Facile synthesis of salmochelin S1, S2, MGE, DGE, and TGE

Electrochemical Glycosylation via Halogen-Atom-Transfer for C-Glycoside Assembly

Glycosyl donor activation emerged as an enabling technology for anomeric functionalization, but aimed primarily at O-glycosylation. In contrast, we herein disclose mechanistically distinct electrochemical glycosyl bromide donor activations via halogen-atom transfer and anomeric C-glycosylation. The anomeric radical addition to alkenes led to C-alkyl glycoside synthesis under precious metal-free reaction conditions from readily available glycosyl bromides. The robustness of our e-XAT strategy was further mirrored by C-aryl and C-acyl glycosides assembly through nickela-electrocatalysis. Our approach provides an orthogonal strategy for glycosyl donor activation with expedient scope, hence representing a general method for direct C-glycosides assembly.

Zirconaaziridine-Mediated Ni-Catalyzed Diastereoselective C(sp2)-Glycosylation

In the realm of organic synthesis, the catalytic and stereoselective formation of C-glycosidic bonds is a pivotal process, bridging carbohydrates with aglycones. However, the inherent chirality of the saccharide scaffold often has a substantial impact on the stereoinduction imposed by a chiral ligand. In this study, we have established an unprecedented zirconaaziridine-mediated asymmetric nickel catalysis, enabling the diastereoselective coupling of bench-stable glycosyl phosphates with a range of (hetero)aromatic and glycal iodides as feasible coupling electrophiles. Our developed method showcases a broad scope and a high tolerance for various functional groups. More importantly, precise stereocontrol toward both anomeric configurations of forming C(sp2)-glycosides can be realized by simply utilizing the popular chiral bioxazoline (biOx) ligands in this reductive Ni catalysis. Regarding the operating mechanism, both experimental and computational studies support the occurrence of a redox transmetalation process, leading to the formation of a transient, bimetallic Ni-Zr species that acts as a potent and efficient single-electron reductant in the catalytic process.

A novel radiolabelled salmochelin derivative for bacteria-specific PET imaging: synthesis, radiolabelling and evaluation

For specific imaging of bacterial infections we aimed at targeting the exclusive bacterial iron transport system via siderophore-based radiotracers. De novo synthesis and radiolabeling yielded the salmochelin-based PET radiotracer [68Ga]Ga-RMA693, which showed a favourable biodistribution and a bacteria-specific uptake in an animal model of Escherichia coli infection.

Skeleton Synthesis of a Plant-Derived Radioprotective Alkaloid Born to Produce a Novel Fused Heterocycle

Alkaloids are a material treasure bestowed on humans by nature owing to their numerous biological activities. Orychophragine D, an alkaloid isolated from the seeds of Orychophragmus violaceus was identified as bearing a novel skeleton and proved to have an excellent radioprotective effect. Different from the common alkaloid structure, the main block of orychophragine D is constructed of an oxotriazine and an oxopiperazine, which are connected in parallel by a C-N bond. In this paper, a preparation method for the novel heterocycle skeleton of orychophragine D is proposed for the first time. N-Boc-L-serine was utilized as the original material to complete the preparation with 11 steps in a 13% overall yield. A hydroxyl group was established on the side chain of the skeleton as the reaction site for researchers to conduct further structural modification or derivatization.

Advances in the Synthesis of Enterobactin, Artificial Analogues, and Enterobactin-Derived Antimicrobial Drug Conjugates and Imaging Tools for Infection Diagnosis

Iron is an essential growth factor for bacteria, but although highly abundant in nature, its bioavailability during infection in the human host or the environment is limited. Therefore, bacteria produce and secrete siderophores to ensure their supply of iron. The triscatecholate siderophore enterobactin and its glycosylated derivatives, the salmochelins, play a crucial role for iron acquisition in several bacteria. As these compounds can serve as carrier molecules for the design of antimicrobial siderophore drug conjugates as well as siderophore-derived tool compounds for the detection of infections with bacteria, their synthesis and the design of artificial analogues is of interest. In this review, we give an overview on the synthesis of enterobactin, biomimetic as well as totally artificial analogues, and related drug-conjugates covering up to 12/2021.

1 Introduction

2 Antibiotic Crisis and Sideromycins as Natural Templates for New Antimicrobial Drugs

3 Biosynthesis of Enterobactin, Salmochelins, and Microcins

4 Total Synthesis of Enterobactin and Salmochelins

5 Chemoenzymatic Semi-synthesis of Salmochelins and Microcin E492m Derivatives

6 Synthesis of Biomimetic Enterobactin Derivatives with Natural Tris-lactone Backbone

7 Synthesis of Artificial Enterobactin Derivatives without Tris-lactone Backbone

8 Conclusions

Synthetic strategies in construction of organic low molecular-weight carrier-drug conjugates

Inefficient transportation of polar metabolic inhibitors through cell membranes of eukaryotic and prokaryotic cells precludes their direct use as drug candidates in chemotherapy. One of the possible solutions to this problem is application of the 'Trojan horse' strategy, i.e. conjugation of an active substance with a molecular carrier of organic or inorganic nature, facilitating membrane penetration. In this work, the synthetic strategies used in rational design and preparation of conjugates of bioactive agents with three types of organic low molecular-weight carriers have been reviewed. These include iron-chelating agents, siderophores and cell-penetrating peptides. Moreover, a less known but very promising "molecular umbrella" conjugation strategy has been presented. Special attention has been paid on appropriate linking strategies, especially these allowing intracellular drug release after internalisation of a conjugate.

A Salmochelin S4-Inspired Ciprofloxacin Trojan Horse Conjugate

A novel ciprofloxacin-siderophore Trojan Horse antimicrobial was prepared by incorporating key design features of salmochelin, a stealth siderophore that evades mammalian siderocalin capture via its glycosylated catechol units. Assessment of the antimicrobial activity of the conjugate revealed that attachment of the salmochelin mimic resulted in decreased potency, compared to ciprofloxacin, against two Escherichia coli strains, K12 and Nissle 1917, in both iron replete and deplete conditions. This observation could be attributed to a combination of reduced DNA gyrase inhibition, as confirmed by in vitro DNA gyrase assays, and reduced bacterial uptake. Uptake was monitored using radiolabeling with iron-mimetic ⁶⁷Ga³⁺, which revealed limited cellular uptake in E. coli K12. In contrast, previously reported staphyloferrin-based conjugates displayed a measurable uptake in analogous ⁶⁷Ga³⁺ labeling studies. These results suggest that, in the design of Trojan Horse antimicrobials, the choice of siderophore and the nature and length of the linker remain a significant challenge.

How Many O-Donor Groups in Enterobactin Does It Take to Bind a Metal Cation?

The E. coli siderophore enterobactin, the strongest Fe^III chelator known to date, forms hexacoordinate complexes with Si^IV , Ge^IV , and Ti^IV . Synthetic protocols have been developed to prepare non-symmetric enterobactin analogues with varying denticities. Various benzoic acid residues were coupled to the macrocyclic lactone to afford a diverse library of ligands. These enterobactin analogues were bound to Si^IV , Ge^IV , and Ti^IV , and the complexes were investigated through experimental and computational techniques. The binding behavior of the synthesized chelators enabled assessment of the contribution of each of the phenolic hydroxy groups in enterobactin to metal-ion complexation. It was found that at least four O-donors are needed for enterobactin derivatives to act as metal binders. Density functional theory calculations indicate that the strong binding behavior of enterobactin can be ascribed to a diminished translational entropy penalty, a common feature of the chelate effect, coupled with the structural arrangement of the three catechol moieties, which allows the triseryl base to be installed without distorting the preferred local metal-binding geometry of the catecholate ligands.

Mimicking salmochelin S1 and the interactions of its Fe(III) complex with periplasmic iron siderophore binding proteins CeuE and VctP

A mimic of the tetradentate stealth siderophore salmochelin S1, was synthesised, characterised and shown to form Fe(III) complexes with ligand-to-metal ratios of 1:1 and 3:2. Circular dichroism spectroscopy confirmed that the periplasmic binding proteins CeuE and VctP of Campylobacter jejuni and Vibrio cholerae, respectively, bind the Fe(III) complex of the salmochelin mimic by preferentially selecting Λ-configured Fe(III) complexes. Intrinsic fluorescence quenching studies revealed that VctP binds Fe(III) complexes of the mimic and structurally-related catecholate ligands, such as enterobactin, bis(2, 3-dihydroxybenzoyl-l-serine) and bis(2, 3-dihydroxybenzoyl)-1, 5-pentanediamine with higher affinity than does CeuE. Both CeuE and VctP display a clear preference for the tetradentate bis(catecholates) over the tris(catecholate) siderophore enterobactin. These findings are consistent with reports that V. cholerae and C. jejuni utilise the enterobactin hydrolysis product bis(2, 3-dihydroxybenzoyl)-O-seryl serine for the acquisition of Fe(III) and suggest that the role of salmochelin S1 in the iron uptake of enteric pathogens merits further investigation.

Glycosyl Cross-Coupling of Anomeric Nucleophiles: Scope, Mechanism, and Applications in the Synthesis of Aryl C-Glycosides

Stereoselective manipulations at the C1 anomeric position of saccharides are one of the central goals of preparative carbohydrate chemistry. Historically, the majority of reactions forming a bond with anomeric carbon has focused on reactions of nucleophiles with saccharide donors equipped with a leaving group. Here, we describe a novel approach to stereoselective synthesis of C-aryl glycosides capitalizing on the highly stereospecific reaction of anomeric nucleophiles. First, methods for the preparation of anomeric stannanes have been developed and optimized to afford both anomers of common saccharides in high anomeric selectivities. We established that oligosaccharide stannanes could be prepared from monosaccharide stannanes via O-glycosylation with Schmidt-type donors, glycal epoxides, or under dehydrative conditions with C1 alcohols. Second, we identified a general set of catalytic conditions with Pd₂(dba)₃ (2.5 mol%) and a bulky ligand (JackiePhos, 10 mol%) controlling the β-elimination pathway. We demonstrated that the glycosyl cross-coupling resulted in consistently high anomeric selectivities for both anomers with mono- and oligosaccharides, deoxysugars, saccharides with free hydroxyl groups, pyranose, and furanose substrates. The versatility of the glycosyl cross-coupling reaction was probed in the total synthesis of salmochelins (siderophores) and commercial anti-diabetic drugs (gliflozins). Combined experimental and computational studies revealed that the β-elimination pathway is suppressed for biphenyl-type ligands due to the shielding of Pd(II) by sterically demanding JackiePhos, whereas smaller ligands, which allow for the formation of a Pd-F complex, predominantly result in a glycal product. Similar steric effects account for the diminished rates of cross-couplings of 1,2-cis C1-stannanes with aryl halides. DFT calculations also revealed that the transmetalation occurs via a cyclic transition state with retention of configuration at the anomeric position. Taken together, facile access to both anomers of various glycoside nucleophiles, a broad reaction scope, and uniformly high transfer of anomeric configuration make the glycosyl cross-coupling reaction a practical tool for the synthesis of bioactive natural products, drug candidates, allowing for late-stage glycodiversification studies with small molecules and biologics.

Understanding Tetrahydropyranyl as a Protecting Group in Peptide Chemistry

Tetrahydropyranyl (Thp) is recognized as a useful protecting group for alcohols in organic synthesis. It has several advantages, including low cost, ease of introduction, general stability to most non-acidic reagents, it confers good solubility, and the ease with which it can be removed if the functional group it protects requires manipulation. However, little attention has been paid to Thp in peptide chemistry. Provided here is a concise analysis of the Thp protection of various amino acid functionalities (OH, SH, NH and COOH) and its application to peptide synthesis. Thp is a useful moiety for the side-chain protection of serine, threonine and cysteine and is suitable for the Fmoc/tBu solid-phase peptide synthesis strategy. The immobilized version of 3,4-dihydro-2H-pyran, the so-called Ellman resin, is also discussed as a useful solid support for anchoring the side chains of serine, threonine and tryptophan residues.

Tetrahydropyranyl: A Non-aromatic, Mild-Acid-Labile Group for Hydroxyl Protection in Solid-Phase Peptide Synthesis

The use of the tetrahydropyranyl (Thp) group for the protection of serine and threonine side-chain hydroxyl groups in solid-phase peptide synthesis has not been widely investigated. Ser/Thr side-chain hydroxyl protection with this acid-labile and non-aromatic moiety is presented here. Although Thp reacts with free carboxylic acids, it can be concluded that to introduce Thp ethers at the hydroxyl groups of N-protected Ser and Thr, protection of the C-terminal carboxyl group is unnecessary due to the lability of Thp esters. Thp-protected Ser/Thr-containing tripeptides are synthesized and the removal of Thp studied in low concentrations of trifluoroacetic acid in the presence of cation scavengers. Given its general stability to most non-acidic reagents, improved solubility of its conjugates and ease with which it can be removed, Thp emerges as an effective protecting group for the hydroxyl groups of Ser and Thr in solid-phase peptide synthesis.

C-Glycopyranosyl Arenes and Hetarenes: Synthetic Methods and Bioactivity Focused on Antidiabetic Potential

This Review summarizes close to 500 primary publications and surveys published since 2000 about the syntheses and diverse bioactivities of C-glycopyranosyl (het)arenes. A classification of the preparative routes to these synthetic targets according to methodologies and compound categories is provided. Several of these compounds, regardless of their natural or synthetic origin, display antidiabetic properties due to enzyme inhibition (glycogen phosphorylase, protein tyrosine phosphatase 1B) or by inhibiting renal sodium-dependent glucose cotransporter 2 (SGLT2). The latter class of synthetic inhibitors, very recently approved as antihyperglycemic drugs, opens new perspectives in the pharmacological treatment of type 2 diabetes. Various compounds with the C-glycopyranosyl (het)arene motif were subjected to biological studies displaying among others antioxidant, antiviral, antibiotic, antiadhesive, cytotoxic, and glycoenzyme inhibitory effects.

Bacteria in an intense competition for iron: Key component of the Campylobacter jejuni iron uptake system scavenges enterobactin hydrolysis product

To acquire essential Fe(III), bacteria produce and secrete siderophores with high affinity and selectivity for Fe(III) to mediate its uptake into the cell. Here, we show that the periplasmic binding protein CeuE of Campylobacter jejuni, which was previously thought to bind the Fe(III) complex of the hexadentate siderophore enterobactin (Kd ∼ 0.4 ± 0.1 µM), preferentially binds the Fe(III) complex of the tetradentate enterobactin hydrolysis product bis(2,3-dihydroxybenzoyl-l-Ser) (H5-bisDHBS) (Kd = 10.1 ± 3.8 nM). The protein selects Λ-configured [Fe(bisDHBS)](2-) from a pool of diastereomeric Fe(III)-bisDHBS species that includes complexes with metal-to-ligand ratios of 1:1 and 2:3. Cocrystal structures show that, in addition to electrostatic interactions and hydrogen bonding, [Fe(bisDHBS)](2-) binds through coordination of His227 and Tyr288 to the iron center. Similar binding is observed for the Fe(III) complex of the bidentate hydrolysis product 2,3-dihydroxybenzoyl-l-Ser, [Fe(monoDHBS)2](3-) The mutation of His227 and Tyr288 to noncoordinating residues (H227L/Y288F) resulted in a substantial loss of affinity for [Fe(bisDHBS)](2-) (Kd ∼ 0.5 ± 0.2 µM). These results suggest a previously unidentified role for CeuE within the Fe(III) uptake system of C. jejuni, provide a molecular-level understanding of the underlying binding pocket adaptations, and rationalize reports on the use of enterobactin hydrolysis products by C. jejuni, Vibrio cholerae, and other bacteria with homologous periplasmic binding proteins.

Targeting virulence: salmochelin modification tunes the antibacterial activity spectrum of β-lactams for pathogen-selective killing of Escherichia coli

New antibiotics are required to treat bacterial infections and counteract the emergence of antibiotic resistance. Pathogen-specific antibiotics have several advantages over broad-spectrum drugs, which include minimal perturbation to the commensal microbiota. We present a strategy for targeting antibiotics to bacterial pathogens that utilises the salmochelin-mediated iron uptake machinery of Gram-negative Escherichia coli. Salmochelins are C-glucosylated derivatives of the siderophore enterobactin. The biosynthesis and utilisation of salmochelins are important for virulence because these siderophores allow pathogens to acquire iron and evade the enterobactin-scavenging host-defense protein lipocalin-2. Inspired by the salmochelins, we report the design and chemoenzymatic preparation of glucosylated enterobactin-β-lactam conjugates that harbour the antibiotics ampicillin (Amp) and amoxicillin (Amx), hereafter GlcEnt-Amp/Amx. The GlcEnt scaffolds are based on mono- and diglucosylated Ent where one catechol moiety is functionalized at the C5 position for antibiotic attachment. We demonstrate that GlcEnt-Amp/Amx provide up to 1000-fold enhanced antimicrobial activity against uropathogenic E. coli relative to the parent β-lactams. Moreover, GlcEnt-Amp/Amx based on a diglucosylated Ent (DGE) platform selectively kill uropathogenic E. coli that express the salmochelin receptor IroN in the presence of non-pathogenic E. coli and other bacterial strains that include the commensal microbe Lactobacillus rhamnosus GG. Moreover, GlcEnt-Amp/Amx evade the host-defense protein lipocalin-2, and exhibit low toxicity to mammalian cells. Our work establishes that siderophore-antibiotic conjugates provide a strategy for targeting virulence, narrowing the activity spectrum of antibiotics in clinical use, and achieving selective delivery of antibacterial cargos to pathogenic bacteria on the basis of siderophore receptor expression.

Recent developments in β-C-glycosides: synthesis and applications

In the last few years, considerable progress has been made in the synthesis of C-glycosides. Despite its challenging chemistry, due to its versatility, C-glycosides play a pivotal role in developing novel materials, surfactants and bioactive molecules. In this review, we present snapshots of various synthetic methodologies developed for C-glycosides in the recent years and the potential application of C-glycosides derived from β-C-glycosidic ketones.

Siderophore-mediated cargo delivery to the cytoplasm of Escherichia coli and Pseudomonas aeruginosa: syntheses of monofunctionalized enterobactin scaffolds and evaluation of enterobactin-cargo conjugate uptake

The design and syntheses of monofunctionalized enterobactin (Ent, L- and D-isomers) scaffolds where one catecholate moiety of enterobactin houses an alkene, aldehyde, or carboxylic acid at the C5 position are described. These molecules are key precursors to a family of 10 enterobactin-cargo conjugates presented in this work, which were designed to probe the extent to which the Gram-negative ferric enterobactin uptake and processing machinery recognizes, transports, and utilizes derivatized enterobactin scaffolds. A series of growth recovery assays employing enterobactin-deficient E. coli ATCC 33475 (ent-) revealed that six conjugates based on L-Ent having relatively small cargos promoted E. coli growth under iron-limiting conditions whereas negligible-to-no growth recovery was observed for four conjugates with relatively large cargos. No growth recovery was observed for the enterobactin receptor-deficient strain of E. coli H1187 (fepA-) or the enterobactin esterase-deficient derivative of E. coli K-12 JW0576 (fes-), or when the D-isomer of enterobactin was employed. These results demonstrate that the E. coli ferric enterobactin transport machinery identifies and delivers select cargo-modified scaffolds to the E. coli cytoplasm. Pseudomonas aeruginosa PAO1 K648 (pvd-, pch-) exhibited greater promiscuity than that of E. coli for the uptake and utilization of the enterobactin-cargo conjugates, and growth promotion was observed for eight conjugates under iron-limiting conditions. Enterobactin may be utilized for delivering molecular cargos via its transport machinery to the cytoplasm of E. coli and P. aeruginosa thereby providing a means to overcome the Gram-negative outer membrane permeability barrier.