Chemical Diversity and Bioactivities of Monoterpene Indole Alkaloids (MIAs) from Six Apocynaceae Genera

Vincarostine A, a novel anti-malarial trimeric monoterpenoid indole alkaloid from Catharanthus roseus

A novel trimeric monoterpenoid indole alkaloid, vincarostine A (1) consisting of an aspidosperma-iboga-aspidosperma type skeleton, was isolated from the whole plant of Catharanthus roseus. The structure including absolute stereochemistry was elucidated on the basis of 2D NMR data and CD spectrum. Vincarostine A (1) showed anti-malarial activity.

Plant synthetic biology for human health: advances in producing medicines in heterologous expression systems

Plant synthetic biology has the capability to provide solutions to global challenges in the production and supply of medicines. Recent advances in 'omics' technologies have accelerated gene discoveries in medicinal plant research so that even multistep biosynthetic pathways for bioactive plant natural products with high structural complexity can be reconstituted in heterologous plant expression systems more rapidly. This review provides an overview of concept and strategies used to produce high-value plant natural products in heterologous plant systems and highlights recent successes in engineering the biosynthesis of conventional and new medicines in alternative plant hosts.

Modular Divergent Synthesis of Indole Alkaloid Derivatives by an Atypical Ugi Multicomponent Reaction

We present an Ugi multicomponent approach to explore the chemical space around Aspidosperma-type monoterpene indole alkaloids. By variation of the isocyanide and carboxylic acid inputs we demonstrate the rapid generation of molecular diversity and the possibility to introduce handles for further modification. The key Ugi three-component reaction showed full diastereoselectivity towards the cis-fused ring system, which can be rationalized by DFT calculations that moreover indicate that the reaction proceeds via a Passerini-type hydrogen bonding mechanism. Several post-Ugi modifications were also performed, including Pictet-Spengler cyclization to highly complex nonacyclic natural product hybrid scaffolds.

Vallesamidine and schizozygane alkaloids: rearranged monoterpene indole alkaloids and synthetic endeavours

Covering 1963 to 2023Monoterpene indole alkaloids are the main sub-family of indole alkaloids with fascinating structures, stereochemistry, and diverse bioactivities (e.g., anticancer, anti-malarial and anti-arrhythmic etc.). Vallesamidine alkaloids and structurally more complex schizozygane alkaloids are small groups of rearranged monoterpene indole alkaloids with a unique 2,2,3-trialkylated indoline scaffold, while schizozygane alkaloids can generate a further rearranged skeleton, isoschizozygane, possessing a tetra-substituted, bridged tetrahydroquinoline core. In this review, the origin and structural features of vallesamidine and schizozygane alkaloids are introduced, and a discussion on the relationship of these alkaloids with aspidosperma alkaloids and a structural rearrangement hypothesis based on published studies is followed. Moreover, uncommon skeletons and potential bioactivities, such as anti-malarial and anti-tumour activities, make such alkaloids important synthetic targets, attracting research groups globally to accomplish total synthesis, resulting in impressive works on novel total synthesis, formal synthesis, and construction of key intermediates. These synthetic endeavours are systematically reviewed and highlighted with key strategies and efficiencies, providing different viewpoints on molecular structures and promoting the extension of chemical space and mining of new active scaffolds.

Chemical composition and antioxidant activity of the Amazonian fruit Ambelania acida Aubl

Aqueous and hydroalcoholic extracts from the pulp of Ambelania acida Aubl. (Apocynaceae) fruits were subjected to analysis through UHPLC-HRMS and antioxidant potential using the TPC, DPPH, ABTS, FRAP, and ORAC assays. A putative identification of the compounds carried out by comparison of the fragmentation spectra revealed the predominance of the monoterpene indole alkaloids tabersonine, pseudocopsinine, ajmalicine, and strictosidine. Additionally, gallic acid, caffeic acid, citric acid, 3-O-p-coumaroylquinic acid, chlorogenic acid, catechin, ellagic acid, eschweilenol C (ellagic acid deoxyhexoside), and sucrose were identified. In face of the phenolic compounds observed, hydroalcoholic extract showed a higher antioxidant activity compared to the aqueous extract, observed at TPC (108.85 mg GAE/100g), FRAP (0.73 µmol Fe2SO4/g), DPPH (1221.76 µmol TE/g), ABTS (3460.00 µmol TE/g), and ORAC assays (120.47 µmol TE/g). These findings underscore the abundant presence of bioactive compounds, including phenolics and alkaloids, in an edible Amazonian fruit.

Chromosome-level Alstonia scholaris genome unveils evolutionary insights into biosynthesis of monoterpenoid indole alkaloids

Alstonia scholaris of the Apocynaceae family is a medicinal plant with a rich source of bioactive monoterpenoid indole alkaloids (MIAs), which possess anti-cancer activity like vinca alkaloids. To gain genomic insights into MIA biosynthesis, we assembled a high-quality chromosome-level genome for A. scholaris using nanopore and Hi-C data. The 444.95 Mb genome contained 35,488 protein-coding genes. A total of 20 chromosomes were assembled with a scaffold N50 of 21.75 Mb. The genome contained a cluster of strictosidine synthases and tryptophan decarboxylases with synteny to other species and a saccharide-terpene cluster involved in the monoterpenoid biosynthesis pathway of the MIA upstream pathway. The multi-omics data of A. scholaris provide a valuable resource for understanding the evolutionary origins of MIAs and for discovering biosynthetic pathways and synthetic biology efforts for producing pharmaceutically useful alkaloids.

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

A natural small molecule aspidosperma-type alkaloid, hecubine, as a new TREM2 activator for alleviating lipopolysaccharide-induced neuroinflammation in vitro and in vivo

Neuroinflammation and oxidative stress play a crucial role in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease. The triggering receptor expressed on myeloid cells 2 (TREM2), highly expressed by microglia in the central nervous system (CNS), can modulate neuroinflammatory responses. Currently, there are no approved drugs specifically targeting TREM2 for CNS diseases. Aspidosperma alkaloids have shown potential as anti-inflammatory and neuroprotective agents. This study aimed to elucidate the potential therapeutic effect of Hecubine, a natural aspidosperma-type alkaloid, as a TREM2 activator in lipopolysaccharide (LPS)-stimulated neuroinflammation in in vitro and in vivo models. In this study, molecular docking and cellular thermal shift assay (CTSA) were employed to investigate the interaction between Hecubine and TREM2. Enzyme-linked immunosorbent assay (ELISA), quantitative PCR, immunofluorescence, Western blotting, and shRNA gene knockdown were used to assess the anti-neuroinflammatory and antioxidant effects of Hecubine in microglial cells and zebrafish. Our results revealed that Hecubine directly interacted with TREM2, leading to its activation. Knockdown of TREM2 mRNA expression significantly abolished the anti-inflammatory and antioxidant effects of Hecubine on LPS-stimulated proinflammatory mediators (NO, TNF-α, IL-6, and IL-1β) and oxidative stress in microglia cells. Furthermore, Hecubine upregulated Nrf2 expression levels while downregulating TLR4 signaling expression levels both in vivo and in vitro. Silencing TREM2 upregulated TLR4 and downregulated Nrf2 signaling pathways, mimicking the effect of Hecubine, further supporting TREM2 as the drug target by which Hecubine inhibits neuroinflammation. In conclusion, this is the first study to identify a small molecule, namely Hecubine directly targeting TREM2 to mediate anti-neuroinflammation and anti-oxidative effects, which serves as a potential therapeutic agent for the treatment of neural inflammation-associated CNS diseases.

The Madagascar palm genome provides new insights on the evolution of Apocynaceae specialized metabolism

Specialized metabolites possess diverse interesting biological activities and some cardenolides- and monoterpene indole alkaloids- (MIAs) derived pharmaceuticals are currently used to treat human diseases such as cancers or hypertension. While these two families of biocompounds are produced by specific subfamilies of Apocynaceae, one member of this medicinal plant family, the succulent tree Pachypodium lamerei Drake (also known as Madagascar palm), does not produce such specialized metabolites. To explore the evolutionary paths that have led to the emergence and loss of cardenolide and MIA biosynthesis in Apocynaceae, we sequenced and assembled the P. lamerei genome by combining Oxford Nanopore Technologies long-reads and Illumina short-reads. Phylogenomics revealed that, among the Apocynaceae whose genomes have been sequenced, the Madagascar palm is so far the species closest to the common ancestor between MIA producers/non-MIA producers. Transposable elements, constituting 72.48% of the genome, emerge as potential key players in shaping genomic architecture and influencing specialized metabolic pathways. The absence of crucial MIA biosynthetic genes such as strictosidine synthase in P. lamerei and non-Rauvolfioideae species hints at a transposon-mediated mechanism behind gene loss. Phylogenetic analysis not only showcases the evolutionary divergence of specialized metabolite biosynthesis within Apocynaceae but also underscores the role of transposable elements in this intricate process. Moreover, we shed light on the low conservation of enzymes involved in the final stages of MIA biosynthesis in the distinct MIA-producing plant families, inferring independent gains of these specialized enzymes along the evolution of these medicinal plant clades. Overall, this study marks a leap forward in understanding the genomic dynamics underpinning the evolution of specialized metabolites biosynthesis in the Apocynaceae family, with transposons emerging as potential architects of genomics restructuring and gene loss.

Alstoboonine, an Ulean-Type Indole Alkaloid from Alstonia boonei Leaves

Alstonia boonei De Wild is a common plant in West Africa used in traditional medicine for various indications. While the stem bark has frequently been investigated, not much is known about the phytochemistry and bioactivity of the leaves. Within the current study, the major alkaloids of a hydroethanolic leaf extract were therefore isolated and characterized by MS, NMR, and ECD. This led to the identification of alstoboonine 1, a new ulean-type alkaloid, along with eight previously reported indole alkaloids, 15-hydroxyangustilobine A (2), 6,7-seco-angustilobine B (3), 6,7-seco-19,20-α-epoxyangustilobine B (4), alstrostine E (5), alstrostine C (6), alstrostine D (7), 12-methoxyechitamidine (8), and 19-oxo-12-methoxyechitamidine (9). 1 was moderately active in vitro against Plasmodium falciparum NF54 (IC50 6.9 μM), but inactive against other protozoan parasites (Trypanosoma brucei, Trypanosoma cruzi, Leishmania donovani). No significant cytotoxic effects were observed in L6 rat skeletal myoblast cells and MCF-7 breast cancer cells. Similarly, compounds 3 to 9 did not show cytotoxicity in MCF-7 cells. Due to the reported traditional use of the plant as an anthelmintic, the major alkaloids 2, 5, 6, and 8 were tested against the nematode Caenorhabditis elegans. Nematicidal effects were observed for 6 (LC50 400 μM), whereas 2, 5, and 8 were inactive.

Toward a More Sustainable Sample Preparation in Phytochemistry: Case Studies in Four Subclasses of Alkaloids

The fact that alkaloids are bases has been the most explored chemical feature of their extraction and purification procedures. The main drawback of these procedures is that they employ undesirable chemicals, with HCl and CH2Cl2 probably being the most commonly employed chemicals in their subsequent steps. This work tested the hypothesis that advantages in recovery efficiency support this common practice. Experiments were conducted in three laboratories, monitoring the alkaloids harmine (1), boldine (2), vincamine (3), and mescaline (4) extracted from Banisteriopsis caapi, Peumus boldus, Vinca minor, and Trichocereus macrogonus var. pachanoi, respectively. The research demonstrated that HCl could be replaced with citric acid (CA) without loss or even better extraction performance. The recommended EtOAc could completely replace CH2Cl2 in three out of four study cases and partially in the fourth case without harming the extraction efficiency. In addition, the alternative solvents tert-amyl methyl ether (TAME) and n-butyl acetate (BuOAc) could enhance the extraction of alkaloids. These results might incentivize natural products laboratories to consider sustainability more routinely, thus being closer to current practices in the pharmaceutical industry, which has been replacing solvents and processes with greener ones.

Studies on Secondary Metabolites and In vitro and In silico Anticholinesterases Activities of the Sea Urchin Echinometra mathaei Crude Venoms from the Persian Gulf-Bushehr

Background:

Echinoderms are a unique source of amazing secondary metabolites with a wide spectrum of biological activities. Several species of sea urchins contain various toxins and biologically active metabolites. One of the most attractive approaches to treat Alzheimer's disease is searching for effective marine natural products with cholinesterase inhibitory activities.

Objective:

The current study is designed to investigate the in vitro and in silico acetylcholinesterase and butyrylcholinesterase inhibitory activities of the Persian Gulf echinoderm sea urchin Echinometra mathaei venom and related chemical compounds.

Methods:

The experiments for LD50, total protein, protein bands, in vitro cholinesterase inhibitory activities, the identity of secondary metabolites, and the in silico evaluations, respectively, were performed by Spearman-Karber, Lowry, SDS-PAGE, Ellman's spectroscopic, GC-MS, and docking methods.

Results:

The LD50 (IV rat) of the spine, gonad, and coelomic fluid from sea urchin samples were 2.231 ± 0.09, 1.03 ± 0.05, and 1.12 ± 0.13 mg/ml, respectively. The SDS-PAGE and total protein studies showed that at least a portion of the venom is protein in nature. GC-MS analysis of the identified samples revealed 12, 23, and 21 compounds with different chemical types, including alkaloids, terpenes, and steroids, respectively. According to the results, all samples act as significant inhibitors of both enzymes. In silico data for the identified compounds also confirmed the experimental results.

Conclusion:

The alkaloid compound 6H-Indolo[3,2,1-de] [1,5] naphthyridine-6-one,1,2,3a,4,5- hexahydro-8-hydroxy-3-methyl (C7) had the highest affinity for both enzymes. Further research is needed to determine whether C7 could be a therapeutic candidate for Alzheimer's disease.

Vincazalidine A, a unique bisindole alkaloid from Catharanthus roseus

A new dimeric indole alkaloid, vincazalidine A consisting of an aspidosperma type and a modified iboga type with 1-azatricyclo ring system consisting of one azepane and two piperidine rings coupled with an oxazolidine ring was isolated from Catharanthus roseus, and the structure including absolute stereochemistry was elucidated on the basis of spectroscopic data as well as DP4 statistical analysis. Vincazalidine A induced G2 arrest and subsequent apoptosis in human lung carcinoma cell line, A549 cells.

Evolution and diversification of carboxylesterase-like [4+2] cyclases in aspidosperma and iboga alkaloid biosynthesis

Monoterpene indole alkaloids (MIAs) are a large and diverse class of plant natural products, and their biosynthetic construction has been a subject of intensive study for many years. The enzymatic basis for the production of aspidosperma and iboga alkaloids, which are produced exclusively by members of the Apocynaceae plant family, has recently been discovered. Three carboxylesterase (CXE)-like enzymes from Catharanthus roseus and Tabernanthe iboga catalyze regio- and enantiodivergent [4+2] cycloaddition reactions to generate the aspidosperma (tabersonine synthase, TS) and iboga (coronaridine synthase, CorS; catharanthine synthase, CS) scaffolds from a common biosynthetic intermediate. Here, we use a combined phylogenetic and biochemical approach to investigate the evolution and functional diversification of these cyclase enzymes. Through ancestral sequence reconstruction, we provide evidence for initial evolution of TS from an ancestral CXE followed by emergence of CorS in two separate lineages, leading in turn to CS exclusively in the Catharanthus genus. This progression from aspidosperma to iboga alkaloid biosynthesis is consistent with the chemotaxonomic distribution of these MIAs. We subsequently generate and test a panel of chimeras based on the ancestral cyclases to probe the molecular basis for differential cyclization activity. Finally, we show through partial heterologous reconstitution of tabersonine biosynthesis using non-pathway enzymes how aspidosperma alkaloids could have first appeared as "underground metabolites" via recruitment of promiscuous enzymes from common protein families. Our results provide insight into the evolution of biosynthetic enzymes and how new secondary metabolic pathways can emerge through small but important sequence changes following co-option of preexisting enzymatic functions.

Asymmetric Synthesis of Arboduridine

The first asymmetric total synthesis of the monoterpenoid indole alkaloid arboduridine has been accomplished. The tricyclic A/B/D ring system was constructed by an enantioselective Michael reaction followed by intramolecular nucleophilic addition. Intramolecular α-amination of a ketone forged the piperidine ring, while a Horner-Wadsworth-Emmons (HWE) reaction was used to form the pyrrolidine ring. A reduction cyclization cascade led to formation of the tetrahydrofuran ring.

Engineered biosynthesis of plant heteroyohimbine and corynantheine alkaloids in Saccharomyces cerevisiae

Monoterpene indole alkaloids (MIAs) are a class of natural products comprised of thousands of structurally unique bioactive compounds with significant therapeutic values. Due to difficulties associated with isolation from native plant species and organic synthesis of these structurally complex molecules, microbial production of MIAs using engineered hosts are highly desired. In this work, we report the engineering of fully integrated Saccharomyces cerevisiae strains that allow de novo access to strictosidine, the universal precursor to thousands of MIAs at 30-40 mg/L. The optimization efforts were based on a previously reported yeast strain that is engineered to produce high titers of the monoterpene precursor geraniol through compartmentalization of mevalonate pathway in the mitochondria. Our approaches here included the use of CRISPR-dCas9 interference to identify mitochondria diphosphate transporters that negatively impact the titer of the monoterpene, followed by genetic inactivation; the overexpression of transcriptional regulators that increase cellular respiration and mitochondria biogenesis. Strain construction included the strategic integration of genes encoding both MIA biosynthetic and accessory enzymes into the genome under a variety of constitutive and inducible promoters. Following successful de novo production of strictosidine, complex alkaloids belonging to heteroyohimbine and corynantheine families were reconstituted in the host with introduction of additional downstream enzymes. We demonstrate that the serpentine/alstonine pair can be produced at ∼5 mg/L titer, while corynantheidine, the precursor to mitragynine can be produced at ∼1 mg/L titer. Feeding of halogenated tryptamine led to the biosynthesis of analogs of alkaloids in both families. Collectively, our yeast strain represents an excellent starting point to further engineer biosynthetic bottlenecks in this pathway and to access additional MIAs and analogs through microbial fermentation.

ONE SENTENCE SUMMARY

An Saccharomyces cerevisiae-based microbial platform was developed for the biosynthesis of monoterpene indole alkaloids, including the universal precursor strictosidine and further modified heteroyohimbine and corynantheidine alkaloids.

Polycyclic pyrroloindoline-containing natural products with a unique 3-heptyl-2a,4a-diazapentaleno[1,6-ab]indene core isolated from Alstonia scholaris

Alscholarine C (1), featuring an unprecedented pyrroloindoline-containing natural product (PiNP) with a 6/5/5/5 tetracyclic carbon skeleton, and four known PiNPs (2-5), namely demethylalstoscholarinine E (2), Nb-demethylechitamine (3), winphylline A (4), and echitamine (5), were isolated from Alstonia scholaris. Compound 1 was characterized by a hexahydropyrrolo[2,3-b] indole (HPI) core fused to a unique 4-heptylimidazolidine motif, forming an unparalleled 3-heptyl-2a,4a-diazapentaleno[1,6-ab]indene ring system. Their structures were established by spectroscopic analysis, quantum-chemical calculated 13C NMR data with DP4+ probability analyses, and ECD calculations and comparison. A plausible biosynthetic pathway of 1 was proposed. Compound 1 exhibited potential anti-inflammatory activity against LPS-stimulated NO production in RAW264.7 cells.

Isovincathicine from Catharanthus roseus induces apoptosis in A549 cells

A dimeric indole alkaloid, isovincathicine consisting of an aspidosperma type and modified iboga with C-7-C-20 connection type skeletons was first isolated from Catharanthus roseus, and the structure including stereochemistry was elucidated on the basis of spectroscopic data as well as DP4 statistical analysis. Isovincathicine inhibited cell proliferation in A549 cells. We investigated the detailed mode of action of isovincathicine-induced inhibitory effects on cell proliferation in A549 cells. Flow cytometric analysis showed that isovincathicine-treated cells accumulated in the G2 phase after 24 h, and the percentage of cells showing cell death increased after 48 h. Western blotting also showed increased expression of BimEL, an apoptosis-related protein, and decreased expression of Mcl-1 and Bcl-xL. Isovincathicine was suggested to induce apoptosis in A549 cells by a mechanism is similar to that of vinblastine.

Dearomative Spirocyclization of Tryptamine-Derived Isocyanides via Iron-Catalyzed Carbene Transfer

Tryptamine-derived isocyanides are valuable building blocks in the construction of spirocyclic indolenines and indolines via dearomatization of the indole moiety. We report the Bu4N[Fe(CO)3NO]-catalyzed carbene transfer of α-diazo esters to 3-(2-isocyanoethyl)indoles, leading to ketenimine intermediates that undergo spontaneous dearomative spirocyclization. The utility of this iron-catalyzed carbene transfer/spirocyclization cascade was demonstrated by its use as a key step in the formal total synthesis of monoterpenoid indole alkaloids (±)-aspidofractinine, (±)-limaspermidine, (±)-aspidospermidine, and (±)-17-demethoxy-N-acetylcylindrocarine.

Gold(I)-Catalyzed Cascade Cyclization of Alkynyl Indoles for the Stereoselective Construction of the Quaternary Carbon Center of Akuammiline Alkaloids

A gold-catalyzed cyclization reaction of alkynyl-indoles has been developed for the stereoselective construction of the quaternary carbon center of fused indolines. This reaction efficiently produces fused indolines via diastereoselective 6-endo-dig cyclization controlled by a bulky TIPS group, followed by nucleophilic attack of the carboxy group on the resulting imine. The lactone moiety of the fused indoline can be reductively cleaved to produce a tricyclic indoline, which could be useful for the synthesis of akuammiline alkaloids.

Synthetic Matching of Complex Monoterpene Indole Alkaloid Chemical Space

Monoterpene indole alkaloids (MIAs) are endowed with high structural and spatial complexity and characterized by diverse biological activities. Given this complexity-activity combination in MIAs, rapid and efficient access to chemical matter related to and with complexity similar to these alkaloids would be highly desirable, since such compound classes might display novel bioactivity. We describe the design and synthesis of a pseudo-natural product (pseudo-NP) collection obtained by the unprecedented combination of MIA fragments through complexity-generating transformations, resulting in arrangements not currently accessible by biosynthetic pathways. Cheminformatic analyses revealed that both the pseudo-NPs and the MIAs reside in a unique and common area of chemical space with high spatial complexity-density that is only sparsely populated by other natural products and drugs. Investigation of bioactivity guided by morphological profiling identified pseudo-NPs that inhibit DNA synthesis and modulate tubulin. These results demonstrate that the pseudo-NP collection occupies similar biologically relevant chemical space that Nature has endowed MIAs with.

BiBr3 -Mediated Intramolecular Aza-Prins Cyclization of Aza-Achmatowicz Rearrangement Products: Asymmetric Total Synthesis of Suaveoline and Sarpagine Alkaloids

An intramolecular aza-Prins cyclization of aza-Achmatowicz rearrangement products was developed in which bismuth tribromide (BiBr3 ) plays a dual role as an efficient Lewis acid and source of the bromide nucleophile. This approach enables the facile construction of highly functionalized 9-azabicyclo[3.3.1]nonanes (9-ABNs), which are valuable synthetic building blocks and a powerful platform for the synthesis of a variety of alkaloid natural products and drug molecules. Suitable substrates for the aza-Prins cyclization include 1,1-disubstituted alkenes, 1,2-disubstituted alkenes, alkynes, and allenes, with good to excellent yields observed. Finally, we showcase the application of this new approach to the enantioselective total synthesis of six indole alkaloids: (-)-suaveoline (1), (-)-norsuaveoline (2), (-)-macrophylline (3), (+)-normacusine B (4), (+)-Na -methyl-16-epipericyclivine (5) and (+)-affinisine (6) in a total of 9-14 steps. This study significantly expands the synthetic utility of the aza-Achmatowicz rearrangement, and the strategy (aza-Achmatowicz/aza-Prins) is expected to be applicable to the total synthesis of other members of the big family of macroline and sarpagine indole alkaloids.

Effects and mechanisms of plant bioactive compounds in preventing fungal spoilage and mycotoxin contamination in postharvest fruits: A review

Spoilage and mycotoxin contamination of fruits cause significant economic losses and food safety issues. Synthetic chemical fungicide treatment as primary postharvest management has attracted increasing public concern in recent years, because it may cause negative effects on the environment and human health. Numerous bioactive compounds from plants have demonstrated excellent control effects on fruit spoilage and mycotoxin contamination. Plant bioactive compounds have been considered one of the most promising alternatives, because they are generally regarded as safe and environmentally friendly. Here, we reviewed the most recent advances in plant bioactive compounds in the prevention of fungal spoilage and mycotoxin contamination in fruits. The control effects of these compounds and the mechanisms involved were summarized, and current limitations and future perspectives were discussed.

Hunteriasines A - D, tryptamine-derived alkaloids from Hunteria umbellata

Four undescribed tryptamine-derived alkaloids, hunteriasines A - D, were isolated and identified from Hunteria umbellata (Apocynaceae), together with fifteen known indole alkaloids. The chemical structure and absolute configuration of hunteriasine A were determined by spectroscopic and X-ray crystallographic data analyses. Hunteriasine A, featuring with a unique scaffold comprised of tryptamine and an unprecedented "12-carbon unit" moiety, is a zwitterionic indole-derived and pyridinium-containing alkaloid. Hunteriasines B - D were identified by spectroscopic data analyses and theoretical calculations. A plausible biogenetic pathway for hunteriasines A and B was proposed. The lipopolysaccharide-stimulated mouse macrophage cell line J774A.1 cell-based bioactivity assays revealed that (+)-eburnamine, strictosidinic acid, and (S)-decarbomethoxydihydrogambirtannine enhance the release of interleukin-1β.

Synthesis, Structure and Biological Activity of Indole-Imidazole Complexes with ZnCl2: Can Coordination Enhance the Functionality of Bioactive Ligands?

The ability of the indole-imidazole hybrid ligands to coordinate with the Zn(II) ion and the resulting structures of this new class of coordination compounds were analyzed in order to determine their structural properties and biological functionalities. For this purpose, six novel Zn(II) complexes, [Zn(InIm)₂Cl₂] (1), [Zn(InMeIm)₂Cl₂] (2), [Zn(IniPrIm)₂Cl₂] (3), [Zn(InEtMeIm)₂Cl₂] (4), [Zn(InPhIm)₂Cl₂] (5) and [Zn₂(InBzIm)₂Cl₂] (6) (where InIm is 3-((1H-imidazol-1-yl)methyl)-1H-indole), were synthesized by the reactions of ZnCl₂ and the corresponding ligand in a 1:2 molar ratio in methanol solvent at an ambient temperature. The structural and spectral characterization of these complexes was performed using NMR, FT-IR and ESI-MS spectrometry and elemental analysis, and the crystal structures of 1-5 were determined using single-crystal X-ray diffraction. Complexes 1-5 form polar supramolecular aggregates by utilizing, for this purpose, the N-H(indole)∙∙∙Cl(chloride) intermolecular hydrogen bonds. The assemblies thus formed differ depending on the distinctive molecular shape, which can be either compact or extended. All complexes were screened for their hemolytic, cytoprotective, antifungal, and antibacterial activities. The results show that the cytoprotective activity of the indole/imidazole ligand significantly increases upon its complexation with ZnCl₂ up to a value comparable with the standard antioxidant Trolox, while the response of its substituted analogues is diverse and less pronounced.

Quantum Mechanical Prediction and Experimental Verification of Au(I)-Catalyzed Substitution-Controlled Syntheses of 1H-Pyrido[4,3-b]indole and Spiro[indoline-3,3'-pyridine] Derivatives

Density functional theory calculations were applied to predict the pathways of gold(I)-catalyzed cycloisomerization of the indole substrates with 1,6-enynes, which were consistent with the ensuing experimental results. The substitution-controlled synthesis led to the formation of 1H-pyrido[4,3-b]indole and spiro[indoline-3,3'-pyridine] derivatives in a tunable way. The reactions had good functional group tolerances, and a possible mechanism was proposed based on the computational and experimental results.

Terpenoid Transport in Plants: How Far from the Final Picture?

Contrary to the biosynthetic pathways of many terpenoids, which are well characterized and elucidated, their transport inside subcellular compartments and the secretion of reaction intermediates and final products at the short- (cell-to-cell), medium- (tissue-to-tissue), and long-distance (organ-to-organ) levels are still poorly understood, with some limited exceptions. In this review, we aim to describe the state of the art of the transport of several terpene classes that have important physiological and ecological roles or that represent high-value bioactive molecules. Among the tens of thousands of terpenoids identified in the plant kingdom, only less than 20 have been characterized from the point of view of their transport and localization. Most terpenoids are secreted in the apoplast or stored in the vacuoles by the action of ATP-binding cassette (ABC) transporters. However, little information is available regarding the movement of terpenoid biosynthetic intermediates from plastids and the endoplasmic reticulum to the cytosol. Through a description of the transport mechanisms of cytosol- or plastid-synthesized terpenes, we attempt to provide some hypotheses, suggestions, and general schemes about the trafficking of different substrates, intermediates, and final products, which might help develop novel strategies and approaches to allow for the future identification of terpenoid transporters that are still uncharacterized.

Gene coexpression networks allow the discovery of two strictosidine synthases underlying monoterpene indole alkaloid biosynthesis in Uncaria rhynchophylla

Plant-derived monoterpene indole alkaloids (MIAs) from Uncaria rhynchophylla (UR) have huge medicinal properties in treating Alzheimer's disease, Parkinson's disease, and depression. Although many bioactive UR-MIA products have been isolated as drugs, their biosynthetic pathway remains largely unexplored. In this study, untargeted metabolome identified 79 MIA features in UR tissues (leaf, branch stem, hook stem, and stem), of which 30 MIAs were differentially accumulated among different tissues. Short time series expression analysis captured 58 pathway genes and 12 hub regulators responsible for UR-MIA biosynthesis and regulation, which were strong links with main UR-MIA features. Coexpression networks further pointed to two strictosidine synthases (UrSTR1/5) that were coregulated with multiple MIA-related genes and highly correlated with UR-MIA features (r > 0.7, P < 0.005). Both UrSTR1/5 catalyzed the formation of strictosidine with tryptamine and secologanin as substrates, highlighting the importance of key residues (UrSTR1: Glu309, Tyr155; UrSTR5: Glu295, Tyr141). Further, overexpression of UrSTR1/5 in UR hairy roots constitutively increased the biosynthesis of bioactive UR-MIAs (rhynchophylline, isorhynchophylline, corynoxeine, etc), whereas RNAi of UrSTR1/5 significantly decreased UR-MIA biosynthesis. Collectively, our work not only provides candidates for reconstituting the biosynthesis of bioactive UR-MIAs in heterologous hosts but also highlights a powerful strategy for mining natural product biosynthesis in medicinal plants.

Natural Product-Oriented Photo-Induced Denitrogenative Annulations of 1-Alkenylbenzotriazoles

The photo-induced denitrogenative annulations of a variety of 1-alkenylbenzotriazoles were investigated. By judiciously manipulating the structural variations of 1-alkenylbenzotriazoles, two characteristic polycyclic skeletons associated with monoterpene indole alkaloids were constructed through a diverted and controllable manner. The present work not only enriches the photochemistry of 1-alkenylbenzotriazoles, but also offers a unified approach to access skeletally diverse indole alkaloid scaffolds.

Chemical and nutritional characterization of Ambelania duckei (Apocynaceae) an unexplored fruit from the Amazon region

Ambelania duckei Markgr is a species of the Apocynaceae family, native to the Amazon region that is unexplored from a nutritional point of view and studied in relation to its chemical constituents. This work presents an unprecedented study of the proximate composition, lipid profile, a chromatographic analysis, and the antioxidant activity of extracts obtained from the pulp, peel and seeds of the fruit. The results showed that potassium, calcium, and magnesium stood out as the most abundant key minerals in the fruit peel and pulp, with an emphasis on the potassium present in the fruit pulp at 1750.0 mg/100 g. The peel had the highest content of total phenolics (374.86 mg/g), flavonoids (15.54 mg/g), tannins (27.45 mg/g) and O-diphenols (379.36 mg/g; 645.71 mg/g). The antioxidant activity (AA) was highest in the peel compared to the pulp in the DPPH, ABTS, and ORAC tests showing: IC₅₀ of 29.82; 43.67; and 407.13 µg/mL, respectively but a lower activity for the Fe²⁺ chelator. The analysis of the lipid fractions from the peel, pulp, and seeds of the A. duckei fruit resulted in 14 types of fatty acids. The major fatty acids found in the three parts of the fruit were oleic acid (peel, 22.52 %), palmitic acid (pulp, 17.34 %), and linoleic acid (seeds, 47.99 %). The lipid profile and nutritional aspects had a PUFA/SFA ratio (0.4-1.8) in the different parts of the A. duckei fruit; the atherogenic and thrombogenic indexes were higher in the peel (1.23) and pulp (0.62), respectively. The ratio between the hypocholesterolemic and hypercholesterolemic fatty acids (0.5 - 3.8) calculated for the fruit are within the desirable range for a nutritious food. The chromatographic analysis of the volatile organic compounds (VOCs) from the peel and pulp of the fruit, identified 74 VOCs, of which 60.9 % are related to terpenes, and emit notes such as cucumber, green, fatty, floral, and mint, due to the presence of substances with OAVs > 10, especially α-ionone, 1,8-cineole, 2,4-decadienal, and dodecanal. The analysis of the MS and MS/MS spectra of the chromatograms obtained by LC- QTOF-HRMS led to the identification of 26 compounds in the peel, seeds and pulp of A. duckei, such as fatty acids, phenolic acid, flavonoids, proanthocyanidins, alkaloids, and terpenoids. The results show that the pulp of A. duckei has potential as nourishing food and the nutritional and chemical aspects of the peel can be applied to commercial applications.

Increasing the Efficacy of Treatment of Staphylococcus aureus-Candida albicans Mixed Infections with Myrtenol

Infectious diseases caused by various nosocomial microorganisms affect worldwide both immunocompromised and relatively healthy persons. Bacteria and fungi have different tools to evade antimicrobials, such as hydrolysis damaging the drug, efflux systems, and the formation of biofilm that significantly complicates the treatment of the infection. Here, we show that myrtenol potentiates the antimicrobial and biofilm-preventing activity of conventional drugs against S. aureus and C. albicans mono- and dual-species cultures. In our study, the two optical isomers, (-)-myrtenol and (+)-myrtenol, have been tested as either antibacterials, antifungals, or enhancers of conventional drugs. (+)-Myrtenol demonstrated a synergistic effect with amikacin, fluconazole, and benzalkonium chloride on 64-81% of the clinical isolates of S. aureus and C. albicans, including MRSA and fluconazole-resistant fungi, while (-)-myrtenol increased the properties of amikacin and fluconazole to repress biofilm formation in half of the S. aureus and C. albicans isolates. Furthermore, myrtenol was able to potentiate benzalkonium chloride up to sixteen-fold against planktonic cells in an S. aureus-C. albicans mixed culture and repressed the adhesion of S. aureus. The mechanism of both (-)-myrtenol and (+)-myrtenol synergy with conventional drugs was apparently driven by membrane damage since the treatment with both terpenes led to a significant drop in membrane potential similar to the action of benzalkonium chloride. Thus, due to the low toxicity of myrtenol, it seems to be a promising agent to increase the efficiency of the treatment of infections caused by bacteria and be fungi of the genus Candida as well as mixed fungal-bacterial infections, including resistant strains.

First genomic insights into the Mandevilla genus

Mandevilla (Apocynaceae) is a greatly appreciated genus in the world ornamental market. In this study, we attempted to address the poor genetic knowledge and the huge taxonomic gaps existing in this genus by analyzing a collection of 55 accessions. After cytometrically determining the triploid genome size (1,512.64 Mb) of a reference sample (variety "Mandevilla 2001"), the plastidial genome (cpDNA, 0.18 Mb) and a draft of the nuclear genome (nuDNA, 207 Mb) were assembled. While cpDNA was effective in reconstructing the phylogenesis of the Apocynaceae family based on a DNA superbarcoding approach, the nuDNA assembly length was found to be only 41% of the haploid genome size (506 Mb, predicted based on the K-mer frequency distribution). Its annotation enabled the prediction of 37,811 amino acid sequences, of which 10,562 resulted full length proteins. Among them, we identified nine proteins whose orthologs (in Catharanthus roseus) are involved in the biosynthesis of monoterpene indole alkaloids (MIAs), including catharanthine, tabersonine, and vincadifformine. The nuclear genome draft was also useful to develop a highly informative (average polymorphism information content, PIC = 0.62) set of 23 simple sequence repeat (SSR) markers that was validated on the Mandevilla collection. These results were integrated with cytometric measurements, nuclear ITS1 haplotyping and chloroplast DNA barcoding analyses to assess the origin, divergence and relationships existing among the 55 accessions object of the study. As expected, based on the scarce information available in the literature, the scenario was extremely intricate. A reasonable hypothesis is that most of the accessions represent interspecific hybrids sharing the same species as maternal parent (i.e., Mandevilla sanderi).

Anti-Alphaviral Alkaloids: Focus on Some Isoquinolines, Indoles and Quinolizidines

The discovery and the development of safe and efficient therapeutics against arthritogenic alphaviruses (e.g., chikungunya virus) remain a continuous challenge. Alkaloids are structurally diverse and naturally occurring compounds in plants, with a wide range of biological activities including beneficial effects against prominent pathogenic viruses and inflammation. In this short review, we discuss the effects of some alkaloids of three biologically relevant structural classes (isoquinolines, indoles and quinolizidines). Based on various experimental models (viral infections and chronic diseases), we highlight the immunomodulatory effects of these alkaloids. The data established the capacity of these alkaloids to interfere in host antiviral and inflammatory responses through key components (antiviral interferon response, ROS production, inflammatory signaling pathways and pro- and anti-inflammatory cytokines production) also involved in alphavirus infection and resulting inflammation. Thus, these data may provide a convincing perspective of research for the use of alkaloids as immunomodulators against arthritogenic alphavirus infection and induced inflammation.

Two new aspidosperma-type monoterpenoid indole alkaloids from Ervatamia officinalis

Two new aspidosperma-type monoterpenoid indole alkaloids, 16-O-methylvoafinine (1) and 14,15-diepi-voafinidine (2) were isolated from the aerial parts of Ervatamia officinalis. Their structures were established by comprehensive spectroscopic analysis including 1D and 2D NMR, HR-ESI-MS, and electronic circular dichroism calculation. The isolated compounds were evaluated for cytotoxic activities against HepG2, MCF-7, and A549 cell lines by CCK-8 assay.

The synthesis of indole-3-carbinols (I3C) and their application to access unsymmetrical bis(3-indolyl)methanes (BIMs) bearing a quaternary sp3-carbon

In the present study, the novel synthesis of tert-indole-3-carbinols is reported through the DDQ-mediated oxidation of the allylic C-H bond/aromatization/hydroxylation at the indolyl carbon using water as the hydroxyl source. The reaction is highly efficient and high yielding and it works under mild reaction conditions. Furthermore, the synthetic value of such indole-based tert-carbinols is explored through their use as excellent electrophilic methylene surrogates to develop medicinally important unsymmetrical bis(3-indolyl)methanes containing an all carbon quaternary center.

Iboga-type alkaloids with Indolizidino[8,7-b]Indole scaffold and bisindole alkaloids from Tabernaemontana bufalina Lour

Phytochemical investigation on the aerial parts of Tabernaemontana bufalina Lour. (Apocynaceae) led to the identification of four undescribed monoterpenoid indole alkaloids named taberbufamines A-D, an undescribed natural product, and fourteen known indole alkaloids. The structures of the undescribed alkaloids were established by spectroscopic and computational methods, and their absolute configurations were further determined by quantum chemical TDDFT calculations and the experimental ECD spectra. Taberbufamines A and B possessed an uncommon skeleton incorporating an indolizidino [8,7-b]indole motif with a 2-hydroxymethyl-butyl group attached at the pyrrolidine ring. Biosynthetically, Taberbufamines A and B might be derived from iboga-type alkaloid through rearrangement. Vobatensine C showed significant bioactivity against A-549, Bel-7402, and HCT-116 cells with IC₅₀ values of 2.61, 1.19, and 1.74 μM, respectively. Ervahanine A showed antimicrobial activity against Bacillus subtilis, Mycobacterium smegmatis, and Helicobacter pylori with MIC values of 4, 8, and 16 μg/mL, respectively. 19(S)-hydroxyibogamine was shown as butyrylcholinesterase inhibitor (IC₅₀ of 20.06 μM) and α-glycosidase inhibitor (IC₅₀ of 17.18 μM), while tabernamine, ervahanine B, and ervadivaricatine B only showed α-glycosidase inhibitory activities with IC₅₀ values in the range of 0.95-4.61 μM.

Monoterpene Indole Alkaloids from the Aerial Parts of Rhazya stricta Induce Cytotoxicity and Apoptosis in Human Adenocarcinoma Cells

Chromatographic investigation of the aerial parts of the Rhazya stricta (Apocynaceae) resulted in the isolation of two new monoterpene indole alkaloids, 6-nor-antirhine-N₁-methyl (1) and razyamide (2), along with six known compounds, eburenine (3), epi-rhazyaminine (4), rhazizine (5), 20-epi-sitsirikine (6), antirhine (7), and 16-epi-stemmadenine-N-oxide (8). The chemical structures were established by various spectroscopic experiments. Compounds 1–8 exhibited cytotoxic effects against three cancer cells with IC₅₀ values ranging between 5.1 ± 0.10 and 93.2 ± 9.73 µM against MCF-7; 5.1 ± 0.28 and 290.2 ± 7.50 µM against HepG2, and 3.1 ± 0.17 and 55.7 ± 4.29 µM against HeLa cells. Compound 2 showed the most potent cytotoxic effect against all cancer cell lines (MCF-7, HepG2 and HeLa with IC₅₀ values = 5.1 ± 0.10, 5.1 ± 0.28, and 3.1 ± 0.17 µM, respectively). Furthermore, compound 2 revealed a significant increase in the apoptotic cell population of MCF-7, HepG2, and HeLa cells, with 31.4 ± 0.2%, 29.2 ± 0.5%, and 34.9 ± 0.6%, respectively. Compound 2 decreased the percentage of the phagocytic pathway on HepG2 cells by 15.0 ± 0.1%. These findings can explain the antiproliferative effect of compound 2.

Synthesis of pyrimidine-containing alkaloids

This review deals with the synthesis of naturally occurring alkaloids containing partially or completely saturated pyrimidine nuclei. The interest in these compounds is associated with their structural diversity, high biological activity and toxicity. The review is divided into four parts, each of which describes a number of synthetic methodologies toward structurally different naturally occurring alkaloids containing saturated cyclic six-membered amidine, guanidine, aminal and urea (thiourea) moieties, respectively. The development of various synthetic strategies for the preparation of these compounds has remarkably increased during the past few decades. This is primarily due to the fact that some of these compounds are isolated only in limited quantities, which makes it practically impossible to study their full structural characteristics and biological activity.

Isolation and biological activity of azocine and azocane alkaloids

Thousands of known alkaloids contain a nitrogen (N) heterocycle. While five-, six- and seven-membered N-heterocycles (ie: pyrroles, imidazoles, indoles, pyridines and azepines and their saturated variants) are common, those with an eight-membered N-heterocycle are comparatively rare. This review discusses the structure and bioactivity of alkaloids that contain an azocine (or saturated azocane) ring, and the array of sources whence they originate.

Therapeutic potential of indole alkaloids in respiratory diseases: A comprehensive review

BACKGROUND

Indole alkaloids are very promising for potential therapeutic purposes and appear to be particularly effective against respiratory diseases. Several experimental studies have been performed, both in vivo and in vitro, to evaluate the effectiveness of indole alkaloids for the management of respiratory disorders, including asthma, emphysema, tuberculosis, cancer, and pulmonary fibrosis.

PURPOSE

The fundamental objective of this review was to summarize the in-depth therapeutic potential of indole alkaloids against various respiratory disorders.

STUDY DESIGN

In addition to describing the therapeutic potential, this review also evaluates the toxicity of these alkaloids, which have been utilized for therapeutic benefits but have demonstrated toxic consequences. Some indole alkaloids, including scholaricine, 19-epischolaricine, vallesamine, and picrinine, which are derived from the plant Alstonia scholaris, have shown toxic effects in non-rodent models.

METHODS

This review also discusses clinical studies exploring the therapeutic efficacy of indole alkaloids, which have confirmed the promising benefits observed in vivo and in vitro.

RESULTS

The indole alkaloidal compounds have shown efficacy in subjects with respiratory diseases.

CONCLUSION

The available data established both preclinical and clinical studies confirm the potential of indole alkaloids to treat the respiratory disorders.

Bisindole Alkaloids from the Alstonia Species: Recent Isolation, Bioactivity, Biosynthesis, and Synthesis

Bisindoles are structurally complex dimers and are intriguing targets for partial and total synthesis. They exhibit stronger biological activity than their corresponding monomeric units. Alkaloids, including those containing C-19 methyl-substitution in their monomeric units, their synthetic derivatives, and their mismatched pairs can be attractive targets for synthesis and may unlock better drug targets. We herein discuss the isolation of bisindoles from various Alstonia species, their bioactivity, putative biosynthesis, and synthesis. The total synthesis of macralstonidine, macralstonine, O-acetylmacralstonine, and dispegatrine, as well as the partial synthesis of alstonisidine, villalstonine, and macrocarpamine are also discussed in this review. The completion of the total synthesis of pleiocarpamine by Sato et al. completes the formal synthesis of the latter two bisindoles.

4,21-Secovincanol, a Novel Immunosuppressive Monoterpenoid Indole Alkaloid from Kopsia Hainanensis

4,21-Secovincanol (1), a novel C-21/N-4 cleavage monoterpenoid indole alkaloid, along with four analogs (2-5), were obtained from the aerial parts of Kopsia hainanensis. Structurally, compound 1 might be a derivative of epivincanol (2) via C-21/N-4 cleavage. Their structures were confirmed by means of comprehensive spectroscopic data analysis and comparison with the reported data. All isolates significantly inhibited Con A-stimulated mice splenocytes proliferation at 10-40 μM in a dose-dependent manner in vitro. Especially, compound 3 exhibited potent activities comparable to positive control (Dexamethasone, DXM).