Svetamycins A-G, Unusual Piperazic Acid-Containing Peptides from Streptomyces sp

Identification of the Cirratiomycin Biosynthesis Gene Cluster in Streptomyces Cirratus: Elucidation of the Biosynthetic Pathways for 2,3-Diaminobutyric Acid and Hydroxymethylserine

Cirratiomycin, a heptapeptide with antibacterial activity, was isolated and characterized in 1981; however, its biosynthetic pathway has not been elucidated. It contains several interesting nonproteinogenic amino acids, such as (2S,3S)-2,3-diaminobutyric acid ((2S,3S)-DABA) and α-(hydroxymethyl)serine, as building blocks. Here, we report the identification of a cirratiomycin biosynthetic gene cluster in Streptomyces cirratus. Bioinformatic analysis revealed that several Streptomyces viridifaciens and Kitasatospora aureofaciens strains also have this cluster. One S. viridifaciens strain was confirmed to produce cirratiomycin. The biosynthetic gene cluster was shown to be responsible for cirratiomycin biosynthesis in S. cirratus in a gene inactivation experiment using CRISPR-cBEST. Interestingly, this cluster encodes a nonribosomal peptide synthetase (NRPS) composed of 12 proteins, including those with an unusual domain organization: a stand-alone adenylation domain, two stand-alone condensation domains, two type II thioesterases, and two NRPS modules that have no adenylation domain. Using heterologous expression and in vitro analysis of recombinant enzymes, we revealed the biosynthetic pathway of (2S,3S)-DABA: (2S,3S)-DABA is synthesized from l-threonine by four enzymes, CirR, CirS, CirQ, and CirB. In addition, CirH, a glycine/serine hydroxymethyltransferase homolog, was shown to synthesize α-(hydroxymethyl)serine from d-serine in vitro. These findings broaden our knowledge of nonproteinogenic amino acid biosynthesis.

Biodiversity and Bioactive Potential of Actinomycetes from Unexplored High Altitude Regions of Kargil, India

The effectiveness of currently available antimicrobials and anticancer medications is steadily declining due to the emergence of drug resistance. Since actinobacteria are important producers of bioactive substances, we have isolated them from the soil samples of exotic North-Western Himalayan terrains. Out of 128 isolates, 39 strains were prioritized based on their bioactive potential. The diversity analysis revealed higher abundance distribution of actinomycetes in the soil of an open field (68.7%), followed by the mountainside (34.9%), from which most of the bioactive strains were obtained. The extract of the strain S26-11 was found to be highly active against Gram-positive Staphylococcus aureus and Bacillus subtilis with a MIC of 0.5 μg/mL and 1 μg/mL respectively. A cytotoxicity assay (sulforhodamine B) was performed on a series of cancer cell lines (PC-3, MCF-7, A-549, and HCT-116). The extract of the strain S26-11 showed cytotoxic activity against all cancer cell lines with an IC50 of 2 µg/mL against PC-3, 1.9 µg/mL against MCF-7, 0.52 µg/mL against A-549, and 0.83 µg/mL against HCT-116. Moreover, the antioxidant activity was assessed using a DPPH-based assay and the results revealed that the S17-8 isolate showed the highest antioxidant activity with IC50 of 114.136 μg/mL. The Response Surface Methodology (RSM) had helped to optimize the physical parameters for scaling up of the bioactive strain S26-11. The unexplored soil niches of Kargil (UT, Ladakh), India, is rich in actinomycetes which are having potential bioactivities, would be worth to explore for the discovery of bioactive compounds.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-023-01133-1.

Actinobacteria as Microbial Cell Factories and Biocatalysts in The Synthesis of Chiral Intermediates and Bioactive Molecules; Insights and Applications

Actinobacteria are one of the most intriguing bacterial phyla in terms of chemical diversity and bioactivities of their reported biomolecules and natural products, including various types of chiral molecules. Actinobacterial genera such as Detzia, Mycobacterium, and Streptomyces are among the microbial sources targeted for selective reactions such as asymmetric biocatalysis catalyzed by whole cells or enzymes induced in their cell niche. Remarkably, stereoselective reactions catalyzed by actinobacterial whole cells or their enzymes include stereoselective oxidation, stereoselective reduction, kinetic resolution, asymmetric hydrolysis, and selective transamination, among others. Species of actinobacteria function with high chemo-, regio-, and enantio-selectivity under benign conditions, which could help current industrial processing. Numerous selective enzymes were either isolated from actinobacteria or expressed from actinobacteria in other microbes and hence exploited in the production of pure organic compounds difficult to obtain chemically. In addition, different species of actinobacteria, especially Streptomyces species, function as natural producers of chiral molecules of therapeutic importance. Herein, we discuss some of the most outstanding contributions of actinobacteria to asymmetric biocatalysis, which are important in the organic and/or pharmaceutical industries. In addition, we highlight the role of actinobacteria as microbial cell factories for chiral natural products with insights into their various biological potentialities.

Recent advances on cyclodepsipeptides: biologically active compounds for drug research

Cyclodepsipeptides are a large family of peptide-related natural products consisting of hydroxy and amino acids linked by amide and ester bonds. A number of cyclodepsipeptides have been isolated and characterized from fungi and bacteria. Most of them showed antitumor, antifungal, antiviral, antimalarial, and antitrypanosomal properties. Herein, this review summarizes the recent literatures (2010-2022) on the progress of cyclodepsipeptides from fungi and bacteria except for those of marine origin, in order to enrich our knowledge about their structural features and biological sources.

Targeted and Logical Discovery of Piperazic Acid-Bearing Natural Products Based on Genomic and Spectroscopic Signatures

A targeted and logical discovery method was devised for natural products containing piperazic acid (Piz), which is biosynthesized from ornithine by l-ornithine N-hydroxylase (KtzI) and N-N bond formation enzyme (KtzT). Genomic signature-based screening of a bacterial DNA library (2020 strains) using polymerase chain reaction (PCR) primers targeting ktzT identified 62 strains (3.1%). The PCR amplicons of KtzT-encoding genes were phylogenetically analyzed to classify the 23 clades into two monophyletic groups, I and II. Cultivating hit strains in media supplemented with 15NH4Cl and applying 1H-15N heteronuclear multiple bond correlation (HMBC) along with 1H-15N heteronuclear single quantum coherence (HSQC) and 1H-15N HSQC-total correlation spectroscopy (HSQC-TOCSY) NMR experiments detected the spectroscopic signatures of Piz and modified Piz. Chemical investigation of the hit strains prioritized by genomic and spectroscopic signatures led to the identification of a new azinothricin congener, polyoxyperuin B seco acid (1), previously reported chloptosin (2) in group I, depsidomycin D (3) incorporating two dehydropiperazic acids (Dpz), and lenziamides A and B (4 and 5), structurally novel 31-membered cyclic decapeptides in group II. By consolidating the phylogenetic and chemical analyses, clade-structure relationships were elucidated for 19 of the 23 clades. Lenziamide A (4) inhibited STAT3 activation and induced G2/M cell cycle arrest, apoptotic cell death, and tumor growth suppression in human colorectal cancer cells. Moreover, lenziamide A (4) resensitized 5-fluorouracil (5-FU) activity in both in vitro cell cultures and the in vivo 5-FU-resistant tumor xenograft mouse model. This work demonstrates that the genomic and spectroscopic signature-based searches provide an efficient and general strategy for new bioactive natural products containing specific structural motifs.

PhI(OAc)2-mediated aminoacyloxylation of β,γ-unsaturated hydrazones using Togni reagent II as an acyloxyl precursor

A metal-free protocol for the direct construction of C(sp2)-N and C-O bonds via a PhI(OAc)2-mediated dehydrogenative aminoacyloxylation of β,γ-unsaturated hydrazones with Togni reagent II is reported. Initiated by the carboxyl-containing species generated in situ from Togni reagent II, this method offers a new solution for regioselective functionalization at a remote site on β,γ-unsaturated hydrazones, thus providing a straightforward method for the synthesis of acyloxyl-substituted pyridazines. This reaction features a broad substrate scope and mild conditions.

Annulative Aminosulfonylation of Unactivated Alkenes for Accessing Pyrazolines via Multicomponent SO2 Insertion

A direct arylsulfonylation of β,γ-unsaturated hydrazones method, in which sulfonated pyrazolines are accessed by a three-component reaction of β,γ-unsaturated hydrazones, DABSO, and aryldiazonium tetrafluoroborates, has been developed without external oxidants or catalysts. This transformation is triggered by the formation of arylsulfonyl radicals in situ from the reaction of aryldiazonium tetrafluoroborates and DABSO, and is enabled by controllable generation of C center radical, in which DABSO was utilized as the sulfone source and an oxidant in this radical-mediated cascaded reaction. A wide range of substrates can be applied in this process to afford pyrazolines in good yield, and it is amenable for gram-scale synthesis.

Bioactive Metabolites from Terrestrial and Marine Actinomycetes

Actinomycetes inhabit both terrestrial and marine ecosystems and are highly proficient in producing a wide range of natural products with diverse biological functions, including antitumor, immunosuppressive, antimicrobial, and antiviral activities. In this review, we delve into the life cycle, ecology, taxonomy, and classification of actinomycetes, as well as their varied bioactive metabolites recently discovered between 2015 and 2023. Additionally, we explore promising strategies to unveil and investigate new bioactive metabolites, encompassing genome mining, activation of silent genes through signal molecules, and co-cultivation approaches. By presenting this comprehensive and up-to-date review, we hope to offer a potential solution to uncover novel bioactive compounds with essential activities.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Computational 1 H and 13 C NMR in structural and stereochemical studies

Present review outlines the advances and perspectives of computational ¹ H and ¹³ C NMR applied to the stereochemical studies of inorganic, organic, and bioorganic compounds, involving in particular natural products, carbohydrates, and carbonium ions. The first part of the review briefly outlines theoretical background of the modern computational methods applied to the calculation of chemical shifts and spin-spin coupling constants at the DFT and the non-empirical levels. The second part of the review deals with the achievements of the computational ¹ H and ¹³ C NMR in the stereochemical investigation of a variety of inorganic, organic, and bioorganic compounds, providing in an abridged form the material partly discussed by the author in a series of parent reviews. Major attention is focused herewith on the publications of the recent years, which were not reviewed elsewhere.

Zelkovamycins F and G, Cyclopeptides with Cα-Methyl-threonine Residues, from an Endophytic Kitasatospora sp

Zelkovamycins F and G (1 and 2), two new natural cyclic octapeptides possessing the unprecedented nonproteinogenic amino acid residues l-α-methyl-threonine and l-α-methyl-allo-threonine, respectively, along with four new analogues, zelkovamycins H-K (3-6), were identified from the endophytic Kitasatospora sp. CPCC 204717. Their structures were elucidated by detailed analysis of NMR and HRESIMS/MS spectroscopic data. The configurations of amino acid residues were determined by Marfey's analysis combined with NMR calculations. Compounds 1, 2, and 4 showed potent antibacterial activity against methicillin-resistant Staphylococcus aureus and Staphylococcus epidermidis. The structure-activity relationship study revealed that the 2-methyl-3-oxobutyrine and sarcosine residues played important roles in their antibacterial activities. Zelkovamycin (7) and zelkovamycin E (8) exhibited significant antiviral activity against the hepatitis C virus.

Synthetic and biosynthetic routes to nitrogen-nitrogen bonds

The nitrogen-nitrogen bond is a core feature of diverse functional groups like hydrazines, nitrosamines, diazos, and pyrazoles. Such functional groups are found in >300 known natural products. Such N-N bond-containing functional groups are also found in significant percentage of clinical drugs. Therefore, there is wide interest in synthetic and enzymatic methods to form nitrogen-nitrogen bonds. In this review, we summarize synthetic and biosynthetic approaches to diverse nitrogen-nitrogen-bond-containing functional groups, with a focus on biosynthetic pathways and enzymes.

Inverse Electron-Demand Aza-Diels-Alder Reaction of Cyclic Enamides with 1,2-Diaza-1,3-dienes in Situ Generated from α-Halogeno Hydrazones: Access to Fused Polycyclic Tetrahydropyridazine Derivatives

An efficient inverse electron-demand aza-Diels-Alder reaction of cyclic enamides and 1,2-diaza-1,3-dienes, which could be readily formed in situ from α-halogeno hydrazones and a base, has been successfully developed. With the developed approach, a wide range of fused polycyclic tetrahydropyridazines were smoothly obtained in up to 99% yield under benign reaction conditions. This reaction concept was also extended to acyclic enamide substrates for accessing 1,4,5,6-tetrahydropyridazines. A gram-scale experiment and further derivatizations of the polycyclic tetrahydropyridazine products were also conducted to verify the practicability of the methodology.

A critical review on the use of DP4+ in the structural elucidation of natural products: the good, the bad and the ugly. A practical guide

Covering: 2015 up to the end of 2020Even in the golden age of NMR, the number of natural products being incorrectly assigned is becoming larger every day. The use of quantum NMR calculations coupled with sophisticated data analysis provides ideal complementary tools to facilitate the elucidation process in challenging cases. Among the current computational methodologies to perform this task, the DP4+ probability is a popular and widely used method. This updated version of Goodman's DP4 synergistically combines NMR calculations at higher levels of theory with the Bayesian analysis of both scaled and unscaled data. Since its publication in late 2015, the use of DP4+ to solve controversial natural products has substantially grown, with several predictions being confirmed by total synthesis. To date, the structures of more than 200 natural products were determined with the aid of DP4+. However, all that glitters is not gold. Besides its intrinsic limitations, on many occasions it has been improperly used with potentially important consequences on the quality of the assignment. Herein we present a critical revision on how the scientific community has been using DP4+, exploring the strengths of the method and how to obtain optimal results from it. We also analyze the weaknesses of DP4+, and the paths to by-pass them to maximize the confidence in the structural elucidation.

Bioproduction of L-piperazic acid in gram scale using Aureobasidium melanogenum

Currently, piperazic acid is chemically synthesized using ecologically unfriendly processes. Microbial synthesis from glucose is an attractive alternative to chemical synthesis. In this study, we report the production of L-piperazic acid via microbial fermentation with the first engineered fungal strain of Aureobasidium melanogenum; this strain was constructed by chassis development, genetic element reconstitution and optimization, synthetic rewiring and constitutive genetic circuit reconstitution, to build a robust L-piperazic acid synthetic cascade. These genetic modifications enable A. melanogenum to directly convert glucose to L-piperazic acid without relying on the use of either chemically synthesized precursors or harsh conditions. This bio-based process overcomes the shortcomings of the conventional synthesis routes. The ultimately engineered strain is a very high-efficient cell factory that can excrete 1.12 ± 0.05 g l-1 of L-piperazic acid after a 120-h 10.0-l fed-batch fermentation; this is the highest titre of L-piperazic acid reported using a microbial cell factory.

Chlorinated metabolites from Streptomyces sp. highlight the role of biosynthetic mosaics and superclusters in the evolution of chemical diversity

LCMS-guided screening of a library of biosynthetically talented bacteria and fungi identified Streptomyces sp. MST- as a prolific producer of chlorinated metabolites. We isolated and characterised six new and nine reported compounds from MST-, belonging to three discrete classes - the depsipeptide svetamycins, the indolocarbazole borregomycins and the aromatic polyketide anthrabenzoxocinones. Following genome sequencing of MST-, we describe, for the first time, the svetamycin biosynthetic gene cluster (sve), its mosaic structure and its relationship to several distantly related gene clusters. Our analysis of the sve cluster suggested that the reported stereostructures of the svetamycins may be incorrect. This was confirmed by single-crystal X-ray diffraction analysis, allowing us to formally revise the absolute configurations of svetamycins A-G. We also show that the borregomycins and anthrabenzoxocinones are encoded by a single supercluster (bab) implicating superclusters as potential nucleation points for the evolution of biosynthetic gene clusters. These clusters highlight how individual enzymes and functional subclusters can be co-opted during the formation of biosynthetic gene clusters, providing a rare insight into the poorly understood mechanisms underpinning the evolution of chemical diversity.

Structural characterisation of natural products by means of quantum chemical calculations of NMR parameters: new insights

In this review, we focus in all aspects of NMR simulation of natural products, from the fundamentals to the new computational toolboxes available, combining advanced quantum chemical calculations with upstream data processing and machine learning.

DFT/NMR Approach for the Configuration Assignment of Groups of Stereoisomers by the Combination and Comparison of Experimental and Predicted Sets of Data

Quantum mechanical/nuclear magnetic resonance (NMR) approaches are widely used for the configuration assignment of organic compounds generally comparing one cluster of experimentally determined data (e.g., 13C NMR chemical shifts) with those predicted for all possible theoretical stereoisomers. More than one set of experimental data, each related to a specific stereoisomer, may occur in some cases, and the accurate stereoassignments can be obtained by combining the experimental and computed data. We introduce here a straightforward methodology based on the simultaneous analysis, combination, and comparison of all sets of experimental/calculated 13C chemical shifts for aiding the correct configuration assignment of groups of stereoisomers. The comparison of the differences between the calculated/experimental chemical shifts instead of the shifts themselves led to the advantage of avoiding errors arising from calibration procedures, reducing systematic errors, and highlighting the most diagnostic differences between calculated and experimental data. This methodology was applied on a tetrad of synthesized cladosporin stereoisomers (cladologs) and further corroborated on a tetrad of pochonicine stereoisomers, obtaining the correct correspondences between experimental and calculated sets of data. The new MAEΔΔδ parameter, useful for indicating the best fit between sets of experimental and calculated data, is here introduced for facilitating the stereochemical assignment of groups of stereoisomers.

Computational 1 H NMR: Part 2. Chemical applications

This is the second one of three closely interrelated reviews dealing with computation of ¹ H NMR chemical shifts and ¹ H-¹ H spin-spin coupling constants prepared for Magnetic Resonance in Chemistry. Presented in this review are some basic notes and illustrative examples of how modern computational ¹ H NMR could be used for structural elucidation and stereoelectronic studies of the medium-sized organic molecules involving saturated, unsaturated, aromatic, and heteroaromatic compounds together with their functional derivatives and coordination complexes to get deeper insight into their stereochemical structure and stereodynamic behavior.

Biosynthetic Pathways to Nonproteinogenic α-Amino Acids

Natural nonproteinogenic amino acids vastly outnumber the well-known 22 proteinogenic amino acids. Such amino acids are generated in specialized metabolic pathways. In these pathways, diverse biosynthetic transformations, ranging from isomerizations to the stereospecific functionalization of C-H bonds, are employed to generate structural diversity. The resulting nonproteinogenic amino acids can be integrated into more complex natural products. Here we review recently discovered biosynthetic routes to freestanding nonproteinogenic α-amino acids, with an emphasis on work reported between 2013 and mid-2019.

Stereospecific Copper(II)-Catalyzed Tandem Ring Opening/Oxidative Alkylation of Donor-Acceptor Cyclopropanes with Hydrazones: Synthesis of Tetrahydropyridazines

Aerobic copper(II)-catalyzed tandem ring opening and oxidative C-H alkylation of donor-acceptor cyclopropanes with bisaryl hydrazones is accomplished to produce tetrahydropyridazines, in which copper(II) plays dual role as a Lewis acid as well as redox catalyst. The reaction is stereospecific, and optically active cyclopropanes can be reacted with high optical purities (89-98% enantiomeric excess). The substrate scope, functional group tolerance, dual role of the copper(II) catalyst, and the use of air as an oxidant are the important practical features. A product bearing a 3-bromoaryl group can be subjected to Pd-catalyzed Suzuki coupling with boronic acid in high yield.

Piperazic acid containing peptides produced by an endophytic Streptomyces sp. isolated from the medicinal plant Atropa belladonna

The culture broth of endophytic Streptomyces sp. AB100, isolated from the shoots of medicinal plant Atropa belladonna (L.) was investigated for the presence of antibacterial compounds. After initial testing followed by bioactivity-guided fractionation, six new piperazic acid (PA)-containing congeners of two known peptides, JBIR-39 and JBIR-40, were identified by HR-MS/MS and NMR analyses. Only the dehydroxylated hexapeptidic derivatives with unusual incorporation of four PA moieties exhibited weak antibacterial activity against Gram-positive test organism Bacillus subtilis. A 16S rDNA-based phylogenetic tree of known Streptomyces spp. producing PA-containing hexapeptides isolated from different habitats and endophyte Streptomyces AB100 showed considerable diversity, suggesting that these metabolites may play an important environmental role beyond their antibacterial activity.

Antibiotic-Producing Beneficial Bacteria in the Gut of the Burying Beetle Nicrophorus vespilloides

The increasing prevalence of antibiotic-resistant human pathogens is a growing public concern and there is intense pressure to identify new antibacterial compounds that can be developed into antibiotics with novel mode of action. Evolutionary theory predicts that insects that have evolved to occupy sophisticated ecological niches by feeding and reproducing on carcasses will depend on their gut microbiome to prevent colonization by invading pathogens taken up with the diet. This inspired our hypothesis that the complex interactions between the core microbiome and the more flexible microbial communities dependent on the environment may promote the outsourcing of antibiotic synthesis to beneficial microbes. We tested this hypothesis by cultivating and characterizing bacteria isolated from the gut of the burying beetle Nicrophorus vespilloides, which feeds and reproduces on small vertebrate carcasses buried in the soil to avoid competitors such as fly maggots. The extracts of isolated bacteria were screened for activity against human pathogens such as Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Candida albicans. More than 400 strains were isolated, among which the crude extract of Serratia marcescens 2MH3-2 displayed promising activity against Staphylococcus aureus. Bioactivity-guided fractionation enabled purification of the primary antimicrobial compound of the extract. By LC-MS and NMR experiments, it was identified as serrawettin W2 (C₃₈H₆₁N₅O₉), the antibacterial and nematostatic activity of which was corroborated in our study. We postulate that this antibiotic could contribute to the control of both bacteria and phoretic nematodes in the gut, which compete for food when transferred to the carcass. Our study shows that the gut microbiome of N. vespilloides is a promising resource for the screening of antibiotic-producing bacteria.

Piperazic acid-containing natural products: structures and biosynthesis

Piperazic acid is a cyclic hydrazine and a non-proteinogenic amino acid found in diverse non-ribosomal peptide (NRP) and hybrid NRP–polyketide (PK) structures.

Chemical shift assignment of mono- and di-bromo triimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine derivatives by DFT/NMR integrated approach

Mono- and di-bromo derivatives of triimidazo[1,2-a:1',2'-c:1″,2″-e][1,3,5]triazine have been proposed as new organic molecules presenting a very rich and complex photophysical behavior. Thus, we afforded the correct chemical shift assignment by integrating the experimental data with DFT calculation of NMR parameters. Our findings lay foundation for a structural reference in the organic synthesis and characterization of new congeners of this intriguing class of molecules.

Copper-Mediated Aminoazidation, Aminohalogenation, and Aminothiocyanation of β,γ-Unsaturated Hydrazones: Synthesis of Versatile Functionalized Pyrazolines

A versatile method for the rapid synthesis of diverse functionalized pyrazolines has been developed based on copper-mediated aminofunctionalization of β,γ-unsaturated hydrazones. The scope of this strategy encompasses a range of difunctionalization reactions: aminoazidation, aminohalogenation, and aminothiocyanation. These reactions provide straightforward access to a series of useful pyrazoline building blocks containing various functional groups that are hard to access traditionally.

Hypervalent iodine-mediated Ritter-type amidation of terminal alkenes: The synthesis of isoxazoline and pyrazoline cores

Hypervalent iodine-mediated olefin functionalization provides a rapid gateway towards accessing both various heterocyclic cores and functional groups. In this regard, we have developed a Ritter-type alkene functionalization utilizing a PhI(OAc)₂ ((diacetoxyiodo)benzene, PIDA)/Lewis acid combination in order to access isoxazoline and pyrazoline cores. Based on allyl ketone oximes and allyl ketone tosylhydrazones, we have developed an alkene oxyamidation and amido-amidation protocol en route to accessing both isoxazoline and pyrazoline cores. Additionally, acetonitrile serves as both the solvent and an amine source in the presence of this PIDA/Lewis acid combination. This operationally straightforward and metal-free protocol provides an easy access to isoxazoline and pyrazoline derivatives.

Actinobacteria-Derived peptide antibiotics since 2000

Members of the Actinobacteria, including Streptomyces spp., Kutzneria sp. Actinoplanes spp., Actinomycete sp., Nocardia sp., Brevibacteriumsp.,Actinomadura spp., Micromonospora sp., Amycolatopsis spp., Nonomuraea spp., Nocardiopsis spp., Marinactinospora sp., Rhodococcus sp., Lentzea sp., Actinokineospora sp., Planomonospora sp., Streptomonospora sp., and Microbacterium sp., are an important source of structurally diverse classes of short peptides of ∼30 residues or fewer that will likely play an important role in new antibiotic development and discovery. Additionally, many have unique structures that make them recalcitrant to traditional modes of drug resistance via novel mechanisms, and these are ideal therapeutic tools and potential alternatives to current antibiotics. The need for novel antibiotic is urgent, and this review summarizes 199 Actinobacteria compounds published since 2000, including 35 cyclic lipopeptides containing piperazic or pipecolic acids, eight aromatic peptides, five glycopeptides, 21 bicyclic peptides, 44 other cyclic lipopeptides, five linear lipopeptides, six 2,5-diketopiperazines, one dimeric peptide, four nucleosidyl peptides, two thioamide-containing peptides, 25 thiopeptides, nine lasso peptides, and 34 typical cyclic peptides. The current and potential therapeutic applications of these peptides, including their structure, antituberculotic, antibacterial, antifungal, antiviral, anti-brugia, anti-plasmodial, and anti-trypanosomal activities, are discussed.

Transition metal-free aminofluorination of β,γ-unsaturated hydrazones: base-controlled regioselective synthesis of fluorinated dihydropyrazole and tetrahydropyridazine derivatives

Various base-controlled regioselective reactions of β,γ-unsaturated hydrazones with Selectfluor were achieved under transition metal-free conditions.

Iodine(iii)-mediated intramolecular sulfeno- and selenofunctionalization of β,γ-unsaturated tosyl hydrazones and oximes

A cascade radical cyclization/sulfenylation or selenylation of β,γ-unsaturated hydrazones and oximes was realized under mild conditions with phenyliodine(iii) diacetate (PIDA) as the sole oxidant, leading to the construction of diversely functionalized heteroatom-containing pyrazoline and isoxazoline derivatives.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as svetamycin B from a Streptomyces species.