Monoterpenoid Bisindole Alkaloids

Alstomaphylines A-K, monoterpenoid bisindole alkaloids from Alstonia macrophylla with AChE inhibitory activity and cytotoxicity

Eleven undescribed monoterpenoid bisindole alkaloids, alstomaphyines A-K (1-11), along with three known analogues were isolated from the leaves and stem bark of the Alstonia macrophylla. Compounds 1-3 were unprecedented dimerization alkaloids incorporating a macroline-type motif with an ajmaline-type motif via a C-C linkage. Their structures and absolute configurations were elucidated by extensive spectroscopic analysis, electronic circular dichroism (ECD) calculation, and CD exciton chirality method. Compounds 1-3 displayed potential inhibitory bioactivity against AChE with IC50 values of 4.44 ± 0.35, 3.59 ± 0.18, and 3.71 ± 0.23 μM, respectively. Enzyme kinetic study revealed compounds 1-3 as mixed competitive AChE inhibitors. Besides, compounds 8 and 12-14 exhibited better cytotoxicity against human cancer cell line HT-29 than cisplatin. Flow cytometry data revealed that compounds 8, 13, and 14 significantly induced the HT-29 cells arrest in G0/G1 phase in a concentration-dependent manner.

Implementation of an MS/MS Spectral Library for Monoterpene Indole Alkaloids

In less than 10 years, molecular networking (MN) strategy has revolutionized the art of Natural Products (NP) isolation to enter a rational workflow greatly increasing the probabilities of isolating new chemical entities. To pinpoint and streamline the isolation of new Monoterpene Indole Alkaloids (MIAs) in producing plants, we rendered publicly available the MIA database (MIADB), comprising MS² data for ca. 200 structurally diverse MIA, by uploading it to the Global Natural Products Social Molecular Networking (GNPS) platform. Here, we describe the key experimental aspects underlying data collection, data curation, and their subsequent upload to the GNPS libraries as a database. Practical tips are also provided at the end of this chapter to help optimizing the efficiency of the dereplication of MIA-containing plants against the MIADB-implemented GNPS library.

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.

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.

The Bisindole Alkaloids Angustilongines M and A from Alstonia penangiana Induce Mitochondrial Apoptosis and G0/G1 Cell Cycle Arrest in HT-29 Cells through Promotion of Tubulin Polymerization

A new linearly fused macroline-sarpagine bisindole, angustilongine M (1), was isolated from the methanolic extract of Alstonia penangiana. The structure of the alkaloid was elucidated based on analysis of the spectroscopic data, and its biological activity was evaluated together with another previously reported macroline-akuammiline bisindole from the same plant, angustilongine A (2). Compounds 1 and 2 showed pronounced in vitro growth inhibitory activity against a wide panel of human cancer cell lines. In particular, the two compounds showed potent and selective antiproliferative activity against HT-29 cells, as well as strong growth inhibitory effects against HT-29 spheroids. Cell death mechanistic studies revealed that the compounds induced mitochondrial apoptosis and G0/G1 cell cycle arrest in HT-29 cells in a time-dependent manner, while in vitro tubulin polymerization assays and molecular docking analysis showed that the compounds are microtubule-stabilizing agents, which are predicted to bind at the β-tubulin subunit at the Taxol-binding site.

Alkaloids with Anti-Onchocercal Activity from Voacanga africana Stapf (Apocynaceae): Identification and Molecular Modeling

A new iboga-vobasine-type isomeric bisindole alkaloid named voacamine A (1), along with eight known compounds-voacangine (2), voacristine (3), coronaridine (4), tabernanthine (5), iboxygaine (6), voacamine (7), voacorine (8) and conoduramine (9)-were isolated from the stem bark of Voacangaafricana. The structures of the compounds were determined by comprehensive spectroscopic analyses. Compounds 1, 2, 3, 4, 6, 7 and 8 were found to inhibit the motility of both the microfilariae (Mf) and adult male worms of Onchocerca ochengi, in a dose-dependent manner, but were only moderately active on the adult female worms upon biochemical assessment at 30 μM drug concentrations. The IC50 values of the isolates are 2.49-5.49 µM for microfilariae and 3.45-17.87 µM for adult males. Homology modeling was used to generate a 3D model of the O. ochengi thioredoxin reductase target and docking simulation, followed by molecular dynamics and binding free energy calculations attempted to offer an explanation of the anti-onchocercal structure-activity relationship (SAR) of the isolated compounds. These alkaloids are new potential leads for the development of antifilarial drugs. The results of this study validate the traditional use of V. africana in the treatment of human onchocerciasis.

Discovery of 3-(benzofuran-2-ylmethyl)-1H-indole derivatives as potential autophagy inducers in cervical cancer cells

In this manuscript we have documented the identification of a novel anticancer scaffold 3-(benzofuran-2-ylmethyl)-1H-indole. This scaffold has been designed by tweaking the known bisindolylmethane scaffold of natural products that display a wide range of biological activities. A series of 24 new conjugates have been synthesized and among them 5 derivatives exhibited IC₅₀ values less than 40 µM against two cervical cancer cell lines SiHa and C33a. Further mechanistic studies of two compounds 3eb and 3ec revealed that the toxicity of these compounds was due to the effective induction of autophagy mediated cell death. The autophagy induction was confirmed by the progressive conversion of LC3I to LC3II and downregulation of p62 in cervical cancer cells.

Cytotoxic Monoterpenoid Indole Alkaloids from Tabernaemontana corymbosa as Potent Autophagy Inhibitors by the Attenuation of Lysosomal Acidification

Twenty-six alkaloids, including the new taberines A-I (1-9), were obtained from Tabernaemontana corymbosa. The structures and absolute configurations were elucidated via MS, NMR, and ECD spectroscopic data analyses. Alkaloids 1-4 are new vobasinyl-ibogan alkaloids, and 1 is characterized by an unusual 1,3-oxazinane moiety. Alkaloids 4 and 16 exhibited moderate cytotoxic potency against various human cancer cell lines, while 4, 10, 11, 13, 14, and 16 showed attenuation of lysosomal acidification activity (EC₅₀: 12.9-29.8 μM), thereby inhibiting autophagic flux.

Macroline-Sarpagine Bisindole Alkaloids with Antiproliferative Activity from Alstonia penangiana

A methanol extract of the stem bark of the Malayan Alstonia penangiana provided seven new bisindole alkaloids, comprising six macroline-sarpagine alkaloids (angustilongines E-K, 1-6) and one macroline-pleiocarpamine bisindole alkaloid (angustilongine L, 7). Analysis of the spectroscopic data (NMR and MS) of these compounds led to the proposed structures of these alkaloids. The macroline-sarpagine alkaloids (1-6) showed in vitro growth inhibitory activity against a panel of human cancer cell lines, inclusive of KB, vincristine-resistant KB, PC-3, LNCaP, MCF7, MDA-MB-231, HT-29, HCT 116, and A549 cells (IC₅₀ values: 0.02-9.0 μM).

Ervadivamines A and B, Two Unusual Trimeric Monoterpenoid Indole Alkaloids from Ervatamia divaricata

Ervadivamines A (1) and B (2), two unprecedented trimeric monoterpenoid indole alkaloids, were isolated from Ervatamia divaricata. They are the first examples of vobasine-iboga-vobasine-type alkaloid with both C-C and C-N linkage patterns. Their structures including absolute configurations were fully accomplished by extensive spectroscopic analysis, single-crystal X-ray diffraction, and electric circular dichroism methods. The plausible biogenetic pathways of these trimeric alkaloids were also proposed. In addition, compound 1 exhibited significant cytotoxicity against four cancer cells.

Gelsecorydines A-E, Five Gelsedine-Corynanthe-Type Bisindole Alkaloids from the Fruits of Gelsemium elegans

Five monoterpenoid bisindole alkaloids with new carbon skeletons, gelsecorydines A-E (1-5), together with their biogenetic precursors were isolated from the fruits of Gelsemium elegans. Compounds 1-5 represent the first examples of heterodimeric frameworks composed of a gelsedine-type alkaloid and a modified corynanthe-type one. Notably, compound 2 featured an unprecedented caged skeleton with a 6/5/7/6/5/6 heterohexacyclic ring system, which possessed a pyridine ring that linked the two monomers. Their structures and absolute configurations were elucidated by spectroscopic analysis, X-ray diffraction, and electronic circular dichroism (ECD) calculation. A plausible biosynthetic pathway for compounds 1-5 is proposed. Compounds 1, 3, 4, and 5 exhibited a significant inhibitory effect against nitric oxide (NO) production in macrophages.

Synthesis of Bisindole Alkaloids from the Apocynaceae Which Contain a Macroline or Sarpagine Unit: A Review

Bisindole natural products consist of two monomeric indole alkaloid units as their obligate constituents. Bisindoles are more potent with respect to their biological activity than their corresponding monomeric units. In addition, the synthesis of bisindoles are far more challenging than the synthesis of monomeric indole alkaloids. Herein is reviewed the enantiospecific total and partial synthesis of bisindole alkaloids isolated primarily from the Alstonia genus of the Apocynaceae family. The monomeric units belong to the sarpagine, ajmaline, macroline, vobasine, and pleiocarpamine series. An up-to-date discussion of their isolation, characterization, biological activity as well as approaches to their partial and total synthesis by means of both synthetic and biosynthetic strategies are presented.