Oxa-spirocycles: synthesis,properties and applications

A general approach to a new generation of spirocyclic molecules – oxa-spirocycles – was developed. The key synthetic step was iodocyclization. More than 150 oxa-spirocyclic compounds were prepared. Incorporation of an oxygen atom into the spirocyclic unit dramatically improved water solubility (by up to 40 times) and lowered lipophilicity. More potent oxa-spirocyclic analogues of antihypertensive drug terazosin were synthesized and studied in vivo.

A general practical approach to a new generation of spirocyclic molecules – oxa-spirocycles – is developed.     

Synthesis of C-acyl furanosides via the cross-coupling of glycosyl esters with carboxylic acids

C-Acyl furanosides are versatile synthetic precursors to a variety of natural products, nucleoside analogues, and pharmaceutical molecules. This report addresses the unmet challenge in preparing C-acyl furanosides by developing a cross-coupling reaction between glycosyl esters and carboxylic acids. A key step is the photoredox activation of the glycosyl ester, which promotes the homolysis of the strong anomeric C–O bond through CO₂ evolution to afford glycosyl radicals. This method embraces a large scope of furanoses, pyranoses, and carboxylic acids, and is readily applicable to the synthesis of a thymidine analogue and diplobifuranylone B, as well as the late-stage modification of (+)-sclareolide. The convenient preparation of the redox active glycosyl ester from native sugars and the compatibility with common furanoses exemplifies the potential of this method in medicinal chemistry.

A cross-coupling of glycosyl esters with carboxylic acids to prepare C-acyl furanosides and pyranosides. The reaction proceeds through photoredox activation of the glycosyl ester to afford glycosyl radicals.     

‘Reverse biomimetic’ synthesis of l-arogenate and its stabilized analogues from l-tyrosine

l-Arogenate (also known as l-pretyrosine) is a primary metabolite on a branch of the shikimate biosynthetic pathway to aromatic amino acids. It plays a key role in the synthesis of plant secondary metabolites including alkaloids and the phenylpropanoids that are the key to carbon fixation. Yet understanding the control of arogenate metabolism has been hampered by its extreme instability and the lack of a versatile synthetic route to arogenate and its analogues. We now report a practical synthesis of l-arogenate in seven steps from O-benzyl l-tyrosine methyl ester in an overall yield of 20%. The synthetic route also delivers the fungal metabolite spiroarogenate, as well as a range of stable saturated and substituted analogues of arogenate. The key step in the synthesis is a carboxylative dearomatization by intramolecular electrophilic capture of tyrosine''s phenolic ring using an N-chloroformylimidazolidinone moiety, generating a versatile, functionalizable spirodienone intermediate.

l-Tyrosine provides a precursor for a practical synthesis of the unstable primary metabolite l-arogenate and some stabilised arogenate analogues.     

Improvement of the antimicrobial potency,pharmacokinetic and pharmacodynamic properties of albicidin by incorporation of nitrogen atoms

The worrisome development and spread of multidrug-resistant bacteria demands new antibacterial agents with strong bioactivities particularly against Gram-negative bacteria. Albicidins were recently structurally characterized as highly active antibacterial natural products from the bacterium Xanthomonas albilineans. Albicidin, which effectively targets the bacterial DNA-gyrase, is a lipophilic hexapeptide mostly consisting of para amino benzoic acid units and only one α-amino acid. In this study, we report on the design and synthesis of new albicidins, containing N-atoms on each of the 5 different phenyl rings. We systematically introduced N-atoms into the aromatic backbone to monitor intramolecular H-bonds and for one derivative correlated them with a significant enhancement of the antibacterial activity and activity spectrum, particularly also towards Gram-positive bacteria. In parallel we conducted DFT calculations to find the most stable conformation of each derivative. A drastic angle-change was observed for the lead compound and shows a preferred planarity through H-bonding with the introduced N-atom at the D-fragment of albicidin. Finally, we went to the next level and conducted the first in vivo experiments with an albicidin analogue. Our lead compound was evaluated in two different mouse experiments: In the first we show a promising PK profile and the absence of toxicity and in the second very good efficiency and reduction of the bacterial titre in an E. coli infection model with FQ-resistant clinically relevant strains. These results qualify albicidins as active antibacterial substances with the potential to be developed as a drug for treatment of infections caused by Gram-negative and Gram-positive bacteria.

A systematic pyridine-scan of the albicidin molecule provides a new lead structure with improved antimicrobial properties.     

An N-capping asparagine–lysine–proline (NKP) motif contributes to a hybrid flexible/stable multifunctional peptide scaffold

Structural diversity drives multiple biological activities and mechanisms of action in linear peptides. Here we describe an unusual N-capping asparagine-lysine-proline (NKP) motif that confers a hybrid multifunctional scaffold to a computationally designed peptide (PaDBS1R7). PaDBS1R7 has a shorter α-helix segment than other computationally designed peptides of similar sequence but with key residue substitutions. Although this motif acts as an α-helix breaker in PaDBS1R7, the Asn5 presents exclusive N-capping effects, forming a belt to establish hydrogen bonds for an amphipathic α-helix stabilization. The combination of these different structural profiles was described as a coil/N-cap/α-helix scaffold, which was also observed in diverse computational peptide mutants. Biological studies revealed that all peptides displayed antibacterial activities. However, only PaDBS1R7 displayed anticancer properties, eradicated Pseudomonas aeruginosa biofilms, decreased bacterial counts by 100–1000-fold in vivo, reduced lipopolysaccharide-induced macrophages stress, and stimulated fibroblast migration for wound healing. This study extends our understanding of an N-capping NKP motif to engineering hybrid multifunctional peptide drug candidates with potent anti-infective and immunomodulatory properties.

An unusual N-capping asparagine-lysine-proline (5NKP7) motif yields a coil/N-cap/α-helix multifunctional scaffold in a computer-made peptide selective for anionic surfaces and with anticancer, antibacterial, antibiofilm, anti-infective (in vivo), and immunomodulatory potential.     

Catalytic asymmetric hydrometallation of cyclobutenes with salicylaldehydes

Chiral, substituted cyclobutanes are common motifs in bioactive compounds and intermediates in organic synthesis but few asymmetric routes for their synthesis are known. Herein we report the Rh-catalyzed asymmetric hydrometallation of a range of meso-cyclobutenes with salicylaldehydes. The ortho-phenolic group promotes hydroacylation and can be used as a handle for subsequent transformations. The reaction proceeds via asymmetric hydrometallation of the weakly activated cyclobutene, followed by a C–C bond forming reductive elimination. A prochiral, spirocyclic cyclobutene undergoes a highly regioselective hydroacylation. This report will likely inspire the development of other asymmetric addition reactions to cyclobutenes via hydrometallation pathways.

Chiral, substituted cyclobutanes are common motifs in bioactive compounds and intermediates in organic synthesis but few asymmetric routes for their synthesis are known.     

Mechanistic insights into the C55-P targeting lipopeptide antibiotics revealed by structure–activity studies and high-resolution crystal structures

The continued rise of antibiotic resistance is a global concern that threatens to undermine many aspects of modern medical practice. Key to addressing this threat is the discovery and development of new antibiotics that operate by unexploited modes of action. The so-called calcium-dependent lipopeptide antibiotics (CDAs) are an important emerging class of natural products that provides a source of new antibiotic agents rich in structural and mechanistic diversity. Notable in this regard is the subset of CDAs comprising the laspartomycins and amphomycins/friulimicins that specifically target the bacterial cell wall precursor undecaprenyl phosphate (C₅₅-P). In this study we describe the design and synthesis of new C₅₅-P-targeting CDAs with structural features drawn from both the laspartomycin and amphomycin/friulimicin classes. Assessment of these lipopeptides revealed previously unknown and surprisingly subtle structural features that are required for antibacterial activity. High-resolution crystal structures further indicate that the amphomycin/friulimicin-like lipopeptides adopt a unique crystal packing that governs their interaction with C₅₅-P and provides an explanation for their antibacterial effect. In addition, live-cell microscopy studies provide further insights into the biological activity of the C₅₅-P targeting CDAs highlighting their unique mechanism of action relative to the clinically used CDA daptomycin.

Structural and mechanistic studies give new insights into calcium-dependent lipopeptide antibiotics that target C₅₅-P.     

Local detection of pH-induced disaggregation of biocompatible micelles by fluorescence switch ON

Fluorogenic nanoparticles (NPs) able to sense different physiological environments and respond with disaggregation and fluorescence switching OFF/ON are powerful tools in nanomedicine as they can combine diagnostics with therapeutic action. pH-responsive NPs are particularly interesting as they can differentiate cancer tissues from healthy ones, they can drive selective intracellular drug release and they can act as pH biosensors. Controlled polymerization techniques are the basis of such materials as they provide solid routes towards the synthesis of pH-responsive block copolymers that are able to assemble/disassemble following protonation/deprotonation. Ring opening metathesis polymerization (ROMP), in particular, has been recently exploited for the development of experimental nanomedicines owing to the efficient direct polymerization of both natural and synthetic functionalities. Here, we capitalize on these features and provide synthetic routes for the design of pH-responsive fluorogenic micelles via the assembly of ROMP block-copolymers. While detailed photophysical characterization validates the pH response, a proof of concept experiment in a model cancer cell line confirmed the activity of the biocompatible micelles in relevant biological environments, therefore pointing out the potential of this approach in the development of novel nano-theranostic agents.

pH-responsive micelles disassembly, upon acidification during lysosomal uptake, leads to fluorescence switch ON. These nanoparticles are promising candidates for the design of novel stimuli-responsive drug delivery systems.     

Catalysis with cycloruthenated complexes

Cycloruthenated complexes have been studied extensively over the last few decades. Many accounts of their synthesis, characterisation, and catalytic activity in a wide variety of transformations have been reported to date. Compared with their non-cyclometallated analogues, cycloruthenated complexes may display enhanced catalytic activities in known transformations or possess entirely new reactivity. In other instances, these complexes can be chiral, and capable of catalysing stereoselective reactions. In this review, we aim to highlight the catalytic applications of cycloruthenated complexes in organic synthesis, emphasising the recent advancements in this field.

We discuss recent advances in the applications of cycloruthenated complexes in organic synthesis, comprising C–H activation, chiral-at-metal catalysis, Z-selective olefin metathesis, transfer hydrogenation, enantioselective cyclopropanations and cycloadditions.     

Galbofloxacin: a xenometal-antibiotic with potent in vitro and in vivo efficacy against S. aureus

Siderophore-antibiotic drug conjugates are considered potent tools to deliver and potentiate the antibacterial activity of antibiotics, but only few have seen preclinical and clinical success. Here, we introduce the gallium(iii) complex of a ciprofloxacin-functionalized linear desferrichrome, Galbofloxacin, with a cleavable serine linker as a potent therapeutic for S. aureus bacterial infections. We employed characterization using in vitro inhibitory assays, radiochemical, tracer-based uptake and pharmacokinetic assessment of our lead compound, culminating in in vivo efficacy studies in a soft tissue model of infection. Galbofloxacin exhibits a minimum inhibitory concentration of (MIC₉₈) 93 nM in wt S. aureus, exceeding the potency of the parent antibiotic ciprofloxacin (0.9 μM). Galbofloxacin is a protease substrate that can release the antibiotic payload in the bacterial cytoplasm. Radiochemical experiments with wt bacterial strains reveal that ⁶⁷Galbofloxacin is taken up efficiently using siderophore mediated, active uptake. Biodistribution of ⁶⁷Galbofloxacin in a mouse model of intramuscular S. aureus infection revealed renal clearance and enhanced uptake in infected muscle when compared to ⁶⁷Ga-citrate, which showed no selectivity. A subsequent in vivo drug therapy study reveals efficient reduction in S. aureus infection burden and sustained survival with Galbofloxacin for 7 days. Ciprofloxacin had no treatment efficacy at identical molecular dose (9.3 μmol kg⁻¹) and resulted in death of all study animals in <24 hours. Taken together, the favorable bacterial growth inhibitory, pharmacokinetic and in vivo efficacy properties qualify Galbofloxacin as the first rationally designed Ga-coordination complex for the management of S. aureus bacterial infections.

Galbofloxacin, a novel theranostic xenosiderophore antibiotic, exhibits unparalleled potency in combating S. aureus infections in vivo.     

Enzyme-activatable fluorescent probes for β-galactosidase: from design to biological applications

β-Galactosidase (β-gal), a typical hydrolytic enzyme, is a vital biomarker for cell senescence and primary ovarian cancers. Developing precise and rapid methods to monitor β-gal activity is crucial for early cancer diagnoses and biological research. Over the past decade, activatable optical probes have become a powerful tool for real-time tracking and in vivo visualization with high sensitivity and specificity. In this review, we summarize the latest advances in the design of β-gal-activatable probes via spectral characteristics and responsiveness regulation for biological applications, and particularly focus on the molecular design strategy from turn-on mode to ratiometric mode, from aggregation-caused quenching (ACQ) probes to aggregation-induced emission (AIE)-active probes, from near-infrared-I (NIR-I) imaging to NIR-II imaging, and from one-mode to dual-mode of chemo-fluoro-luminescence sensing β-gal activity.

This review highlights the molecular design strategy of β-galactosidase-activatable probes from turn-on mode to ratiometric mode, from ACQ to AIE-active probes, from NIR-I to NIR-II imaging and dual-mode of chemo-fluoro-luminescence imaging.     

Precisely synthesized segmented polyurethanes toward block sequence-controlled drug delivery

The construction of polyurethanes (PUs) with sequence-controlled block structures remains a serious challenge. Here, we report the precise synthesis of PUs with desirable molecular weight, narrow molecular weight distribution, and controlled block sequences from commercially available monomers. The synthetic procedure is derived from a liquid-phase synthetic methodology, which involves diisocyanate-based iterative protocols in combination with a convergent strategy. Furthermore, a pair of multifunctional PUs with different sequence orders of cationic and anion segments were prepared. We show that the sequence order of functional segments presents an impact on the self-assembly behavior and results in unexpected surface charges of assembled micelles, thereby affecting the protein absorption, cell internalization, biodistribution and antitumor effect of the nanocarriers in vitro and in vivo. This work provides a versatile platform for the development of precise multiblock PUs with structural complexity and functional diversity, and will greatly facilitate the clinical translation of PUs in biomedicine.

Polyurethanes (PUs) with different segment sequence orders are prepared using a precise synthetic platform, and the sequence order of functional segments plays a crucial role in regulating the surface charges and performance of nano-drug carriers.     

Easily accessible non-aromatic heterocycles with handles: 4-bromo-2,3-dihydrofurans from 1,2-dibromohomoallylic alcohols

The first general preparation of 4-bromo-2,3-dihydrofurans is reported. These non-aromatic heterocycles containing a useful coupling handle are accessed via Cu-catalyzed intramolecular cyclization of 1,2-dibromohomoallylic alcohols, which are themselves available in just two steps from aromatic and aliphatic aldehydes and ketones. Molecular dynamics simulations using the simple substrates and key geometric parameters provide a rationale for the selectivities observed. The synthetic utility of the 4-bromodihydrofurans is also demonstrated.

The first general preparation of 4-bromo-2,3-dihydrofurans is reported.     

Generation of oligonucleotide conjugates via one-pot diselenide-selenoester ligation–deselenization/alkylation

A breadth of strategies are needed to efficiently modify oligonucleotides with peptides or lipids to capitalize on their therapeutic and diagnostic potential, including the modulation of in vivo chemical stability and for applications in cell-targeting and cell-permeability. The chemical linkages typically used in peptide oligonucleotide conjugates (POCs) have limitations in terms of stability and/or ease of synthesis. Herein, we report an efficient method for POC synthesis using a diselenide-selenoester ligation (DSL)-deselenization strategy that rapidly generates a stable amide linkage between the two biomolecules. This conjugation strategy is underpinned by a novel selenide phosphoramidite building block that can be incorporated into an oligonucleotide by solid-phase synthesis to generate diselenide dimer molecules. These can be rapidly ligated with peptide selenoesters and, following in situ deselenization, lead to the efficient generation of POCs. The diselenide within the oligonucleotide also serves as a flexible functionalisation handle that can be leveraged for fluorescent labelling, as well as for alkylation to generate micelles.

An efficient and versatile approach for the late-stage generation of oligonucleotide conjugates by diselenide-selenoester ligation (DSL)–deselenization/alkylation was developed.     

Expanded porphyrins: functional photoacoustic imaging agents that operate in the NIR-II region

Photoacoustic imaging (PAI) relies on the use of contrast agents with high molar absorptivity in the NIR-I/NIR-II region. Expanded porphyrins, synthetic analogues of natural tetrapyrrolic pigments (e.g. heme and chlorophyll), constitute as potentially attractive platforms due to their NIR-II absorptivity and their ability to respond to stimuli. Here, we evaluate two expanded porphyrins, naphthorosarin (1) and octaphyrin (4), as stimuli responsive PA contrast agents for functional PAI. Both undergo proton-coupled electron transfer to produce species that absorb well in the NIR-II region. Octaphyrin (4) was successfully encapsulated into 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-poly(ethylene glycol) (DSPE-PEG₂₀₀₀) nanoparticles to afford OctaNPs. In combination with PAI, OctaNPs allowed changes in the acidic environment of the stomach to be visualized and cancerous versus healthy tissues to be discriminated.

In this study, two expanded porphyrins, octaphyrin and naphthorosarin were evaluated as potential PA agents. The nanoparticle encapsulation of octaphyrin successfully enabled the visualization of acidic environments and the discrimination between cancerous and healthy tissues.     

The convergent total synthesis and antibacterial profile of the natural product streptothricin F

A convergent, diversity-enabling total synthesis of the natural product streptothricin F has been achieved. Herein, we describe the potent antimicrobial activity of streptothricin F and highlight the importance of a total synthesis that allows for the installation of practical divergent steps for medicinal chemistry exploits. Key features of our synthesis include a Burgess reagent-mediated 1,2-anti-diamine installation, diastereoselective azidation of a lactam enolate, and a mercury(ii) chloride-mediated desulfurization-guanidination. The development of this chemistry enables the synthesis and structure–activity studies of streptothricin F analogs.

The second ever total synthesis of streptothricin F and the first achieved through a diversity-enabling convergent route. The synthesis is achieved in 35 total steps, with a longest linear sequence of 19 steps, and 0.40% overall yield.     

Ring-opening and ring-expansion reactions of carborane-fused borirane

Though the reaction chemistry of three-membered ring molecules such as cyclopropanes and their heteroatom-containing analogues has been extensively studied, the chemical properties of their boron analogues, boriranes, are little known thus far. This work describes the diverse reactivity patterns of carborane-fused borirane 2. This borirane engages in ring-opening reactions with different types of Lewis acids, such as BBr₃, GeCl₂, GaCl₃, BH₃(SMe₂) and HBpin, affording a series of ring-opening products, in which M–X or B–H bonds add across the B–C(cage) bond of the three-membered ring in 2. On the other hand, borirane 2 can undergo ring-expansion reactions with unsaturated molecules such as PhCHO, CO₂ and PhCN to give ring-expansion products, five-membered boracycles, via a concerted reaction mechanism as supported by DFT calculations. The results of this work not only enrich the reaction chemistry of boriranes, but also offer new routes to boron-containing compounds and heterocycles.

Carborane-fused borirane can not only engage in ring-opening reactions with different types of Lewis acids, but also undergo ring-expansion reactions with unsaturated molecules such as PhCHO, CO₂ and PhCN to give five-membered boracycles.     

Fully synthetic Mincle-dependent self-adjuvanting cancer vaccines elicit robust humoral and T cell-dependent immune responses and protect mice from tumor development

A new strategy based on a macrophage-inducible C-type lectin (Mincle) agonist was established to construct synthetic cancer vaccines. Using sialyl-Tn (STn) as a model antigen, four conjugates with the Mincle agonist as a built-in adjuvant were designed and synthesized through a facile and efficient method. All conjugates could induce BMDMs to produce inflammatory cytokines in a Mincle-dependent manner and were found to elicit robust humoral and T cell-dependent immune responses alone in mice. The corresponding antibodies could recognize, bind and exhibit complement-dependent cytotoxicity to STn-positive cancer cells, leading to tumor cell lysis. Moreover, all conjugates could effectively inhibit tumor growth and prolong the mice survival time in vivo, with therapeutic effects better than STn-CRM197/Al. Notably, compared to conventional glycoprotein conjugate vaccines, these fully synthetic conjugate vaccines do not cause “epitope suppression.” Mincle ligands thus hold great potential as a platform for the development of new vaccine carriers with self-adjuvanting properties for cancer treatment. Preliminary structure–activity relationship analysis shows that a vaccine containing one STn antigen carried by vizantin exhibits the best efficacy, providing support for further optimization and additional investigation into Mincle agonists as the carrier of self-adjuvanting cancer vaccines.

A new strategy based on a Macrophage-inducible C-type lectin (Mincle) agonist was established to construct synthetic cancer vaccines.     

Controlling the pyridinium–zwitterionic ligand ratio on atomically precise gold nanoclusters allowing for eradicating Gram-positive drug-resistant bacteria and retaining biocompatibility

Infections caused by multidrug-resistant (MDR) bacteria are an increasing global healthcare concern. In this study, we developed a dual-ligand-functionalised Au₂₅(SR₁)_x(SR₂)_18−x-type gold nanocluster and determined its antibacterial activity against MDR bacterial strains. The pyridinium ligand (SR₁) provided bactericidal potency and the zwitterionic ligand (SR₂) enhanced the stability and biocompatibility. By optimising the ligand ratio, our gold nanocluster could effectively kill MDR Gram-positive bacteria via multiple antibacterial actions, including inducing bacterial aggregation, disrupting bacterial membrane integrity and potential, and generating reactive oxygen species. Moreover, combining the optimised gold nanocluster with common antibiotics could significantly enhance the antibacterial activity against MDR bacteria both in in vitro and animal models of skin infections. Furthermore, the fluorescence of the gold nanocluster at the second near-infrared (NIR-II) biological window allowed for the monitoring of its biodistribution and body clearance, which confirmed that the gold nanoclusters had good renal clearance and biocompatibility. This study provides a new strategy to combat the MDR challenge using multifunctional gold nanomaterials.

A NIR-II fluorescent Au₂₅ nanocluster capped with optimal dual functional ligands can effectively aggregate and kill Gram-positive drug-resistant bacteria while maintaining good biocompatibility.     

Hypervalent iodine promoted the synthesis of cycloheptatrienes and cyclopropanes

A new strategy is reported for intramolecular Buchner-type reactions using PIDA as a promotor. Traditionally, the Buchner reaction is achieved via Rh-carbenoids derived from Rh^II catalysts with diazo compounds. Herein, the first metal-free Buchner-type reaction to construct highly strained cycloheptatriene- and cyclopropane-fused lactams is presented. The advantage of these transformations is in their mild reaction conditions, simple operation, broad functional group compatibility and rapid synthetic protocol. In addition, scaled-up experiments and a series of follow-up synthetic procedures were performed to clarify the flexibility and practicability of this method. DFT calculations were carried out to clarify the mechanism.

A new strategy is reported for intramolecular Buchner-type reactions using PIDA as a promotor.