Structure Elucidation at the Nanomole Scale. 2. Hemi-phorboxazole A from Phorbas sp

Palladium-Catalyzed Hydrocyanation of Ynoates: En Route to the Stereodivergent Synthesis of β-Cyanated α,β-Unsaturated Esters via Ligand Controlled Regio- and Stereoselectivity

A Pd-catalyzed highly regio- and stereoselective hydrocyanation was developed, providing a novel approach to the stereodivergent synthesis of β-cyano-substituted acrylates in good yields with a wide substrate scope. The judicious selection of ligands was crucial for elegant control over the stereodivergence. Furthermore, the success of the E-hydrocyanation hinges on the right matching of Pd and L1, which not only ensured the catalytic activity but also prevented the formation of α-cyanation products.

Marine Natural Products from Flora and Fauna of the Western Australian Coast: Taxonomy, Isolation and Biological Activity

Marine natural products occurring along the Western Australian coastline are the focus of this review. Western Australia covers one-third of the Australian coast, from tropical waters in the far north of the state to cooler temperate and Antarctic waters in the south. Over 40 years of research has resulted in the identification of a number of different types of secondary metabolites including terpenoids, alkaloids, polyketides, fatty acid derivatives, peptides and arsenic-containing natural products. Many of these compounds have been reported to display a variety of bioactivities. A description of the compound classes and their associated bioactivities from marine organisms found along the Western Australian coastline is presented.

Natural Products Containing the Nitrile Functional Group and Their Biological Activities

The importance of nitriles as a key class of chemicals with applications across the sciences is widely appreciated. The natural world is an underappreciated source of chemically diverse nitriles. With this in mind, this review describes novel nitrile-containing molecules isolated from natural sources from 1998 to 2021, as well as a discussion of the biological activity of these compounds. This study gathers 192 molecules from varied origins across the plant, animal, and microbial worlds. Their biological activity is extremely diverse, with many potential medicinal applications.

An Overview of Bioactive 1,3-Oxazole-Containing Alkaloids from Marine Organisms

1,3-Oxazole chemicals are a unique class of five-membered monocyclic heteroarenes, containing a nitrogen atom and an oxygen. These alkaloids have attracted extensive attention from medicinal chemists and pharmacologists owing to their diverse arrays of chemical structures and biological activities, and a series of 1,3-oxazole derivatives has been developed into therapeutic agents (e.g., almoxatone, befloxatone, cabotegravir, delpazolid, fenpipalone, haloxazolam, inavolisib). A growing amount of evidence indicates that marine organisms are one of important sources of 1,3-oxazole-containing alkaloids. To improve our knowledge regarding these marine-derived substances, as many as 285 compounds are summarized in this review, which, for the first time, highlights their sources, structural features and biological properties, as well as their biosynthesis and chemical synthesis. Perspective for the future discovery of new 1,3-oxazole compounds from marine organisms is also provided.

Exploring Chemical Diversity of Phorbas Sponges as a Source of Novel Lead Compounds in Drug Discovery

Porifera, commonly referred to as marine sponges, are acknowledged as major producers of marine natural products (MNPs). Sponges of the genus Phorbas have attracted much attention over the years. They are widespread in all continents, and several structurally unique compounds have been identified from this species. Terpenes, mainly sesterterpenoids, are the major secondary metabolites isolated from Phorbas species, even though several alkaloids and steroids have also been reported. Many of these compounds have presented interesting biological activities. Particularly, Phorbas sponges have been demonstrated to be a source of cytotoxic metabolites. In addition, MNPs exhibiting cytostatic, antimicrobial, and anti-inflammatory activities have been isolated and structurally characterized. This review provides an overview of almost 130 secondary metabolites from Phorbas sponges and their biological activities, and it covers the literature since the first study published in 1993 until November 2021, including approximately 60 records. The synthetic routes to the most interesting compounds are briefly outlined.

Lessons in Organic Fluorescent Probe Discovery

Fluorescent probes have gained profound use in biotechnology, drug discovery, medical diagnostics, molecular and cell biology. The development of methods for the translation of fluorophores into fluorescent probes continues to be a robust field for medicinal chemists and chemical biologists, alike. Access to new experimental designs has enabled molecular diversification and led to the identification of new approaches to probe discovery. This review provides a synopsis of the recent lessons in modern fluorescent probe discovery.

Integrated analytical workflow for chromatographic profiling and metabolite annotation of a cytotoxic Phorbas amaranthus extract

Phorbas is a widely studied genus of marine sponge and produce structurally rich cytotoxic metabolites. Still, only few studies have assessed metabolites present in Brazilian species. To circumvent redundancy, in this work, we applied and herein report the use of a scouting liquid chromatographic system associate to the design of experiment produced by the DryLab® software to obtain a fast and efficient chromatographic separation of the active hexane fraction, further enabling untargeted high-resolution mass spectrometry (HRMS) data. To this end, a crude hydroalcoholic extract of the sponge Phorbas amaranthus collected in Brazilian coast was prepared and partitioned. The cytotoxicity of the crude extract and the fractions was evaluated using tumor cell culture models. Fragmentation pathways assembled from HRMS data allowed the annotation of 18 known Phorbas metabolites, while 17 metabolites were inferred based on Global Natural Product Social Molecular Networking (GNPS), matching with a further 29 metabolites annotated through molecular subnetwork. The workflow employed demonstrates that chromatographic method development can be accelerated by the use of automated scouting systems and DryLab®, which is useful for profiling natural product libraries, as well as data curation by molecular clusters and should be incorporated to the tools of natural product chemists.

Synchronous bond molecular dynamics of conjugated chlorocyclopropyl alk-yn-enes revealed by ECD and UV-vis

Chlorocyclopropanes (CCPs) conjugated to alk-yn-enes occur in a unique family of polyketide natural products from marine sponges. Synthesis of several optically enriched analogs of CCPs and measurement of their UV-vis spectra and electronic circular dichroism (ECD) spectra reveal unusually strong hyperconjugation that constrains and aligns the cyclopropyl C-C bond with the π-plane of the distal ene-bond. This alignment imposes a barrier to rotation of at least 5.0 kcal·mol-1 . Comparison of red-shifted Cotton effects in chiral CCPs show the barrier is independent of alkene substituent and establishes an empirical rule for assignment of other CCP-containing natural products.

E- and Z-, di- and tri-substituted alkenyl nitriles through catalytic cross-metathesis

Nitriles are found in many bioactive compounds, and are among the most versatile functional groups in organic chemistry. Despite many notable recent advances, however, there are no approaches that may be used for preparation of di- or trisubstituted alkenyl nitriles. Related approaches which are broad in scope and can deliver the desired products in high stereoisomeric purity are especially scarce. Here, we describe the development of several efficient catalytic cross-metathesis strategies, which provide direct access to a considerable range of Z- or E-disubstituted cyano-substituted alkenes or their corresponding trisubstituted variants. Depending on the reaction type, a molybdenum-based monoaryloxide pyrrolide (MAP) or chloride (MAC) complex may be the optimal choice. The utility of the approach, enhanced by an easy-to-apply protocol for utilization of substrates bearing an alcohol or a carboxylic acid moiety, is highlighted in the context of applications to synthesis of biologically active compounds.

Recent developments and applications of cyanamides in electrophilic cyanation

This review summarizes the recent developments and applications of readily accessible cyanamides in the electrophilic cyanation of various nucleophiles.

Structure of Echivulgarine, a Pyrrolizidine Alkaloid Isolated from the Pollen of Echium vulgare

1,2-Dehydropyrrolizidine alkaloids are common toxic metabolites isolated from plants within the Boraginaceae, in particular from the genera Heliotropium and Echium. Previous studies have deduced the structures of these often potent bioactives based upon mass spectrometric evidence, but these identifications have not established conclusive connectivity and configurational data. Herein, we describe the isolation and full structural characterization of echivulgarine, occurring in the pollen of Echium vulgare and correct the structure previously ascribed to the compound, using a comprehensive combination of both experimental and calculated nuclear magnetic resonance and electronic circular dichroism spectroscopic data.

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.

Cytotoxic diterpenoid pseudodimers from the Korean sponge Phorbas gukhulensis

Four new cytotoxic diterpenoid pseudodimers (2-5), along with a previously reported one, gukulenin A (1), were isolated from the marine sponge Phorbas gukhulensis collected off the coast of Gagu-do, Korea. These novel compounds, designated gukulenins C-F (2-5), were determined by extensive spectroscopic analyses to be pseudodimers of the gagunins, like gukulenin A. The termini of the tropolone-containing side chains in gukulenins C-E (2-4) were found to have diverse modifications involving acetamides or taurine, whereas gukulenin F (5) was formed from 1 by the ring-opening of a cyclic hemiketal. The relative and absolute configurations were assigned by Murata's and modified Snatzke's methods using a HETLOC experiment and a CD measurement of a dimolybdenum complex, respectively. All of these compounds exhibited significant cytotoxicity against the K562 and A549 cell lines.

Determining and reporting purity of organic molecules: why qNMR

Although NMR has been routinely used to determine/estimate relative number of protons for structure elucidation, it has been rarely used to determine and report the purity of organic compounds. Through this paper, we want to emphasize on routine use of quantitative NMR (qNMR) for this purpose. The results of qNMR can be routinely considered as documentation of purity much like other established methods (HPLC, elemental analysis and differential scanning calorimetry). qNMR is a fast, easy, accurate and non-destructive alternate to speed up the whole analytical process and serves the purpose of both identification and purity determination of compounds using single technique.

Photochemical oxazole-nitrile conversion downstream of rhizoxin biosynthesis and its impact on antimitotic activity

Through metabolic profiling of mutants and wild type of the endofungal bacterium Burkholderia rhizoxinica two novel rhizoxin derivatives with unusual nitrile substitutions were discovered. The nitrile groups result from a photochemical oxidative cleavage of the oxazolyl moiety. In vitro studies revealed that the photooxidation by singlet oxygen also takes place in the absence of a photosensitizer, and that also a thiazolyl-substituted rhizoxin analogue undergoes the same transformation. The resulting nitriles have antimitotic properties but are significantly less active than the parent compounds. These results highlight the impact of photoreactions onto the antiproliferative agent and encourage the introduction of bioisosteric groups that render the compound less susceptible towards photooxidation.

Marine natural products: a new wave of drugs?

The largely unexplored marine world that presumably harbors the most biodiversity may be the vastest resource to discover novel 'validated' structures with novel modes of action that cover biologically relevant chemical space. Several challenges, including the supply problem and target identification, need to be met for successful drug development of these often complex molecules; however, approaches are available to overcome the hurdles. Advances in technologies such as sampling strategies, nanoscale NMR for structure determination, total chemical synthesis, fermentation and biotechnology are all crucial to the success of marine natural products as drug leads. We illustrate the high degree of innovation in the field of marine natural products, which in our view will lead to a new wave of drugs that flow into the market and pharmacies in the future.

Phorboxazole Synthetic Studies: Design, Synthesis and Biological Evaluation of Phorboxazole A and Hemi-Phorboxazole A Related Analogues

The design, synthesis and biological evaluation of a new phorboxazole analogue, comprising an acetal replacement for the C-ring tetrahdropyran of the natural product and carrying a potency-enhancing C(45-46) vinyl chloride side chain, is described. In addition, the synthesis of (+)-hemi-phorboxazole A and a series of related hemi-phorboxazole A analogues has been achieved. The new acetal ring replacement analogue displayed activity comparable to that of the parent natural product against HCT-116 (colon) cells (IC(50) 2.25 ng/mL). Equally important, the phorboxazole analogue and two related hemiphorboxazole A congeners exhibited significant antifungal activity when assayed against pathogenic Candida albicans strains.

Microscale methodology for structure elucidation of natural products

Advances in microscale spectroscopic techniques, particularly microcryoprobe NMR, allow discovery and structure elucidation of new molecules down to only a few nanomole. Newer methods for utilizing circular dichroism (CD) have pushed the limits of detection to picomole levels. NMR and CD methods are complementary to the task of elucidation of complete stereostructures of complex natural products. Together, integrated microprobe NMR spectroscopy, microscale degradation and synthesis, are synergistic tools for the discovery of bioactive natural products and have opened new realms for discovery among extreme sources including compounds from uncultured microbes, rare invertebrates and environmental samples.

Structure elucidation at the nanomole scale. 3. Phorbasides G-I from Phorbas sp

Three new phorbasides (G-I), chlorocyclopropyl ene-yne macrolide glycosides, were isolated from the sponge Phorbas sp. in yields of 7-9.5 mug and fully characterized by MS, CD, and microcryoprobe NMR. The structures of the new compounds differ only in the nature of the sugar residues. The absolute configurations of the new compounds were correlated by ROESY and CD with the parent compounds phorbasides A and B.

Methodology for the Synthesis of Substituted 1,3-Oxazoles

The halogen dance isomerization is a facile and preparatively effective pathway for the synthesis of 2,4,5-trisubstituted-1,3-oxazoles.

Hemi-phorboxazole a: structure confirmation, analogue design and biological evaluation

A synthesis providing totally synthetic (+)-hemi-phorboxazole A (1), proceeding in two steps (85% yield) from known vinyl iodide precursor (+)-2, has been achieved in conjunction with the design, synthesis, and biological evaluation of two hemi-phorboxazole analogues [(+)-3 and (-)-4] featuring ring replacements inscribed within the macrolide. Although hemi-phorboxazole A (1) displayed no activity when tested against Candida albicans and two human cancer cell lines, analogue (-)-4 exhibited significant tumor cell growth inhibitory activity in the nanomolar range against HCT-116 (colon) and SK-BR-3 (breast), while (+)-3 displayed promising antifungal activity against C. albicans.