A Likely Biogenetic Gateway Linking 2-Aminoimidazolinone Metabolites of Sponges to Proline: Spontaneous Oxidative Conversion of the Pyrrole-Proline-Guanidine Pseudo-peptide to Dispacamide A

A base-mediated aerobic oxidative synthesis of cyclopent-2-enol derivatives from doubly activated cyclopropanes and substituted acetonitriles

We report here that polysubstituted cyclopent-2-enols can be constructed by the one-pot reaction of doubly activated cyclopropanes and α-EWG substituted acetonitriles under mild basic conditions via a domino-ring-opening-cyclization/deacylation/oxidation sequence. Moreover, the synthetic applications of these cyclopent-2-enols have been demonstrated in the late-stage derivatization into functionalized cyclopentapyrimidin-4-ones and 2-hydroxy cyclopentanones with good yields.

A Review of the Synthetic Strategies toward Dihydropyrrolo[1,2-a]Pyrazinones

Dihydropyrrolo[1,2-a]pyrazinone rings are a class of heterocycles present in a wide range of bioactive natural products and analogues thereof. As a direct result of their bioactivity, the synthesis of this privileged class of compounds has been extensively studied. This review provides an overview of these synthetic pathways. The literature is covered up until 2020 and is organized according to the specific strategies used to construct the scaffold: fusing a pyrazinone to an existing pyrrole, employing a pyrazinone-first strategy, an array of multicomponent reactions and some miscellaneous reactions.

Precursor-Guided Mining of Marine Sponge Metabolomes Lends Insight into Biosynthesis of Pyrrole-Imidazole Alkaloids

Pyrrole-imidazole alkaloids are natural products isolated from marine sponges, holobiont metazoans that are associated with symbiotic microbiomes. Pyrrole-imidazole alkaloids have attracted attention due to their chemical complexity and their favorable pharmacological properties. However, insights into how these molecules are biosynthesized within the sponge holobionts are scarce. Here, we provide a multiomic profiling of the microbiome and metabolomic architectures of three sponge genera that are prolific producers of pyrrole-imidazole alkaloids. Using a retrobiosynthetic scheme as a guide, we mine the metabolomes of these sponges to detect intermediates in pyrrole-imidazole alkaloid biosynthesis. Our findings reveal that the nonproteinogenic amino acid homoarginine is a critical branch point that connects primary metabolite lysine to the production of pyrrole-imidazole alkaloids. These insights are derived from the polar metabolomes of these sponges which additionally reveal the presence of zwitterionic betaines that may serve important ecological roles in marine habitats. We also establish that metabolomic richness does not correlate with microbial diversity of the sponge holobiont for neither the polar nor the nonpolar metabolomes. Our findings now provide the biochemical foundation for genomic interrogation of the sponge holobiont to establish biogenetic routes for pyrrole-imidazole alkaloid production.

Marine-Derived 2-Aminoimidazolone Alkaloids. Leucettamine B-Related Polyandrocarpamines Inhibit Mammalian and Protozoan DYRK & CLK Kinases

A large diversity of 2-aminoimidazolone alkaloids is produced by various marine invertebrates, especially by the marine Calcareous sponges Leucetta and Clathrina. The phylogeny of these sponges and the wide scope of 2-aminoimidazolone alkaloids they produce are reviewed in this article. The origin (invertebrate cells, associated microorganisms, or filtered plankton), physiological functions, and natural molecular targets of these alkaloids are largely unknown. Following the identification of leucettamine B as an inhibitor of selected protein kinases, we synthesized a family of analogues, collectively named leucettines, as potent inhibitors of DYRKs (dual-specificity, tyrosine phosphorylation regulated kinases) and CLKs (cdc2-like kinases) and potential pharmacological leads for the treatment of several diseases, including Alzheimer's disease and Down syndrome. We assembled a small library of marine sponge- and ascidian-derived 2-aminoimidazolone alkaloids, along with several synthetic analogues, and tested them on a panel of mammalian and protozoan kinases. Polyandrocarpamines A and B were found to be potent and selective inhibitors of DYRKs and CLKs. They inhibited cyclin D1 phosphorylation on a DYRK1A phosphosite in cultured cells. 2-Aminoimidazolones thus represent a promising chemical scaffold for the design of potential therapeutic drug candidates acting as specific inhibitors of disease-relevant kinases, and possibly other disease-relevant targets.

Discovery of 5-(5,5-Dimethylbutenolide-3-ethylidene)-2-amino-imidazolinone Derivatives as Fungicidal Agents

The novel fungicidal agents 5-(5,5-dimethylbutenolide-3-ethylidene)-2-amino-imidazolinone derivatives, were designed and synthesized in moderate to excellent yields in four steps by α-hydroxyketone and diketene as raw materials and characterized by HR-ESI-MS and ¹H-NMR. The preliminary bioassay showed that some of these compounds, such as 4a, 4e and 5g exhibit 94.9%, 92.8% and 81.4% inhibition rates against Sclerotiniascleotiorum at the concentration of 50 µg/mL, respectively. The EC₅₀ values of compounds 4e and 4i were 4.14 and 3.27 µM against Alternaria Solani, and 5g had EC₅₀ value of 3.23 µM against S. scleotiorum. Compounds 4d and 4g displayed 98.0% and 97.8% control of spore germination against Botrytiscinerea at the concentration of 100 µg/mL, respectively.

Dimeric pyrrole-imidazole alkaloids: synthetic approaches and biosynthetic hypotheses

The pyrrole-imidazole alkaloids are a group of structurally unique and biologically interesting marine sponge metabolites. Among them, the cyclic dimers have caught synthetic chemists' attention particularly. Numerous synthetic strategies have been developed and various biosynthetic hypotheses have been proposed for these fascinating natural products. We discuss herein the synthetic approaches and the biosynthetic insights obtained from these studies.

Concise synthesis of didebromohamacanthin A and demethylaplysinopsine: addition of ethylenediamine and guanidine derivatives to the pyrrole-amino acid diketopiperazines in oxidative conditions

Oxidative nucleophilic addition of ethylenediamine and guanidine derivatives to pyrrole-amino acid diketopiperazines was shown to provide substituted 5,6-dihydro-2(1H)-piperazinones, quinoxalinones, and 2-aminoimidazolones. On the basis of this methodology, a concise approach to natural products didebromohamacanthin A and demethylaplysinopsine has been demonstrated.

Selectivity, cocrystal structures, and neuroprotective properties of leucettines, a family of protein kinase inhibitors derived from the marine sponge alkaloid leucettamine B

DYRKs (dual specificity, tyrosine phosphorylation regulated kinases) and CLKs (cdc2-like kinases) are implicated in the onset and development of Alzheimer's disease and Down syndrome. The marine sponge alkaloid leucettamine B was recently identified as an inhibitor of DYRKs/CLKs. Synthesis of analogues (leucettines) led to an optimized product, leucettine L41. Leucettines were cocrystallized with DYRK1A, DYRK2, CLK3, PIM1, and GSK-3β. The selectivity of L41 was studied by activity and interaction assays of recombinant kinases and affinity chromatography and competition affinity assays. These approaches revealed unexpected potential secondary targets such as CK2, SLK, and the lipid kinase PIKfyve/Vac14/Fig4. L41 displayed neuroprotective effects on glutamate-induced HT22 cell death. L41 also reduced amyloid precursor protein-induced cell death in cultured rat brain slices. The unusual multitarget selectivity of leucettines may account for their neuroprotective effects. This family of kinase inhibitors deserves further optimization as potential therapeutics against neurodegenerative diseases such as Alzheimer's disease.

Asymmetric synthesis of ageliferin

We describe herein an asymmetric synthesis of ageliferin. A Mn(III)-mediated oxidative radical cyclization reaction was used as the key step to construct the core skeleton of this pyrrole-imidazole dimer. This approach resembles the biogenic [4 + 2] dimerization in an intramolecular fashion.

Biosynthesis, asymmetric synthesis, and pharmacology, including cellular targets, of the pyrrole-2-aminoimidazole marine alkaloids

The pyrrole-2-aminoimidazole (P-2-AI) alkaloids are a growing family of marine alkaloids, now numbering well over 150 members, with high topographical and biological information content. Their intriguing structural complexity, rich and compact stereochemical content, high N to C ratio (~1 : 2), and increasingly studied biological activities are attracting a growing number of researchers from numerous disciplines world-wide. This review surveys advances in this area with a focus on the structural diversity, biosynthetic hypotheses with increasing, but still rare, verifying experimental studies, asymmetric syntheses, and biological studies, including cellular target receptor isolation studies, of this stimulating and exciting alkaloid family.

Leucettines, a class of potent inhibitors of cdc2-like kinases and dual specificity, tyrosine phosphorylation regulated kinases derived from the marine sponge leucettamine B: modulation of alternative pre-RNA splicing

We here report on the synthesis, optimization, and biological characterization of leucettines, a family of kinase inhibitors derived from the marine sponge leucettamine B. Stepwise synthesis of analogues starting from the natural structure, guided by activity testing on eight purified kinases, led to highly potent inhibitors of CLKs and DYRKs, two families of kinases involved in alternative pre-mRNA splicing and Alzheimer's disease/Down syndrome. Leucettine L41 was cocrystallized with CLK3. It interacts with key residues located within the ATP-binding pocket of the kinase. Leucettine L41 inhibits the phosphorylation of serine/arginine-rich proteins (SRp), a family of proteins regulating pre-RNA splicing. Indeed leucettine L41 was demonstrated to modulate alternative pre-mRNA splicing, in a cell-based reporting system. Leucettines should be further explored as pharmacological tools to study and modulate pre-RNA splicing. Leucettines may also be investigated as potential therapeutic drugs in Alzheimer's disease (AD) and in diseases involving abnormal pre-mRNA splicing.

Cylindradines A and B: novel bromopyrrole alkaloids from the marine sponge Axinella cylindratus

The novel alkaloids cylindradines A and B were isolated from Axinella cylindratus, and their structures were elucidated by spectroscopic analyses. Stereochemistries of these compounds were determined by X-ray analysis. Cylindradines showed moderate inhibitory activity against the murine leukemia cell line P388.

A fluorescein-containing, small-molecule, water-soluble receptor for cytosine free bases

In this study, we synthesized small-molecule, water-soluble, fluorescein-containing ureido compounds 6 and 8 as target receptors for cytosine free bases and then investigated the binding of cytosine free bases with the receptors using (15)N NMR spectroscopy and partially labeled cytosine-2,4-(13)C-1,3,4-(15)N-cytosine. Binding with the receptor 6a (the disodium form of 6) caused the chemical shift of the nitrogen atom of the amino group of cytosine to move downfield; binding of the receptor 8a (the disodium form of 8), which is possessing no corresponding aryl nitrogen atom, had no effect on this signal. Fluorescence spectroscopy revealed that binding of cytosine and its derivatives led to quenching of the fluorescence of receptor 6a; in contrast, the quenching of receptor 8a was only slightly affected by cytosine. Because the fluorescence of 6a was not quenched by either deoxycytidine or uracil, it appears that this receptor is a specific for cytosine among the DNA bases. We used the fluorescence of 6a to measure the apparent binding constants for various cytosine derivatives, including the anticancer prodrug 5-fluorocytosine. Receptor 6a is the first small-molecule, water-soluble fluorescent receptor for the specific binding of cytosine free bases in aqueous solution.

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.

Exploring symmetry-based logic for a synthesis of palau'amine

A synthetic approach to palau'amine is described that exploits veiled symmetry in the structure. Bis-alkylidenes i have been prepared and found susceptible to halogenative desymmetrization using t-BuOCl. This oxidation forms the imbedded spirocyclopentane motif observed in the natural product. A host of atypical reactions and processes developed during these studies are discussed, as are plans for completing total syntheses of this compound class.

Verpacamides A−D, a Sequence of C11N5 Diketopiperazines Relating Cyclo(Pro-Pro) to Cyclo(Pro-Arg), from the Marine Sponge Axinella vaceleti: Possible Biogenetic Precursors of Pyrrole-2-aminoimidazole Alkaloids

[reaction: see text] Four C(11)N(5) diketopiperazine metabolites named verpacamides A (6), B (7), C (8), and D (9) consisting of a proline-arginine dipeptide skeleton have been isolated from the marine sponge Axinella vaceleti. Verpacamides A-D are a sequence of metabolites showing the transformation of proline and arginine into the oxidized guanidinyl-cyclo(Pro-Pro) 8 and 9. Compounds 6-9 are structurally and chemically related to C(11)N(5) pyrrole-2-aminoimidazole metabolites also isolated from the Axinellidae and Agelasidae families of sponges and exemplified by dispacamide A (4) and dibromophakellin (10).

A New Glycociamidine Ring Precursor: Syntheses of (Z)-Hymenialdisine, (Z)-2-Debromohymenialdisine, and (±)-endo-2-Debromohymenialdisine†

[chemical reaction: see text]. The synthesis of the C11H5 marine sponge alkaloids, (Z)-hymenialdisine and (Z)-2-debromohymenialdisine, is described. A key step was the condensation between aldisine or its monobromo derivative and a new, efficient imidazolinone-based glycociamidine precursor. In the first case, the main product turned out to be the unprecedented (+/-)-endo-2-debromohymenialdisine.