Protein kinase C inhibitors: novel spirosesquiterpene aldehydes from a marine sponge Aka (= Siphonodictyon) coralliphagum

Total Synthesis of Marine Natural Products (+)-Strongylin A and Corallidictyal D by Regio- and Stereoselective Cyclization of Alkenyl Benzenes

An expeditious access to marine natural products (+)-strongylin A and corallidictyal D is described. A TFA/Et₃SiH-induced reductive isomerization of enols I to alkenyl benzenes II followed by a selectivity-controlled cyclization in the presence of HCl and BF₃·Et₂O affords benzofuran III and benzopyran IV, respectively. The applicability of this HCl-induced cyclization is showcased by a regio- and stereoselective synthesis of corallidictyal D, while BF₃·Et₂O-promoted cyclization posterior to rearrangement of an alkenyl benzene provides a regioselectively different benzopyran, (+)-strongylin A.

Biomimetic Dearomatization Strategies in the Total Synthesis of Meroterpenoid Natural Products

Natural products are biosynthesized from a limited pool of starting materials via pathways that obey the same chemical logic as textbook organic reactions. Given the structure of a natural product, it is therefore often possible to predict its likely biosynthesis. Although biosynthesis mainly occurs in the highly specific chemical environments of enzymes, the field of biomimetic total synthesis attempts to replicate predisposed pathways using chemical reagents.We have followed several guidelines in our biomimetic approach to total synthesis. The overarching aim is to construct the same skeletal C-C and C-heteroatom bonds and in the same order as our biosynthetic hypothesis. In order to explore the innate reactivity of (bio)synthetic intermediates, the use of protecting groups is avoided or at least minimized. The key step, which is usually a cascade reaction, should be predisposed to selectively generate molecular complexity under substrate control (e.g., cycloadditions, radical cyclizations, carbocation rearrangements). In general, simple reagents and mild conditions are used; many of the total syntheses presented in this Account could be achieved using pre-1980s methodology. We have focused almost exclusively on the synthesis of meroterpenoids, that is, natural products of mixed terpene and aromatic polyketide origin, using commercially available terpenes and electron-rich aromatic compounds as starting materials. Finally, all of the syntheses in this Account involve a dearomatization step as a means to trigger a cascade reaction or to construct stereochemical complexity from a planar, aromatic intermediate.A biomimetic strategy can offer several advantages to a total synthesis project. Most obviously, successful biomimetic syntheses are usually concise and efficient, naturally adhering to the atom, step, and redox economies of synthesis. For example, in this Account, we describe a four-step synthesis of garcibracteatone and a three-step synthesis of nyingchinoid A. It is difficult to imagine shorter, non-biomimetic syntheses of these intricate molecules. Furthermore, biomimetic synthesis gives insight into biosynthesis by revealing the chemical relationships between biosynthetic intermediates. Access to these natural substrates allows collaboration with biochemists to help uncover the function of newly discovered enzymes and elucidate biosynthetic pathways, as demonstrated in our work on the napyradiomycin family. Third, by making biosynthetic connections between natural products, we can sometimes highlight incorrect structural assignments, and herein we discuss structure revisions of siphonodictyal B, rasumatranin D, and furoerioaustralasine. Last, biomimetic synthesis motivates the prediction of "undiscovered natural products" (i.e., missing links in biosynthesis), which inspired the isolation of prenylbruceol A and isobruceol.

Marine natural products

Covering: 2016. Previous review: Nat. Prod. Rep., 2017, 34, 235-294This review covers the literature published in 2016 for marine natural products (MNPs), with 757 citations (643 for the period January to December 2016) 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 (1277 in 432 papers for 2016), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included.

Regioselective 1,2-Diol Rearrangement by Controlling the Loading of BF3·Et2O and Its Application to the Synthesis of Related Nor-Sesquiterene- and Sesquiterene-Type Marine Natural Products

The regiocontrolled rearrangement of 1,2-diols has been achieved by controlling the loading of BF₃·Et₂O. Its applicability is showcased by the divergent synthesis of austrodoral, austrodoric acid, and 8-epi-11-nordriman-9-one, as well as a formal synthesis of siphonodictyal B and liphagal. A new light is shed on piancol-type rearrangements that will be useful in diversity-oriented synthesis of related natural products.

Marine Sponges and Bacteria as Challenging Sources of Enzyme Inhibitors for Pharmacological Applications

Enzymes play key roles in different cellular processes, for example, in signal transduction, cell differentiation and proliferation, metabolic processes, DNA damage repair, apoptosis, and response to stress. A deregulation of enzymes has been considered one of the first causes of several diseases, including cancers. In the last several years, enzyme inhibitors, being good candidates as drugs in the pathogenic processes, have received an increasing amount of attention for their potential application in pharmacology. The marine environment is considered a challenging source of enzyme inhibitors for pharmacological applications. In this review, we report on secondary metabolites with enzyme inhibitory activity, focusing our attention on marine sponges and bacteria as promising sources. In the case of sponges, we only reported the kinase inhibitors, because this class was the most representative isolated so far from these marine organisms.

Enantiospecific total syntheses of meroterpenoids (−)-F1839-I and (−)-corallidictyals B and D

Enantiospecific total syntheses of spiromeroterpenoid natural products (−)-F1839-I and (−)-corallidictyals B and D were achieved using the environmentally benign and highly atom economical Lewis acid catalysed Friedel–Crafts reaction and a highly regio- and stereoselective spirocyclic C–O bond formation reaction.

Chermesins A-D: Meroterpenoids with a Drimane-Type Spirosesquiterpene Skeleton from the Marine Algal-Derived Endophytic Fungus Penicillium chermesinum EN-480

Chermesins A-D (1-4), four new spiromeroterpenoids containing a drimane-type sesquiterpene skeleton, were isolated and identified from the culture extract of Penicillium chermesinum EN-480, an endophytic fungus obtained from the inner tissue of the marine red alga Pterocladiella tenuis. The structures of these new spiromeroterpenoids were elucidated based on detailed spectroscopic analyses, and their absolute configurations were confirmed by single-crystal X-ray diffraction experiments and by ECD data. This is the first report of the crystal structures of spiromeroterpenoids that contain a drimane-type sesquiterpene skeleton with a rare cyclohexa-2,5-dienone unit, which resulted in the unambiguous assignment of their relative and absolute configurations. Compounds 1 and 2 exhibited antibacterial activity against the opportunistic pathogen Micrococcus luteus, with an MIC value of 8 μg/mL.

Some chemical speculation on the biosynthesis of corallidictyals A–D

The efficient conversion of siphonodictyal B into the spirocyclic natural products corallidictyals A–D has been achieved via oxidative and acid catalyzed cyclizations.

A marine sponge associated strain of Bacillus subtilis and other marine bacteria can produce anticholinesterase compounds

Background

Acetylcholinesterase (AChE) inhibitors or anticholinesterases reduce the activity of enzyme acetylcholinesterase that degrades the neurotransmitter acetylcholine in the brain. The inhibitors have a significant pharmacological role in neurodegenerative diseases like Alzheimer’s and Parkinson’s etc. Although plants have been a significant source of these compounds, there are very few sporadic reports of microorganisms producing such inhibitors. Anticholinesterase activity in bacterial associates of marine soft corals and sponges were not previously reported.

Results

We screened 887 marine bacteria for the presence of acetylcholinesterase inhibitors, in a microplate based assay, and found that 140 (15.8%) of them inhibit the electric eel enzyme, acetylcholinesterase. Majority of the active isolates were bacterial associates of soft corals followed by sediment isolates while most of the potent inhibitors belonged to the bacterial associates of marine sponges. Maximum inhibition (54%) was exhibited by a bacterial strain M18SP4P (ii), isolated from the marine sponge Fasciospongia cavernosa. Based on phenotypic characterization and 16S rDNA sequencing, the strain was identified as Bacillus subtilis - revealing yet another activity in a strain of the model organism that is considered to be a cell factory. TLC bioautography of the methanol extract of this culture, showed the presence of two major components having this activity, when compared to Galanthamine, the positive control.

Conclusion

From the results of our study, we conclude that acetylcholinesterase inhibitors are quite prevalent in marine bacteria, particularly the bacterial associates of marine invertebrates. Several potential AChE inhibitors in marine bacteria are waiting to be discovered to provide easily manipulable natural sources for the mass production of these therapeutic compounds.

Meroterpenes from marine invertebrates: structures, occurrence, and ecological implications

Meroterpenes are widely distributed among marine organisms; they are particularly abundant within brown algae, but other important sources include microorganisms and invertebrates. In the present review the structures and bioactivities of meroterpenes from marine invertebrates, mainly sponges and tunicates, are summarized. More than 300 molecules, often complex and with unique skeletons originating from intra- and inter-molecular cyclizations, and/or rearrangements, are illustrated. The reported syntheses are mentioned. The issue of a potential microbial link to their biosynthesis is also shortly outlined.

Kinase inhibitors from marine sponges

Protein kinases play a critical role in cell regulation and their deregulation is a contributing factor in an increasing list of diseases including cancer. Marine sponges have yielded over 70 novel compounds to date that exhibit significant inhibitory activity towards a range of protein kinases. These compounds, which belong to diverse structural classes, are reviewed herein, and ordered based upon the kinase that they inhibit. Relevant synthetic studies on the marine natural product kinase inhibitors have also been included.

Comprehensive structural and functional characterization of the human kinome by protein structure modeling and ligand virtual screening

The growing interest in the identification of kinase inhibitors, promising therapeutics in the treatment of many diseases, has created a demand for the structural characterization of the entire human kinome. At the outset of the drug development process, the lead-finding stage, approaches that enrich the screening library with bioactive compounds are needed. Here, protein structure based methods can play an important role, but despite structural genomics efforts, it is unlikely that the three-dimensional structures of the entire kinome will be available soon. Therefore, at the proteome level, structure-based approaches must rely on predicted models, with a key issue being their utility in virtual ligand screening. In this study, we employ the recently developed FINDSITE/Q-Dock ligand homology modeling approach, which is well-suited for proteome-scale applications using predicted structures, to provide extensive structural and functional characterization of the human kinome. Specifically, we construct structure models for the human kinome; these are subsequently subject to virtual screening against a library of more than 2 million compounds. To rank the compounds, we employ a hierarchical approach that combines ligand- and structure-based filters. Modeling accuracy is carefully validated using available experimental data with particularly encouraging results found for the ability to identify, without prior knowledge, specific kinase inhibitors. More generally, the modeling procedure results in a large number of predicted molecular interactions between kinases and small ligands that should be of practical use in the development of novel inhibitors. The data set is freely available to the academic community via a user-friendly Web interface at http://cssb.biology.gatech.edu/kinomelhm/ as well as at the ZINC Web site ( http://zinc.docking.org/applications/2010Apr/Brylinski-2010.tar.gz ).

Cytotoxic terpene quinones from marine sponges

The 1,4-benzoquinone moiety is a common structural feature in a large number of compounds that have received considerable attention owing to their broad spectrum of biological activities. The cytotoxic and antiproliferative properties of many natural sesquiterpene quinones and hydroquinones from sponges of the order Dictyoceratida, such as avarol, avarone, illimaquinone, nakijiquinone and bolinaquinone, offer promising opportunities for the development of new antitumor agents. The present review summarizes the structure and cytotoxicity of natural terpenequinones/hydroquinones and their bioactive analogues and derivatives.

Application of a radioreceptor assay to the screening and characterisation of compounds from marine organisms with activity at the phorbol ester binding site of protein kinase C

Protein kinase C is a ubiquitous enzyme with a key role in cellular function, making it an attractive drug target. Utilising a competitive binding assay for the phorbol ester binding site of the enzyme in a rat brain membrane preparation, screening was undertaken on 686 marine macroorganisms representing a broad range of taxa and environments from throughout Australasia. Of these extracts from 28 organisms significantly inhibited [3H]phorbol dibutyrate binding, while two samples enhanced binding. Sponges and echinoderms were particularly well represented in the active specimens. A combination of taxonomic and elution information for individual leads provided a rationale for dereplication and prioritisation. Utilising assay-guided purification, the identity of active compounds from the sponge Agelas axifera was examined in detail. The previously described compounds, the agelasines, were identified. The screening and characterisation methods described provide a method for readily identifying novel probes for protein kinase C.

Fishing for drugs from the sea: status and strategies

Marine organisms represent an enormous, essentially unexploited, resource of natural products. Globally, the race to develop marine-derived drugs is well under way with many pharmaceutical companies positioning themselves to reap large profits by the exploitation of the ocean's rich chemical diversity. Targeted strategies, often in combination with high-throughput screening, are being employed in this hunt for novel pharmacotherapeutic agents. David de Vries and Phil Beart examine the potential, problems and technologies of an international pharmaceutical search that has important ethical considerations.