New monoterpenoid indole alkaloids from the stems of Tabernaemontana bovina Lour (Apocynaceae)

Two new monoterpenoid indole alkaloids, named taberibogines E and F (1 and 2), together with three known ones (3-5) were isolated from the stems of Tabernaemontana bovina Lour (Apocynaceae). Their structures including absolute configurations were elucidated from a combination of NMR and HRESIMS data and NMR calculations as well as DP4+ probability analyses. Compounds 1 and 2 exhibited inhibitory effects on LPS-induced nitric oxide (NO) production in RAW 264.7 macrophages.

Synthesis of Isoquinuclidines via Dearomative Diels-Alder Reaction of Cyclic Amidines with Indoles

The development and utilization of new dienes and dienophiles for the controlled synthesis of isoquinuclidines is highly appealing. Herein, we describe a novel strategy for diastereoselective synthesis of indoline-fused isoquinuclidines via copper-catalyzed dearomative Diels-Alder reaction of cyclic amidines with indoles. This protocol avoids the use of unstable DHPs and activated alkenes, offering a more efficient and selective approach to synthesize isoquinuclidines.

Comparative morpho-anatomical standardization and chemical profiling of root drugs for distinction of fourteen species of family Apocynaceae

BACKGROUND

The root drugs of the family Apocynaceae are medicinally important and used in Indian Systems of Medicine (ISM). There is often a problem of misidentification and adulteration of genuine samples with other samples in the market trade. Keeping in view the adulteration problem of raw drug material, comparative macroscopic and microscopic (qualitative and quantitative) characterisation and chemical analysis (TLC and LC-MS profiling) of a total of 14 economically important root drugs of family Apocynaceae were done for practical and rapid identification. A total of 33 qualitative botanical characteristics of root samples were subjected to Principal Component Analysis (PCA) and Cluster analysis to identify taxonomically significant characteristics in the distinction of root drug samples at the species level.

RESULTS

Comparative qualitative and quantitative data on morphological, macroscopic, and microscopic characters were generated for the studied 14 species. Despite the similarity in some root characters, a combined study involving the surface, anatomical, and powder features helped distinguish root samples at the species level. The relative relationship between selected species was represented as clustering or grouping in the dendrogram. PCA analysis determined significant characters leading to species grouping and identification. Results showed that clustering of xylem vessels in cross-section, pore size, and distribution in the cut root, the shape of starch grains, the thickness of cork zone were among the most notable characters in species distinction. Chemical profiling revealed unique fingerprints and content of chemical compounds, which were significant in identification of root drug samples.

CONCLUSIONS

The comparative botanical standards and chemical profiles developed in the present study can be used as future reference standards for the quick, easy, and correct identification of root drug samples to be used in the herbal drug industry. Further, the identified significant microscopic characters have the potential for taxonomic studies in species delimitation.

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.

A General Iridium-Catalyzed Reductive Dienamine Synthesis Allows a Five-Step Synthesis of Catharanthine via the Elusive Dehydrosecodine

A new reductive strategy for the stereo- and regioselective synthesis of functionalized isoquinuclidines has been developed. Pivoting on the chemoselective iridium(I)-catalyzed reductive activation of β,γ-unsaturated δ-lactams, the efficiently produced reactive dienamine intermediates readily undergo [4 + 2] cycloaddition reactions with a wide range of dienophiles, resulting in the formation of bridged bicyclic amine products. This new synthetic approach was extended to aliphatic starting materials, resulting in the efficient formation of cyclohexenamine products, and readily applied as the key step in the shortest (five-step) total synthesis of vinca alkaloid catharanthine to date, proceeding via its elusive biosynthetic precursor, dehydrosecodine.

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

By the end of the twentieth century, the interest in natural compounds as probable sources of drugs has declined and was replaced by other strategies such as molecular target-based drug discovery. However, in the recent times, natural compounds regained their position as extremely important source drug leads. Indole-containing compounds are under clinical use which includes vinblastine and vincristine (anticancer), atevirdine (anti-HIV), yohimbine (erectile dysfunction), reserpine (antihypertension), ajmalicine (vascular disorders), ajmaline (anti-arrhythmic), vincamine (vasodilator), etc. Monoterpene Indole Alkaloids (MIAs) deserve the curiosity and attention of researchers due to their chemical diversity and biological activities. These compounds were considered as an impending source of drug-lead. In this review 444 compounds, were identified from six genera belonging to the family Apocynaceae, will be discussed. These genera (Alstonia, Rauvolfia, Kopsia, Ervatamia, and Tabernaemontana, and Rhazya) consist of 400 members and represent 20% of Apocynaceae species. Only 30 (7.5%) species were investigated, whereas the rest are promising to be investigated. Eleven bioactivities, including antibacterial, antifungal, anti-inflammatory and immunosuppressant activities, were reported. Whereas cytotoxic effect represents 47% of the reported activities. Convincingly, the genera selected in this review are a wealthy source for future anticancer drug lead.

The iboga enigma: the chemistry and neuropharmacology of iboga alkaloids and related analogs

Covering: 2000 up to 2020 Few classes of natural products have inspired as many chemists and biologists as have the iboga alkaloids. This family of monoterpenoid indole alkaloids includes the anti-addictive compound ibogaine as well as catharanthine, a precursor to the chemotherapeutic vinblastine. Despite being known for over 120 years, these small molecules continue to challenge our assumptions about biosynthetic pathways, catalyze our creativity for constructing complex architectures, and embolden new approaches for treating mental illness. This review will cover recent advances in both the biosynthesis and chemical synthesis of iboga alkaloids as well as their use as next-generation neurotherapeutics. Whenever appropriate, we provide historical context for the discoveries of the past decade and indicate areas that have yet to be resolved. While significant progress regarding their chemistry and pharmacology has been made since the 1960s, it is clear that the iboga alkaloids will continue to stoke scientific innovation for years to come.

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.

Asymmetric Formal Synthesis of (+)-Catharanthine via Desymmetrization of Isoquinuclidine

Although (+)-catharanthine is an attractive alkaloid for both clinical research and organic synthetic chemistry, only a limited number of approaches for its catalytic asymmetric synthesis exist. Herein, we describe a novel strategy for synthesizing a chiral intermediate of (+)-catharanthine via phosphoric acid-catalyzed asymmetric desymmetrization of a meso-isoquinuclidine possessing a 1,3-diol unit that was synthesized by a formal amide insertion reaction.

Three New Monoterpenoid Indole Alkaloids from Ervatamia pandacaqui

Three new monoterpenoid indole alkaloids, ervapandine A (1), (3R)-hydroxyibogaine (3), 12-hydroxyakuammicine N(4)-oxide (6), along with four known ones, were isolated from the twigs and leaves of Ervatamia pandacaqui. The structures of the new alkaloids were elucidated by spectroscopic methods and sugar hydrolysis experiment. All of the compounds were evaluated for their cytotoxicity against three human cell lines. Compound 7 showed moderate cytotoxic activity.

Bisindole alkaloids from Tabernaemontana corymbosa

Continued study in bioactive monoterpenoid alkaloids led to the isolation of nine undescribed alkaloids, taberyunines A-I, together with 32 known ones from the aerial parts of Tabernaemontana corymbosa Roxb. ex Wall (Apocynaceae). Among the undescribed alkaloids, taberyunines A-G and H-I were assigned to Aspidosperma-Aspidosperma and Vobasinyl-Ibogan type bisindoles, respectively. Their structures were determined by NMR spectra, MS data and X-ray diffraction. Taberyunine B showed significant cytotoxicity against three cancer cell lines.

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.

The Iboga Alkaloids

Iboga alkaloids are a particular class of indolomonoterpenes most often characterized by an isoquinuclidine nucleus. Their first occurrence was detected in the roots of Tabernanthe iboga, a sacred plant to the people of Gabon, which made it cult object. Ibogaine is the main representative of this class of alkaloids and its psychoactive properties are well documented. It has been proposed as a drug cessation treatment and has a wide range of activities in targeting opioids, cocaine, and alcohol. The purpose of this chapter is to provide a background on this molecule and related compounds and to update knowledge on the most recent advances made. Difficulties linked to the status of ibogaine as a drug in several countries have hampered its development, but 18-methoxycoronaridine is currently under evaluation for the same purposes and for the treatment of leishmaniasis. The chapter is divided into six parts: an introduction aiming at defining what is called an iboga alkaloid, and this is followed by current knowledge on their biosynthesis, which unfortunately remains a "black box" as far as the key construction step is concerned. Many of these alkaloids are still being discovered and the third and fourth parts of the chapter discuss the analytical tools in use for this purpose and give lists of new monomeric and dimeric alkaloids belonging to this class. When necessary, the structures are discussed especially with regard to absolute configuration determinations, which remain a point of weakness in their assignments. Part V gives an account of progress made in the synthesis, partial and total, which the authors believe is key to providing solid solutions to the industrial development of the most promising molecules. The last part of the chapter is devoted to the biological properties of iboga alkaloids, with particular emphasis on ibogaine and 18-methoxycoronaridine.

Ervaoffines E–G, three iboga-type alkaloids featuring ring C cleavage and rearrangement from Ervatamia officinalis

Three novel alkaloids (1–3) reveal the high structural plasticity of ring C in iboga-type alkaloids.

Melohemsines A-I, melodinus-type alkaloids from Melodinus hemsleyanus

Melohemsines A-I (1–9) were obtained from Melodinus hemsleyanus. 1 is the first melodinus-type alkaloid possessing a 6/6/5/5/6/3 six ring skeleton system. 2 and 3 are the first natural melodinus-type alkaloids with a reduction of two carbon units at C-20.