Solid-phase synthesis of an O-linked glycopeptide based on a benzyl-protected glycan approach

Chemical synthesis of the rare D-Fuc3NAc containing tetrasaccharide repeating unit of the O-antigenic polysaccharide from E. coli O74

Chemical synthesis of the tetrasaccharide repeating unit of the O-antigen from E. coli O74 is accomplished by a convergent [2 + 2] block synthesis strategy. The challenging rare D-Fuc3NAc has been prepared using DTBP and TIPST mediated deoxygenation reaction. Other monosaccharide synthons are prepared through rational protecting group manipulations and the stereoselective glycosylations are achieved either by the activation of thioglycoside or glycosyl trichloroacetimidate. The target tetrasaccharide is made in the form of its 2-aminoethyl glycoside to facilitate further glycoconjugate formation without affecting the anomeric stereochemistry.

Chemical Synthesis and Biological Applications of O-GlcNAcylated Peptides and Proteins

All human cells use O-GlcNAc protein modifications (O-linked N-acetylglucosamine) to rapidly adapt to changing nutrient and stress conditions through signaling, epigenetic, and proteostasis mechanisms. A key challenge for biologists in defining precise roles for specific O-GlcNAc sites is synthetic access to homogenous isoforms of O-GlcNAc proteins, a result of the non-genetically templated, transient, and heterogeneous nature of O-GlcNAc modifications. Toward a solution, this review details the state of the art of two strategies for O-GlcNAc protein modification: advances in "bottom-up" O-GlcNAc peptide synthesis and direct "top-down" installation of O-GlcNAc on full proteins. We also describe key applications of synthetic O-GlcNAc peptide and protein tools as therapeutics, biophysical structure-function studies, biomarkers, and as disease mechanistic probes to advance translational O-GlcNAc biology.

Chemical synthesis of the 4-amino-4,6-dideoxy-d-glucose containing pentasaccharide repeating unit of the O-specific polysaccharide from Aeromonas hydrophila strain K691 in the form of its 2-aminoethyl glycoside

Chemical synthesis of the pentasaccharide repeating unit of the O-specific polysaccharide from Aeromonas hydrophilastrain K691 is reported. Synthesis of the pentasaccharide is accomplished by using a common disaccharide in sequence and finally attaching the rare sugar unit. The target structure was made in the form of its 2-aminoethyl glycoside which is essential for further glycconjugate formation. Stereoselective glycosylations were achieved by the activation of thioglycosides in the presence of H₂SO₄-silica in conjunction with N-iodosuccinimide.

Convergent chemical synthesis of the pentasaccharide repeating unit of the O-antigen from E. coli O158

Synthesis of the pentasaccharide repeating unit of the O-antigen fromE. coliO158 through a convergent [3 + 2] strategy is reported.

Solid-Phase Synthesis of Core 8O-Glycan-Linked MUC5AC Glycopeptide

The benzyl-protected disaccharide building blocks of core 8 O-glycan (15a/15b) for glycopeptide were stereoselectively synthesized by two glycosidation reactions with the glycosyl fluoride method. The building blocks were utilized in the solid-phase synthesis of a glycopeptide carrying two O-glycans with the consensus sequence of the tandem-repeat domain of MUC5AC. The synthetic glycopeptide was detached from the resin with reagent K, and subsequent debenzylation under conditions of low-acidity TfOH afforded glycopeptide 2. The synthetic sample will be used as a suitable standard in studies of the physicochemical or immunochemical characterization of mucin glycoforms.

Chemoenzymatic Synthesis of a MUC1 Glycopeptide Carrying Non-Natural Sialyl TF-βO-Glycan

A MUC1 type of glycopeptide was synthesized by the 9-fluorenylmethoxycarbonyl (Fmoc) solid-phase peptide synthesis (SPPS) protocol using benzyl and benzylidene-protected beta-D-Gal-(1-->3)-beta-D-GalNAc-Ser/Thr (TF-beta: a stereoisomer of the Thomsen-Friedenreich antigen). The synthetic glycopeptide was released from the resin with reagent K, and the resulting benzylated glycopeptide was deprotected under conditions of low-acidity trifluoromethanesulfonic acid (TfOH). The glycopeptide carrying duplicate non-natural O-glycans was dominant in the products, but was accompanied by a considerable amount of the glycopeptide missing one of the O-glycans. In contrast, the native alpha-glycoside was relatively stable under the acidic debenzylation conditions as shown by a parallel experiment with the glycopeptide involving alpha-D-GalNAc-Ser/Thr linkage. Enzymatic glycosylation with CMP-NeuAc was successful with both natural and non-natural O-glycans of the synthetic glycopeptide.

Expedient synthesis of a pentasaccharide related to the O-specific polysaccharide of Escherichia coli O117:K98:H4 strain

A convenient synthetic strategy has been developed for the synthesis of a pentasaccharide, related to the O-specific polysaccharide ofEscherichia coliO117:K98:H4 strain, using sequential glycosylations of functionalized monosaccharide moieties.

Divergent behavior of glycosylated threonine and serine derivatives in solid phase peptide synthesis

Solid phase peptide coupling of glycosylated threonine derivatives was systematically evaluated. In contrast to glycosylated serine derivatives which are highly prone to epimerization, glycosylated threonine derivatives produce only negligible amounts of epimerization. Under forcing conditions, glycosylated threonine analogs undergo β-elimination, rather than epimerization. Mechanistic studies and molecular modeling were used to understand the origin of the differences in reactivity.

Enhanced epimerization of glycosylated amino acids during solid-phase peptide synthesis

Glycopeptides are extremely useful for basic research and clinical applications, but access to structurally defined glycopeptides is limited by the difficulties in synthesizing this class of compounds. In this study, we demonstrate that many common peptide coupling conditions used to prepare O-linked glycopeptides result in substantial amounts of epimerization at the α position. In fact, epimerization resulted in up to 80% of the non-natural epimer, indicating that it can be the major product in some reactions. Through a series of mechanistic studies, we demonstrate that the enhanced epimerization relative to nonglycosylated amino acids is due to a combination of factors, including a faster rate of epimerization, an energetic preference for the unnatural epimer over the natural epimer, and a slower overall rate of peptide coupling. In addition, we demonstrate that use of 2,4,6-trimethylpyridine (TMP) as the base in peptide couplings produces glycopeptides with high efficiency and low epimerization. The information and improved reaction conditions will facilitate the preparation of glycopeptides as therapeutic compounds and vaccine antigens.

Chemical glycosylation in the synthesis of glycoconjugate antitumour vaccines

Therapeutic vaccines derived from carbohydrate antigen-adjuvant combinations are a promising approach for cancer immunotherapy. One of the critical limitations in this area is access to sufficient quantities of tumour-associated carbohydrate antigens and glycoconjugate adjuvants. At present, availability of the complex oligosaccharide constructs that are needed for the systematic design and evaluation of novel vaccine formulations relies on de novo chemical synthesis. The use of both state-of-the-art and emerging glycosylation technologies has led to significant advances in this field, allowing the clinical exploration of carbohydrate-based antigens in the treatment of cancer.

Site-Specific Incorporation of Glycosylated Serine and Tyrosine Derivatives into Proteins

Glycosylation of proteins can have a dramatic effect on their physical, chemical, and biological properties. Analogues of dihydrofolate reductase and firefly luciferase containing glycosylated amino acids at single, predetermined sites have been elaborated. Misacylated suppressor tRNAs activated with glycosylated serine and tyrosine derivatives were used for suppression of the nonsense codons in a cell-free protein biosynthesizing system, thereby permitting the preparation of the desired glycosylated proteins. In this fashion, it was possible to obtain proteins containing both mono- and diglycosylated amino acids, including glycosylated serine and tyrosine moieties. For the modified firefly luciferases, the effect of these substitutions on the wavelength of the light emitted by firefly luciferase was investigated. The maximum wavelength for mutants containing peracetylated glycosylated serine derivatives at position 284 showed a red shift in the emission spectra. For mutants containing glycosylated tyrosines, the red shift was observed only when the carbohydrate moiety was fully deacetylated.

Organic azides: an exploding diversity of a unique class of compounds

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aubé rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Solution-Phase Synthesis with Solid-State Workup of an O-Glycopeptide with a Cluster of Cancer-Related T Antigens

An N-terminal glycopeptide of asialoglycophorin AM with three O-linked T antigens was prepared by "solution-phase synthesis with solid-state workup" using unprotected glycosyl amino acids as building blocks. For the glycopeptide assembly, all reactions were conducted in homogeneous NMP solutions, while the product of each reaction was readily isolated as solid precipitates upon addition of diethyl ether. In the preparation of building blocks, a robust approach was established to selectively alpha-glycosylate Ser and Thr derivatives. [reaction: see text]

A New Scaffold for the Stereoselective Synthesis of α-O-Linked Glycopeptide Mimetics

alpha-O-Linked glycohomoglutamates are obtained as diastereomerically pure compounds by chemo-, regio-, and stereoselective cycloadditions between glycals and aspartic acid derivatives. The latter constitute orthogonally functionalized scaffolds for glycopeptide mimetics.

Acetylated methyl glucopyranuronate trichloroacetimidate as a glycosyl donor for efficient synthesis of disaccharides

Allyl (methyl 2,3,4-tri-O-acetyl-beta-D-glucopyranosyl uronate)-(1-->3)-4,6-O-benzylidene-2-deoxy-2-phthalimido-beta-D-glucopyranoside (4) and benzyl (methyl 2,3,4-tri-O-acetyl-beta-D-glucopyranosyl uronate)-(1-->3)-4,6-O-benzylidene-2-deoxy-2-phthalimido-beta-D-glucopyranoside (5) have been efficiently synthesized by coupling allyl 4,6-O-benzylidene-2-deoxy-2-phthalimido-beta-D-glucopyranoside (2) or benzyl 4,6-O-benzylidene-2-deoxy-2-phthalimido-beta-D-glucopyranoside (3) with methyl (2,3,4-tri-O-acetyl-1-O-trichloroacetimidoyl)-alpha-D-glucopyranuronate (1), respectively, using trimethylsilyl triflate as promoter.

Synthesis of the pentasaccharide related to the repeating unit of the antigen from Shigella dysenteriae type 4 in the form of its methyl ester 2-(trimethylsilyl)ethyl glycoside

Starting from D-mannose, D-glucose and L-fucose, the pentasaccharide derivative methyl 2,3,4-tri-O-benzyl-alpha-L-fucopyranosyl-(1-->3)-2-O-acetyl-4,6-O-benzylidene-alpha-D-mannopyranosyl-(1-->3)-2-O-acetyl-6-O-benzyl-4-O-(2,3,4-tri-O-benzyl-alpha-L-fucopyranosyl)-alpha-D-mannopyranosyl-(1-->4)-[2-(trimethylsilyl)ethyl 2,3-di-O-benzyl-beta-D-glucopyranosid]uronate was synthesized. This compound with two alpha-mannopyranosyl units was transformed, via Walden inversion and subsequent deprotection, into the alpha-D-glucosamine-type target compound, namely methyl alpha-L-fucopyranosyl-(1-->3)-2-acetamido-2-deoxy-alpha-D-glucopyranosyl-(1-->3)-2-acetamido-2-deoxy-4-O-(alpha-L-fucopyranosyl)-alpha-D-glucopyranosyl-(1-->4)-[2-(trimethylsilyl)ethyl beta-D-glucopyranosid]uronate which is related to the repeating unit of the O-antigen from Shigella dysenteriae type 4.

Silyl linker-based approach to the solid-phase synthesis of Fmoc glycopeptide thioesters

An efficient solid-phase synthesis of Fmoc (glyco)peptide thioesters is described. Fmoc x Ser x OAll and Fmoc x Thr x OAll bound to resin with a silyl ether linker were deallylated by Pd(0) catalysis and condensed with thiophenol, benzyl mercaptane, and ethyl 3-mercaptopropionate by activation with DCC/HOBt. The thioesters were released from the resin either by treatment with CsF-AcOH or by acidic hydrolysis. The effectiveness of this silyl linker strategy is further demonstrated by the synthesis of more complex (glyco)peptide thioesters 25, 26 and 27 involving N-->C and C-->N peptide elongation.

Unprotected oligosaccharides as phase tags: solution-phase synthesis of glycopeptides with solid-phase workups

[reaction--see text] N-Linked glycopeptides were synthesized from glycosyl asparagines containing unprotected oligosaccharides and other simple amino acids by an Fmoc method. The free oligosaccharide chains were used as phase tags to facilitate the product isolation by a precipitation method. Thus, while the elongation of glycopeptides was achieved in a solution of N-methylpyrrolidinone (NMP), the product of each step could be precipitated by adding ether to the reaction mixtures. The strategy also eliminated the final step of carbohydrate deprotection in glycopeptide synthesis.

Use of fluorobenzoyl protective groups in synthesis of glycopeptides: beta-elimination of O-linked carbohydrates is suppressed

Fluorobenzoyl groups have been investigated as alternatives to acetyl and benzoyl protective groups in carbohydrate and glycopeptide synthesis. D-Glucose and lactose were protected with different fluorobenzoyl groups and then converted into glycosyl bromides in high yields (>80% over two steps). Glycosylation of protected derivatives of serine with these donors gave 1,2-trans glycosides in good yields (approximately 60--70%) and excellent stereoselectivity without formation of ortho esters. The resulting glycosylated amino acid building blocks were then used in solid-phase synthesis of two model O-linked glycopeptides known to be unusually sensitive to beta-elimination on base-catalyzed deacylation. When either a 3-fluoro- or a 2,5-difluorobenzoyl group was used for protection of each of the two model glycopeptides the extent of beta-elimination decreased from 80% to 10% and from 50% to 0%, respectively, as compared to when using the ordinary benzoyl group. Fluorobenzoyl groups thus combine the advantages of the benzoyl group in formation of glycosidic bonds (i.e., high stereoselectivity and low levels of ortho ester formation) with the ease of removal characteristic of the acetyl group.

Solid-phase synthesis of O-linked glycopeptide analogues of enkephalin

The synthesis of 18 N-alpha-FMOC-amino acid glycosides for solid-phase glycopeptide assembly is reported. The glycosides were synthesized either from the corresponding O'Donnell Schiff bases or from N-alpha-FMOC-amino protected serine or threonine and the appropriate glycosyl bromide using Hanessian's modification of the Koenigs-Knorr reaction. Reaction rates of D-glycosyl bromides (e.g., acetobromoglucose) with the L- and D-forms of serine and threonine are distinctly different and can be rationalized in terms of the steric interactions within the two types of diastereomeric transition states for the D/L and D/D reactant pairs. The N-alpha-FMOC-protected glycosides [monosaccharides Xyl, Glc, Gal, Man, GlcNAc, and GalNAc; disaccharides Gal-beta(1-4)-Glc (lactose), Glc-beta(1-4)-Glc (cellobiose), and Gal-alpha(1-6)-Glc (melibiose)] were incorporated into 22 enkephalin glycopeptide analogues. These peptide opiates bearing the pharmacophore H-Tyr-c[DCys-Gly-Phe-DCys]- were designed to probe the significance of the glycoside moiety and the carbohydrate-peptide linkage region in blood-brain barrier (BBB) transport, opiate receptor binding, and analgesia.

Studies toward the site specific incorporation of sugars into proteins: synthesis of glycosylated aminoacyl-tRNAs

A series of glycosylated serine derivatives was synthesized from peracetylated sugars and Fmoc-protected serine; these were chemically esterified with the tris-(tetrabutylammonium) salt of pdCpA. The fully protected and deprotected glycosylated aminoacyl pdCpAs were ligated enzymatically to an abbreviated tRNA (tRNA-C(OH)) to provide the title compounds that are key intermediates in the elaboration of glycoproteins using readthrough of a nonsense codon.

Solid-phase synthesis of the glycopeptide of human glycophorin AM bearing the consecutive sialyl-T antigen

Fmoc solid-phase synthesis of the N-terminal glycopentapeptide of human glycophorin AM, bearing the consecutive sialyl-T antigen, was accomplished using glycosylated amino acid building blocks on a weakly acid-labile resin with high efficiency. The benzylated glycopeptide was treated with TMSOTf-thioanisole in TFA and then with aq NaHCO3 to afford the deprotected glycopeptide in good yield.

X-Ray diffraction and high resolution NMR analysis of methyl D-glucopyranuronate derivatives

X-Ray diffraction and high resolution 1H and 13C NMR spectral data for methyl 2,3,4,-tri-O-acetyl-alpha-D-glucopyranuronate and methyl (allyl 2,3,4-tri-O-acetyl- beta-D-glucopyranosid)uronate are presented. Both compounds adopt the 4C1 conformation.