatile glycosylating agents for the synthesis of beta-glucosidic, beta-galactosidic, alpha-fucosidic, alpha-mannosidic, beta-glucuronic acid, and beta-glucosamine linkages upon activation with trimethylsilyl trifluoromethanesulfonate (TMSOTf). In addition to serving as efficient donors for O-glycosylations, glycosyl phosphates are effective in the preparation of S-glycosides and C-glycosides. Furthermore, the acid-catalyzed coupling of glycosyl phosphates with silylated acceptors is also discussed. Glycosyl dithiophosphates are synthesized and are also used as glycosyl donors. This alternate method offers compatibility with acceptors containing glycals to form beta-glycosides. To minimize protecting group manipulations, orthogonal and regioselective glycosylation strategies with glycosyl phosphates are reported. An orthogonal glycosylation method involving the activation of a glycosyl phosphate donor in the presence of a thioglycoside acceptor is described, as is an acceptor-mediated regioselective glycosylation strategy. Additionally, a unique glycosylation strategy exploiting the difference in reactivity of alpha- and beta-glycosyl phosphates is disclosed. The procedures outlined here provide the basis for the assembly of complex oligosaccharides in solution and by automated solid-phase synthesis with glycosyl phosphate building blocks exclusively or in concert with other donors.     

Synthesis and use of glycosyl phosphates as glycosyl donors

Differentially protected glycosyl phosphates prepared by a straightforward synthesis from glycal precursors are used as powerful glycosyl donors. Activation of beta-glycosyl phosphates by TMSOTf at -78 degrees C achieves the selective formation of beta-glycosidic linkages in excellent yields with complete stereoselectivity. Reaction with thiols results in the conversion of glycosyl phosphates into thioglycosides in nearly quantitative yield. An orthogonal coupling strategy using glycosyl phosphate donors and thioethyl glycoside acceptors allows for the rapid synthesis of a trisaccharide.     

A Practical method for preparation of β-glycosides of N-acetylglucosamine

A mild and efficient method for preparation of GlcNAc derivatives by reaction of glycosyl acceptors with glycosyl oxazolines has been developed. The key feature is use of Yb(OTf)₃ as promoter, requiring moderate reaction times and equivalence stoichiometry. We anticipate application in the preparation of a wide range of derivatives of this biologically important class of building blocks.     

Synthesis of a Tetra- and a Hexasaccharide Donor Corresponding to the Non-Reducing Termini of Mycobacterial 3-O-Methylmannose Polysaccharide (MMP)

Abstract

The blockwise synthesis of the title compounds, namely the tetra- and the hexasaccharide trichloroacetimidates (20) and (23), is described. Both acetates and imidates were employed as glycosyl donors in most of the coupling reactions. As nearly all of the synthetic intermediates contain one or more OCH₃ groups, they are easily identified by NMR spectroscopy the methyl signals. The fully functionalized compounds 20 and 23 correspond to the non-reducing terminal fragments of mycobacterial 3-O-methylmannose polysaccharide (MMP), and can serve as suitable building blocks for the synthesis of higher-order structures of MMP.     

Synthesis, Antigenicity Against Human Sera and Structure-Activity Relationships of Carbohydrate Moieties from <em>Toxocara</em> larvae and Their Analogues

Stereocontrolled syntheses of biotin-labeled oligosaccharide portions containing the Galβ1-3GalNAc core of the TES-glycoprotein antigen obtained from larvae of the parasite Toxocara and their analogues have been accomplished. Trisaccharides Fuc2Meα1-2Gal4Meβ1-3GalNAcα1-OR (A), Fucα1-2Gal4Meβ1-3GalNAcα1-OR (B), Fuc2Meα1-2Galβ1-3GalNAcα1-OR (C), Fucα1-2Galβ1-3GalNAcα1-OR (D) and a disaccharide Fuc2Meα1-2Gal4Meβ1-OR (E) (R = biotinylated probe) were synthesized by block synthesis using 5-(methoxycarbonyl)pentyl-2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl-(1?3)-2-azide-4-O-benzyl-2-deoxy-α-D-galactopyranoside as a common glycosyl acceptor. We examined the antigenicity of these five oligosaccharides by enzyme linked immunosorbent assay (ELISA). Our results demonstrate that the O-methyl groups in these oligosaccharides are important for their antigenicity and the biotinylated oligosaccharides A, B, C and E have high serodiagnostic potential to detect infections caused by Toxocara larvae.     

Solid-phase synthesis of core 2 O-linked glycopeptide and its enzymatic sialylation

The core 2-type tetrasaccharide building blocks (1a/1b) for solid-phase synthesis of glycopeptide were synthesized via stereoselective glycosylation of the disaccharyl Ser/Thr (3a/3b) with a glycosyl fluoride (2) carrying the 2-trichloroacetamido group that was readily converted into a 2-acetamido group by reduction. A segment of glycoprotein leukosialin (215-224) was synthesized by the solid-phase protocol, the building block (1b) being utilized. Cleavage of the synthetic glycopeptide from resin was effected with reagent K and subsequent treatment of the product with a cocktail for the ‘low-acidity TfOH’ facilitated complete removal of the benzyl groups with minimum loss of glycosidic linkages. To the deprotected glycopeptide (21), were enzymatically introduced N-acetylneuraminic acid (sialic acid) residues in remarkably high efficiency by using the specific sialyltransferases.     

Toward the total synthesis of vineomycin B2: application of an efficient glycosylation methodology using 2,3-unsaturated sugars

The application of an efficient glycosylation methodology using 2,3-unsaturated sugars to synthesize critical precursors required for the total synthesis of an antibiotic, vineomycin B₂ (1), was demonstrated. The required disaccharide, the acurosyl rhodinose derivative of 1, was prepared by chemoselective glycosylation using a 2,3-saturated glycosyl acetate corresponding to the rhodinose moiety and a 2,3-unsaturated glycosyl acetate corresponding to the acurose portion. Further, the right-hand side chain of 1, consisting of β-oxo-tert-alcohol and rhodinose, was constructed by a powerful glycosylation approach using a 2,3-unsaturated glycosyl acetate in an ionic liquid under reduced pressure.     

Ring-opening polymerization of ε-caprolactone catalysed by (pyridyl)benzoazole Zn(II) and Cu(II) complexes

This paper describes the synthesis of (pyridyl)benzoazole Zn(II) and Cu(II) complexes and their applications as catalysts in ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). Reactions of 2-(3-pyridyl)-1H-benzimidazole (L1), 2-(2-pyridyl)-1H-benzothiazole (L2) and 2-(2-pyridyl)-1H-benzimidazole (L3) with Zn(II) and Cu(II) acetates produced the corresponding complexes; [Zn₂(L1)₂(OAc)₄)] (1), [Cu₂(L1)₂(OAc)₄] (2), [Zn(L2)(OAc)₂)] (3), [Zn(L3)(OAc)₂)] (4) and [Cu(L3), (OAc)₂)] (5). Molecular structures of complexes 2 and 5a revealed that while L1 adopts a monodentate binding mode, through the pyridyl nitrogen atom, L3 exhibits a bidentate coordination mode. All the complexes formed active catalysts in the ROP of ε-CL to afford moderate molecular weight polymers. The kinetics of the ROP reactions of ε-CL were pseudo-first-order with respect to monomer and catalysts.     

Linear synthesis of a protected H-type II pentasaccharide using glycosyl phosphate building blocks.

A linear synthesis of a fully protected H-type II blood group determinant pentasaccharide utilizing glycosyl phosphate and glycosyl trichloroacetimidate building blocks is reported. Envisioning an automated solid-phase synthesis of blood group determinants, the utility of glycosyl phosphates in the stepwise construction of complex oligosaccharides, such as the H-type II antigen, is demonstrated. Installation of the central glucosamine building block required the screening of a variety of nitrogen protecting groups to ensure good glucosamine donor reactivity and protecting group compatibility. The challenge to differentiate C2 of the terminal galactose in the presence of other hydroxyl and amine protecting groups prompted us to introduce the 2-(azidomethyl)benzoyl group as a novel mode of protection for carbohydrate synthesis. The compatibility of this group with traditionally employed protecting groups was examined, as well as its use as a C2 stereodirecting group in glycosylations. The application of the 2-(azidomethyl)benzoyl group along with a systematic evaluation of glycosyl donors allowed for the completion of the pentasaccharide and provides a synthetic strategy that is expected to be generally amenable to the solid support synthesis of blood group determinants.     

Synthesis of a N-glycan nonasaccharide of the bisecting type with additional core-fucose

A chemical synthesis was developed for N-glycans substituted with core-fucose and an additional bisecting GlcNAc (LEC10 motif). The synthesis was conducted using a suitably functionalized N-glycan pentasaccharide assembled from modular building blocks. Selective introduction of the bisecting GlcNAc residue was followed by attachment of the α1,6-arm and core-fucose. After introduction of an aminohexanoyl spacer the nonasaccharide was completely deprotected providing a substrate for enzymatic elongation and conjugation to proteins for further biological studies.     

Chemoselective Removal of Protecting Groups from O-Glycosyl Amino Acid and Peptide (Methoxyethoxy)ethyl Esters Using Lipases and Papain

The selective C-terminal deprotection of O-glycopeptide (methoxyethoxy)ethyl esters is achieved under mild conditions (pH 6.6, 37 degrees C) by enzymatic hydrolysis using papain or lipase M from Mucor javanicus to give building blocks useful for chain-extending glycopeptide synthesis. On the other hand, the selective removal of acetyl protecting groups from the saccharide portion of glycopeptides is accomplished by alternative enzymatic hydrolysis with lipase WG from wheat germ to furnish model substrates for enzymatic glycosyl transfer reactions in order to extend the carbohydrate side chain of these conjugates.     

Synthesis of an aryl 2-deoxy-β-glycosyl tetrasaccharide to probe retaining endo-glycosidase mechanism

The synthesis of the 1,3–1,4-β-glucanase substrate analogue 4-nitrophenyl O-β-d-glucopyranosyl-(1→4)-O-β-d-glucopyranosyl-(1→4)-O-β-d-glucopyranosyl-(1→3)-2-desoxi-β-d-glucopyranoside 2 is reported. Starting from the main tetrasaccharide obtained by enzymatic depolymerization of barley β-glucan, the synthetic scheme involves preparation of the corresponding 3-O-substituted glycal which was converted into a 2-deoxy-α-glycosyl iodide as a glycosyl donor. The key glycosylation step was successfully achieved by nucleophilic substitution of the iodide donor with 4-nitrophenolate with high β-selectivity.     

Zum verhalten konjugierter diene gegenüber C6H5J(OAc)2-(CH3)3SiN3

IR-spectroscopic measurements between ?60° and 0°C show the existence of C₆H₅J(OAc)_2-n(N₃)_n generated by the reaction of C₆H₅J(OAc)₂ with (CH₃)₃SiN₃. C₆H₅J(OAc)_2-n(N₃)_n reacts with 2,3-dimethylbutad to give substances C₁₂H₂₀N₆ (1), having the probable structure of 2,3,5-trimethyl-3,6-di-azidomethyl-heptadiene-1,5 and 3,4-diazido-3-methylbutanone-2 (2). Likewise we get from 1,3-cyclohexadiene the diazide C₁₂H₁₆N₆ (3), and 4-azido-cyclohexene-2-on-1 (4). Analogously from cyclopentadiene we get the carbonylcompound 4-azido-cyclopentene-2-on-1 (5). E-5-azido-hexene-3-on (8) results from 2,4-E,Z-hexadiene. 1,3-cyclooctadiene gives as main product 6 (4-azido-cyclooctene-2-on-1) but also some 7 (8-azido-cyclooctene-2-on-1) emerging from an attack on positions 1 and 2.Δ^1,3, Δ^3,5 and Δ^4,6-cholestadiene yield in a regio- and stereo-specific manner the products 9 (1α-azidocholestene-2-on-4), 10 (6β-azidocholestene-4-on-3) and 11 (4β-azidocholestene-5-on-7). These last examples confirm our suggestions from the preponderance of the 1,4-functionalisation.     

New synthesis of β-lactams

α-Methylene-β-lactams 3 were synthesized from the various 2-bromoallylamine derivatives 2 using a catalytic amount of Pd(OAc)₂ or PD(acac)₂ and PPh₃, under 1–4 atom pressure of CO in good yields. Similarly, α-alkylidene-β-lactams 20 were synthesized from 3-alkyl-2-bromoallylamines 19, which were easily prepared from the olefins 14, in the same manner.     

A versatile approach to pyrrolidine azasugars and homoazasugars based on a highly diastereoselective reductive benzyloxymethylation of protected tartarimide

A highly diastereoselective synthesis of enantio-enriched all trans-3,4-dibenzyloxyl-5-benzyloxymethyl-2-pyrrolidinone 13a was developed based on SmI₂-mediated benzyloxymethylation of O,O′-dibenzyltartarimide. The versatility of 13a and its antipode as the key building blocks for the asymmetric synthesis of pyrrolidine azasugars and homoazasugars has been demonstrated by elaborating them into naturally occurring DAB 1 (1), LAB 1 (2), N-hydroxyethyl-DAB 1 (4), 6-deoxy-DMDP 7, and 5-epi-radicamine B 36 as well as the reductive ring-opening product 35.     

Cellulose Oligomers: Preparation from Cellulose Triacetate, Chemical Transformations and Reactions

Cellulose oligomers obtained by a new degradation method (pivaloylysis) are used as starting materials in organic synthesis. On the one hand these oligomers are functionalized to potent glycosyl donors, on the other hand several methods are shown to generate different types of hydroxy compounds (glycosyl acceptors). Glycosidation reactions performed with these two types of building blocks allow an access to linear products (unidisperse celluloses) as well as to new branched cellulose units.     

Conversion of Complex Sialooligosaccharides into Polymeric Conjugates and their Anti-Influenza Virus Inhibitory Potency

ABSTRACT

To investigate the specificity of various influenza virus strains we have prepared polyacrylic type conjugates of undecasaccharide (Neu5Acα2-6Galβ1-4GlcNAcβ1-2Manα1)₂-3,6Manβ1-4GlcNAcβ1-4GlcNAc (YDS), and trisaccharides 6‵-sialyl-N-acetyllactosamine (6‵SLN), 6‵-sialyllactose (6‵SL), and 3‵-sialyllactose (3‵SL). Free oligosaccharides were transformed to glycosylamine-1-N-glycyl derivatives by sequential action of NH₄HCO₃, chloroacetic anhydride, and aqueous NH₃. The known derivatization protocol has been optimized for these sialooligosaccharides. Coupling of obtained amino-spacered derivatives with poly(4-nitrophenyl acrylate) gave rise to two types of conjugates, namely with polyacrylic acid and polyacrylamide backbones; the conversion proceeded quantitatively and without destruction of the oligosaccharides. The content of oligosaccharides in the conjugates was 10, 20, and 30% mol for 3‵SL, 6‵SL, 6‵SLN, and 2, 5 and 10% mol for YDS. Free oligosaccharides and the glycoconjugates were tested as inhibitors of influenza virus adhesion, and also as blockers of virus infectivity in MDCK cell culture. Biantennary YDS demonstrated similar activity to trisaccharide 6‵SLN both as the free form and neoglycoconjugate.