Lajollamycins, nitro group-bearing spiro-β-lactone-γ-lactams obtained from a marine-derived Streptomyces sp

Nitric oxide synthase-guided genome mining identifies a cytochrome P450 enzyme for olefin nitration in bacterial specialized metabolism

The biological signaling molecule nitric oxide (NO) has recently emerged as a metabolic precursor for the creation of microbial natural products with diversified structures and biological activities. Within the biosynthetic gene clusters (BGCs) of these compounds, genes associated with NO production pathways have been pinpointed. In this study, we employ a nitric oxide synthase (NOS)-guided genome mining strategy for the targeted discovery of NO-derived bacterial natural products and NO-utilizing biocatalysts. We show that a conserved NOS-containing BGC, distributed across several actinobacterial genomes, is responsible for the biosynthesis of lajollamycin, a unique nitro-tetraene-containing antibiotic whose biosynthetic mechanism remains elusive. Through a combination of in vivo and in vitro studies, we unveil the first cytochrome P450 enzyme capable of catalyzing olefin nitration in natural product biosynthesis. These results not only expand the current knowledge about biosynthetic nitration processes but also offer an efficient way for targeted identification of NO-utilizing metabolic pathways and novel nitrating biocatalysts.

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.

Transposon-based identification of genes involved in the rimocidin biosynthesis in Streptomyces rimosus M527

The transposon mutagenesis strategy has been employed to generate random insertion mutants and analyze the correlation between genes and secondary metabolites in the genus Streptomyces. In this study, our primary objective was to identify an unknown gene involved in rimocidin biosynthesis and elucidate its role in rimocidin production in Streptomyces rimosus M527. To achieve this, we established a random mutant library of S. rimosus M527 using a Tn5 transposon-mediated random mutagenesis strategy. Among the 137 isolated mutants, M527-G10 and M527-W5 exhibited the most significant variations in antagonistic activity against the plant pathogenic fungus Fusarium oxysporum f. sp. cucumerinum. Specifically, M527-G10 displayed a 72.93% reduction, while M527-W5 showed a 49.8% increase in rimocidin production compared to the wild-type (WT) strain S. rimosus M527. Subsequently, we employed a plasmid rescue strategy to identify the insertion loci of the transposon in the genomes of mutants M527-G10 and M527-W5, revealing a response regulator transcription factor (rrt) and a hypothetical protein (hyp), respectively. The roles of rrt and hyp in rimocidin biosynthesis were determined through gene deletion, overexpression in the WT strain, and complemented expression in the transposon mutants. Notably, the gene-deletion mutants M527-ΔRRT and M527-ΔHYP exhibited similar behavior in rimocidin production compared to the corresponding transposon mutants M527-G10 and M527-W5, suggesting that transposon insertions in genes rrt and hyp led to alterations in rimocidin production. Furthermore, both gene deletion and overexpression of rrt and hyp had no discernible effects on cell growth. These results reveal that genes rrt and hyp have positive and negative impacts on rimocidin production in S. rimosus M527, respectively.

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.

Photoredox-Catalyzed Tandem Radical Cyclization/Hydroxylation for the Synthesis of 4-Hydroxyalkyl-3,3-difluoro-γ-lactams

The photoredox-catalyzed radical difluoroalkylation/cyclization/hydroxylation cascade reaction of various 2-bromo-2,2-difluoro-N-arylacetamides containing unactivated alkene moieties has been developed, providing green and efficient access to various 4-hydroxyalkyl-3,3-difluoro-γ-lactams. Control experiments confirmed a radical process, and inexpensive air acted as the sole hydroxy resource. In addition, the highlights of this protocol include good tolerance for a variety functional groups, lower photocatalyst loading, and ease of operation.

Diastereoselective and Enantioselective Synthesis of α-p-Methoxyphenoxy-β-Lactones: Dependence on the Stereoelectronic Properties of the β-Hydroxy-α-p-Methoxyphenoxycarboxylic Acid Precursors

Treatment of β-hydroxy-α-p-methoxyphenoxy carboxylic acids derived from the asymmetric glycolate aldol addition reaction with p-nitrobenzenesulfonyl chloride yielded divergent results depending on the nature of the β-substituent of the carboxylic acid. Substrates bearing either alkyl substituents (R = -n-butyl, -n-octyl, -benzyl, isopropyl, -tert-butyl) or aryl systems bearing electron-withdrawing substituents (R = -p-C₆H₄Cl, -p-C₆H₄Br, -p-C₆H₄NO₂) yielded β-lactones. In contrast, α-p-methoxyphenoxy-β-hydroxycarboxylic acids bearing electron-donating aryl groups or the sterically demanding 2-naphthyl group formed (Z)-alkenes.

Asymmetric Total Synthesis of Oxazolomycins B and C

Efforts towards the first total synthesis of (-)-oxazolomycin B and (+)-oxazolomycin C from the intermediate of our previous synthesis of (+)-neoxazolomycin are reported. The syntheses were achieved in a longest linear sequence of 25 steps from the amino acid serine in 3.6 and 2.7 % overall yields, respectively. The efficiency of our approach is derived from silyl triflate-mediated reductive oxazolidine ring-opening and Fürstner's Ru-catalyzed hydrosilylation and protodesilylation reactions. The obtained spectra and optical rotations were in good agreement with those of natural products, thus confirming the structures.

Oxazolomycins produced by Streptomyces glaucus and their cytotoxic activity

Six oxazolomycins (1–6) were isolated from the fermentation broth of a soil-borne bacterial strain, Streptomyces glaucus. The structures of the new compounds, oxazolomycins D–F (1–3) and glaucumycins A, B (6a/6b), were elucidated by detailed spectroscopic data analysis. Oxazolomycins 1, 2, 4, and 5 demonstrated weak or modest cytotoxic activities against four human cancer cell lines, with IC₅₀ values ranging from 10.6 ± 1.7 to 89.5 ± 6.6 μM (or >100 μM). Further study showed that 4 caused S phase cell cycle arrest in SMMC7721 cells through down-regulating the protein expression of cyclin A2, CDK2. Meanwhile, 4 induced apoptosis in SMMC7721 cells through down-regulating the protein levels of Bcl-2, up-regulating the levels of Bax, and activating the cleavage of caspase-3.

Six oxazolomycins (1–6) were isolated from the fermentation broth of a soil-borne bacterial strain, Streptomyces glaucus.

Enantioselective, Organocatalytic Strategy for the Oxazolomycin Core: Formal Synthesis of (+)-Neooxazolomycin

A concise, organocatalytic, enantioselective route to the γ-lactam core of the oxazolomycins was developed. Key steps include a Lewis base-catalyzed, Michael proton transfer-lactamization organocascade, a one-pot N-methylation and diastereoselective α-alkylation, a diastereotopic group-selective reduction, a substrate-directed allylic hydroxylation, and a lanthanide-mediated organolithium addition to append the side chain. A formal synthesis of (+)-neooxazolomycin via interception of a Kende intermediate, accessed in 10 steps (previously 24 steps from α-d-glucose), enabled confirmation of the relative and absolute stereochemistry.

Visible-Light-Induced, Catalyst-Free Radical Cross-Coupling Cyclization of N-Allylbromodifluoroacetamides with Disulfides or Diselenides

A visible-light-induced, catalyst-free radical cross-coupling cyclization of diselenides or disulfides with N-allylbromodifluoroacetamide has been developed. This developed protocol exhibits good functional group tolerance and affords a variety of 4-thio- and 4-seleno-substituted 3,3-difluoro-γ-lactams in moderate to good yields. Based on control experiments, a plausible radical-radical cross-coupling pathway is proposed.

The Biological and Chemical Diversity of Tetramic Acid Compounds from Marine-Derived Microorganisms

Tetramic acid (pyrrolidine-2,4-dione) compounds, isolated from a variety of marine and terrestrial organisms, have attracted considerable attention for their diverse, challenging structural complexity and promising bioactivities. In the past decade, marine-derived microorganisms have become great repositories of novel tetramic acids. Here, we discuss the biological activities of 277 tetramic acids of eight classifications (simple 3-acyl tetramic acids, 3-oligoenoyltetramic acids, 3-decalinoyltetramic acid, 3-spirotetramic acids, macrocyclic tetramic acids, N-acylated tetramic acids, α-cyclopiazonic acid-type tetramic acids, and other tetramic acids) from marine-derived microbes, including fungi, actinobacteria, bacteria, and cyanobacteria, as reported in 195 research studies up to 2019.

Marine Actinomycetes-derived Natural Products

Actinomycetes is an abundant resource for discovering a large number of lead compounds, which play an important role in microbial drug discovery. Compared to terrestrial microorganisms, marine actinomycetes have unique metabolic pathways because of their special living environment, which has the potential to produce a variety of bioactive substances. In this paper, secondary metabolites isolated from marine actinomycetes are reviewed (2013-2018), most of which exhibited cytotoxic, antibacterial, and antiviral biological activities.

The oxazolomycin family: a review of current knowledge

Oxazolomycin A and neooxazolomycin were firstly isolated in 1985 by the group of Uemura et al. from the Streptomyces sp. bacteria.

Spirocyclic Motifs in Natural Products

Spirocyclic motifs are emerging privileged structures for drug discovery. They are also omnipresent in the natural products domain. However, until today, no attempt to analyze the structural diversity of various spirocyclic motifs occurring in natural products and their relative populations with unique compounds reported in the literature has been undertaken. This review aims to fill that void and analyze the diversity of structurally unique natural products containing spirocyclic moieties of various sizes.

Asymmetric Total Synthesis of (+)-Neooxazolomycin Using a Chirality-Transfer Strategy

The total synthesis of (+)-neooxazolomycin was achieved from the amino-acid d-serine. The efficiency of this approach is derived from the use of principles of memory of chirality and dynamic kinetic resolution in the intramolecular aldol reaction of a serine derivative to build the densely functionalized lactam framework and to install three contiguous stereocenters. The key intermediate was readily elaborated to the target natural product.

Strepchazolins A and B: Two New Alkaloids from a Marine Streptomyces chartreusis NA02069

Two new alkaloids, strepchazolins A (1) and B (2), together with a previously reported compound, streptazolin (3), were isolated from a marine actinomycete, Streptomyces chartreusis NA02069, collected in the Coast of Hainan Island, China. The structures of new compounds were determined by extensive NMR, mass spectroscopic and X-ray crystallographic analysis, as well as modified Mosher’s method. Compound 1 showed weak anti-Bacillus subtilis activity with the MIC value of 64.0 μM, and weak inhibitory activity against acetylcholinesterase (AChE) in vitro with IC₅₀ value of 50.6 μM, while its diastereoisomer, Compound 2, is almost inactive.

A new phenylpropanoid glucoside and a chain compound from the roots of Allium tuberosum

A new phenylpropanoid glucoside tuberosinine D (1) and a chain compound (Z)-11R,12S,13S-trihydroxy-9-octadecenoate (2) were isolated from the roots of Allium tuberosum. The absolute configuration of 1 was established by comparing of experimental and calculated electronic circular dichroism. The absolute configuration of 2 was determined using the modified Mosher's method for the first time.

Marine natural products

This review covers the literature published in 2014 for marine natural products (MNPs), with 1116 citations (753 for the period January to December 2014) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1378 in 456 papers for 2014), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Signaling preferences of substituted pyrrole coupled six-membered rhodamine spirocyclic probes for Hg2+ ion detection

Six-membered spiro-ring expanded probes in rhodamine scaffolds exhibit Hg²⁺ ion specific dual mode signaling responses; specificity is achieved with tuning substituents on the appended pyrrole receptor.

Enantioselective synthesis of chiral α,β-unsaturated γ-substituted butyrolactams by organocatalyzed direct asymmetric vinylogous Michael addition of α,β-unsaturated γ-butyrolactam to 2-enoylpyridines

Approach providing a series of optically active α,β-unsaturated γ-substituted butyrolactams in high yields with excellent diastereo- and enantioselectivities.

Suncheonosides A-D, Benzothioate Glycosides from a Marine-Derived Streptomyces sp

A marine-derived Streptomyces strain, SSC21, was isolated from the sediment of Suncheon Bay, Republic of Korea. Chemical analysis of the bacterial strain resulted in the isolation of four new metabolites, suncheonosides A-D (1-4, respectively), each bearing a sulfur atom. The planar structures of the suncheonosides were identified as hexasubstituted benzothioate glycosides by combined spectroscopic analyses. Analysis of the configuration of the sugar moieties based on ROESY nuclear magnetic resonance correlations, one-bond (1)H-(13)C coupling constant analysis, and chemical derivatizations indicated that the suncheonosides incorporate only l-rhamnose. Suncheonosides A, B, and D promoted adiponectin production in a concentration-dependent manner during adipogenesis in human mesenchymal stem cells, suggesting antidiabetic potential.

Pd(ii)-catalyzed asymmetric addition of arylboronic acids to cyclic N-sulfonyl ketimine esters and a DFT study of its mechanism

A highly efficient palladium-catalyzed asymmetric arylation of cyclic ketimine esters is developed, which provides the desired product in up to 99% yield with up to 99% ee. The mechanism of enantioselection is studied using DFT calculation.