Araiosamines A-D: tris-bromoindole cyclic guanidine alkaloids from the marine sponge Clathria (Thalysias) araiosa

Advances in the total synthesis of bis- and tris-indole alkaloids containing N-heterocyclic linker moieties

The past several years have seen an increase in the discovery and isolation of natural products of the indole alkaloid class. Bis- and tris-indole alkaloids are classes of natural products that have been shown to display diverse, potent biological activities. Of particular interest are bis- and tris-indole alkaloids containing N-heterocyclic linker moieties. It has been reported that more than 85% of biologically active compounds contain one or more heterocyclic moieties; of these, N-heterocycles have been identified as the most prevalent. The goal of this review is to provide a detailed overview of the recent advances in isolation and total synthesis of bis- and tris-indole alkaloids that contain N-heterocyclic linker moieties. The known biological activities of these natural products will also be discussed.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Samarium(ii) iodide-mediated reactions applied to natural product total synthesis

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated. Due to the demanding structures and complex functionality of many natural products, only powerful and selective methods and reagents will be highlighted in this proceeding. Since its introduction by Henri Kagan, samarium(ii) iodide (SmI₂, Kagan's reagent) has found increasing use in chemical synthesis. Over the years, many reviews have been published on the application of SmI₂ in numerous reductive coupling procedures as well as in natural product total synthesis. This review highlights recent advances in SmI₂-mediated synthetic strategies, as applied in the total synthesis of natural products since 2004.

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated.

Synthesis of pyrimidine-containing alkaloids

This review deals with the synthesis of naturally occurring alkaloids containing partially or completely saturated pyrimidine nuclei. The interest in these compounds is associated with their structural diversity, high biological activity and toxicity. The review is divided into four parts, each of which describes a number of synthetic methodologies toward structurally different naturally occurring alkaloids containing saturated cyclic six-membered amidine, guanidine, aminal and urea (thiourea) moieties, respectively. The development of various synthetic strategies for the preparation of these compounds has remarkably increased during the past few decades. This is primarily due to the fact that some of these compounds are isolated only in limited quantities, which makes it practically impossible to study their full structural characteristics and biological activity.

The application of C–H bond functionalization in the total syntheses of indole natural products

The recent advances in total synthesis of indole natural products focusing on the application of C–H bond functionalization are summarized.

Conjugated Polyene Ketones From the Marine Sponge Clathria (Thalysias) Reinwardti (Vosmaer, 1880) and Their Cytotoxic Activity

Five conjugated polyene ketones (1-5) were isolated from the methanol extract of the marine sponge Clathria ( Thalysias) reinwardti (Vosmaer, 1880) living in the coastal waters of Vietnam. Their structures were determined to be 8-(2′,3′,4′-trimethylphenyl)-6-methyl-oct-3( E),5( E),7( E)-trien-2-one (1), 13-apoastaxanthinone (2), 9-apoastaxanthinone (3), 2,3-dehydro-4-oxo- β-ionone (4), and 4-(2′,3′,4′-trimethylphenyl)-but-3( E)-en-2-one (5), by extensive analysis of high-resolution electron spray ionization mass spectrum (HR-ESI-MS), one-dimensional, and two-dimensional (2D) nuclear magnetic resonance (NMR) spectra, as well as by comparison of the spectral data with those reported in the literature. Compound 1 was new, compounds 2 to 4 were isolated from nature for the first time, and the chemical structure as well as the NMR assignments, of 5 were indicated by 2D NMR for the first time. Additionally, compound 5 exhibited cytotoxic activity against the human cancer cells SK-LU-1, SK-Mel-2, MCF-7, and Hep-G2 with half-minimal inhibitory concentration (IC50) values of 15.12 ± 3.43, 17.41 ± 2.83, 33.12 ± 3.39, and 34.38 ± 3.52 µM, respectively, but displayed only a weak cytotoxic effect on the normal HEK-239A cells (IC50 64.67 ± 3.67 µM). Compound 5 also significantly increased Caspase-3 activity in SK-LU-1 cells at concentrations of 10, 15, and 20 µM.

Ideality in Context: Motivations for Total Synthesis

Total synthesis-the ultimate proving ground for the invention and field-testing of new methods, exploration of disruptive strategies, final structure confirmation, and empowerment of medicinal chemistry on natural products-is one of the oldest and most enduring subfields of organic chemistry. In the early days of this field, its sole emphasis focused on debunking the concept of vitalism, that living organisms could create forms of matter accessible only to them. Emphasis then turned to the use of synthesis to degrade and reconstitute natural products to establish structure and answer questions about biosynthesis. It then evolved to not only an intricate science but also a celebrated form of art. As the field progressed, a more orderly and logical approach emerged that served to standardize the process. These developments even opened up the possibility of computer-aided design using retrosynthetic analysis. Finally, the elevation of this field to even higher levels of sophistication showed that it was feasible to synthesize any natural product, regardless of complexity, in a laboratory. During this remarkable evolution, as has been reviewed elsewhere, many of the principles and methods of organic synthesis were refined and galvanized. In the modern era, students and practitioners are still magnetically attracted to this field due to the excitement of the journey, the exhilaration of creation, and the opportunity to invent solutions to challenges that still persist. Contemporary total synthesis is less concerned with demonstrating a proof of concept or a feasible approach but rather aims for increased efficiency, scalability, and even "ideality." In general, the molecules of Nature are created biosynthetically with levels of practicality that are still unimaginable using the tools of modern synthesis. Thus, as the community strives to do more with less (i.e., innovation), total synthesis is now focused on a pursuit for simplicity rather than a competition for maximal complexity. In doing so, the practitioner must devise outside-the-box strategies supplemented with forgotten or newly invented methods to reduce step count and increase the overall economy of the approach. The downstream applications of this pursuit not only empower students who often go on to apply these skills in the private sector but also lead to new discoveries that can impact numerous disciplines of societal importance. This account traces some select case studies from our laboratory over the past five years that vividly demonstrate our own motivation for dedicating so much effort to this classic field. In aiming for simplicity, we focus on the elusive goal of achieving ideality, a term that, when taken in the proper context, can serve as a guiding light to point the way to furthering progress in organic synthesis.

6-Bromoindole Derivatives from the Icelandic Marine Sponge Geodia barretti: Isolation and Anti-Inflammatory Activity

An UPLC-qTOF-MS-based dereplication study led to the targeted isolation of seven bromoindole alkaloids from the sub-Arctic sponge Geodia barretti. This includes three new metabolites, namely geobarrettin A–C (1–3) and four known compounds, barettin (4), 8,9-dihydrobarettin (5), 6-bromoconicamin (6), and l-6-bromohypaphorine (7). The chemical structures of compounds 1–7 were elucidated by extensive analysis of the NMR and HRESIMS data. The absolute stereochemistry of geobarrettin A (1) was assigned by ECD analysis and Marfey’s method employing the new reagent l-N^α-(1-fluoro-2,4-dinitrophenyl)tryptophanamide (l-FDTA). The isolated compounds were screened for anti-inflammatory activity using human dendritic cells (DCs). Both 2 and 3 reduced DC secretion of IL-12p40, but 3 concomitantly increased IL-10 production. Maturing DCs treated with 2 or 3 before co-culturing with allogeneic CD4⁺ T cells decreased T cell secretion of IFN-γ, indicating a reduction in Th1 differentiation. Although barettin (4) reduced DC secretion of IL-12p40 and IL-10 (IC₅₀ values 11.8 and 21.0 μM for IL-10 and IL-12p40, respectively), maturing DCs in the presence of 4 did not affect the ability of T cells to secrete IFN-γ or IL-17, but reduced their secretion of IL-10. These results indicate that 2 and 3 may be useful for the treatment of inflammation, mainly of the Th1 type.

Chemistry and Biological Activities of the Marine Sponges of the Genera Mycale (Arenochalina), Biemna and Clathria

Over the past seven decades, particularly since the discovery of the first marine-derived nucleosides, spongothymidine and spongouridine, from the Caribbean sponge Cryptotethya crypta in the early 1950s, marine natural products have emerged as unique, renewable and yet under-investigated pools for discovery of new drug leads with distinct structural features, and myriad interesting biological activities. Marine sponges are the most primitive and simplest multicellular animals, with approximately 8900 known described species, although more than 15,000 species are thought to exist worldwide today. These marine organisms potentially represent the richest pipeline for novel drug leads. Mycale (Arenochalina) and Clathria are recognized marine sponge genera belonging to the order Poecilosclerida, whereas Biemna was more recently reclassified, based on molecular genetics, as a new order Biemnida. Together, these sponge genera contribute to the production of physiologically active molecular entities with diverse structural features and a wide range of medicinal and therapeutic potentialities. In this review, we provide a comprehensive insight and up-to-date literature survey over the period of 1976⁻2018, focusing on the chemistry of the isolated compounds from members of these three genera, as well as their biological and pharmacological activities, whenever available.

New Crambescidin-Type Alkaloids from the Indonesian Marine Sponge Clathria bulbotoxa

A crude methanolic extract of the Indonesian sponge Clathria bulbotoxa showed a potent cytotoxic activity against the human epidermoid carcinoma A431 cells. An investigation of the active components led to the isolation of three new compounds named crambescidins 345 (1), 361 (2), and 373 (3), together with the known related metabolites crambescidins 359 (4), 657 (5), and 800 (6). The structures of the compounds were determined by spectroscopic analysis. These compounds 1–4 that possess a simple pentacyclic guanidine core exhibited moderate cytotoxicity against the A431 cells with the IC₅₀ values of 7.0, 2.5, 0.94, and 3.1 μM, respectively, while the known compounds 5 and 6 that possess a long aliphatic side chain were found to be significantly cytotoxic. On the other hand, in an anti-oomycete activity test against the fungus-like plant pathogen Phytophthora capsici, 1–4 showed a higher activity than that of 5 and 6, suggesting that the long aliphatic side chain plays a significant role for cytotoxicity, but is not effective or suppressive for anti-oomycete activity.

Recent Advances in the Isolation, Synthesis and Biological Activity of Marine Guanidine Alkaloids

Marine organisms are prolific resources of guanidine-containing natural products with intriguing structures and promising biological activities. These molecules have therefore attracted the attention of chemists and biologists for their further studies towards potential drug leads. This review focused on the guanidine alkaloids derived from marine sources and discussed the recent progress on their isolation, synthesis and biological activities, covering the literature from the year 2010 to the present.

The chemistry and biology of guanidine natural products

The chemistry and biology of natural guanidines isolated from microbial culture media, from marine invertebrates, as well as from terrestrial plants and animals, are reviewed.

11-Step Total Synthesis of Araiosamines

A concise route to a small family of exotic marine alkaloids known as the araiosamines has been developed, and their absolute configuration has been assigned. The dense array of functionality, high polarity, and rich stereochemistry coupled with equilibrating topologies present an unusual challenge for chemical synthesis and an opportunity for innovation. Key steps involve the use of a new reagent for guanidine installation, a remarkably selective C-H functionalization, and a surprisingly simple final step that intersects a presumed biosynthetic intermediate. Synthetic araiosamines were shown to exhibit potency against Gram-positive and -negative bacteria despite a contrary report of no activity.

Rhodocomatulin-Type Anthraquinones from the Australian Marine Invertebrates Clathria hirsuta and Comatula rotalaria

Chemical investigations of an Australian sponge, Clathria hirsuta, from the Great Barrier Reef, have resulted in the isolation of two known anthraquinones, rhodocomatulin 5,7-dimethyl ether (1) and rhodocomatulin 7-methyl ether (2). Additionally, four new anthraquinone metabolites, 6-methoxyrhodocomatulin 7-methyl ether, 3-bromo-6-methoxy-12-desethylrhodocomatulin 7-methyl ether, 3-bromo-6-methoxyrhodocomatulin 7-methyl ether, and 3-bromorhodocomatulin 7-methyl ether (3-6), were also isolated and characterized. This is the first report of the rhodocomatulin-type anthraquinones from a marine sponge, as 1 and 2 were previously isolated from the marine crinoid genus Comatula. An additional chemical investigation of the marine crinoid Comatula rotalaria enabled the isolation of further quantities of 1 and 2, as well as two additional new crinoid metabolites, 12-desethylrhodocomatulin 5,7-dimethyl ether and 12-desethylrhodocomatulin 7-methyl ether (7 and 8). An NMR spectroscopic analysis of compounds 7 and 8 provided further insight into the rhodocomatulin planar structure and, together with the successful implementation of DFT-NMR calculations, confirmed that the rhodocomatulin metabolites existed as para rather than ortho quinones.

Marine Indole Alkaloids

Marine indole alkaloids comprise a large and steadily growing group of secondary metabolites. Their diverse biological activities make many compounds of this class attractive starting points for pharmaceutical development. Several marine-derived indoles were found to possess cytotoxic, antineoplastic, antibacterial and antimicrobial activities, in addition to the action on human enzymes and receptors. The newly isolated indole alkaloids of marine origin since the last comprehensive review in 2003 are reported, and biological aspects will be discussed.

Additional sesterterpenes and a nortriterpene saponin from the sponge Clathria gombawuiensis

Three new terpene metabolites (1-3) were isolated from the marine sponge Clathria gombawuiensis collected from Korean waters. On the basis of the results of combined spectroscopic analyses, the structures of phorone B (1) and ansellone C (2) were determined to be the sesterterpenes of the phorone and ansellone classes, respectively, whereas the saponin gombaside A (3) was a nortriterpene sodium O-sulfonato-glucuronide of the rare 4,4,14-trimethylpregnane class. The absolute configuration of the glucuronate of 3 was assigned by an application of the phenylglycine methyl ester (PGME) method. The new compounds exhibited moderate cytotoxicity against A549 and K562 cell lines, and compound 3 showed antibacterial activity. The cytotoxicity of 1 may be related to the presence of a free phenolic -OH group, as the corresponding O-methoxy derivative 4 is inactive.

Marine Bi-, Bis-, and Trisindole Alkaloids

This chapter, covering the chemistry literature up until June 2013 and comprising 142 references, records the chemical structures of 130 bi-, bis-, and trisindole alkaloids isolated from a plethora of marine phyla including bacteria, algae, bryozoans, sponges, mollusks, hard corals, and ascidians. While the vast majority of bisindoles have been isolated from marine sponges, biindoles are more commonly found in red algae species than sponges. Trisindoles are far less common than bisindoles in the marine environment and have been limited to two species of sponge and a single species of marine microbe. Antimicrobial activity and cytotoxicity dominate the bioactivities explored for selected members of this family of alkaloids. Synthetic approaches to 28 natural products are presented in 33 schemes, and in the absence of any in vivo biosynthetic studies, the putative biosyntheses of eight bisindole metabolites are presented.

Synthesis of cyclic guanidines via Pd-catalyzed alkene carboamination

A new approach to the synthesis of substituted 5-membered cyclic guanidines is described. Palladium-catalyzed alkene carboamination reactions between acyclic N-allyl guanidines and aryl or alkenyl halides provide these products in good yield. This method allows access to a number of different cyclic guanidine derivatives in only two steps from readily available allylic amines.

Marine natural products

This review covers the literature published in 2011 for marine natural products, with 870 citations (558 for the period January to December 2011) 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 (1152 for 2011), together with the 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.

The chemistry and biology of organic guanidine derivatives

The chemistry and biology of organic natural guanidines are reviewed, including the isolation, structure determination, synthesis, biosynthesis and biological activities of alkaloids, non-ribosomal peptides, guanidine-bearing terpenes, polyketides and shikimic acid derivatives from natural sources.

Bicyclic C21 terpenoids from the marine sponge Clathria compressa

Three new bicyclic C(21) terpenoids, clathric acid (1) and two N-acyl taurine derivatives, clathrimides A (2) and B (3), were isolated from the marine sponge Clathria compressa. The structures of these compounds were elucidated by interpretation of spectroscopic data. Clathric acid showed mild antibacterial activity against several Gram-positive bacteria.