Preparation of building blocks for glycopeptide synthesis by glycosylation of Fmoc amino acids having unprotected carboxyl groups

Proline Analogues

Within the canonical repertoire of the amino acid involved in protein biogenesis, proline plays a unique role as an amino acid presenting a modified backbone rather than a side-chain. Chemical structures that mimic proline but introduce changes into its specific molecular features are defined as proline analogues. This review article summarizes the existing chemical, physicochemical, and biochemical knowledge about this peculiar family of structures. We group proline analogues from the following compounds: substituted prolines, unsaturated and fused structures, ring size homologues, heterocyclic, e.g., pseudoproline, and bridged proline-resembling structures. We overview (1) the occurrence of proline analogues in nature and their chemical synthesis, (2) physicochemical properties including ring conformation and cis/trans amide isomerization, (3) use in commercial drugs such as nirmatrelvir recently approved against COVID-19, (4) peptide and protein synthesis involving proline analogues, (5) specific opportunities created in peptide engineering, and (6) cases of protein engineering with the analogues. The review aims to provide a summary to anyone interested in using proline analogues in systems ranging from specific biochemical setups to complex biological systems.

An identification method to distinguish monomeric sugar isomers on glycopeptides

A one-step protocol for the automated flow synthesis of protected glycosylated amino acids is described using pumps with open-source controls in overall yields of 21-50%. The resulting glycosylated amino acids could be used directly in solid-phase peptide synthesis (SPPS) protocols to quickly produce glycopeptide standards. Access to a variety of stereoisomers of the sugar enabled the development of an LC-MS/MS protocol that can distinguish between peptides modified with carbohydrates having the same exact mass. This method could definitively identify fucose in an O-glycosylation site on the transmembrane protein, Notch1.

A Hitchhiker's Guide to Problem Selection in Carbohydrate Synthesis

Oligosaccharides are ubiquitous in molecular biology and are used for functions ranging from governing protein folding to intercellular communication. Perhaps paradoxically, the exact role of the glycan in most of these settings is not well understood. One reason for this contradiction concerns the fact that carbohydrates often appear in heterogeneous form in nature. These mixtures complicate the isolation of pure material and characterization of structure-activity relationships. As a result, a major bottleneck in glycoscience research is the synthesis and modification of pure materials. While synthetic and chemoenzymatic methods have enabled access to homogeneous tool compounds, a central problem, particularly for newer synthetic chemists, is the matter of problem selection. This outlook aims to provide an entry level overview of fundamental principles in carbohydrate chemistry with an eye toward enabling solutions to frontier challenges.

Solvent free, catalytic and diastereoselective preparation of aryl and alkyl thioglycosides as key components for oligosaccharide synthesis

A new and environmentally friendly protocol for the conversion of sugar per-acetates into thioglycosides under solvent free and catalytic conditions is presented. The procedure involves heating in the presence of InCl₃ and various aryl thiols. For alkyl thioglycoside synthesis, cyclohexane thiol was found to give good results and yield a glycosyl donor with reactivity similar to a thioethyl congener. The established optimum reaction conditions were found to provide the desired thioglycoside products in an easy and highly diastereoselective manner even when conducted on a multigram scale.

Synthesis of C-terminal glycopeptidesviaoxime resin aminolysis

We developed a general solid-phase approach to complex C-terminal glycopeptides.

Solid-Phase O-Glycosylation with a Glucosamine Derivative for the Synthesis of a Glycopeptide

An efficient synthesis of the O-linked glycosylamino acid Fmoc–l-Ser((Ac)3–β-d-GlcNAc)-OH building block is described. The utility of the method was demonstrated with direct solid-phase O-glycosylation of the hydroxyl group on the amino acid (Ser) side chain of a human α-A crystallin-derived peptide (AIPVSREEK) in nearly quantitative glycosylation yield.

Preparation of S-glycoside surfactants and cysteine thioglycosides using minimally competent Lewis acid catalysis

Here we report a method for the preparation of anomerically pure β-S-glycopyranosides (1,2-trans-glycosides) from the corresponding peracetate donors. S-glycosylation was performed in CHCl3 at reflux in the presence of a catalytic amount of InBr3. Deacylation of the intermediate peracetates were achieved under Zemplén conditions. Five pyranose examples, monosaccharides D-glucose and D-galactose and disaccharides cellobiose, maltose, and lactose, were used as donors, and five thiols including an α/ω dithiol and Fmoc-L-cysteine were used as acceptors. Melting points, high res MS, [α]D and NMR data ((1)H and (13)C, including COSY, HSQC and HMBC) are reported for compounds not previously described.

Synthesis aided structural determination of amyloid-β(1-15) glycopeptides, new biomarkers for Alzheimer's disease

Unique tyrosine glycosylated amyloid-β(1-15) glycopeptides were synthesized with well-defined stereochemistry at the glycosidic linkages. Aided by these glycopeptides and tandem mass spectrometry analysis, the naturally existing amyloid-β glycopeptides, isolated from Alzheimer's disease patients, were determined to contain an α-linked N-acetyl galactosamine at the modified tyrosine 10 residue. Glycosylation can significantly impact the properties of amyloid-β as the glycopeptide has much lower affinity for Cu(+) ions.

Novel glycosylated endomorphin-2 analog produces potent centrally-mediated antinociception in mice after peripheral administration

We report the synthesis and pharmacological characterization of a novel glycosylated analog of a potent and selective endogenous μ-opioid receptor (MOP) agonist, endomorphin-2 (Tyr-Pro-Phe-Phe-NH2, EM-2), obtained by the introduction in position 3 of the tyrosine residue possessing the glucose moiety attached to the phenolic function via a β-glycosidic bond. The improved blood-brain barrier permeability and enhanced antinociceptive effect of the novel glycosylated analog suggest that it may be a promising template for design of potent analgesics. Furthermore, the described methodology may be useful for increasing the bioavailability and delivery of opioid peptides to the CNS.

Optimization of physicochemical and pharmacological properties of peptide drugs by glycosylation

Many biological interactions and functions are mediated by glycans, leading to the emerging importance of carbohydrate and glycoconjugate chemistry in the design of novel drug therapeutics. In addition to direct effects on biological activity, sugar addition appears to alter many physicochemical and pharmacological properties of the peptide backbone. Consequently, glycosylation has been often used to improve various less than optimal features of peptide drug leads.In order to study the effects that naturally occurring and/or nonnatural glycans have on peptide drug solubility, conformation, proteolytic resistance, membrane permeability, and toxicity, it is essential to have convenient synthetic access toward synthesis of glycopeptide analogs. The crucial step in the synthesis of glycopeptides is the introduction of the carbohydrate group. The preformed glycosyl amino acid building block is the most commonly employed approach used in glycopeptide synthesis.In this review, we will describe various synthetic approaches to prepare N- and O-glycopeptides bearing simple monosaccharides as a tool to improve peptide therapeutic efficacy by glycosylation.

Glycosylation of α-amino acids by sugar acetate donors with InBr3. Minimally competent Lewis acids

A simplified method for the preparation of Fmoc-serine and Fmoc-threonine glycosides for use in O-linked glycopeptide synthesis is described. Lewis acids promote glycoside formation, but also promote undesired reactions of the glycoside products. Use of 'minimally competent' Lewis acids such as InBr(3) promotes the desired activation catalytically, and with greatly reduced side products from sugar peracetates.

Synthesis of Peptides from α- and β-Tubulin Containing Glutamic Acid Side-Chain Linked Oligo-Glu with Defined Length

Side-chain oligo- and polyglutamylation represents an important posttranslational modification in tubulin physiology. The particular number of glutamate units is related to specific regulatory functions. In this work, we present a method for the synthesis of building blocks for the Fmoc synthesis of peptides containing main chain glutamic acid residues that carry side-chain branching with oligo-glutamic acid. The two model peptide sequences CYEEVGVDSVEGEG-E(E_x)-EEGEEY and CQDATADEQG-E(E_x)-FEEEEGEDEA from the C-termini of mammalian α1- and β1-tubulin, respectively, containing oligo-glutamic acid side-chain branching with lengths of 1 to 5 amino acids were assembled in good yield and purity. The products may lead to the generation of specific antibodies which should be important tools for a more detailed investigation of polyglutamylation processes.

Human OGA binds substrates in a conserved peptide recognition groove

Modification of cellular proteins with O-GlcNAc (O-linked N-acetylglucosamine) competes with protein phosphorylation and regulates a plethora of cellular processes. O-GlcNAcylation is orchestrated by two opposing enzymes, O-GlcNAc transferase and OGA (O-GlcNAcase or β-N-acetylglucosaminidase), which recognize their target proteins via as yet unidentified mechanisms. In the present study, we uncovered the first insights into the mechanism of substrate recognition by human OGA. The structure of a novel bacterial OGA orthologue reveals a putative substrate-binding groove, conserved in metazoan OGAs. Guided by this structure, conserved amino acids lining this groove in human OGA were mutated and the activity on three different substrate proteins [TAB1 (transforming growth factor-β-activated protein kinase 1-binding protein 1), FoxO1 (forkhead box O1) and CREB (cAMP-response-element-binding protein)] was tested in an in vitro deglycosylation assay. The results provide the first evidence that human OGA may possess a substrate-recognition mechanism that involves interactions with O-GlcNAcylated proteins beyond the GlcNAc-binding site, with possible implications for differential regulation of cycling of O-GlcNAc on different proteins.

Facile synthesis of glycosylated Fmoc amino acid building blocks assisted by microwave irradiation

The synthesis of glycosylated Fmoc amino acids by reaction of mono- and disaccharide peracetates with Fmoc amino acids having free carboxyl groups was rapidly promoted by Lewis acids (SnCl(4), BF(3)·Et(2)O) under microwave irradiation. The products are useful building blocks for the synthesis of glycopeptides.

Unexpected tolerance of glycosylation by UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferase revealed by electron capture dissociation mass spectrometry: carbohydrate as potential protective groups

UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases (ppGalNAcTs, EC 2.4.1.41), a family of key enzymes that initiate posttranslational modification with O-glycans in mucin synthesis by introduction of alpha-GalNAc residues, are structurally composed of a catalytic domain and a lectin domain. It has been known that multiple Ser/Thr residues are assigned in common mucin glycoproteins as potential O-glycosylation sites and more than 20 distinct isoforms of this enzyme family contribute to produce densely O-glycosylated mucin glycoproteins. However, it seems that the functional role of the lectin domain of ppGalNAcTs remains unclear. We considered that electron capture dissociation mass spectrometry (ECD-MS), a promising method for highly selective fragmentation at peptide linkages of glycopeptides to generate unique c and z series of ions, should allow for precise structural characterization to uncover the mechanism in O-glycosylation of mucin peptides by ppGalNAcTs. In the present study, it was demonstrated that a system composed of an electrospray source, a linear RFQ ion trap that isolates precursor ions, the ECD device, and a TOF mass spectrometer is a nice tool to identify the preferential O-glycosylation sites without any decomposition of the carbohydrate moiety. It should be noted that electrons used for ECD are accelerated within a range from 1.75 to 9.75 eV depending on the structures of glycopeptides of interest. We revealed for the first time that additional installation of a alpha-GalNAc residue at potential glycosylation sites by ppGalNAcT2 proceeds smoothly in various unnatural glycopeptides having alpha-Man, alpha-Fuc, and beta-Gal residues as well as alpha-GalNAc residues. The results may suggest that ppGalNAcT2 did not differentiate totally presubstituted sugar residues in terms of configuration of functional groups, d-, l-configuration, and even alpha-, beta-stereochemistry at an anomeric carbon atom when relatively short synthetic peptides were employed for the acceptor substrates. Unexpected characteristics of ppGalNAcT2 motivated us to challenge site-directed installation of alpha-GalNAc residues at desired position(s) by protecting some hydroxyl groups of Thr/Ser residues with selectively removable sugars, notably a novel concept as "carbohydrate as protective groups", toward a goal of the systematic chemical and enzymatic synthesis of biologically important mucin glycopeptides.

Selective formation of glycosidic linkages of N-unsubstituted 4-hydroxyquinolin-2-(1H)-ones

A comparative study for selective glucosylation of N-unsubstituted 4-hydroxyquinolin-2(1H)-ones into 4-(tetra-O-acetyl-beta-D-glucopyranosyloxy)quinolin-2(1H)-ones is reported. Four glycosyl donors including tetra-O-acetyl-alpha-D-glucopyranosyl bromide, beta-D-glucose pentaacetate, glucose tetraacetate and tetra-O-acetyl-alpha-D-glucopyranosyl trichloroacetimidate were tested, along with different promoters and reaction conditions. The best results were obtained with tetra-O-acetyl-alpha-D-glucopyranosyl bromide with Cs(2)CO(3) in CH(3)CN. In some cases the 4-O-glucosylation of the quinolinone ring was accompanied by 2-O-glucosylation yielding the corresponding 2,4-bis(tetra-O-acetyl-beta-D-glucopyranosyloxy)quinoline. Next, 4-(tetra-O-acetyl-beta-D-glucopyranosyloxy)quinolin-2(1H)-ones were deacetylated into 4-(beta-D-glucopyranosyloxy)quinolin-2(1H)-ones with Et(3)N in MeOH. In some instances the deacetylation was accompanied by the sugar-aglycone bond cleavage. Structure elucidation, complete assignment of proton and carbon resonances as well as assignment of anomeric configuration for all the products under investigation were performed by 1D and 2D NMR spectroscopy.

Antibacterial cyclic D,L-alpha-glycopeptides

We report the design, synthesis, membrane activity, biophysical characterization, and in vitro antibacterial activities of cationic cyclic D,L-alpha-glycopeptides bearing d-glucosamine (GlcNH(2)), D-galactose (Gal), or D-mannose (Man) glycosyl side chains.

Carbohydrate-pi interactions: what are they worth?

Protein-carbohydrate interactions play an important role in many biologically important processes. The recognition is mediated by a number of noncovalent interactions, including an interaction between the alpha-face of the carbohydrate and the aromatic side chain of the protein. To elucidate this interaction, it has been studied in the context of a beta-hairpin in aqueous solution, in which the interaction can be investigated in the absence of other cooperative noncovalent interactions. In this beta-hairpin system, both the aromatic side chain and the carbohydrate were varied in an effort to gain greater insight into the driving force and magnitude of the carbohydrate-pi interaction. The magnitude of the interaction was found to vary from -0.5 to -0.8 kcal/mol, depending on the nature of the aromatic ring and the carbohydrate. Replacement of the aromatic ring with an aliphatic group resulted in a decrease in interaction energy to -0.1 kcal/mol, providing evidence for the contribution of CH-pi interactions to the driving force. These findings demonstrate the significance of carbohydrate-pi interactions within biological systems and also their utility as a molecular recognition element in designed systems.

Conformational consequences of protein glycosylation: preparation of O-mannosyl serine and threonine building blocks, and their incorporation into glycopeptide sequences derived from alpha-dystroglycan

With the goal to investigate the structural impact of O-mannosyl glycosylation on alpha-dystroglycan, a glycoprotein that has an important role in the extracellular organization of muscle, glycopeptides derived from its mucin-like sequence have been prepared by solid-phase peptide synthesis. Two approaches have been explored to obtain needed mannosylated serine and threonine building blocks. With the alpha-carboxyl group unprotected, and with tetraaceto-1-fluoro-alpha-D-mannose as the sugar donor, the desired alpha-O-mannosyl-Fmoc-Ser/Thr formed, along with mannosyl ester isomers and the species with mannose attached to both hydroxyl and carboxyl functions. Relevant mechanistic questions and stability issues were elucidated. Alternatively, building blocks were made with the alpha-carboxyl protected/activated as the pentafluorophenyl (Pfp) ester. Glycopeptides synthesized herein contained 5-9 residues, and featured one, two, and four consecutive Ser and/or Thr residues O-glycosylated with mannose. Circular dichroism (CD) spectra for Man-containing glycopeptides recorded in water show them to be not well ordered. For one of the alpha-dystroglycan-derived sequences, the comparative conformational consequences of glycosylation by either Man or GalNAc have been examined by CD and NMR, with both methods showing a more organized structure when GalNAc is present.

An aminooxy-functionalized non-nucleosidic phosphoramidite for the construction of multiantennary oligonucleotide glycoconjugates on a solid support

In this unit, a method is described that allows construction of multiantennary oligonucleotide glycoconjugates on a solid support. A bis(hydroxymethyl)malondiamide-based phosphoramidite that contains two phthaloyl-protected aminooxy groups compatible with normal chain assembly is prepared. The aminooxy functions can be deblocked with a hydrazinium acetate treatment and subsequently oximated on-support with fully acetylated 4-oxobutyl alpha-D-mannopyranoside. The resulting reagent is then used to prepare a conjugate containing two non-nucleosidic building blocks (i.e., four alpha-D-mannopyranosyl units) close to the 5' terminus of the oligonucleotide.

Preparation of glycosylated amino acids suitable for Fmoc solid-phase assembly

Many biological interactions and functions are mediated by glycans, consequently leading to the emerging importance of carbohydrate and glycoconjugate chemistry in the design of novel drug therapeutics. Despite the challenges that carbohydrate moieties bring into the synthesis of glycopeptides and glycoproteins, considerable progress has been made during recent decades. Glycopeptides carrying many simple glycans have been chemically synthesized, enzymatic approaches have been utilized to introduce more complex glycans, and most recently native chemical ligation has enabled synthesis of glycoproteins from well-designed peptide and glycopeptide building blocks. Currently, general synthetic methodology for glycopeptides relies on preformed glycosylated amino acids for the stepwise solid-phase peptide synthesis. The formation of glycosidic bonds is of fundamental importance in the assembly of glycopeptides. As such, every glycosylation has to be regarded as a unique problem, demanding considerable systematic research. In this chapter we will summarize the most common chemical methods for the stereoselective synthesis of N- and O-glycosylated amino acids. The particular emphasis will be given to the preparation of building blocks for use in solid-phase glycopeptide synthesis based on the 9-fluorenylmethoxycarbonyl (Fmoc) protective group strategy.

A comparative study of different glycosylation methods for the synthesis of d-mannopyranosides of Nα-fluorenylmethoxycarbonyl-trans-4-hydroxy-l-proline allyl ester

The synthesis of Nalpha-fluorenylmethoxycarbonyl-trans-4-hydroxy-4-O-[(2,3,4,6-tetra-O-acetyl)-alpha-d-mannopyranosyl]-l-proline allyl ester and Nalpha-fluorenylmethoxycarbonyl-trans-4-hydroxy-4-O-[(2,3,4,6-tetra-O-benzoyl)-alpha-d-mannopyranosyl]-l-proline allyl ester is described. Glycosylation using Königs-Knorr conditions with a benzoyl protected glycosyl donor provided the optimum method. Removal of the allyl ester gave two mannosylated building blocks suitable for solid phase glycopeptide synthesis.

Synthesis of fused bicyclic thioglycosides of N-acylated glucosamine as analogues of mycothiol

The synthesis of a fused bicyclic thioglycoside analogue of mycothiol, (3R)-3-acetylamino-4-one-6,7-dihydro-(1',2'-dideoxy-beta-D-glucopyranoso)[2',1'-f]-1,5-thiazepane (5), is reported. Treatment of phthalimido-protected peracetylated glucosamine with N-acetyl-cysteine and boron trifluoride-etherate gave the beta-linked thioglycoside, which was deprotected and cyclized, using HOBt and EDCl to form the lactam and giving the target structure. This mycothiol mimic and its tri-O-acetate will be investigated as potential inhibitors of enzymes involved in the biosynthesis of mycothiol. The protected derivative also has the potential to be an alpha-selective N-cysteinyl glucosamine donor; however, initial glycosylation attempts failed due to the apparent stability of the fused bicyclic system.

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.

Improved synthesis of O-linked, and first synthesis of S- linked, carbohydrate functionalised N-carboxyanhydrides (glycoNCAs)

An improved method for the synthesis of glycosylated N-carboxyanhydrides, which are monomers for glycopeptide synthesis, is presented.

Evaluation of a carbohydrate–π interaction in a peptide model system

A carbohydrate-pi interaction contributes -0.8 kcal mol(-1) to the stabilization of a beta-hairpin peptide.

Glycosylated foldamers: synthesis of carbohydrate-modified β3hSer and incorporation into β-peptides

Fmoc-protected beta(3)hserine (beta(3)hSer) was prepared and O-linked to suitably protected N-acetylgalactosamine (GalNAc) and N-acetylglucosamine (GlcNAc) derivatives. Glycosylation of beta(3)hSer was made by two independent routes: either by direct glycosyl linkage to the beta(3)hSer, or linkage to natural L-Ser and then utilizing the carbohydrate moiety as a protecting group in an Arndt-Eistert homologation. Both procedures gave the novel glycosylated beta(3)-amino acids Fmoc-beta(3)hSer(alpha-D-GalNAc(Ac)(3))-OH (1a), its beta-anomer (1b), and Fmoc-beta(3)hSer(beta-D-GlcNAc(Ac)(3))-OH (2), which were utilized in the solid-phase peptide synthesis of four glycosylated dipeptides (3a-d) and two heptapeptides (4a-b). The preparation of beta-amino acids bearing common post-translational modifiers represents an important step towards functionalized foldamers with broad applications in biomedical research.

Covalent modification of a melanoma-derived antigenic peptide with a natural quinone methide. Preliminary chemical, molecular modelling and immunological evaluation studies

A LigandFit shape-directed docking methodology was used to identify the best position at which the melanoma-derived MHC class-I HLA-A2-binding antigenic peptide ELAGIGILTV could be modified by attaching a small molecule capable of fitting at the interface of complementary determining regional (CDR) loops of a T-cell receptor (TCR) while triggering T-cell responses. The small molecule selected here for determining the feasibility of this alternative track to chemical alteration of antigenic peptides was the electrophilic quinone methide (+)-puupehenone (), a natural product that belongs to a family of marine metabolites capable of expressing immunomodulatory activities. A preliminary chemical reactivity model study revealed the efficacy of the thiol group of a cysteine (C) side-chain in its nucleophilic addition reaction with in a regio- and diastereoselective manner. The best TCR/HLA-A2 ligand [i.e., ELAGCGILTV-S-puupehenol ()] then identified by the LigandFit docking procedure was synthesized and used to pulse HLA-A2(+) T2 cells for T-cell stimulation. Among the ELAGIGILTV-specific T-cell clones we tested, five of them recognized the conjugate in spite of its low binding affinity for the HLA-A2 molecules. The resulting T-cell stimulation was determined through the intracytoplasmic secretion of IFN-gamma and the percentage of T-cells thus activated. These highly encouraging results indicate that small non-peptidic natural product-derived molecules attached onto the central part of an antigenic peptide can fit at the TCR/HLA-A2 interface with induction of T-cell responses.

Macrolactamization of glycosylated peptide thioesters by the thioesterase domain of tyrocidine synthetase

The 35 kDa thioesterase (TE) domain excised from the megadalton tyrocidine synthetase (Tyc Syn) retains autonomous capacity to macrocyclize peptidyl thioesters to D-Phe1-L-Leu10-macrolactams. Since a number of nonribosomal peptides undergo O-glycosylation events during tailoring to gain biological activity, the Tyc Syn TE domain was evaluated for cyclization capacity with glycosylated peptidyl-S-NAC substrates. First, Tyr7 was replaced with Tyr(beta-D-Gal) and Tyr(beta-D-Glc) as well as with Ser-containing beta-linked D-Gal, D-Glc, D-GlcNAc, and D-GlcNH2, and these new analogs were shown to be cyclized with comparable kcat/Km catalytic efficiency. Similarly, Gal- or tetra-O-acetyl-Gal-Ser could also be substituted at residues 5, 6, and 8 in the linear decapeptidyl-S-NAC sequences and cyclized without substantial loss in catalytic efficiency by Tyc Syn TE. The cyclic glycopeptides retained antibiotic activity as membrane perturbants in MIC assays, opening the possibility for library construction of cyclic glycopeptides by enzymatic macrocyclization.

Microwave-assisted glycosylation for the synthesis of glycopeptides

An efficient one-step synthesis of O-linked glycosylamino acids is described. This methodology converts commercially available peracetylated mono- and disaccharides activated by cheap and environmentally safe FeCl(3) under microwave irradiation with Fmoc-Ser-OBn to the corresponding beta-glycosides in short reaction times and moderate yields.