Two imidazole alkaloids from a sponge

Natural Product-Derived Phytochemicals for Influenza A Virus (H1N1) Prevention and Treatment

Influenza A (H1N1) viruses are prone to antigenic mutations and are more variable than other influenza viruses. Therefore, they have caused continuous harm to human public health since the pandemic in 2009 and in recent times. Influenza A (H1N1) can be prevented and treated in various ways, such as direct inhibition of the virus and regulation of human immunity. Among antiviral drugs, the use of natural products in treating influenza has a long history, and natural medicine has been widely considered the focus of development programs for new, safe anti-influenza drugs. In this paper, we focus on influenza A (H1N1) and summarize the natural product-derived phytochemicals for influenza A virus (H1N1) prevention and treatment, including marine natural products, flavonoids, alkaloids, terpenoids and their derivatives, phenols and their derivatives, polysaccharides, and derivatives of natural products for prevention and treatment of influenza A (H1N1) virus. We further discuss the toxicity and antiviral mechanism against influenza A (H1N1) as well as the druggability of natural products. We hope that this review will facilitate the study of the role of natural products against influenza A (H1N1) activity and provide a promising alternative for further anti-influenza A drug development.

Advances in catalytic and protecting-group-free total synthesis of natural products: a recent update

Catalytic processes in protecting-group-free syntheses of natural products are fast emerging towards achieving the goal of efficiency and economy in total synthesis. Present day sustainable development in synthesis of natural products does not permit the luxury of using stoichiometric reagents and protecting groups. Catalysis and step-economy can contribute significantly toward economy and efficiency of synthesis. This feature article details the ingenious efforts by many researchers in the last couple of years toward concise total syntheses, based on catalytic steps and protecting-group-free-strategies. These would again serve as guidelines in future development of reagents and catalysts aimed at achieving higher efficiency and chemoselectivity to the point that catalysis and protecting-group-free synthesis will be an accepted common practice.

Synthesis of Imidazolidinone, Imidazolone, and Benzimidazolone Derivatives through Oxidation Using Copper and Air

A new synthetic method of urea derivatives using copper and air was developed. These mild conditions provided moderate to very good yields for 15 examples (53-93%), while low yields were obtained with sterically hindered substrates (3 examples). The reaction was found to go through an in situ generated copper- N-heterocyclic carbene, which was then oxidized into cyclic urea derivatives possessing alkyl, benzyl, aryl, hydroxy, Boc-protected, and tertiary amine groups.

Applications of Friedel-Crafts reactions in total synthesis of natural products

Over the years, Friedel-Crafts (FC) reactions have been acknowledged as the most useful and powerful synthetic tools for the construction of a special kind of carbon-carbon bond involving an aromatic moiety. Its stoichiometric and, more recently, its catalytic procedures have extensively been studied. This reaction in recent years has frequently been used as a key step (steps) in the total synthesis of natural products and targeted complex bioactive molecules. In this review, we try to underscore the applications of intermolecular and intramolecular FC reactions in the total syntheses of natural products and complex molecules, exhibiting diverse biological properties.

Protecting-Group-Free Total Synthesis and Biological Evaluation of 3-Methylkealiiquinone and Structural Analogues

The modular protecting-group-free total synthesis of 3-methylkealiiquinone, an analogue of the marine alkaloid kealiiquinone, was accomplished in seven steps. A regioselectively constructed functionalized arylbenzimidazolone moiety and dimethyl squarate were used as the only two building blocks. A thermal ring expansion via 6π-conrotatory ring closure to build the quinone fragment gave rise to the desired linear analogue of the natural compound along with a nondescribed structurally attractive angular naphtho[1,2- d]imidazole regioisomer. The IC₅₀ values for the compounds were determined on three cancer cell lines.

Naamines and Naamidines as Novel Agents against a Plant Virus and Phytopathogenic Fungi

Naamines, naamidines and various derivatives of these marine natural products were synthesized and characterized by means of nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry. The activities of these alkaloids against a plant virus and phytopathogenic fungi were evaluated for the first time. A benzyloxy naamine derivative 15d displayed excellent in vivo activity against tobacco mosaic virus at 500 μg/mL (inactivation activity, 46%; curative activity, 49%; and protective activity, 41%); its activities were higher than the corresponding activities of the commercial plant virucide ribavirin (32%, 35%, and 34%, respectively), making it a promising new lead compound for antiviral research. In vitro assays revealed that the test compounds exhibited very good antifungal activity against 14 kinds of phytopathogenic fungi. Again, the benzyloxy naamine derivative 15d exhibited broad-spectrum fungicidal activity, emerging as a new lead compound for fungicidal research. Additional in vivo assays indicated that many of the compounds displayed inhibitory effects >30%.

Imidazole Alkaloids and Their Zinc Complexes from the Calcareous Marine Sponge Leucetta chagosensis

Five new imidazole derivatives (1-5), together with eight related known alkaloids, were isolated from a calcareous marine sponge, Leucetta chagosensis, collected from the South China Sea. Their structures were fully characterized by spectroscopic methods. Structurally, 1 possesses an unusual skeleton featuring imidazole and oxazolone rings linked via a nitrogen atom, whereas 2 bears an intriguing guanylurea-substituted imidazole ring. Compounds 4 and 5 were identified as zinc complexes; they represent the metal complex analogues of naamidine J (6) and pyronaamidine (7), respectively. Among the isolated compounds, 2 and 5 showed significant inhibitory activities toward the LPS-induced production of IL-6 in the human acute monocytic leukemia cell line THP-1, and 7 displayed cytotoxicity against MCF-7, PC9, A549, and breast cancer stem cells (MCF-7-Oct4-GFP) with IC₅₀ values of 5.2, 5.6, 7.8, and 10 μM, respectively.

Chagosendines A - C, New Metal Complexes of Imidazole Alkaloids from the Calcareous Sponge Leucetta chagosensis

Chemical examination of the bright yellow sponge Leucetta chagosensis resulted in the isolation of three new imidazole-based alkaloid complexes namely chagosendines A - C (1 - 3), together with known analogues pyronaamidine, naamidine J, and naamine C. Their structures were determined on the basis of extensive spectroscopic (IR, MS, NMR, and single-crystal X-ray diffraction) analysis in association with the chemical conversion. The isolated alkaloids together with three synthesized new homodimer complexes were evaluated for the cytotoxic activities against a panel of tumor cell lines. The copper complexes of imidazole alkaloids 2 and 3, as found from nature for the first time, exerted selective and remarkable activities against the tumor cell lines K562, HepG2, and HeLa.

Total synthesis of kealiiquinone: the regio-controlled strategy for accessing its 1-methyl-4-arylbenzimidazolone core

A practical, concise and straightforward total synthesis of kealiiquinone 1, a naphtho[2,3-d]imidazole alkaloid obtained from the Micronesian marine sponge Leucetta sp. was accomplished. The squaric acid chemistry to construct the 1,4-quinoid ring and the regioselective N-methylation through a benzo[c][1,2,5]selenadiazolium heterocycle are the key features in this report. The full details of the representative approaches involving the different attempted synthetic strategies are also presented. Finally a successful total synthesis of this complex secondary metabolite is described.

A practical, concise and straightforward total synthesis of kealiiquinone 1, a naphtho[2,3-d]imidazole alkaloid obtained from the Micronesian marine sponge Leucetta sp. was accomplished.

Total synthesis and cytotoxicity of Leucetta alkaloids

The total synthesis of a number of representative natural products isolated from Leucetta and Clathrina sponges containing a polysubstituted 2-aminoimidazole are described. These syntheses take advantage of the site specific metallation reactions of 4,5-diiodoimidazoles resulting in the syntheses of three different classes of Leucetta derived natural products. The cytotoxicities of these natural products, along with several precursors in MCF7 cells were determined through an MTT growth assay. For comparative purposes a series of naphthimidazole-containing family members are included.

Imidazole Alkaloids from the South China Sea Sponge Pericharax heteroraphis and Their Cytotoxic and Antiviral Activities

Marine sponges continue to serve as a rich source of alkaloids possessing interesting biological activities and often exhibiting unique structural frameworks. In the current study, chemical investigation on the marine sponge Pericharax heteroraphis collected from the South China Sea yielded one new imidazole alkaloid named naamidine J (1) along with four known ones (2-5). Their structures were established by extensive spectroscopic methods and comparison of their data with those of the related known compounds. All the isolates possessed a central 2-aminoimidazole ring, substituted by one or two functionalized benzyl groups in some combination of the C4 and C5 positions. The cytotoxicities against selected HL-60, HeLa, A549 and K562 tumor cell lines and anti-H1N1 (Influenza a virus (IAV)) activity for the isolates were evaluated. Compounds 1 and 2 exhibited cytotoxicities against the K562 cell line with IC50 values of 11.3 and 9.4 μM, respectively. Compound 5 exhibited weak anti-H1N1 (influenza a virus, IAV) activity with an inhibition ratio of 33%.

Studies Towards the Leucetta-derived Alkaloids Spirocalcaridine A and B - Possible Biosynthetic Implications

An exploration of an abiotic approach to spirocalcaridines A and B is described centered on electrophile-induced dearomatizing spirocyclization of aryl enyne derivatives. Elaboration of the α-iodoenone via an Ullmann-like, copper-catalyzed amidation provided a formamide which upon treatment with methylamine undergoes a dienol-arene rearrangement, providing the corresponding kealiinine-like framework. This observation suggests a possible biosynthetic links between the spirocalcaridines and the naphthimidazole group of Leucetta alkaloids.

A bioassay for the detection of benzimidazoles reveals their presence in a range of environmental samples

Cobamides are a family of enzyme cofactors that include vitamin B₁₂ (cobalamin) and are produced solely by prokaryotes. Structural variability in the lower axial ligand has been observed in cobamides produced by diverse organisms. Of the three classes of lower ligands, the benzimidazoles are uniquely found in cobamides, whereas the purine and phenolic bases have additional biological functions. Many organisms acquire cobamides by salvaging and remodeling cobamides or their precursors from the environment. These processes require free benzimidazoles for incorporation as lower ligands, though the presence of benzimidazoles in the environment has not been previously investigated. Here, we report a new purification method and bioassay to measure the total free benzimidazole content of samples from microbial communities and laboratory media components. The bioassay relies on the “calcofluor-bright” phenotype of a bluB mutant of the model cobalamin-producing bacterium Sinorhizobium meliloti. The concentrations of individual benzimidazoles in these samples were measured by liquid chromatography-tandem mass spectrometry. Several benzimidazoles were detected in subpicomolar to subnanomolar concentrations in host-associated and environmental samples. In addition, benzimidazoles were found to be common contaminants of laboratory media components. These results suggest that benzimidazoles present in the environment and in laboratory media have the potential to influence microbial metabolic activities.

Total synthesis of 7'-desmethylkealiiquinone, 4'-desmethoxykealiiquinone, and 2-deoxykealiiquinone

Synthetic approaches to the imidazonaphthoquinone core of kealiiquinone and related Leucetta-derived alkaloids are described. The polysubstituted benzimidazole framework can be constructed through intramolecular Diels-Alder reactions of propiolate-derived enynes followed by oxidation. Adjustment of the oxidation state of the thus formed lactone allows introduction of the 2,3-dihydroxybenzoquinone moiety through a presumed benzoin-like condensation between a phthaldehyde derivative and a masked glyoxal equivalent catalyzed by a cyanide ion. Oxidation of the C2-position can be accomplished through application of an operationally simple treatment of an imidazolium salt with bleach, thus producing the corresponding 2-imidazolone. Debenzylation of a late stage intermediate en route to kealiiquinone was compromised by concomitant O-demethylation upon treatment with triflic acid resulting in the formation of non-natural 7'-desmethylkealiiquinone. Other endgame strategies were evaluated; however, these efforts did not lead to completion of a synthesis of kealiiquinone but did provide access to other closely related analogues.

Total syntheses and cytotoxicity of kealiiquinone, 2-deoxy-2-aminokealiiquinone and analogs

Concise syntheses of two Leucetta-derived naphthimidazole alkaloids, kealiiquinone and 2-deoxy-2-aminokealiiquinone, are described based on a biosynthetic-guided hypothesis. Advanced intermediates containing the full naphthimidazole framework are constructed through Friedel-Crafts chemistry followed by oxidation of the electron rich C-ring with hydrogen peroxide. The cytotoxicity of these alkaloids in a breast cancer cell line along with several closely related marine-derived natural products kealiinines A-C and analogs are reported.

Total syntheses of kealiinines A-C

Short total syntheses of the Leucetta-derived alkaloids, kealiinines A-C, have been accomplished using an intramolecular Friedel-Crafts-dehydration sequence of a bis benzylic diol. The precursor diol was obtained through a series of position-specific Grignard reactions from 1-methyl-4,5-diiodoimidazole. C2-Azidation and hydrogenation of the azide then provided the reported structures of kealiinines A-C. While the (1)H NMR data did not completely match for these materials, the HPLC data were consistent with the assigned structure of these alkaloids.

Synthesis of the reported structures for kealiinines B and C

Syntheses of the reported structures of kealiinines B and C have been executed. An intermolecular electrophile-induced cyclization of a pendant arene on an ene-guanidine affords the tetracyclic, oxidized naphthimidazole cores.

Assessing calcareous sponges and their associated bacteria for the discovery of new bioactive natural products

An overview of the chemistry and microbiology of calcareous sponges (Calcispongiae) is provided, highlighting the potential of these sessile filter-feeding marine invertebrates and their associated bacteria for the discovery of new bioactive natural products. 103 compounds are presented and 116 references cited.

Total synthesis of 7'-desmethylkealiiquinone

The total synthesis of an analogue of the marine alkaloid kealiiquinone has been completed through application of an intramolecular Diels-Alder reaction of an imidazole-containing enyne. Oxidative aromatization of the lactone adduct and N-methylation facilitates C2-oxidation via the imidazolium salt. Conversion of the lactone to the phthalaldehyde derivative and then to the dihydroxybenzoquinone was achieved via a reaction with glyoxal in the presence of KCN. Esterification of the vinylogous diacid and deprotection provided 7'-desmethylkealiiquinone.

Synthesis of 2-imidazolones and 2-iminoimidazoles

Convenient methods for the direct conversion of imidazolium salts to the corresponding 2-imidazolone or 2-imino imidazole derivatives have been developed. Treatment of the salt with commercial bleach leads to effective oxidation at C2 and the formation of the corresponding imidazolone. Alternatively, treatment of the salt with an N-chloro amide affords the corresponding protected 2-amino derivative in good yield.

Studies for the enantiocontrolled preparation of substituted tetrahydropyrans: Applications for the synthesis of leucascandrolide A macrolactone

Strategies for the stereocontrolled preparations of 2,6-cis-and 2,6-trans-substituted tetrahydropyrans have been devised. These studies have explored methodology for asymmetric induction in SE' reactions using chiral 1,3,2-diazaborolidine controllers. Reactions with aldehydes at -78 °C yield nonracemic 1,5-diols for chemoselective internal backside displacements. This concept is developed as a flexible and reliable strategy in studies toward leucascandrolide A macrolactone 2 via the sequential applications of SE' reactions leading to the C1-C9 aldehyde 14, and the bis-tetrahydropyran 59, respectively.

Cellular localization of clathridimine, an antimicrobial 2-aminoimidazole alkaloid produced by the Mediterranean calcareous sponge Clathrina clathrus

Chemical investigation of the Mediterranean calcareous sponge Clathrina clathrus led to the isolation of large amounts of a new 2-aminoimidazole alkaloid, named clathridimine (1), along with the known clathridine (2) and its zinc complex (3). The structure of the new metabolite was assigned by detailed spectroscopic analysis. Clathridimine (1) displayed selective anti-Escherichia coli and anti-Candida albicans activities. Clathridine (2) showed only anti-Candida albicans activity, and its zinc complex (3) exhibited selective anti-Staphylococcus aureus activity. The isolation of analogues of 2-amino-imidazole derivatives from several Leucetta species from various sites in the Pacific Ocean and the Red Sea raises the question of their biosynthetic origin. Microscopic studies revealed abundant extracellular bacteria located in the mesohyl of the sponge, with two predominant morphotypes including spiral bacteria and long, narrow bacilli. Chemical analysis with HPLC/UV/ELSD profiles of sponge cells separated from bacteria by differential centrifugation and trypsinization of the sponge cell surface revealed that clathridine (2) was localized in the sponge cells.

Total Syntheses of Naamidine G and 14-Methoxynaamidine G

Simple total syntheses of two Leucetta-derived marine alkaloids have been developed using position specific halogen-metal exchange of polyhaloimidazoles to introduce the benzyl substituted sidechains. Introduction of the C2 amine group by lithiation and trapping with tosyl azide provides amines on catalytic hydrogenation, which can be converted to naamidine G and 14-methoxynaamidine G using a procedure described in the literature.

Two new imidazole alkaloids from Leucetta chagosensis sponge

Chemical investigation of the sponge Leucetta chagosensis, has led to the isolation of two new imidazole alkaloids, methyldorimidazole (1), preclathridine B (2) along with the known compounds naamine E (3) and leucettamine C (4). The structures of the newly compounds (1 and 2) were elucidated by employing spectroscopic techniques ((1)H NMR, MS and UV). The structures of the known compounds 3 and 4 were determined by comparison of their (1)H NMR and Mass spectroscopic data with published values.