Indolo[3,2-a]carbazoles from a deep-water sponge of the genus Asteropus

π-Extension of Indoles Using Acrolein Linker: Synthesis of Indolo[3,2-a]carbazole-6-carbaldehydes and Racemosin B

A simple method for the synthesis of indolo[3,2-a]carbazole-6-carbaldehydes by the π-extension of indoles with acrolein is reported. The scope of the method is demonstrated with 33 examples. The products exhibit high activities toward advanced synthesis and are proved to be able to produce valuable chemicals, such as natural products, dyes, and organic fluorescent materials. In addition, the alkaloid racemosin B can be prepared by this method in two steps in ∼50% overall yield.

Catecholamine Derivatives: Natural Occurrence, Structural Diversity, and Biological Activity

Catecholamines (CAs) are aromatic amines containing a 3,4-dihydroxyphenyl nucleus and an amine side chain. Representative CAs included the endogenous neurotransmitters epinephrine, norepinephrine, and dopamine. CAs and their derivatives are good resources for the development of sympathomimetic or central nervous system drugs, while they also provide ligands important for G-protein coupled receptor (GPCR) research. CAs are of broad interest in the fields of chemical, biological, medical, and material sciences due to their high adhesive capacities, chemical reactivities, metal-chelating abilities, redox activities, excellent biocompatibilities, and ease of degradability. Herein, we summarize CAs derivatives isolated and identified from microorganisms, plants, insects, and marine invertebrates in recent decades, alongside their wide range of reported biological activities. The aim of this review is to provide an overview of the structural and biological diversities of CAs, the regularity of their natural occurrences, and insights toward future research and development pertinent to this important class of naturally occurring compounds.

Trends in carbazole synthesis - an update (2013-2023)

The interest of scientists in the carbazole core has risen steadily over the last 30 years, particularly over the last decade given its presence in several active pharmaceutical ingredients, functional materials and a wide range of biologically active natural products. The continuous development of more efficient, more (regio-)selective and "greener" methodologies to access the carbazole core is thus imperative. This review compares and evaluates synthetic strategies towards the carbazole core that have been reported since 2013, with a focus on their applicability towards the total synthesis of carbazole-containing natural products.

Organocatalytic atroposelective synthesis of axially chiral N,N'-pyrrolylindoles via de novo indole formation

The first organocatalytic atroposelective synthesis of axially chiral N,N'-pyrrolylindoles based on o-alkynylanilines was successfully established via de novo indole formation catalyzed by chiral phosphoric acid (CPA). This new synthetic strategy introduced CPA-catalyzed asymmetric 5-endo-dig cyclization of new well-designed o-alkynylanilines containing a pyrrolyl unit, resulting in a wide range of axially chiral N,N'-pyrrolylindoles in high yields with exclusive regioselectivity and excellent enantioselectivity (up to 99% yield, >20 : 1 rr, 95 : 5 er). Considering the potential biological significance of N-N atropisomers, preliminary biological activity studies were performed and revealed that these structurally important N,N'-pyrrolylindoles had a low IC50 value with promising impressive cytotoxicity against several kinds of cancer cell lines. DFT studies reveal that the N-nucleophilic cyclization mediated by CPA is the rate- and stereo-determining step, in which ligand-substrate dispersion interactions facilitate the axial chirality of the target products.

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.

Good Practices in Sponge Natural Product Studies: Revising Vouchers with Isomalabaricane Triterpenes

Species misidentification in the field of natural products is an acknowledged problem. These errors are especially widespread in sponge studies, albeit rarely assessed and documented. As a case study, we aim to revisit reports of isomalabaricane triterpenes, isolated from four demosponge genera: Jaspis, Geodia, Stelletta and Rhabdastrella. From a total of 44 articles (1981–2022), 27 unique vouchers were listed, 21 of which were accessed and re-examined here: 11 (52.4%) of these were misidentified. Overall, 65.9% of the studies published an incorrect species name: previously identified Jaspis and Stelletta species were all in fact Rhabdastrella globostellata. We conclude that isomalabaricane triterpenes were isolated from only two Rhabdastrella species and possibly one Geodia species. In addition to shedding a new light on the distribution of isomalabaricane triterpenes, this study is an opportunity to highlight the crucial importance of vouchers in natural product studies. Doing so, we discuss the impact of species misidentification and poor accessibility of vouchers in the field of sponge natural products. We advocate for stricter voucher guidelines in natural product journals and propose a common protocol of good practice, in the hope of reducing misidentifications in sponge studies, ensure reproducibility of studies, and facilitate follow-up work on the original material.

Synthesis of indole derivatives as prevalent moieties present in selected alkaloids

Indoles are a significant heterocyclic system in natural products and drugs. They are important types of molecules and natural products and play a main role in cell biology. The application of indole derivatives as biologically active compounds for the treatment of cancer cells, microbes, and different types of disorders in the human body has attracted increasing attention in recent years. Indoles, both natural and synthetic, show various biologically vital properties. Owing to the importance of this significant ring system, the investigation of novel methods of synthesis have attracted the attention of the chemical community. In this review, we aim to highlight the construction of indoles as a moiety in selected alkaloids.

Recent progress on anti-Candida natural products

Candida is an intractable life-threatening pathogen. Candida infection is extremely difficult to eradicate, and thus is the major cause of morbidity and mortality in immunocompromised individuals. Morevover, the rapid spread of drug-resistant fungi has led to significant decreases in the therapeutic effects of clinical drugs. New anti-Candida agents are urgently needed to solve the complicated medical problem. Natural products with intricate structures have attracted great attention of researchers who make every endeavor to discover leading compounds for antifungal agents. Their novel mechanisms and diverse modes of action expand the variety of fungistatic agents and reduce the emergence of drug resistance. In recent decades, considerable effort has been devoted to finding unique antifungal agents from nature and revealing their unusual mechanisms, which results in important progress on the development of new antifungals, such as the novel cell wall inhibitors YW3548 and SCY-078 which are being tested in clinical trials. This review will present a brief summary on the landscape of anti-Candida natural products within the last decade. We will also discuss in-depth the research progress on diverse natural fungistatic agents along with their novel mechanisms.

NMR characterization of rearranged staurosporine aglycone analogues from the marine sponge Damiria sp

The indolocarbazole family of bisindole alkaloids is best known for the natural product staurosporine, a protein kinase C inhibitor that belongs to the indolo[2,3-a]carbazole structural class. A large number of other indolo[2,3-a]carbazoles have subsequently been isolated and identified, but other isomeric forms of indolocarbazole natural products have rarely been reported. An extract of the marine sponge Damiria sp., which represents an understudied genus, provided two novel alkaloids named damirines A (1) and B (2). Their structures were assigned by comprehensive NMR spectroscopic analyses, and for compound 2, this included application of the LR-HSQMBC pulse sequence, a long-range heteronuclear correlation experiment that has particular utility for defining proton-deficient scaffolds. The damirines represent a new hexacyclic carbon-nitrogen framework comprised of an indolo[3,2-a]carbazole fused with either an aminoimidazole or a imidazolone ring. Compound 1 showed selective cytotoxic properties toward six different cell lines in the NCI-60 cancer screen.

Revisiting Furodiindolines: One-Pot Synthesis of Furodiindolines Using Indole 2,3-Epoxide Surrogates and Their Synthetic Applications

Furodiindolines have emerged as versatile intermediates in various transformations, which are easily obtained from indole 2,3-epoxide surrogates via a one-pot procedure and allowed access to heterocyclic compounds with structural diversity and complexity. Four types of transformations from the furodiindolines have been achieved: (i) dehydrative rearrangement to afford 2,3'-biindoles, (ii) hydrolysis/cyclization to give 3,4-disubstituted quinolines, (iii) ring-opening/cyclization to give pyrroloindolines, and (iv) aminal cleavage to give a 3,3-disubstituted 2-oxindole.

Novel racemosin B derivatives as new therapeutic agents for aggressive breast cancer

Carbazole derivatives show anti-cancer activity and are of great interest for drug development. In this study, we synthesized and analyzed several new alkylamide derivatives of racemocin B, a natural indolo[3,2-a]carbazole molecule originally isolated from the green alga Caulerpa racemose. Several alkylamide derivatives were found to exhibit moderate to strong growth inhibition against human breast cancer cell lines. They induced G2/M cell cycle arrest and apoptosis in the aggressive triple-negative breast cancer cell line MDA-MB-231. Among these derivatives, compound 25 with the lowest IC₅₀ induced cell death by suppressing autophagy. This was accompanied by inhibition of autophagic flux and accumulation of autophagy protein 1 light chain 3, LC3II, and p62. The novel alkylamide derivative offers a potential new treatment for human breast cancer.

Organocatalytic Asymmetric Synthesis of Spiro-Bridged and Spiro-Fused Heterocyclic Compounds Containing Chromane, Indole, and Oxindole Moieties

Following the reactivity inversion strategy, two different two-step sequences were designed and successfully applied to the asymmetric synthesis of spiro-bridged and spiro-fused heterocyclic compounds, which combined chromane, indole, and oxindole, three potential pharmacophores, in one molecule. The power of these two organocatalytic pathways is underscored by mild reaction conditions and high efficiency in the production of synthetically challenging, but biologically important heterocyclic products, which could be transformed into more biologically interesting heterocyclic structures.

Chemistry and Properties of Indolocarbazoles

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

Metal-Free Oxidative Cross Coupling of Indoles with Electron-Rich (Hetero)arenes

A new method for the synthesis of bi-heteroaryls is reported, based on the umpolung of indoles with benziodoxol(on)e hypervalent iodine reagents (IndoleBX). The oxidative coupling of IndoleBX with an equimolar amount of electron-rich benzenes, indoles, pyrroles, and thiophenes proceeded under mild transition-metal-free conditions. Functionalized non-symmetrical bi-indolyl heterocycles were accessed efficiently. Introduction of a new type of C2-substituted indole benziodoxole reagents further allowed extending the scope of the reaction to NH unprotected and C3-alkylated indoles. The obtained bi-heterocycles are important building blocks in synthetic and medicinal chemistry, and could be easily transformed into more complex heterocyclic systems.

Facile synthesis of indolo[3,2-a]carbazoles via Pd-catalyzed twofold oxidative cyclization

A rapid and efficient route has been developed for the synthesis of 9-methoxycarbonylindolo[3,2-a]carbazole derivatives. The key steps in this approach involved an aromatic amination and an oxidative biaryl coupling. Via the present route, indolo[3,2-a]carbazole derivatives are available in 3-4 steps based on commercially available starting materials.

Double Palladium Catalyzed Reductive Cyclizations. Synthesis of 2,2'-, 2,3'-, and 3,3'-Bi-1H-indoles, Indolo[3,2-b]indoles, and Indolo[2,3-b]indoles

A palladium catalyzed, carbon monoxide mediated, double reductive cyclization of 1,4-, 1,3-, and 2,3-bis(2-nitroaryl)-1,3-butadienes to afford 2,2'-, 2,3'-, and 3,3'-biindoles, respectively, was developed. In contrast, reductive cyclizations of 1,2-bis(2-nitroaryl)ethenes were nonselective, affording mixtures of monocyclized indoles, indolo[3,2-b]indole, indolo[1,2-c]quinazolin-6(5H)-ones, and 5,11-dihydro-6H-indolo[3,2-c]quinolin-6-ones. Nonselective product formation was also observed from reductive cyclization of 1,1-bis(2-nitroaryl)ethenes, producing indolo[2,3-b]indoles and indolo[2,3-c]quinolin-6-ones. Carbon monoxide insertion to give the carbonyl containing products was the major or sole reaction path starting from 1,1- or 1,2-bis(2-nitroaryl)ethenes.

Characterization of New Ent-kaurane Diterpenoids of Yunnan Arabica Coffee Beans

Five new ent-kaurane diterpenoids, named mascaroside III-V (1-3), and 20-nor-cofaryloside I-II (4-5), together with seven known diterpenoids, were isolated from methanol extracts of the green coffee beans of Yunnan Arabica Coffee. Their chemical structures were elucidated by extensive spectroscopic analyses. Meanwhile, cytotoxicity assay against HL-60, A-549, SMMC-7721, MCF-7 and SW480 cell lines showed that they have not evident inhibition of cytotoxicity.

Marine natural products

This review covers the literature published in 2013 for marine natural products (MNPs), with 982 citations (644 for the period January to December 2013) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1163 for 2013), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Biological Activity of Recently Discovered Halogenated Marine Natural Products

This review presents the biological activity-antibacterial, antifungal, anti-parasitic, antiviral, antitumor, antiinflammatory, antioxidant, and enzymatic activity-of halogenated marine natural products discovered in the past five years. Newly discovered examples that do not report biological activity are not included.

Simple indole alkaloids and those with a nonrearranged monoterpenoid unit

This review summarizes the isolation, structure determination, total syntheses and biological activities of simple indole alkaloids and those with a nonrearranged monoterpenoid unit, with literature coverage from 2012 to 2013.

Characterization of diterpenoid glucosides in roasted puer coffee beans

Five new diterpenoid glucosides, named mascaroside I (1), mascaroside II (2), paniculoside VI (3), cofaryloside I (4), and villanovane I (5), along with seven known ent-kaurane diterpenoid glucosides (6-12) were isolated from acetone extracts of the roasted coffee beans of Coffea arabica var. yunnanensis. Their structures were established by extensive spectroscopic analysis including 1D and 2D NMR (HSQC, HMBC, COSY, and ROESY) and by comparison with published data. Cytotoxicities evaluation of the isolates showed that they were inactive against HL-60, SMMC-7721, A-549, MCF-7, and SW480 cells.