Antimalarial Isocyano and Isothiocyanato Sesquiterpenes with Tri- and Bicyclic Skeletons from the Nudibranch Phyllidia ocellata

Recent advances on marine mollusk-derived natural products: chemistry, chemical ecology and therapeutical potential

Covering: 2011-2021Marine mollusks, which are well known as rich sources of diverse and biologically active natural products, have attracted significant attention from researchers due to their chemical and pharmacological properties. The occurrence of some of these marine mollusk-derived natural products in their preys, predators, and associated microorganisms has also gained interest in chemical ecology research. Based on previous reviews, herein, we present a comprehensive summary of the recent advances of interesting secondary metabolites from marine mollusks, focusing on their structural features, possible chemo-ecological significance, and promising biological activities, covering the literature from 2011 to 2021.

Marine organisms as potential sources of natural products for the prevention and treatment of malaria

Vector-borne diseases (VBDs) are a worldwide critical concern accounting for 17% of the estimated global burden of all infectious diseases in 2020. Despite the various medicines available for the management, the deadliest VBD malaria, caused by Plasmodium sp., has resulted in hundreds of thousands of deaths in sub-Saharan Africa only. This finding may be explained by the progressive loss of antimalarial medication efficacy, inherent toxicity, the rise of drug resistance, or a lack of treatment adherence. As a result, new drug discoveries from uncommon sources are desperately needed, especially against multi-drug resistant strains. Marine organisms have been investigated, including sponges, soft corals, algae, and cyanobacteria. They have been shown to produce many bioactive compounds that potentially affect the causative organism at different stages of its life cycle, including the chloroquine (CQ)-resistant strains of P. falciparum. These compounds also showed diverse chemical structures belonging to various phytochemical classes, including alkaloids, terpenoids, polyketides, macrolides, and others. The current article presents a comprehensive review of marine-derived natural products with antimalarial activity as potential candidates for targeting different stages and species of Plasmodium in both in vitro and in vivo and in comparison with the commercially available and terrestrial plant-derived products, i.e., quinine and artemisinin.

Stereoselective Synthesis of (E)- and (Z)-Isocyanoalkenes

(E)- and (Z)-isocyanoalkenes were selectively synthesized via the sequential cross coupling of vinyl iodides with formamide, followed by dehydration. The optimal catalyst, generated in situ from CuII and trans-N,N'-dimethyl-1,2-cyclohexanediamine, rapidly coupled (E)- or (Z)-vinyl iodides with formamide, which minimized the isomerization of the resultant vinyl formamide. The method efficiently provided a range of acyclic, carbocyclic, and heterocyclic isocyanoalkenes; the versatility is illustrated with the selective, stereodivergent syntheses of the diastereomeric isocyanoalkene antibiotics, B371 and E-B371.

Total Synthesis of Clovan-2,9-dione via [3 + 2 + 1] Cycloaddition and Hydroformylation/Aldol Reaction

Here we report the total synthesis of clovan-2,9-dione via Rh-catalyzed [3 + 2 + 1] cycloaddition/hydroformylation/aldol reaction. The [3 + 2 + 1] reaction of 1-yne-vinylcyclopropane and CO was used for the generation of a 5/6 bicyclic skeleton with a bridgehead vinyl group. The hydroformylation reaction converted the congested olefin of the [3 + 2 + 1] cycloadduct to a one-carbon elongated aldehyde, which underwent in situ aldol reaction, with the carbonyl group in the [3 + 2 + 1] cycloadduct, to generate the tricyclic bridged-ring skeleton of the target molecule.

Phyllidiidae (Nudibranchia, Heterobranchia, Gastropoda): an integrative taxonomic approach including chemical analyses

Members of the widely distributed and common nudibranch family Phyllidiidae are often easily spotted in the marine environment because of their conspicuous colours and obvious presence on the reef. They are interesting with regard to their defensive chemical compounds that may lead to new drug discoveries. Despite their abundance, the family is also well known for its taxonomic problems and the difficulties in species identification due to very similarly coloured species and lack of morphological characters. In this study, phyllidiid species were analysed using an integrative approach. Molecular analysis of the mitochondrial genes 16S and CO1 was utilised, running phylogenetic analyses, species delimitation tests, and haplotype network analyses. Additionally, for the first time, external morphological characters were analysed, museum material was re-analysed, and chemical profiles were applied for characterising species. The analyses are based on sequences of 598 specimens collected in Indonesia by the team, with the addition of published sequences available on GenBank. This study comprises 11 species of Phyllidia, seven species of Phyllidiopsis, and at least 14 species of Phyllidiella. Moreover, 11 species belonging to these three genera are probably new to science, Phyllidiopsis pipeki is synonymised with P. krempfi, and Phyllidiella albonigra is resurrected. Some of the external colouration previously used for species identification is shown to not be valid, but alternative characters are provided for most species. Chemical analyses led to species characterisation in a few examples, indicating that these species use particular sponge species as food; however, many species show a broad array of compounds and are therefore characterised more by their composition or profile than by distinct or unique compounds.

Antimalarial Natural Products

Natural products have made a crucial and unique contribution to human health, and this is especially true in the case of malaria, where the natural products quinine and artemisinin and their derivatives and analogues, have saved millions of lives. The need for new drugs to treat malaria is still urgent, since the most dangerous malaria parasite, Plasmodium falciparum, has become resistant to quinine and most of its derivatives and is becoming resistant to artemisinin and its derivatives. This volume begins with a short history of malaria and follows this with a summary of its biology. It then traces the fascinating history of the discovery of quinine for malaria treatment and then describes quinine's biosynthesis, its mechanism of action, and its clinical use, concluding with a discussion of synthetic antimalarial agents based on quinine's structure. The volume then covers the discovery of artemisinin and its development as the source of the most effective current antimalarial drug, including summaries of its synthesis and biosynthesis, its mechanism of action, and its clinical use and resistance. A short discussion of other clinically used antimalarial natural products leads to a detailed treatment of other natural products with significant antiplasmodial activity, classified by compound type. Although the search for new antimalarial natural products from Nature's combinatorial library is challenging, it is very likely to yield new antimalarial drugs. The chapter thus ends by identifying over ten natural products with development potential as clinical antimalarial agents.

Trends of antimalarial marine natural products: progresses, challenges and opportunities

This review provides an overview of the antimalarial marine natural products, focusing on their chemistry, malaria-related targets and mechanisms, and highlighting their potential for drug development.

Medicinal Chemistry of Isocyanides

In eons of evolution, isocyanides carved out a niche in the ecological systems probably thanks to their metal coordinating properties. In 1859 the first isocyanide was synthesized by humans and in 1950 the first natural isocyanide was discovered. Now, at the beginning of XXI century, hundreds of isocyanides have been isolated both in prokaryotes and eukaryotes and thousands have been synthesized in the laboratory. For some of them their ecological role is known, and their potent biological activity as antibacterial, antifungal, antimalarial, antifouling, and antitumoral compounds has been described. Notwithstanding, the isocyanides have not gained a good reputation among medicinal chemists who have erroneously considered them either too reactive or metabolically unstable, and this has restricted their main use to technical applications as ligands in coordination chemistry. The aim of this review is therefore to show the richness in biological activity of the isocyanide-containing molecules, to support the idea of using the isocyanide functional group as an unconventional pharmacophore especially useful as a metal coordinating warhead. The unhidden hope is to convince the skeptical medicinal chemists of the isocyanide potential in many areas of drug discovery and considering them in the design of future drugs.

Bioactive Compounds from Marine Heterobranchs

The natural products of heterobranch molluscs display a huge variability both in structure and in their bioactivity. Despite the considerable lack of information, it can be observed from the recent literature that this group of animals possesses an astonishing arsenal of molecules from different origins that provide the molluscs with potent chemicals that are ecologically and pharmacologically relevant. In this review, we analyze the bioactivity of more than 450 compounds from ca. 400 species of heterobranch molluscs that are useful for the snails to protect themselves in different ways and/or that may be useful to us because of their pharmacological activities. Their ecological activities include predator avoidance, toxicity, antimicrobials, antifouling, trail-following and alarm pheromones, sunscreens and UV protection, tissue regeneration, and others. The most studied ecological activity is predation avoidance, followed by toxicity. Their pharmacological activities consist of cytotoxicity and antitumoral activity; antibiotic, antiparasitic, antiviral, and anti-inflammatory activity; and activity against neurodegenerative diseases and others. The most studied pharmacological activities are cytotoxicity and anticancer activities, followed by antibiotic activity. Overall, it can be observed that heterobranch molluscs are extremely interesting in regard to the study of marine natural products in terms of both chemical ecology and biotechnology studies, providing many leads for further detailed research in these fields in the near future.

Metabolome of the Phyllidiella pustulosa Species Complex (Nudibranchia, Heterobranchia, Gastropoda) Reveals Rare Dichloroimidic Sesquiterpene Derivatives from a Phylogenetically Distinct and Undescribed Clade

Phyllidiid nudibranchs are brightly colored gastropod mollusks, frequently encountered in coral reefs of the tropical Indo-Pacific. The lack of a protective shell is suggested to be compensated by toxic secondary metabolites that are sequestered from specific prey sponges. Our ongoing reconstruction of phyllidiid phylogeny using molecular data of more than 700 specimens, based on published data and newly collected specimens in various seasons and localities around North Sulawesi (Indonesia), demonstrates that Phyllidiella pustulosa is a species complex with at least seven well-supported clades. A metabolomic analysis of 52 specimens from all seven clades of P. pustulosa was performed. Secondary metabolite profiles were found to correlate with the phylogenetic study and not the prevailing food sponges as expected. GNPS molecular networking revealed a unique chemotype in clade 6. Detailed chemical analysis of a specimen from this chemically and genetically distinct P. pustulosa clade led to the identification of seven new sesquiterpenoids with a rare dichloroimidic moiety (1 and 4) and derivatives thereof (2, 3, 5-7). Our findings suggest that P. pustulosa clades should be raised to the species level.

A Sesquiterpene Isonitrile with a New Tricyclic Skeleton from the Indo-Pacific Nudibranch Phyllidiella pustulosa: Spectroscopic and Computational Studies

The sesquiterpene isonitrile, 9-isocyanoneoallopupukeanane 1, has been obtained from the Indo-Pacific nudibranch Phyllidiella pustulosa. The structure of 1, which was investigated by extensive NMR experiments, molecular modelling studies, and density functional calculations, has a different arrangement of the tricyclic ring system compared with other isonitrile metabolites isolated from nudibranchs or sponges. The viability of a biosynthetic pathway leading to 1, proposed to involve a series of carbocation rearrangements, is explored in a computational study. Isonitrile 1 exhibited micromolar antimalarial activity when screened against Plasmodium falciparum infected erythrocytes.

Marine Pharmacology in 2014-2015: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis, Antiviral, and Anthelmintic Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action

The systematic review of the marine pharmacology literature from 2014 to 2015 was completed in a manner consistent with the 1998-2013 reviews of this series. Research in marine pharmacology during 2014-2015, which was reported by investigators in 43 countries, described novel findings on the preclinical pharmacology of 301 marine compounds. These observations included antibacterial, antifungal, antiprotozoal, antituberculosis, antiviral, and anthelmintic pharmacological activities for 133 marine natural products, 85 marine compounds with antidiabetic, and anti-inflammatory activities, as well as those that affected the immune and nervous system, and 83 marine compounds that displayed miscellaneous mechanisms of action, and may probably contribute to novel pharmacological classes upon further research. Thus, in 2014-2015, the preclinical marine natural product pharmacology pipeline provided novel pharmacology as well as new lead compounds for the clinical marine pharmaceutical pipeline, and thus continued to contribute to ongoing global research for alternative therapeutic approaches to many disease categories.

Antiplasmodial natural products: an update

Background

Malaria remains a significant public health challenge in regions of the world where it is endemic. An unprecedented decline in malaria incidences was recorded during the last decade due to the availability of effective control interventions, such as the deployment of artemisinin-based combination therapy and insecticide-treated nets. However, according to the World Health Organization, malaria is staging a comeback, in part due to the development of drug resistance. Therefore, there is an urgent need to discover new anti-malarial drugs. This article reviews the literature on natural products with antiplasmodial activity that was reported between 2010 and 2017.

Methods

Relevant literature was sourced by searching the major scientific databases, including Web of Science, ScienceDirect, Scopus, SciFinder, Pubmed, and Google Scholar, using appropriate keyword combinations.

Results and Discussion

A total of 1524 compounds from 397 relevant references, assayed against at least one strain of Plasmodium, were reported in the period under review. Out of these, 39% were described as new natural products, and 29% of the compounds had IC₅₀ ≤ 3.0 µM against at least one strain of Plasmodium. Several of these compounds have the potential to be developed into viable anti-malarial drugs. Also, some of these compounds could play a role in malaria eradication by targeting gametocytes. However, the research into natural products with potential for blocking the transmission of malaria is still in its infancy stage and needs to be vigorously pursued.

Elucidation of Relative and Absolute Configurations of Highly Rearranged Diterpenoids and Evidence for a Putative Biosynthetic Intermediate from the Australian Nudibranch Goniobranchus geometricus

A diterpene (1), previously isolated from a Japanese marine sponge, together with two undescribed (2, 3) diterpenes with highly rearranged carbon skeletons have been characterized from the Australian nudibranch species Goniobranchus geometricus. The structures and relative configuration were determined by spectroscopic analyses informed by detailed molecular modeling, as well as by DFT, DP4, and coupling constant predictions. A 13 R,14 R configuration was determined for secoshahamin (1) by chemical correlation with 12-desacetoxyshahamin C (4) and 12-desacetoxypolyrhaphin A (5); each metabolite (1, 4, and 5) was subjected to saponification and lactonization, yielding the same δ-lactone product (6). Secoshahamin has the same carbon skeleton as a putative precursor that may play a key role in the biosynthesis of highly rearranged diterpenoid scaffolds via C-9/C-11 cleavage of a spongian diterpene precursor.

Cytotoxic Nitrogenous Terpenoids from Two South China Sea Nudibranchs Phyllidiella pustulosa, Phyllidia coelestis, and Their Sponge-Prey Acanthella cavernosa

A detailed chemical investigation of two South China Sea nudibranchs Phyllidiella pustulosa and Phyllidia coelestis, as well as their possible sponge-prey Acanthella cavernosa, led to the isolation of one new nitrogenous cadinane-type sesquiterpenoid xidaoisocyanate A (1), one new naturally occurring nitrogen-containing kalihinane-type diterpenoid bisformamidokalihinol A (16), along with 17 known nitrogenous terpenoids (2⁻15, 17⁻19). The structures of all the isolates were elucidated by detailed spectroscopic analysis and by the comparison of their spectroscopic data with those reported in the literature. In addition, the absolute stereochemistry of the previously reported axiriabiline A (5) was determined by X-ray diffraction (XRD) analysis. In a bioassay, the bisabolane-type sesquiterpenoids 8, 10, and 11 exhibited cytotoxicity against several human cancer cell lines.

The chemistry and chemical ecology of nudibranchs

Covering: up to the end of February 2017Nudibranchs have attracted the attention of natural product researchers due to the potential for discovery of bioactive metabolites, in conjunction with the interesting predator-prey chemical ecological interactions that are present. This review covers the literature published on natural products isolated from nudibranchs up to February 2017 with species arranged taxonomically. Selected examples of metabolites obtained from nudibranchs across the full range of taxa are discussed, including their origins (dietary or biosynthetic) if known and biological activity.

The Potential of Indonesian Heterobranchs Found around Bunaken Island for the Production of Bioactive Compounds

The species diversity of marine heterobranch sea slugs found on field trips around Bunaken Island (North Sulawesi, Indonesia) and adjacent islands of the Bunaken National Marine Park forms the basis of this review. In a survey performed in 2015, 80 species from 23 families were collected, including 17 new species. Only three of these have been investigated previously in studies from Indonesia. Combining species diversity with a former study from 2003 reveals in total 140 species from this locality. The diversity of bioactive compounds known and yet to be discovered from these organisms is summarized and related to the producer if known or suspected (might it be down the food chain, de novo synthesised from the slug or an associated bacterium). Additionally, the collection of microorganisms for the discovery of natural products of pharmacological interest from this hotspot of biodiversity that is presented here contains more than 50 species that have never been investigated before in regard to bioactive secondary metabolites. This highlights the great potential of the sea slugs and the associated microorganisms for the discovery of natural products of pharmacological interest from this hotspot of biodiversity.

New sesquiterpenoid isonitriles from three species of phyllidid nudibranchs

Chemical investigation of the two nudibranch species Phyllidiella pustulosa and Phyllidia ocellata collected in Queensland, Australia, provided new stereoisomers of 4-isocyano-9-amorphene (1) and of 10-isocyano-4-amorphene (2), respectively. A specimen of Phyllidia picta collected from Bali, Indonesia, contained the axane sesquiterpenoids pictaisonitrile-1 (3) and pictaisonitrile-2 (4). The planar structures were elucidated using 1D and 2D NMR spectroscopy, while relative configurations were established using NOESY correlations, coupling constant data, and comparison with literature data.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) 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 (1340 in 429 papers for 2015), 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.

Gold-Catalyzed Tandem 1,3-Migration/Double Cyclopropanation of 1-Ene-4,n-diyne Esters to Tetracyclodecene and Tetracycloundecene Derivatives

A synthetic method that relies on Au(I)-catalyzed tandem 1,3-acyloxy migration/double cyclopropanation of 1-ene-4,9-diyne and 1-ene-4,10-diyne esters to construct the respective architecturally challenging tetracyclodecene and tetracycloundecene derivatives is described. Achieved under mild reaction conditions, the transformation was shown to be robust with a wide variety of substitution patterns tolerated to give the two members of the carbocyclic family in good to excellent yields and as a single regio- and diastereomer.

Marine Isonitriles and Their Related Compounds

Marine isonitriles represent the largest group of natural products carrying the remarkable isocyanide moiety. Together with marine isothiocyanates and formamides, which originate from the same biosynthetic pathways, they offer diverse biological activities and in spite of their exotic nature they may constitute potential lead structures for pharmaceutical development. Among other biological activities, several marine isonitriles show antimalarial, antitubercular, antifouling and antiplasmodial effects. In contrast to terrestrial isonitriles, which are mostly derived from α-amino acids, the vast majority of marine representatives are of terpenoid origin. An overview of all known marine isonitriles and their congeners will be given and their biological and chemical aspects will be discussed.