Total synthesis of the putative structure of nagelamide D

Total Synthesis of Nagelamide W

Herein, we report the total synthesis of nagelamide W (1), a pyrrole imidazole alkaloid of the nagelamide family isolated in 2013. The key approach in this work involves the construction of the 2-aminoimidazoline core of nagelamide W from alkene 6 through a cyanamide bromide intermediate. The synthesis of nagelamide W was accomplished with an overall yield of 6.0%.

Dimeric pyrrole-imidazole alkaloids: sources, structures, bioactivities and biosynthesis

Pyrrole-imidazole alkaloids (PIAs) constitute a highly diverse and densely functionalized subclass of marine natural products. Among them, the uncommon dimeric PIAs with ornate molecular architectures, attractive biological properties and interesting biosynthetic origin have spurred a considerable interest of chemists and biologists. The present review comprehensively summarized 84 dimeric PIAs discovered during the period from 1981 to September 2022, covering their source organisms, chemical structures, biological activities as well as biosynthesis. For a better understanding, these structurally intricate PIA dimers are firstly classified and presented according to their carbon skeleton features as well as biosynthesis pathways. Furthermore, relevant summaries focusing on the source organisms and the associated bioactivities of these compounds belonging to different chemical classes are also provided, which will help elucidate the fascinating chemistry and biology of these unusual PIA dimers.

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.

Marine Pyrrole Alkaloids

Nitrogen heterocycles are essential parts of the chemical machinery of life and often reveal intriguing structures. They are not only widespread in terrestrial habitats but can also frequently be found as natural products in the marine environment. This review highlights the important class of marine pyrrole alkaloids, well-known for their diverse biological activities. A broad overview of the marine pyrrole alkaloids with a focus on their isolation, biological activities, chemical synthesis, and derivatization covering the decade from 2010 to 2020 is provided. With relevant structural subclasses categorized, this review shall provide a clear and timely synopsis of this area.

Pyrrole carboxamide introduction in the total synthesis of pyrrole-imidazole alkaloids

In this review various strategies for the incorporation of the signature pyrrole carboxamide moiety in the total syntheses of pyrrole-imidazole alkaloids (PIA) are discussed. These so-called oroidin alkaloids have a broad range of biological activities and display interesting skeletal diversity and complexity. These alkaloids are sponge-derived secondary metabolites and thus far more than 200 members of the PIA family have been isolated over the past few decades. Methods range from classical amide bond forming processes to non-traditional bond formation including the de novo synthesis of the pyrrole itself.

Total Synthesis of the Nagelamides - Synthetic Studies toward the Reported Structure of Nagelamide D and Nagelamide E Framework

The nagelamides are a small subset of the oroidin family of marine sponge-derived alkaloids and are, for the most part, dimeric in nature. As part of our efforts to develop synthetic access to this family, a Stille cross-coupling strategy is used to construct the bis-imidazolyl core skeleton. Reduction of the bis-vinylimidazole delivered the core framework of nagelamide D. Introduction of the 2-amino groups via the corresponding azides and introduction of the pyrrolecarboxamides through a double Mitsunobu reaction using a pyrrole hydantoin provided the putative structure of nagelamide D. The spectroscopic data for the synthetic and sponge-derived materials did not match well, whereas the spectroscopic data were a good match for closely related oroidin alkaloids, supporting the structure of the synthetic material. The structure of the synthetic material was further corroborated by obtaining an X-ray crystal structure of a derivative. Electrocyclization of an advanced precursor affords a dihydrobenzimidazole, which is expected to serve as a key intermediate en route to nagelamide E and ageliferin.

Application of copper(i) salt and fluoride promoted Stille coupling reactions in the synthesis of bioactive molecules

The Stille coupling between organostannanes and organohalides is an effective catalytic method for organic synthesis. Despite the ample amount of published results in this area, finding the optimal conditions for this transformation is often not straightforward. It was observed that this reaction could be accelerated with improved efficiency by the addition of a Cu(i) salt and fluoride. This review summarises the application of this simple protocol in the synthesis of natural products, their analogues and other biologically active molecules, from 2004 to 2018.

Efficient Total Synthesis of Lissodendrin B, 2-Aminoimidazole Marine Alkaloids Isolated from Lissodendoryx (Acanthodoryx) Fibrosa

Lissodendrin B is a 2-aminoimidazole alkaloid bearing a (p-hydroxyphenyl) glyoxal moiety that was isolated from the Indonesian sponge Lissodendoryx (Acanthodoryx) fibrosa. We reported the first efficient total synthesis of Lissodendrin B. The precursor 4,5-disubstituted imidazole was obtained through Suzuki coupling and Sonogashira coupling reactions from 4-iodoimidazole. C2-azidation and reduction of the azide then provided the core structures of Lissodendrin B. Subsequent triple-bond oxidation, demethylation, and deacetylation gave the final product. The synthesis approach consists of ten steps with an overall yield of 1.1% under mild reaction conditions, and it can be applied for future analog synthesis and biological studies.

Current Advances in the Synthesis and Biological Evaluation of Pharmacologically Relevant 1,2,4,5-Tetrasubstituted-1H-Imidazole Derivatives

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In recent years, the synthesis and evaluation of the biological properties of 1,2,4,5-tetrasubstituted-1H-imidazole derivatives have been the subject of a large number of studies by academia and industry. In these studies it has been shown that this large and highly differentiated class of heteroarene derivatives includes high valuable compounds having important biological and pharmacological properties such as antibacterial, antifungal, anthelmintic, anti-inflammatory, anticancer, antiviral, antihypertensive, cholesterol-lowering, antifibrotic, antiuricemic, antidiabetic, antileishmanial and antiulcer activities.

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The present review with 411 references, in which we focused on the literature data published mainly from 2011 to 2017, aims to update the readers on the recent developments on the synthesis and biological evaluation of pharmacologically relevant 1,2,4,5-tetrasubstituted-1H-imidazole derivatives with an emphasis on their different molecular targets and their potential use as drugs to treat various types of diseases. Reference was also made to substantial literature data acquired before 2011 in this burgeoning research area.

Crystal structures of 4-{(E)-3-[(imino-λ5-aza-nyl-idene)amino]-prop-1-en-yl}-N,N-di-methyl-imidazole-1-sulfonamide and 2-[(imino-λ5-aza-nyl-idene)amino]-4-{(E)-3-[(imino-λ5-aza-nyl-idene)amino]-prop-1-en-yl}-N,N-di-methyl-imidazole-1-sulfonamide

The structures of two azide containing imidazole derivatives are reported. Allylic azides are fairly reactive making them attractive starting compounds to convert into amides. The first, C8H12N6O2S, contains one azide group with an Nα-Nβ distance of 1.229 (2) Å and an Nβ-Nγ distance of 1.128 (2) Å. The second, C8H11N9O2S, contains two azide groups with an average Nα-Nβ distance of 1.249 (2) Å and an average Nβ-Nγ distance of 1.132 (2) Å. Each compound contains a bulky protecting group (di-methyl-amino-sulfon-yl) which can be easily removed under mildly acidic conditions.

Regiospecific Cleavage of S-N Bonds in Sulfonyl Azides: Sulfonyl Donors

Sulfonyl azides have been widely used as sulfonamido, diazo, and azido donors, as well as all-nitrogen 1,3-dipoles donors in synthetic chemistry. Here, the sulfonyl azides were used as efficient sulfonyl donors, which is very unusual. Trifluoromethanesulfonic acid-induced formation of the sulfonyl cation reactive species from sulfonyl azides was developed and used for the first time to couple various inactivated arenes to prepare sulfones at ambient temperature.

Thio acid-mediated conversion of azides to amides - exploratory studies en route to oroidin alkaloids

The utility of the thio acid-azide coupling reaction to afford amides is explored in imidazole-containing substrates for application in the total synthesis of examples of oroidin alkaloids. Good yields of the expected amides are obtained in both monomeric and dimeric substrates. Bis azides react preferentially at the 2-azido position but hydrosulfenylation and reduction interfere. 2-Thiophenyl and 2-oxo groups were evaluated as 2-amino surrogates, the thioether delivered the expected amide, whereas 2-imidazolone gave a mixture of the expected amide and the hydrosulfenylation product.

Recent Advances in the Synthesis of Hydantoins: The State of the Art of a Valuable Scaffold

The review highlights the hydantoin syntheses presented from the point of view of the preparation methods. Novel synthetic routes to various hydantoin structures, the advances brought to the classical methods in the aim of producing more sustainable and environmentally friendly procedures for the preparation of these biomolecules, and a critical comparison of the different synthetic approaches developed in the last twelve years are also described. The review is composed of 95 schemes, 8 figures and 528 references for the last 12 years and includes the description of the hydantoin-based marketed drugs and clinical candidates.

The use of the Comins-Meyers Amide in Synthetic Chemistry: An Overview

Formylation reactions are fundamental operations in synthetic chemistry allowing the incorporation into a given structure formyl groups amenable to further derivatization. Conceptually, the introduction of such groups through the reaction between an electrophilic donor and a nucleophilic acceptor ( i.e. organometallic reagent) constitutes a reliable technique with widespread applications. In this Highlight, we summarize the effectiveness of the so called Comins-Meyers amide - [2-( N-methyl- N-formylamino]pyridine – in such a chemistry with vistas to the synthesis of natural products and biologically active substrates.

Preparation and Diels-Alder reactions of 1'-heterosubsituted vinylimidazoles

A Diels-Alder/rearrangement sequence has been pursued in our lab en route to a number of oroidin dimers. In order to access the fully substituted core of these molecules, 1',2'-disubstituted 4-vinylimidazoles were required as dienes. The preparation of a series of a 4-vinylimidazoles containing substituents on the vinyl moiety via hydroalumination/electrophilic trapping or hydrosilylation are described. These derivatives undergo Diels-Alder reactions with N-phenylmaleimide to provide the tetrahydrobenzimidazole derivatives. The cycloadducts derived from halosubstituted systems generally undergo elimination, leading to the corresponding dihydrobenzimidazole, whereas the silyl and stannyl derivatives provide the corresponding 4-substituted tetrahydrobenzimidazole.

Unprecedented biomimetic homodimerization of oroidin and clathrodin marine metabolites in the presence of HMPA or phosphonate salt tweezers

The first biomimetic homodimerization of oroidin and clathrodin was effected in the presence HMPA and diphosphonate salts, strong guanidinium and amide chelating agents. The intermolecular associations probably interfere with the entropically and kinetically favored intramolecular cyclizations. Use of oroidin·(1)/2HCl salt or clathrodin·(1)/2HCl was indicative in the presence of the ambident nucleophilic and electrophilic tautomers of the 2-aminoimidazolic oroidin and clathrodin precursors. Surprisingly, the homodimerization of oroidin led to the nagelamide D skeleton, while the homodimerization of clathrodin gave the benzene para-symmetrical structure 19. The common process was rationalized from tautomeric precursors I and III.

Pyrrole aminoimidazole alkaloid metabiosynthesis with marine sponges Agelas conifera and Stylissa caribica

Game-SET-match: Pyrrole aminoimidazole alkaloids (PAIs) are metabiosynthesized from chlorinated analogues of oroidin by cell-free enzyme preparations from PAI-producing sponges. Evidence and implications for the biosynthesis of PAIs include putative single-electron transfers (SETs) that promote C-C bond-forming reactions of precursors.

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.

The Chemistry of Marine Sponges∗

Marine sponges continue to attract wide attention from marine natural product chemists and pharmacologists alike due to their remarkable diversity of bioactive compounds. Since the early days of marine natural products research in the 1960s, sponges have notoriously yielded the largest number of new metabolites reported per year compared to any other plant or animal phylum known from the marine environment. This not only reflects the remarkable productivity of sponges with regard to biosynthesis and accumulation of structurally diverse compounds but also highlights the continued interest of marine natural product researchers in this fascinating group of marine invertebrates.

Among the numerous classes of natural products reported from marine sponges over the years, alkaloids, peptides, and terpenoids have attracted particularly wide attention due to their unprecedented structural features as well as their pronounced pharmacological activities which make several of these metabolites interesting candidates for drug discovery. This chapter consequently highlights several important groups of sponge-derived alkaloids, peptides, and terpenoids and describes their biological and/or pharmacological properties.

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.

Diversity-oriented approach to pyrrole-imidazole alkaloid frameworks

An exploration of imidazolylpropargyl amides as linchpin synthons for the construction of a diverse array of heterocyclic frameworks, many of which are related to those found in the oroidin derived alkaloids, is described. One such intermediate has been used in a formal total synthesis of cyclooroidin.

Asymmetric total synthesis of ent-cyclooroidin

An enantiospecific total synthesis of the pyrrole-imidazole natural product cyclooroidin from histidine is described. The key N1-C9 bond is constructed through an intramolecular SN2-type of reaction of a chloro ester. Subsequent imidazole azidation at the 2-position, pyrrole bromination, azide reduction, and deprotection leads to the completion of the synthesis.

Synthesis of 7-(15)N-Oroidin and evaluation of utility for biosynthetic studies of pyrrole-imidazole alkaloids by microscale (1)H-(15)N HSQC and FTMS

Numerous marine-derived pyrrole-imidazole alkaloids (PIAs), ostensibly derived from the simple precursor oroidin, 1a, have been reported and have garnered intense synthetic interest due to their complex structures and in some cases biological activity; however very little is known regarding their biosynthesis. We describe a concise synthesis of 7-(15)N-oroidin (1d) from urocanic acid and a direct method for measurement of (15)N incorporation by pulse labeling and analysis by 1D (1)H-(15)N HSQC NMR and FTMS. Using a mock pulse labeling experiment, we estimate the limit of detection (LOD) for incorporation of newly biosynthesized PIA by 1D (1)H-(15)N HSQC to be 0.96 microg equivalent of (15)N-oroidin (2.4 nmole) in a background of 1500 microg of unlabeled oroidin (about 1 part per 1600). 7-(15)N-Oroidin will find utility in biosynthetic feeding experiments with live sponges to provide direct information to clarify the pathways leading to more complex pyrrole-imidazole alkaloids.

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.

A submarine journey: the pyrrole-imidazole alkaloids

In his most celebrated tale "The Picture of Dorian Gray", Oscar Wilde stated that "those who go beneath the surface do so at their peril". This sentence could be a prophetical warning for the practitioner who voluntarily challenges himself with trying to synthesize marine sponge-deriving pyrrole-imidazole alkaloids. This now nearly triple-digit membered community has been growing exponentially in the last 20 years, both in terms of new representatives and topological complexity--from simple, achiral oroidin to the breathtaking 12-ring stylissadines A and B, each possessing 16 stereocenters. While the biosynthesis and the role in the sponge economy of most of these alkaloids still lies in the realm of speculations, significant biological activities for some of them have clearly emerged. This review will account for the progress in achieving the total synthesis of the more biologically enticing members of this class of natural products.