Two rare-class tricyclic diterpenes with antitubercular activity from the Caribbean sponge Svenzea flava. Application of vibrational circular dichroism spectroscopy for determining absolute configuration

Four uncommon cycloamphilectane-type diterpenoids with antibacterial activity from the South China Sea soft coral Sinularia brassica

The chemical investigation on the soft coral Sinularia brassica collected off Xuwen Country, Guangdong Province, China, has resulted in the isolation and characterization of three uncommon cycloamphilectane-type diterpenoids, namely sinucycloamtin A-C (1-3), along with two known analogues (5 and 6). In addition, compounds 2 and 3 were hydrolyzed and their hydrolytic derivative sinucycloamtin D (4) was obtained. The structures of these previously undescribed compounds were established on the basis of extensive spectroscopic analysis, X-ray diffraction analysis, chemical conversion, as well as the comparison with the literature reported data. Compounds 1-3 represented the first examples of benzene-containing cycloamphilectane-type diterpenoids isolated from soft coral of genus Sinularia. In the in vitro bioassays, all the isolated and derived diterpenoids exhibited significant antibacterial activities against the fish pathogenic bacteria Phoyobacterium damselae FP2244 and Streptococcus parauberis SPOF3K with MIC90 values ranging from 3.7 to 9.1 μM.

An Enantiospecific Synthesis of Isoneoamphilectane Confirms Its Strained Tricyclic Structure

We describe a total synthesis of the rare isocyanoterpene natural product isoneoamphilectane and two of its unnatural diastereomers. The significantly strained ring system of the reported natural product─along with a hypothesis about a biosynthetic relationship to related family members─inspired us to consider a potential misassignment in the structure's relative configuration. As a result, we initially targeted two less strained, more accessible, stereoisomers of the reported natural product. When these compounds failed to exhibit spectroscopic data that matched those of isoneoamphilectane, we embarked on a synthesis of the originally proposed strained structure via an approach that hinged on a challenging cis-to-trans decalone epimerization. Ultimately, we implemented a novel cyclic sulfite pinacol-type rearrangement to generate the strained ring system. Additional features of this work include the application of a stereocontrolled Mukaiyama-Michael addition of an acyclic silylketene acetal, an unusual intramolecular alkoxide-mediated regioselective elimination, and an HAT-mediated alkene hydroazidation to forge the C-N bond of the tertiary isonitrile. Throughout this work, our synthetic planning was heavily guided by computational analyses to inform on key issues of stereochemical control.

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.

A Soft Spot for Chemistry-Current Taxonomic and Evolutionary Implications of Sponge Secondary Metabolite Distribution

Marine sponges are the most prolific marine sources for discovery of novel bioactive compounds. Sponge secondary metabolites are sought-after for their potential in pharmaceutical applications, and in the past, they were also used as taxonomic markers alongside the difficult and homoplasy-prone sponge morphology for species delineation (chemotaxonomy). The understanding of phylogenetic distribution and distinctiveness of metabolites to sponge lineages is pivotal to reveal pathways and evolution of compound production in sponges. This benefits the discovery rate and yield of bioprospecting for novel marine natural products by identifying lineages with high potential of being new sources of valuable sponge compounds. In this review, we summarize the current biochemical data on sponges and compare the metabolite distribution against a sponge phylogeny. We assess compound specificity to lineages, potential convergences, and suitability as diagnostic phylogenetic markers. Our study finds compound distribution corroborating current (molecular) phylogenetic hypotheses, which include yet unaccepted polyphyly of several demosponge orders and families. Likewise, several compounds and compound groups display a high degree of lineage specificity, which suggests homologous biosynthetic pathways among their taxa, which identifies yet unstudied species of this lineage as promising bioprospecting targets.

Amphilectene Diterpene Isonitriles and Formamido Derivatives from the Hainan Nudibranch Phyllidia Coelestis

Terpene content of two distinct collections of the nudibranch Phyllidia coelestis from the South China Sea has been chemically analyzed. A series of amphilectene diterpenes, most likely of dietary origin, with isocyano and formamido functionalities have been isolated from both collections and spectroscopically characterized by an exhaustive nuclear magnetic resonance (NMR) analysis. Interestingly, the structural architecture of compounds 5–7 and 9 with both 8,13-cis and 12,13-cis ring junctions is unprecedented in the amphilectene skeleton. Metabolite 3, which was the most abundant in the nudibranch’s mantle, has been shown to deter feeding by a generalist predator, supporting its involvement in chemical defense.

Marine natural products as potential anti-tubercular agents

Tuberculosis has been one of the greatest global health challenges of all time. Although the current first-line anti-tuberculosis (anti-TB) medicines used in the clinic have reduced mortality, multidrug-resistance and extensively drug-resistance forms of the disease have now spread worldwide and become a global problem. Even so, few new clinically approved drugs have emerged during the past 30 years. Highly biodiverse marine organisms have received considerable attention for drug discovery in the past couple of decades, and emerging TB drug resistance has motivated interest in assessing marine natural products (MNPs) in the treatment of this disease. So far, more than 170 compounds have been isolated from marine organisms with anti-TB properties, ten of which exhibit potent activity and have the potential for further development. This review systematically surveys MNPs with anti-TB activity and illustrates the impact of these compounds on drug discovery research against tuberculosis.

Marine Pharmacology in 2012-2013: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis, and Antiviral Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action

The peer-reviewed marine pharmacology literature from 2012 to 2013 was systematically reviewed, consistent with the 1998–2011 reviews of this series. Marine pharmacology research from 2012 to 2013, conducted by scientists from 42 countries in addition to the United States, reported findings on the preclinical pharmacology of 257 marine compounds. The preclinical pharmacology of compounds isolated from marine organisms revealed antibacterial, antifungal, antiprotozoal, antituberculosis, antiviral and anthelmitic pharmacological activities for 113 marine natural products. In addition, 75 marine compounds were reported to have antidiabetic and anti-inflammatory activities and affect the immune and nervous system. Finally, 69 marine compounds were shown to display miscellaneous mechanisms of action which could contribute to novel pharmacological classes. Thus, in 2012–2013, the preclinical marine natural product pharmacology pipeline provided novel pharmacology and lead compounds to the clinical marine pharmaceutical pipeline, and contributed significantly to potentially novel therapeutic approaches to several global disease categories.

Vibrational Circular Dichroism: Recent Advances for the Assignment of the Absolute Configuration of Natural Products

Vibrational circular dichroism (VCD) emerged during the last decade as a reliable tool for the absolute configuration (AC) determination of organic compounds. The principles, instrumentation, and methodology applied prior to early 2013 were recently reviewed by us. Since VCD is a very dynamic field, the aim of this review is to update VCD advances for the AC assignment of terpenoids, aromatic compounds, alkaloids, and other natural products for the 2013–2014 period, when VCD was applied to the AC assignment of some 70 natural products. In addition, although discovered in 2012, a brief introduction to the VCD exciton coupling approach and its applications in natural products AC assignment is presented.

Structure-activity of antifungal compounds inspired by aminobisabolenes from the sponge Halichondria sp

Structure-activity relationships of the antifungal aminobisabolene natural product, 1 isolated from Halichondria sp., and synthetic analogs were explored, in parallel with the antidermatophytic allylamine, Terbinafine®, against a panel of pathogenic fungi: Candida spp., Cryptococcus spp. and Trichophyton rubrum. Interpretation of the results suggest different modes of action in antifungal activity for the two classes of compounds.

Natural product-based synthesis of novel anti-infective isothiocyanate- and isoselenocyanate-functionalized amphilectane diterpenes

The marine natural product (-)-8,15-diisocyano-11(20)-amphilectene (1), isolated from the Caribbean sponge Svenzea flava, was used as scaffold to synthetize five new products, all of which were tested against laboratory strains of Plasmodium falciparum and Mycobacterium tuberculosis H37Rv. The scaffold contains two isocyanide units that are amenable to chemical manipulation, enabling them to be elaborated into a small library of sulfur and selenium compounds. Although most of the analogs prepared were less potent than the parent compound, 5 was nearly equipotent showing IC50 values of 0.0066 μM and 0.0025 μM, respectively, against two strains (Dd2 and 3D7) of the malaria parasite. On the other hand, when assayed against the tuberculosis bacterium, analogs 5 and 6 were found to be more potent than 1.

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.

Synthesis and preliminary biological evaluation of a small library of hybrid compounds based on Ugi isocyanide multicomponent reactions with a marine natural product scaffold

A mixture-based combinatorial library of five Ugi adducts (4-8) incorporating known antitubercular and antimalarial pharmacophores was successfully synthesized, starting from the naturally occurring diisocyanide 3, via parallel Ugi four-center three-component reactions (U-4C-3CR). The novel α-acylamino amides obtained were evaluated for their antiinfective potential against laboratory strains of Mycobacterium tuberculosis H37Rv and chloroquine-susceptible 3D7 Plasmodium falciparum. Interestingly, compounds 4-8 displayed potent in vitro antiparasitic activity with higher cytotoxicity in comparison to their diisocyanide precursor 3, with the best compound exhibiting an IC50 value of 3.6 nM. Additionally, these natural product inspired hybrids potently inhibited in vitro thromboxane B2 (TXB2) and superoxide anion (O2(-)) generation from Escherichia coli lipopolysaccharide (LPS)-activated rat neonatal microglia, with concomitant low short-term toxicity.

Vibrational Circular Dichroism: Recent Advances for the Assignment of the Absolute Configuration of Natural Products

Vibrational circular dichroism (VCD) emerged during the last decade as a reliable tool for the absolute configuration (AC) determination of organic compounds. The principles, instrumentation, and methodology applied prior to early 2013 were recently reviewed by us. Since VCD is a very dynamic field, the aim of this review is to update VCD advances for the AC assignment of terpenoids, aromatic compounds, alkaloids, and other natural products for the 2013-2014 period, when VCD was applied to the AC assignment of some 70 natural products. In addition, although discovered in 2012, a brief introduction to the VCD exciton coupling approach and its applications in natural products AC assignment is presented.

Marine natural products

This review covers the literature published in 2013 for marine natural products (MNPs), with 982 citations (644 for the period January to December 2013) 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 (1163 for 2013), 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.

Syntheses and biological studies of marine terpenoids derived from inorganic cyanide

Isocyanoterpenes (ICTs) are marine natural products biosynthesized through an unusual pathway that adorns terpene scaffolds with nitrogenous functionality derived from cyanide. The appendage of nitrogen functional groups - isonitriles in particular - onto stereochemically-rich carbocyclic ring systems provides enigmatic, bioactive molecules that have required innovative chemical syntheses. This review discusses the challenges inherent to the synthesis of this diverse family and details the development of the field. We also present recent progress in isolation and discuss key aspects of the remarkable biological activity of these compounds.

Recent advances and applications of experimental technologies in marine natural product research

Marine natural products are a rich source of novel and biologically active compounds. The number of identified marine natural compounds has grown 20% over the last five years from 2009 to 2013. Several challenges, including sample collection and structure elucidation, have limited the development of this research field. Nonetheless, new approaches, such as sampling strategies for organisms from extreme ocean environments, nanoscale NMR and computational chemistry for structural determination, are now available to overcome the barriers. In this review, we highlight the experimental technology innovations in the field of marine natural products, which in our view will lead to the development of many new drugs in the future.

Recent advances in the use of vibrational chiroptical spectroscopic methods for stereochemical characterization of natural products

A comprehensive look into application of vibrational optical activity methods for conformational and configurational assignments in natural product molecules over the last 15 years is provided.

Structures, semisyntheses, and absolute configurations of the antiplasmodial α-substituted β-lactam monamphilectines B and C from the sponge Svenzea flava

Bioassay-guided fractionation of the Caribbean sponge Svenzea flava collected near Mona Island, off the west coast of Puerto Rico, led to the isolation of two isocyanide amphilectane-type diterpenes named monamphilectines B and C (2 and 3). Attached to the backbone of each of these compounds is the first α-substituted monocyclic β-lactam ring to be isolated from a marine organism. The molecular structures of 2 and 3 were established by spectroscopic methods and then confirmed unequivocally by chemical correlation and comparison of physical and chemical data with the natural products. The new β-lactams were successfully synthesized in one step, starting from the known diisocyanide 4, via parallel Ugi four-center three-component reactions (U-4C-3CR) that also established their absolute stereostructures. Interestingly, compounds 2 and 3 exhibited activities in the low nanomolar range against the human malaria parasite Plasmodium falciparum.