Total Synthesis of Lamellarin D Trimethyl Ether, Lamellarin D, and Lamellarin H

Single-Pot Reaction of Novel Fused Indolizine Derivatives

An unprecedented fused hexacyclic indolizine derivative generated by a single-pot reaction has been developed. This model overcomes the use of catalysts, inefficient atom economy, low yields, and limitations of lengthy steps. Density functional theory calculations reveal the reaction mechanism in which the oxygen molecule plays a crucial role in trapping the penultimate zwitterionic or biradical intermediate to generate the observed products.

An Overview of the Synthesis of 3,4-Fused Pyrrolocoumarins of Biological Interest

3,4-Fused pyrrolocoumarins, synthetically prepared or naturally occurring, possess interesting biological properties. In this review, the synthetic strategies for the synthesis of the title compounds are presented along with their biological activities. Two routes are followed for that synthesis. In one, the pyrrole ring is formed from coumarin derivatives, such as aminocoumarins or other coumarins. In the other approach, the pyranone moiety is built from an existing pyrrole derivative or through the simultaneous formation of coumarin and pyrrole frameworks. The above syntheses are achieved via 1,3-dipolar cycloaddition reactions, Michael reaction, aza-Claisen rearrangement reactions, multi-component reactions (MCR), as well as metal-catalyzed reactions. Pyrrolocoumarins present cytotoxic, antifungal, antibacterial, α-glucosidase inhibition, antioxidant, lipoxygenase (LOX) inhibition, and fluorescent activities, as well as benzodiazepine receptor ability.

Progress on synthesis and structure-activity relationships of lamellarins over the past decade

Lamellarins are polyaromatic alkaloids isolated from marine organisms, including mollusks, tunicates, and sponges. Currently, over 60 structurally distinct natural lamellarins have been reported, and most of them exhibit promising biological activities, such as topoisomerase inhibition, mitochondrial function inhibition, multidrug resistance reversing, and anti-HIV activity. There has also been a significant progress on the synthetic study of lamellarins which has been regularly updated by numerous medicinal chemists as well. This review provides a detailed summary of the synthesis, pharmacology, and structural modification of lamellarins over the past decades.

Pyrrolo[2,1-a]isoquinoline scaffolds for developing anti-cancer agents

Fused pyrrolo[2,1-a]isoquinolines have emerged as compelling molecules with remarkably potent cytotoxic activity and topoisomerase inhibitors. This comprehensive review delves into the intricate world of this family of compounds, analyzing the natural marine lamellarins known for their diverse and complex chemical structures, exploring structure-activity relationships (SARs), and highlighting their remarkable versatility. The review emphasizes their fundamental role as topoisomerase inhibitors and cytotoxic agents, as well as some crucial aspects of the chemistry of pyrrolo[2,1-a]isoquinolines, exploring synthetic strategies in total synthesis and molecular diversification trends, highlighting their importance in the field of medicinal chemistry and beyond.

Synthesis of Pyrrolo[2,1-a]isoquinolines through Cu-Catalyzed Condensation/Addition/Oxidation/Cyclization Cascade

We have developed a copper-catalyzed synthesis of pyrrolo[2,1-a]isoquinolines with terminal alkynes, aldehydes, and tetrahydroisoquinolines. A variety of pyrrolo[2,1-a]isoquinolines have been prepared in 17-69% yield via a condensation/Mannich-type addition/oxidation/cyclization cascade sequence. Modifications through simple chemical transformations provided highly functionalized molecules containing a privileged framework.

Weakly Coordinating tert-Amide-Assisted Ru(II)-Catalyzed Synthesis of Azacoumestans via Migratory Insertion of Quinoid Carbene: Application in the Total Synthesis of Isolamellarins

A weakly coordinating tert-amide-directed straightforward method was developed for the synthesis of azacoumestans using the corresponding azaheterocycle derivatives and diazonaphthoquinones under cheap Ru(II)-catalyzed conditions. The reaction proceeds via migratory insertion of quinoid carbene and subsequent Brønstead acid-mediated cyclization. The optimized C2-selective method offered a wide scope of important azaheterocycles. Bioactive natural products like isolamellarins A and B were synthesized via the developed protocol. Preliminary mechanistic studies highlighted the probable mechanistic pathway.

A Two-Step Approach to a Hexacyclic Lamellarin Core via 1,3-Dipolar Cycloaddition of Isoquinolinium Ylides to Nitrostilbenes

The 1,3-dipolar cycloaddition reaction of isoquinolinium ylides to nitrostilbenes provides an approach to 1,2-diarylpyrrolo[2,1-a]isoquinolinium-3-carboxylates and then to a complete hexacyclic lamellarin core.

Recent progress in the synthesis of pyrrolo[2,1-a]isoquinolines

Pyrrolo[2,1-a]isoquinolines occur frequently in a large number of bioactive natural products and pharmaceutically important molecules. The synthesis of pyrrolo[2,1-a]isoquinoline derivatives is an easy and useful way to produce artificial molecules with potential applications. A huge number of excellent methodologies for constructing pyrrolo[2,1-a]isoquinolines have been reported in the last decade. This review summarizes the recent advances in this research field covering from 2011 to 2021.

Anticancer Activities of Marine-Derived Phenolic Compounds and Their Derivatives

Since the middle of the last century, marine organisms have been identified as producers of chemically and biologically diverse secondary metabolites which have exerted various biological activities including anticancer, anti-inflammatory, antioxidant, antimicrobial, antifouling and others. This review primarily focuses on the marine phenolic compounds and their derivatives with potent anticancer activity, isolated and/or modified in the last decade. Reports on the elucidation of their structures as well as biosynthetic studies and total synthesis are also covered. Presented phenolic compounds inhibited cancer cells proliferation or migration, at sub-micromolar or nanomolar concentrations (lamellarins D (37), M (38), K (39), aspergiolide B (41), fradimycin B (62), makulavamine J (66), mayamycin (69), N-acetyl-N-demethylmayamycin (70) or norhierridin B (75)). In addition, they exhibited anticancer properties by a diverse biological mechanism including induction of apoptosis or inhibition of cell migration and invasive potential. Finally, phlorotannins 1–7 and bromophenols 12–29 represent the most researched phenolic compounds, of which the former are recognized as protective agents against UVB or gamma radiation-induced skin damages. Finally, phenolic metabolites were assorted into six main classes: phlorotannins, bromophenols, flavonoids, coumarins, terpenophenolics, quinones and hydroquinones. The derivatives that could not be attributed to any of the above-mentioned classes were grouped in a separate class named miscellaneous compounds.

Total Synthesis of Lamellarins U and A3 by Interrupting Halogen Dance

A total synthesis of lamellarins U and A3 is described. The synthesis features the interruption of a halogen dance reaction of a metalated α,β-dibromopyrrole. The pyrrolylmagnesium reagent, generated by deprotonative metalation by using (TMP)MgCl·LiCl (TMP = 2,2,6,6-tetramethylpiperidide) as the base, was transmetalated to the corresponding organozinc species without causing the halogen dance reaction, which underwent a Negishi coupling to incorporate an aryl group onto the pyrrole ring. The arylated α,β-dibromopyrrole was then converted into lamellarins U and A3 through an α-selective halogen–magnesium exchange followed by carboxylation and subsequent palladium-mediated cyclization. The late-stage introduction of another aryl group was performed using a Kosugi–Migita–Stille coupling to provide lamellarins U and A3.

Base-promoted [4 + 2] annulation of pyrrole-2-carbaldehyde derivatives with β,γ-unsaturated α-ketoesters: syntheses of 5,6-dihydroindolizines

A base-promoted [4 + 2] annulation of pyrrole-2-carbaldehyde derivatives with β,γ-unsaturated α-ketoesters for the syntheses of multisubstituted 5,6-dihydroindolizines was developed.

Iodine-Mediated Domino Cyclization for One-Pot Synthesis of Indolizine-Fused Chromones via Metal-Free sp3 C-H Functionalization

An efficient method for the synthesis of new indolizine-fused chromones has been accomplished from ethyl (E)-3-(2-acetylphenoxy)acrylates and pyridines in a "one-pot" manner. Facile operation in open-air, metal-free, and mild conditions renders this protocol particularly practical and attractive. Moreover, this method can simultaneously construct two molecular fragments of chromone and indolizine. Scale-up experiment and the construction of natural products further prove the practicability of this strategy.

An Expeditious Modular Hybrid Strategy for the Diversity-Oriented Synthesis of Lamellarins/Azalamellarins with Anticancer Cytotoxicity

A modular hybrid strategy has been developed for the diversity-oriented synthesis of lamellarins/azalamellarins. The common pentacyclic pyrrolodihydroisoquinoline lactone/lactam core was formed via the Michael addition/ring closure (Mi-RC) and the copper(I) thiophene-2-carboxylate (CuTC)-catalyzed C-O/C-N Ullmann coupling. Subsequent direct functionalization at C1, DDQ-mediated C5═C6 oxidation, and global deprotection of all benzyl-type O- and N-protecting groups furnished the desired lamellarins/azalamellarins. The late-stage functionalization at C1 provided a handle to accommodate a wider scope of functional groups as they need to tolerate only the DDQ oxidation and global deprotection. Moreover, with the C1-H pyrrole as the late-stage common intermediate, it was also possible to divergently exploit not only its nucleophilic nature to react with some electrophilic species but also some transition-metal-catalyzed cross-coupling reactions (via the intermediacy of the C1-iodopyrrole) to incorporate diversity at this position. Overall, this strategy simplifies the preparation of lamellarins/azalamellarins; including the Mi-RC, these C1-structurally diverse analogues could be prepared efficiently in 6-7 steps from the easily accessed 1-acetoxymethyldihydroisoquinoline and β-nitrocinnamate. Some selected azalamellarins were evaluated for their inhibitory effect against HeLa cervical cancer cells. An acute induction of intrinsic apoptosis was detected and may lead to growth suppression of or cytotoxicity against cancer cells.

Total Synthesis of Lamellarins G, J, L, and Z Using One-Pot Halogen Dance/Negishi Coupling

A bottom-up synthesis of lamellarins G, J, L, and Z was achieved via one-pot halogen dance/Negishi coupling of a lithiated dibromopyrrole derivative. The easily accessible dibromopyrrole bearing an ester moiety underwent halogen dance smoothly at -78 °C within 10 min. The resultant α-pyrrolyllithium was transmetalated to the corresponding organozinc species, which was then coupled with an aryl iodide in the presence of catalytic palladium to provide the fully substituted pyrrole. Subsequent halogen-lithium exchange was performed to incorporate a boronate group exclusively at the β position proximal to the ester moiety. This synthetic intermediate allowed stepwise diarylation for the total synthesis of lamellarins G, J, L, and Z.

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.

Marine Heterocyclic Compounds That Modulate Intracellular Calcium Signals: Chemistry and Synthesis Approaches

Intracellular Ca²⁺ plays a pivotal role in the control of a large series of cell functions in all types of cells, from neurotransmitter release and muscle contraction to gene expression, cell proliferation and cell death. Ca²⁺ is transported through specific channels and transporters in the plasma membrane and subcellular organelles such as the endoplasmic reticulum and mitochondria. Therefore, dysregulation of intracellular Ca²⁺ homeostasis may lead to cell dysfunction and disease. Accordingly, chemical compounds from natural origin and/or synthesis targeting directly or indirectly these channels and proteins may be of interest for the treatment of cell dysfunction and disease. In this review, we show an overview of a group of marine drugs that, from the structural point of view, contain one or various heterocyclic units in their core structure, and from the biological side, they have a direct influence on the transport of calcium in the cell. The marine compounds covered in this review are divided into three groups, which correspond with their direct biological activity, such as compounds with a direct influence in the calcium channel, compounds with a direct effect on the cytoskeleton and drugs with an effect on cancer cell proliferation. For each target, we describe its bioactive properties and synthetic approaches. The wide variety of chemical structures compiled in this review and their significant medical properties may attract the attention of many different researchers.

Convergent Total Synthesis of Lamellarins and Their Congeners

A convergent total synthesis of lamellarins S and Z is described. The synthesis features a halogen dance of an easily accessible α,β-dibromopyrrole promoted by an ester moiety. The resultant β,β'-dibromopyrrole undergoes a ligand-controlled Suzuki-Miyaura coupling to provide a range of diarylated pyrrole derivatives. The established synthetic method was also applicable to the synthesis of ningalin B and lukianols A and B.

Facile Synthesis of NH-Free 5-(Hetero)Aryl-Pyrrole-2-Carboxylates by Catalytic C-H Borylation and Suzuki Coupling

A convenient two-step preparation of NH-free 5-aryl-pyrrole-2-carboxylates is described. The synthetic route consists of catalytic borylation of commercially available pyrrole-2-carboxylate ester followed by Suzuki coupling without going through pyrrole N–H protection and deprotection steps. The resulting 5-aryl substituted pyrrole-2-carboxylates were synthesized in good- to excellent yields. This synthetic route can tolerate a variety of functional groups including those with acidic protons on the aryl bromide coupling partner. This methodology is also applicable for cross-coupling with heteroaryl bromides to yield pyrrole-thiophene, pyrrole-pyridine, and 2,3’-bi-pyrrole based bi-heteroaryls.

Lamellarin alkaloids: Isolation, synthesis, and biological activity

Lamellarins are marine alkaloids containing fused 14-phenyl-6H-[1]benzopyrano[4',3':4,5]pyrrolo[2,1-a]isoquinoline or non-fused 3,4-diarylpyrrole-2-carboxylate ring systems. To date, more than 50 lamellarins have been isolated from a variety of marine organisms, such as mollusks, tunicates, and sponges. Many of them, especially fused type I lamellarins, exhibit impressive biological activity, such as potent cytotoxicity, topoisomerase I inhibition, protein kinases inhibition, and anti-HIV-1 activity. Due to their useful biological activity and limited availability from natural sources, a number of synthetic methods have been developed. In this chapter, we present an updated and comprehensive review on lamellarin alkaloids summarizing their isolation, synthesis, and biological activity.

Demethylative Lactonization Provides a Shortcut to High-Yielding Syntheses of Lamellarins

Modular gram-scale syntheses of the trimethyl ethers of lamellarins G (6) and D (7) were achieved from readily accessible precursors in the highest overall yields reported to date (6, six steps, 82%; 7, seven steps, 86%). A novel demethylative lactonization between an aryl methyl ether and a neighboring carboxylic acid was developed for creating the chromenone unit of the targets to avoid the need for additional protection and deprotection steps. The central pyrrole core was constructed in a late-stage [4 + 1] condensation between ethyl bromoacetate and an enaminone possessing the remaining components of the lamellarin skeleton. Exhaustive demethylation of both permethyl ethers 6 and 7 gave the polyphenolic natural lamellarins A4 (3) and H (5), respectively.

Scalable Total Syntheses of Some Natural and Unnatural Lamellarins: Application of a One-Pot Domino Process for Regioselective Access to the Central 1,2,4-Trisubstituted Pyrrole Core

Short and scalable total syntheses of lamellarin G trimethyl ether, lamellarin D trimethyl ether, lamellarin H, lamellarin η, dihydrolamellarin η, and lamellarin U have been realized in four to six linear steps with an overall yield of ≤22%. Highlights of the synthesis include single-step access to the central 1,2,4-trisubstituted pyrrole core in a highly regioselective manner via a one-pot [3+2] cycloaddition/elimination/aromatization sequence-based domino process. Subsequent, palladium-mediated double C-H oxidative coupling in a single-pot operation provides access to the pentacyclic coumarin-fused pyrrolo-dihydroisoquinoline core present in lamellarins.

Synthesis of lamellarin alkaloids using orthoester-masked α-keto acids

Enolate arylation of a protected pyruvate is used as a key step in the short and efficient syntheses of the lamellarins.

Pyruvic acid and other α-keto acids are frequently encountered as intermediates in metabolic pathways, yet their application in total synthesis has met with limited success. In this work, we present a bioinspired strategy that utilizes highly functionalized OBO (oxabicyclo[2.2.2]octyl) orthoester masked α-ketoacids as key intermediates for the construction of both type I and II lamellarin alkaloids. Lamellarin D was synthesized, via a key 1,4-dicarbonyl, in 7 steps and 22% yield from pyruvic acid. Key steps in the synthesis involve one-pot double enolate functionalisation of 1 followed by double annulation to form the target pyrrole/N-vinyl pyrrole core and late-stage direct C–H arylation. Lastly, a novel OBO-masked β-cyano ketone, synthesized from 1, proved to be a valuable intermediate for construction of the type II lamellarin core via HBr-mediated cyclisation. In this way, lamellarin Q was synthesized in 7 steps and 20% yield from pyruvic acid.

Ascidian Toxins with Potential for Drug Development

Ascidians (tunicates) are invertebrate chordates, and prolific producers of a wide variety of biologically active secondary metabolites from cyclic peptides to aromatic alkaloids. Several of these compounds have properties which make them candidates for potential new drugs to treat diseases such as cancer. Many of these natural products are not produced by the ascidians themselves, rather by their associated symbionts. This review will focus mainly on the mechanism of action of important classes of cytotoxic molecules isolated from ascidians. These toxins affect DNA transcription, protein translation, drug efflux pumps, signaling pathways and the cytoskeleton. Two ascidian compounds have already found applications in the treatment of cancer and others are being investigated for their potential in cancer, neurodegenerative and other diseases.

Synthesis of Asthma Drug Zafirlukast (Accolate) Using Intramolecular Oxidative Coupling via sp3 C-H Bond Activation

The FDA-approved drug for the treatment of asthma, zafirlukast, is synthesized engaging multiple catalytic reactions including a new method for the construction of 3-aroylindoles via oxidative cyclization. The highlights include transition-metal and peroxide free C-H bond activation using the stoichiometric amount of sodium persulfate as an oxidizing agent in the construction of 3-aroylindole, avoiding transition metal, with over 28% overall yield. The complete process has a turnaround time of 28 h to get the target molecule starting from substituted aniline, with practically no protecting groups.

Gold-silver catalyzed straightforward one pot synthesis of pyrano[3,4-b]pyrrol-7(1H)-ones

Pyrano[3,4-b]pyrrol-7(1H)-one is a bicyclic structure that is rarely described in the literature but is found in numerous polycyclic natural products as lamellarins. This work presents a one-pot synthesis of pyrano[3,4-b]pyrrol-7(1H)-one substituted in the 2- and 5-position. The reaction proceeds via a one-pot two step 5-endo-dig and 6-endo-dig cyclization catalyzed by a cationic gold complex with high regioselectivity.

Construction of Pentacyclic Lamellarin Skeleton via Grob Reaction: Application to Total Synthesis of Lamellarins H and D

An efficient construction of phenyl-substituted coumarin-pyrrole-isoquinoline-fused pentacycle via base-promoted Grob-type coupling of 3-nitrocoumarin and papaverine in a sealed tube is reported. This reaction is further applied to the total synthesis of lamellarin H in three linear steps and lamellarin D in eight linear steps with overall yields of 31% and 14%, respectively.