Prorocentin, a New Polyketide from the Marine Dinoflagellate Prorocentrum lima

An Efficient and Scalable "Second Generation" Total Synthesis of the Marine Polyketide Limaol Endowed with Antiparasitic Activity

The cluster of four skipped exo-methylene substituents on the "northern" wing of limaol renders this dinoflagellate-derived marine natural product unique in structural terms. This arguably non-thermodynamic array gains kinetic stability by virtue of populating local conformations which impede isomerization to a partly or fully conjugated polyene. This analysis suggested that the difficulties encountered during the late stages of our first total synthesis of this polyketide had not been caused by an overly fragile character of this unusual substructure; rather, an unfavorable steric microenvironment about the spirotricyclic core was identified as the likely cause. To remedy the issue, the protecting groups on this central fragment were changed; in effect, this amendment allowed all strategic and practical problems to be addressed. As a result, the overall yield over the longest linear sequence was multiplied by a factor of almost five and the material throughput increased more than eighty-fold per run. Key-to-success was a gold-catalyzed spirocyclization reaction; the reasons why a Brønsted acid cocatalyst is needed and the origin of the excellent levels of selectivity were delineated. The change of the protecting groups also allowed for much improved fragment coupling processes; most notably, the sequence of a substrate-controlled carbonyl addition reaction followed by Mitsunobu inversion that had originally been necessary to affix the southern tail to the core could be replaced by a reagent controlled asymmetric allylation. Finally, a much-improved route to the "northern" sector was established by leveraging the power of asymmetric hydrogenation of a 2-pyrone derivative. Limaol was found to combine appreciable antiparasitic activity with very modest cytotoxicity.

Gilman reagent toward the synthesis of natural products

With the ever-increasing scope of organocuprates, a well-established Gilman reagent has been considered as an unprecedented synthetic tool in modern organic chemistry. The broad research profile of the Gilman reagent (R2CuLi in THF or Et2O) is owing to its propensity to carry out three kinds of reactions, i.e., epoxide ring opening reactions, 1,4-conjugate addition reactions, and SN2 reactions in a regioselective manner. This review examines the applications of Gilman reagent in the total synthesis of both abundant and scarce natural products of remarkable synthetic pharmaceutical profile reported since 2011. The presented insights will be of a vital roadmap to general organic synthesis and it will contribute to the development of new natural products and their analogues in future drug discovery.

Transcriptomic analysis of polyketide synthesis in dinoflagellate, Prorocentrum lima

The benthic dinoflagellate Prorocentrum lima is among the most common toxic morphospecies with a cosmopolitan distribution. P. lima can produce polyketide compounds, such as okadaic acid (OA), dinophysistoxin (DTX) and their analogues, which are responsible for diarrhetic shellfish poisoning (DSP). Studying the molecular mechanism of DSP toxin biosynthesis is crucial for understanding the environmental driver influencing toxin biosynthesis as well as for better monitoring of marine ecosystems. Commonly, polyketides are produced by polyketide synthases (PKS). However, no gene has been confirmatively assigned to DSP toxin production. Here, we assembled a transcriptome from 94,730,858 Illumina RNAseq reads using Trinity, resulting in 147,527 unigenes with average sequence length of 1035 nt. Using bioinformatics analysis methods, we found 210 unigenes encoding single-domain PKS with sequence similarity to type I PKSs, as reported in other dinoflagellates. In addition, 15 transcripts encoding multi-domain PKS (forming typical type I PKSs modules) and 5 transcripts encoding hybrid nonribosomal peptide synthetase (NRPS)/PKS were found. Using comparative transcriptome and differential expression analysis, a total of 16 PKS genes were identified to be up-regulated in phosphorus-limited cultures, which was related to the up regulation of toxin expression. In concert with other recent transcriptome analyses, this study contributes to the building consensus that dinoflagellates may utilize a combination of Type I multi-domain and single-domain PKS proteins, in an as yet undefined manner, to synthesize polyketides. Our study provides valuable genomic resource for future research in order to understand the complex mechanism of toxin production in this dinoflagellate.

Total Syntheses of Nominal and Actual Prorocentin

The dinoflagellate-derived polyether prorocentin is a co-metabolite of the archetypical serine/threonine phosphatase inhibitor okadaic acid. Whereas a structural relationship cannot be missed and a biosynthetic link was proposed, it is currently unknown whether there is any parallel in the bioactivity profile of these natural products. However, it was insinuated in the past that the structure assigned to prorocentin might need to be revised. Indeed, re-examination of the published spectra cast doubts as to the constitution of the fused/spirotricyclic BCD-ring system in the core. To clarify this issue, a flexible synthesis blueprint was devised that allowed us to obtain the originally proposed structure as well as the most plausible amended structure. The key to success was late-stage gold-catalyzed spirocyclization reactions that furnished the isomeric central segments with excellent selectivity. The lexicon of catalytic transformations used to make the required cyclization precursors comprised a titanium-mediated ester methylenation/metathesis cascade, a rare example of a gold-catalyzed allylic substitution, and chain extensions via organocatalytic asymmetric aldehyde propargylation. A wing sector to be attached to the isomeric cores was obtained by Krische allylation, followed by a superbly selective cobalt-catalyzed oxidative cyclization of the resulting di-unsaturated alcohol with the formation of a 2,5-trans-disubstituted tetrahydrofuran; the remaining terminal alkene was elaborated into an appropriate handle for fragment coupling by platinum-catalyzed asymmetric diboration/oxidation. The assembly of the different building blocks to the envisaged isomeric target compounds proved that the structure of prorocentin needs to be revised as disclosed herein.

Natural Polyether Ionophores and Their Pharmacological Profile

This review is devoted to the study of the biological activity of polyether ionophores produced by bacteria, unicellular marine algae, red seaweeds, marine sponges, and coelenterates. Biological activities have been studied experimentally in various laboratories, as well as data obtained using QSAR (Quantitative Structure-Activity Relationships) algorithms. According to the data obtained, it was shown that polyether toxins exhibit strong antibacterial, antimicrobial, antifungal, antitumor, and other activities. Along with this, it was found that natural polyether ionophores exhibit such properties as antiparasitic, antiprotozoal, cytostatic, anti-mycoplasmal, and antieczema activities. In addition, polyethers have been found to be potential regulators of lipid metabolism or inhibitors of DNA synthesis. Further study of the mechanisms of action and the search for new polyether ionophores and their derivatives may provide more effective therapeutic natural polyether ionophores for the treatment of cancer and other diseases. For some polyether ionophores, 3D graphs are presented, which demonstrate the predicted and calculated activities. The data presented in this review will be of interest to pharmacologists, chemists, practical medicine, and the pharmaceutical industry.

Emerging harmful algal bloom species over the last four decades in China

The first recorded micro-algae bloom in Chinese coastal waters dates back to 1933 and was caused by a mixture of Noctiluca scintillans and Skeletonema costatum sensu lato along the Zhejiang coast (the East China Sea). While well-documented harmful algal blooms (HABs) appeared to be extremely scarce from the 1950s to 1990, both the frequency and intensity have been reportedly increasing since 1990. Among them, the fish-killing HABs, mainly caused by Karenia mikimotoi, Karlodinium digitatum, Karlodinium veneficum, Margalefidinium polykrikoides, and Heterocapsa spp., have intensified. Karenia mikimotoi was responsible for at least two extremely serious events in the Pearl River Estuary in 1998 and the Taiwan Strait (in the East China Sea) in 2012, which appeared to be associated with abnormal climate conditions and excessive nutrients loading. Other major toxic algal blooms have been caused by the species responsible for paralytic shellfish poisoning (including Alexandrium catenella, Alexandrium pacificum, Gymnodinium catenatum) and diarrhetic shellfish poisoning (including Dinophysis spp., and a couple of benthic dinoflagellates). Consequent closures of shellfish farms have resulted in enormous economic losses, while consumption of contaminated shellfish has led to occasional human mortality in the Bohai Sea and the East China Sea. Expansions of these HABs species along the coastline of China have occurred over the last four decades and, due to the projected global changes in the climate and marine environments and other anthropological activities, there is potential for the emergence of new types of HABs in China in the future. This literature review aimed to present an updated overview of HABs species over the last four decades in China.

Polyketides with Anti-neuroinflammatory Activity from Theissenia cinerea

Theissenia cinerea 89091602 is a previously reported plant-derived bioactive fungal strain, and the active principles separated from the extracts of its submerged culture were shown to exhibit potent anti-neuroinflammatory activities in both cellular study and animal testing. In a continuation of our previous investigation on the bioactive entities from this fungus, solid state fermentation was performed in an attempt to diversify the bioactive secondary metabolites. In the present study, five previously unreported polyketides, theissenophenol (1), theissenepoxide (2), theissenolactone D (3), theissenone (4), and theissenisochromanone (5), together with the known theissenolactone B (6), theissenolactone C (7), and arthrinone (8), were isolated and characterized through spectroscopic analysis and comparison with the literature data. The configurations of theissenepoxide (2) and theissenisochromanone (5) were further corroborated by single-crystal X-ray diffraction data analysis. Theissenone (4), theissenolactone B (6), theissenolactone C (7), and arthrinone (8) exhibited potent nitric oxide production inhibitory activities in murine brain microglial BV-2 cells with IC₅₀ values of 5.0 ± 1.0, 4.5 ± 0.6, 1.1 ± 0.1, and 3.2 ± 0.3 μM, respectively, without any significant cytotoxic effects.

Total Synthesis of Limaol

A nonthermodynamic array of four skipped methylene substituents on the hydrophobic tail renders limaol, a C40-polyketide of marine origin, unique in structural terms. This conspicuous segment was assembled by a two-directional approach and finally coupled to the polyether domain by an allyl/alkenyl Stille reaction under neutral conditions. The core region itself was prepared via a 3,3′-dibromo-BINOL-catalyzed asymmetric propargylation, a gold-catalyzed spirocyclization, and introduction of the southern sector via substrate-controlled allylation as the key steps.

The Formosalides: Structure Determination by Total Synthesis

Total synthesis allowed the constitution of the cytotoxic marine macrolides of the formosalide family to be confirmed and their previously unknown stereostructure to be assigned with confidence. The underlying blueprint was inherently modular to ensure that each conceivable isomer could be reached. This flexibility derived from the use of strictly catalyst controlled transformations to set the stereocenters, except for the anomeric position, which is under thermodynamic control; as an extra safety measure, all stereogenic centers were set prior to ring closure to preclude any interference of the conformation adopted by the macrolactone rings of the different diastereomers. Late-stage macrocyclization by ring-closing alkyne metathesis was followed by a platinum-catalyzed transannular 6-exo-dig hydroalkoxylation/ketalization to craft the polycyclic frame. The side chain featuring a very labile unsaturation pattern was finally attached to the core by Stille coupling.

Relative Configurational Assignment of 4-Hydroxyprorocentrolide and Prorocentrolide C Isolated from a Benthic Dinoflagellate ( Prorocentrum lima)

Herein, we clarify the structure and relative configurations of two prorocentrolide analogues (1 and 2) isolated from the benthic marine dinoflagellate Prorocentrum lima. The results of NMR spectroscopy show that 1 is prorocentrolide substituted by a hydroxy group at C-4, while the newly isolated compound 2 can be thought of as 1 lacking one ether ring and having one extra double bond. The relative configurations of all stereogenic centers and the configurations of the double bonds in 1 and 2 were determined utilizing ROESY correlations and J-based configuration analysis. Furthermore, 2 was shown to exhibit cytotoxicity against HCT-116 and Neuro-2a cells (IC₅₀ 2.2 and 5.2 μM, respectively.

Synthetic studies toward the marine metabolite prorocentin-4: synthesis of the C1-C23 fragment

A synthetic study of the construction of the C1-C23 fragment of prorocentin-4, a novel linear polyketide, is described. The synthetic highlights include the acid catalyzed epoxide opening, Gilman reaction, Pd(OH)2 catalyzed transformation of a primary propargylic alcohol into an aldehyde, Oxa-Michael cyclization, and Horner-Wadsworth-Emmons (HWE) olefination reaction as key steps.

Metabolomic Profiles of Dinophysis acuminata and Dinophysis acuta Using Non-Targeted High-Resolution Mass Spectrometry: Effect of Nutritional Status and Prey

Photosynthetic species of the genus Dinophysis are obligate mixotrophs with temporary plastids (kleptoplastids) that are acquired from the ciliate Mesodinium rubrum, which feeds on cryptophytes of the Teleaulax-Plagioselmis-Geminigera clade. A metabolomic study of the three-species food chain Dinophysis-Mesodinium-Teleaulax was carried out using mass spectrometric analysis of extracts of batch-cultured cells of each level of that food chain. The main goal was to compare the metabolomic expression of Galician strains of Dinophysis acuminata and D. acuta that were subjected to different feeding regimes (well-fed and prey-limited) and feeding on two Mesodinium (Spanish and Danish) strains. Both Dinophysis species were able to grow while feeding on both Mesodinium strains, although differences in growth rates were observed. Toxin and metabolomic profiles of the two Dinophysis species were significantly different, and also varied between different feeding regimes and different prey organisms. Furthermore, significantly different metabolomes were expressed by a strain of D. acuminata that was feeding on different strains of the ciliate Mesodinium rubrum. Both species-specific metabolites and those common to D. acuminata and D. acuta were tentatively identified by screening of METLIN and Marine Natural Products Dictionary databases. This first metabolomic study applied to Dinophysis acuminata and D.acuta in culture establishes a basis for the chemical inventory of these species.

Prorocentrolide-A from Cultured Prorocentrum lima Dinoflagellates Collected in Japan Blocks Sub-Types of Nicotinic Acetylcholine Receptors

Prorocentrolides are members of the cyclic imine phycotoxins family. Their chemical structure includes a 26-membered carbo-macrocycle and a 28-membered macrocyclic lactone arranged around a hexahydroisoquinoline that incorporates the characteristic cyclic imine group. Six prorocentrolides are already known. However, their mode of action remains undetermined. The aim of the present work was to explore whether prorocentrolide-A acts on nicotinic acetylcholine receptors (nAChRs), using competition-binding assays and electrophysiological techniques. Prorocentrolide-A displaced [¹²⁵I]α-bungarotoxin binding to Torpedo membranes, expressing the muscle-type (α1₂β1γδ) nAChR, and in HEK-293 cells, expressing the chimeric chick neuronal α7-5HT₃ nAChR. Functional studies revealed that prorocentrolide-A had no agonist action on nAChRs, but inhibited ACh-induced currents in Xenopus oocytes that had incorporated the muscle-type α1₂β1γδ nAChR to their membranes, or that expressed the human α7 nAChR, as revealed by voltage-clamp recordings. Molecular docking calculations showed the absence of the characteristic hydrogen bond between the iminium group of prorocentrolide-A and the backbone carbonyl group of Trp147 in the receptor, explaining its weaker affinity as compared to all other cyclic imine toxins. In conclusion, this is the first study to show that prorocentrolide-A acts on both muscle and neuronal nAChRs, but with higher affinity on the muscle-type nAChR.

Limaol: A Polyketide from the Benthic Marine Dinoflagellate Prorocentrum lima

Limaol (1), along with a dinophysistoxin 1 derivative and an okadaic acid (OA) derivative, was isolated from the large-scale cultivation of the benthic marine dinoflagellate Prorocentrum lima. The structure of 1 was determined by a combination of NMR spectroscopy and mass spectrometry and contained tetrahydropyran, 1,3,5,7-tetra(methylene)heptane, and octahydrospiro[pyran-2,2'-pyrano[3,2-b]pyran] moieties. The absolute configuration of 1 was completely elucidated on the basis of ROESY correlations, J-based configuration analysis, and modified Mosher's ester analysis. Limaol showed moderate cytotoxicity when compared to OA against three cancer cell lines.

Toxicity and Growth Assessments of Three Thermophilic Benthic Dinoflagellates (Ostreopsis cf. ovata, Prorocentrum lima and Coolia monotis) Developing in the Southern Mediterranean Basin

Harmful benthic dinoflagellates, usually developing in tropical areas, are expanding to temperate ecosystems facing water warming. Reports on harmful benthic species are particularly scarce in the Southern Mediterranean Sea. For the first time, three thermophilic benthic dinoflagellates (Ostreopsis cf. ovata, Prorocentrum lima and Coolia monotis) were isolated from Bizerte Bay (Tunisia, Mediterranean) and monoclonal cultures established. The ribotyping confirmed the morphological identification of the three species. Maximum growth rates were 0.59 ± 0.08 d⁻¹ for O. cf. ovata, 0.35 ± 0.01 d⁻¹ for C. monotis and 0.33 ± 0.04 d⁻¹ for P. lima. Toxin analyses revealed the presence of ovatoxin-a and ovatoxin-b in O. cf. ovata cells. Okadaic acid and dinophysistoxin-1 were detected in P. lima cultures. For C. monotis, a chromatographic peak at 5.6 min with a mass m/z = 1061.768 was observed, but did not correspond to a mono-sulfated analogue of the yessotoxin. A comparison of the toxicity and growth characteristics of these dinoflagellates, distributed worldwide, is proposed.

Prorocentrum lima from the South Atlantic: Morphological, molecular and toxicological characterization

Morphological descriptions using light and scanning electron microscopy and molecular characterization of two Prorocentrum lima strains (UNR-01 and UNR-09) isolated from Armação dos Búzios, Rio de Janeiro, Brazil are provided. Okadaic acid (OA), dinophysistoxin-1 (DTX1) and DTX2 production by strain UNR-01 was investigated using liquid chromatography with mass spectrometry. Toxins were extracted from heat-treated (boiled) and non-boiled cell pellets to obtain respective quantities of free and total OA and DTX1. Growth parameters (growth rate and mean generation time) were determined for strain UNR-01. Prorocentrum lima cells were oblong-to-ovate in shape, broad in the middle region, and narrow at the anterior end. The periflagellar area was triangular, set into a V-shaped depression and was composed of eight periflagellar platelets of different sizes. The morphology fits well the characterization of the species isolated from elsewhere. Phylogenetic analysis based on internal transcribed spacer - ITS - and D1-D3 large subunit - LSU - of ribosomal RNA gene sequences revealed that both strains were identical and closely related to P. lima isolates from the Caribbean Sea and USA. The growth rate of strain UNR-01 was 0.24divday^-1. OA concentrations were on average 15.2 and 38.5pg[OA]cell^-1 for heat-treated and non-treated cells respectively, while DTX1 mean concentration was 0.5pg[DTX1]cell^-1 for both heat-treated and non-treated cells. DTX2 was not detected. To date, these are the first strains of P. lima from the south Atlantic that have been characterized.

Regio- and stereoselective synthesis of 2-amino-dienes via decarboxylative amination of 4-(ethoxycarbonyl)-2,3-allenols by TsNCO

A metal-free decarboxylative amination of 4-(ethoxycarbonyl)-2,3-allenols by TsNCO via base-induced aza-Michael addition/elimination has been developed. A variety of substituted N-tosyl 1,3-dien-2-yl amines were obtained in good yields and excellent regio- and stereoselectivity. Moreover, this transformation could be applied in preparation of 2-amino-trienes.

Marine natural products

This review covers the literature published in 2005 for marine natural products, with 704 citations (493 for the period January to December 2005) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (812 for 2005), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.