A Divergent Synthesis of Numerous Pyrroloiminoquinone Alkaloids Identifies Promising Antiprotozoal Agents

On the basis of a streamlined route to the pyrroloiminoquinone (PIQ) core, we made 16 natural products spread across four classes of biosynthetically related alkaloid natural products, and multiple structural analogs, all in ≤8 steps longest linear sequence (LLS). The strategy features a Larock indole synthesis as the key operation in a five-step synthesis of a key methoxy-PIQ intermediate. Critically, this compound was readily diverged via selective methylation of either (or both) of the imine-like or pyrrole nitrogens, which then permitted further divergence by either O-demethylation to o-quinone natural products or displacement of the methoxy group with a range of amine nucleophiles. Based on a single, early report of their potential utility against the malaria parasite, we assayed these compounds against several strains of Plasmodium falciparum, as well as two species of the related protozoan parasite Babesia. In combination with evaluations of their human cytotoxicity, we identified several compounds with potent (low-nM IC50) antimalarial and antibabesial activities that are much less toxic toward mammalian cells and are therefore promising lead compounds for antiprotozoal drug discovery.

Divergent total syntheses of pyrroloiminoquinone alkaloids enabled by the development of a Larock/Buchwald-Hartwig annulation/cyclization

Pyrroloiminoquinone alkaloids are a large class of natural products that display a wide range of biological activities. Synthetic approaches to these natural products typically rely on a common late-stage C10-oxygenated pyrroloiminoquinone intermediate, but these strategies often lead to lengthy synthetic sequences that are not amenable to divergent syntheses. We devised an alternative approach aimed at the early introduction of the C10 nitrogen, which we hypothesized would enable late-stage diversification. This strategy hinged upon a Larock/Buchwald-Hartwig annulation/cyclization to quickly access the core of these alkaloids. We report the development of this cascade process, which was facilitated by a dual ligand system in addition to selective functionalization of the key intermediate, to provide efficient syntheses of makaluvamines A, C, and D and isobatzelline B, and the first total synthesis of makaluvamine N.

An Investigation of Structure-Activity Relationships and Cell Death Mechanisms of the Marine Alkaloids Discorhabdins in Merkel Cell Carcinoma Cells

A library of naturally occurring and semi-synthetic discorhabdins was assessed for their effects on Merkel cell carcinoma (MCC) cell viability. The set included five new natural products and semi-synthetic compounds whose structures were elucidated with NMR, HRMS, and ECD techniques. Several discorhabdins averaged sub-micromolar potency against the MCC cell lines tested and most of the active compounds showed selectivity towards virus-positive MCC cell lines. An investigation of structure-activity relationships resulted in an expanded understanding of the crucial structural features of the discorhabdin scaffold. Mechanistic cell death assays suggested that discorhabdins, unlike many other MCC-active small molecules, do not induce apoptosis, as shown by the lack of caspase activation, annexin V staining, and response to caspase inhibition. Similarly, discorhabdin treatment failed to increase MCC intracellular calcium and ROS levels. In contrast, the rapid loss of cellular reducing potential and mitochondrial membrane potential suggested that discorhabdins induce mitochondrial dysfunction leading to non-apoptotic cell death.

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.

Metal-free dearomatization reactions of naphthol-ynamides for the divergent and enantioselective synthesis of azaspirocycles

An efficient Brønsted acid (BA) catalyzed intramolecular dearomatization cyclization of naphthol-ynamides has been developed, enabling the practical and divergent synthesis of two azaspirocycles in high yields.

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.

Design, synthesis and evaluation of dihydropyranoindole derivatives as potential cholinesterase inhibitors against Alzheimer's disease

A series of novel dihydropyranoindole derivatives containing sulphonamide group were designed, synthesized and evaluated for in-vitro anti-cholinesterase activity. The result showed that all the compounds exhibited potent acetylcholinesterase (AChE) activity (IC₅₀ = 0.41-8.79 µM) while demonstrated moderate to good activity for butyrylcholinesterase (BuChE) (IC₅₀ = 1.17-30.17 µM). The tested compounds exhibited selectivity towards AChE over BuChE. Compound 5o was most potent towards both AChE (IC₅₀ = 0.41 µM) and BuChE (IC₅₀ = 1.17 µM) when compared to standard galantamine and rivastigmine. Enzyme kinetics and molecular docking studies revealed that compound 5o shows mixed type inhibition and binds to peripheral anionic site (PAS) and the catalytic sites (CAS) of both the enzymes. Furthermore, cell viability studies were also performed against N2a cells along with neuroprotection studies against H₂O₂ in the same cell line. Antioxidant studies using DPPH radical and H₂O₂ were also performed which revealed that all compounds possessed some antioxidant activity. Also, DNA damage protection assay for compound 5o was performed implying that compound 5o was protective in nature. ADME studies were also performed which demonstrated good pharmacokinetics. These findings indicated that dihydropyranoindole derivatives could be possible drug lead in the search for new multifunctional AD drugs.

Exploration of the Electrophilic Reactivity of the Cytotoxic Marine Alkaloid Discorhabdin C and Subsequent Discovery of a New Dimeric C-1/N-13-Linked Discorhabdin Natural Product

The cytotoxic marine natural product discorhabdin C contains a 2,6-dibromo-cyclohexa-2,5-diene moiety, previously proposed to be a critical feature required for biological activity. We have determined that the dienone-ring of discorhabdin C is indeed electrophilic, reacting with thiol and amine nucleophiles, affording debrominated adducts. In the case of reaction with 1-aminopentane the product contains an unusual C-2/N-18 ring closed, double-hydrate moiety. This electrophilic reactivity also extends to proteins, with lysozyme-discorhabdin C adducts being detected by ESI mass spectrometry. These results prompted further examination of an extract of discorhabdin C-producing sponge, Latrunculia (Latrunculia) trivetricillata, leading to the isolation and characterisation of a new example of a C-1/N-13 linked discorhabdin dimer that shared structural similarities with the 1-aminopentane-discorhabdin C adduct. To definitively assess the influence of the dienone moiety of discorhabdin C on cytotoxicity, a semi-synthetic hydrogenation derivative was prepared, affording a didebrominated ring-closed carbinolamine that was essentially devoid of tumour cell line cytotoxicity. Antiparasitic activity was assessed for a set of 14 discorhabdin alkaloids composed of natural products and semi-synthetic derivatives. Three compounds, (-)-discorhabdin L, a dimer of discorhabdin B and the discorhabdin C hydrogenation carbinolamine, exhibited pronounced activity towards Plasmodium falciparum K1 (IC₅₀ 30–90 nM) with acceptable to excellent selectivity (selectivity index 19–510) versus a non-malignant cell line.

5,8-Quinolinedione Scaffold as a Promising Moiety of Bioactive Agents

Natural 5,8-quinolinedione antibiotics exhibit a broad spectrum of activities including anticancer, antibacterial, antifungal, and antimalarial activities. The structure-activity research showed that the 5,8-quinolinedione scaffold is responsible for its biological effect. The subject of this review report is a presentation of the pharmacological activity of synthetic 5,8-quinolinedione compounds containing different groups at C-6 and/or C-7 positions. The relationship between the activity and the mechanism of action is included if these data have been included in the original literature. The review mostly covers the period between 2000 and 2019. Previously published literature data were used to present historical points.

New Discorhabdin Alkaloids from the Antarctic Deep-Sea Sponge Latrunculia biformis

The sponge genus Latrunculia is a prolific source of discorhabdin type pyrroloiminoquinone alkaloids. In the continuation of our research interest into this genus, we studied the Antarctic deep-sea sponge Latrunculia biformis that showed potent in vitro anticancer activity. A targeted isolation process guided by bioactivity and molecular networking-based metabolomics yielded three known discorhabdins, (-)-discorhabdin L (1), (+)-discorhabdin A (2), (+)-discorhabdin Q (3), and three new discorhabdin analogs (-)-2-bromo-discorhabdin D (4), (-)-1-acetyl-discorhabdin L (5), and (+)-1-octacosatrienoyl-discorhabdin L (6) from the MeOH-soluble portion of the organic extract. The chemical structures of 1-6 were elucidated by extensive NMR, HR-ESIMS, FT-IR, [α]_D, and ECD (Electronic Circular Dichroism) spectroscopy analyses. Compounds 1, 5, and 6 showed promising anticancer activity with IC₅₀ values of 0.94, 2.71, and 34.0 µM, respectively. Compounds 1-6 and the enantiomer of 1 ((+)-discorhabdin L, 1e) were docked to the active sites of two anticancer targets, topoisomerase I-II and indoleamine 2,3-dioxygenase (IDO1), to reveal, for the first time, the binding potential of discorhabdins to these proteins. Compounds 5 and 6 are the first discorhabdin analogs with an ester function at C-1 and 6 is the first discorhabdin bearing a long-chain fatty acid at this position. This study confirms Latrunculia sponges to be excellent sources of chemically diverse discorhabdin alkaloids.

A Convenient C-H Functionalization Platform for Pyrroloiminoquinone Alkaloid Synthesis

Pyrroloiminoquinone alkaloids represent a structurally intriguing class of natural products that display an array of useful biological properties. Here, we present a versatile and scalable platform for the synthesis of this diverse family - and in particular the antitumor discorhabdins - built upon sequential selective C-H functionalization of tryptamine. The utility of this strategy is showcased through short formal syntheses of damirones A-C, makaluvamines D and I, and discorhadbin E. Additionally, we describe efforts to develop the first catalytic asymmetric entry to the discorhabdin subclass.

Enantioselective Synthesis of Indolines, Benzodihydrothiophenes, and Indanes by C-H Insertion of Donor/Donor Carbenes

We employ a single catalyst/oxidant system to enable the asymmetric syntheses of indolines, benzodihydrothiophenes, and indanes by C-H insertion of donor/donor carbenes. This methodology enables the rapid construction of densely substituted five-membered rings that form the core of many drug targets and natural products. Furthermore, oxidation of hydrazones to the corresponding diazo compounds proceeds in situ, enabling a relatively facile one- or two-pot protocol in which isolation of potentially explosive diazo alkanes is avoided. Regioselectivity studies were performed to determine the impact of sterics and electronics in donor/donor metal carbene C-H insertions to form indolines. This methodology was applied to a variety of substrates in high yield, diastereomeric, and enantiomeric ratios and to the synthesis of a patented indane estrogen receptor agonist with anti-cancer activity.

PIFA-BF3·OEt2 mediated intramolecular regioselective domino cyclization of ynamides: A novel method for the synthesis of tetrahydroisoquinoline-oxazol-2(3H)-ones

The transition metal-free intramolecular regioselective domino cyclization of N-Boc protected ynamides has been developed to provide the corresponding tetrahydroisoquinoline-oxazo-2(3H)-ones in moderate to good yields.

Screening and Biological Effects of Marine Pyrroloiminoquinone Alkaloids: Potential Inhibitors of the HIF-1α/p300 Interaction

Inhibition of the hypoxia-inducible factor 1α (HIF-1α) pathway by disrupting its association with the transcriptional coactivator p300 inhibits angiogenesis and tumor development. Development of HIF-1α/p300 inhibitors has been hampered by preclinical toxicity; therefore, we aimed to identify novel HIF-1α/p300 inhibitors. Using a cell-free assay designed to test compounds that block HIF-1α/p300 binding, 170 298 crude natural product extracts and prefractionated samples were screened, identifying 25 active extracts. One of these extracts, originating from the marine sponge Latrunculia sp., afforded six pyrroloiminoquinone alkaloids that were identified as positive hits (IC50 values: 1-35 μM). Luciferase assays confirmed inhibition of HIF-1α transcriptional activity by discorhabdin B (1) and its dimer (2), 3-dihydrodiscorhabdin C (3), makaluvamine F (5), discorhabdin H (8), discorhabdin L (9), and discorhabdin W (11) in HCT 116 colon cancer cells (0.1-10 μM, p < 0.05). Except for 11, all of these compounds also reduced HIF-1α transcriptional activity in LNCaP prostate cancer cells (0.1-10 μM, p < 0.05). These effects occurred at noncytotoxic concentrations (<50% cell death) under hypoxic conditions. At the downstream HIF-1α target level, compound 8 (0.5 μM) significantly decreased VEGF secretion in LNCaP cells (p < 0.05). In COLO 205 colon cancer cells no activity was shown in the luciferase or cytotoxicity assays. Pyrroloiminoquinone alkaloids are a novel class of HIF-1α inhibitors, which interrupt the protein-protein interaction between HIF-1α and p300 and consequently reduce HIF-related transcription.

SAR of Sponge-Inspired Hemibastadin Congeners Inhibiting Blue Mussel PhenolOxidase

Hemibastadin derivatives, including the synthetically-derived 5,5′-dibromohemibastadin-1 (DBHB), are potent inhibitors of blue mussel phenoloxidase (PO), which is a key enzyme involved in the firm attachment of this invertebrate to substrates and, thus, a promising molecular target for anti-fouling research. For a systematic investigation of the enzyme inhibitory activity of hemibastadin derivatives, we have synthesized nine new congeners, which feature structural variations of the DBHB core structure. These structural modifications include, e.g., different halogen substituents present at the aromatic rings, different amine moieties linked to the (E)-2-(hydroxyimino)-3-(4-hydroxyphenyl)propionic acid, the presence of free vs. substituted aromatic hydroxyl groups and a free vs. methylated oxime group. All compounds were tested for their inhibitory activity towards the target enzyme in vitro, and IC₅₀ values were calculated. Derivatives, which structurally closely resemble sponge-derived hemibastadins, revealed superior enzyme inhibitory properties vs. congeners featuring structural moieties that are absent in the respective natural products. This study suggests that natural selection has yielded structurally-optimized antifouling compounds.

Design, synthesis and biological evaluation of ezrin inhibitors targeting metastatic osteosarcoma

Respiratory failure due to pulmonary metastasis is the major cause of death for patients with osteosarcoma. However, the molecular basis for metastasis of osteosarcoma is poorly understood. Recently, ezrin, a member of the ERM family of proteins, has been associated with osteosarcoma metastasis to the lungs. The small molecule NSC 668394 was identified to bind to ezrin, inhibit in vitro and in vivo cell migration, invasion, and metastatic colony survival. Reported herein are the design and synthesis of analogues of NSC 668394, and subsequent functional ezrin inhibition studies. The binding affinity was characterized by surface plasmon resonance technique. Cell migration and invasion activity was determined by electrical cell impedance methodology. Optimization of a series of heterocyclic-dione analogues led to the discovery of compounds 21k and 21m as potential novel antimetastatic agents.

Development and application of new oxidation systems utilizing oxometalate catalysts

This review describes our recent efforts in the development and application of new oxidation systems utilizing oxometalate catalysts. The novel use of heteropoly acid (HPA), an acidic oxometalate catalyst, in hypervalent iodine-mediated oxidations provided an effective strategy to generate cation radical species that enables a variety of direct C-H functionalizations of aromatic compounds. This strategy brought a facile biaryl synthesis and new spirodienone syntheses in aromatic oxidation chemistry. Moreover, this strategy opens up a facile synthetic route to morphinandienone alkaloids. On the other hand, the use of oxometalate catalyst together with poly(N-isopropylacrylamide) (PNIPAAm)-based polymer in the development of new solid-phase catalyst provided a novel reaction system in water. Due to the characteristic temperature-responsive intelligence of PNIPAAm, this reaction system brought a remarkable acceleration of the reactivity and ease of catalyst recovering in catalytic oxidation that uses hydrogen peroxide or oxygen gas (O<inf>2</inf>) as primary oxidant. In addition, the recovered solid-phase catalyst could be used for consecutive reactions without any significant loss of its catalytic efficacy.

Investigation of the electrophilic reactivity of the cytotoxic marine alkaloid discorhabdin B

The mechanisms of action of the cytotoxic marine pyrroloiminoquinone alkaloids the discorhabdins are unknown. We have determined that discorhabdin B acts as an electrophile towards biomimetic thiol nucleophiles leading to debrominated adducts. In contrast, less potent cytotoxins discorhabdins D and Q failed to react, supporting an SAR model of cytotoxicity requiring an orchestrated combination of an electrophilic Δ(1) carbon centre and a nucleophilic N-18 amine for potent activity. The stereospecific nature of nucleophile trapping exhibited by both enantiomers of discorhabdin B implies the biogenesis of ovothiol A substituted discorhabdins H, H(2), K and K(2) need not be mediated by enzymatic processes.