Progress in the Synthesis of Iboga-alkaloids and their Congeners

Investigation of Small-Molecule Constituents in Voacanga africana Seeds and Mapping of Their Spatial Distributions Using Laser Ablation Direct Analysis in Real-Time Imaging-Mass Spectrometry (LADI-MS)

Plant seeds are a renewable resource that can furnish access to medicinal natural products that can only otherwise be isolated from aerial or root parts, the harvest of which may be destructive to the plant or threaten its viability. However, optimization of the isolation of such compounds from seeds would be greatly assisted if the spatial distribution of the molecules of interest within the plant tissue were known. For example, iboga alkaloids that hold promise for the treatment of opioid use disorder are typically isolated from the leaves, bark, or roots of Tabernanthe or Voacanga spp. trees, but it would be more environmentally sustainable to isolate such compounds from their seeds. Here, we leveraged the unique capabilities of the ambient mass spectral imaging technique termed laser ablation direct analysis in real-time imaging-mass spectrometry (LADI-MS) to reveal the spatial distributions of a range of molecules, including alkaloids within V. africana seeds. In addition to six compounds previously reported in these seeds, namely, tetradecanoic acid, n-hexadecanoic acid, (Z,Z)-9,12-octadecadienoic acid, (Z)-9-octadecenoic acid, octadecanoic acid, and Δ14-vincamine, an additional 31 compounds were newly identified in V. africana seeds. The compound classes included alkaloids, terpenes, and fatty acids. The ion images showed that the fatty acids were localized in the embryo of the seed. The alkaloids, which were mainly localized in the seed endosperm, included strictamine, akuammidine, polyneruidine, vobasine, and Δ14-vincamine. This information can be exploited to enhance the efficiency of secondary metabolite isolation from V. africana seeds while eliminating the destruction of other plant parts.

Asymmetric Total Synthesis of (+)-Epiibogamine Enabled by Three-Component Domino Michael/Michael/Mannich Annulation of N-Sulfinyl Metallosilylenamines

The iboga alkaloids are promising antiaddictive and neuroregeneration candidates for medical treatment. There is a lack of studies for C20-epi iboga alkaloids due to the synthetic difficulties. Herein we report the shortest total synthesis of (+)-epiibogamine in seven steps from trimethyl orthobutyrate. The novel N-sulfinyl silylenamine reagent enabled the key step, with three-component domino Michael/Michael/Mannich annulation providing the 1-amino-2,4-diester scaffold with four new chiral centers, and access to the isoquinuclidine in high yield (84%) and diastereoselectivity (>95:5 dr).

Asymmetric Synthesis of Bridged N-Heterocycles with Tertiary Carbon Center through Barbas Dienamine-Catalysis: Scope and Applications

A direct aza-Diels-Alder reaction between 2-aryl-3H-indolin-3-ones and cyclic-enones has been developed to access chiral indolin-3-one fused polycyclic bridged compounds. This method proceeds via proline-catalyzed Barbas-dienamine intermediate formation from various cyclic-enones such as 2-cyclopenten-1-one, 2-cyclohexene-1-one, and 2-cycloheptene-1-one, followed by a reaction with 2-aryl-3H-indol-3-ones. Several indolin-3-ones fusing [2.2.2], [2.2.1], and [3.2.1] skeletons decorated with a tertiary carbon chiral center have been prepared. Computational studies (DFT) supported the observed stereoselectivity in the method. The synthesized compounds have shown exciting photophysical activities and selective sensing of Pd²⁺ and Fe³⁺ ions through the fluorescence quenching "switch-off" mode.

Total synthesis of the pseudoindoxyl class of natural products

The pseudoindoxyl sub-structural motif, amongst the large set of the indole class of alkaloids, represents a unique subset of the oxygenated indole class of the alkaloid family. A majority of this class of natural products contains complex bridged/polycyclic scaffolds with interesting biological profiles. They are thus attractive synthetic targets. Starting from 1963, twenty-eight natural products having the pseudoindoxyl scaffold have been isolated, among which the synthesis of 13 natural products has been accomplished. In this review, we highlight the completed as well as the formal total synthesis of the natural products with a spiro-pseudoindoxyl ring, with a focus on their development. The challenges and the future perspective based on the recent developments in the field will also be discussed. We strongly believe that this review will not only update but also attract the attention of researchers in dealing with the synthesis of pseudoindoxyl compounds.

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.

Direct Amine-Catalyzed Enantioselective Synthesis of Pentacyclic Dibenzo[b,f][1,4]oxazepine/Thiazepine-Fused Isoquinuclidines along with DFT Calculations

A direct protocol for the asymmetric synthesis of dibenzoxazepine/thiazepine-fused [2.2.2] isoquinuclidines is developed. The reaction proceeds through a proline-catalyzed direct Mannich reaction followed by an intramolecular aza-Michael cascade sequence between 2-cyclohexene-1-one and various tricyclic imines, like dibenzoxazepines/thiazepines, as an overall [4 + 2] aza-Diels-Alder reaction. A series of pentacyclic isoquinuclidines have been prepared, with complete endo-selectivity, in good to high yields and excellent enantioselectivity (>99:1). Density functional theory (DFT) calculations further support the observed high stereochemical outcome of the reaction.

Cascade cyclization and intramolecular nitrone dipolar cycloaddition and formal synthesis of 19-hydroxyibogamine

A cascade or domino sequence of condensation of hydroxylamine and an aldehyde to give an oxime, cyclization to a nitrone, and intramolecular 1,3-dipolar cycloaddition has been successfully employed where there is branching at C-4 as a route to the iboga alkaloids. Cyclization occurs with displacement of chloride as a leaving group and intramolecular cycloaddition occurs with an alkene as a dipolarophile. The reaction gives an azabicyclo[2.2.2]octane product containing a fused isoxazolidine as a single stereoisomer and this was converted to an isoquinuclidine that completed a formal synthesis of the alkaloid (±)-19-hydroxyibogamine.

One-Pot Synthesis of Chiral Tetracyclic Dibenzo[ b, f][1,4]oxazepine-Fused 1,2-Dihydropyridines (DHPs) under Metal-Free Conditions

An efficient protocol for the catalytic asymmetric synthesis of new dibenzo[ b, f][1,4]-oxazepine-fused 1,2-dihydropyridines (DHPs) has been described under metal-free conditions. This reaction proceeds through proline-catalyzed direct Mannich/cyclization between seven-membered dibenzo[ b, f][1,4]-oxazepine-imines and aqueous glutaraldehyde, followed by IBX-mediated site-selective dehydrogenative oxidation in one-pot operation with high yields (up to 92%) and excellent enantioselectivity (up to >99:1 er).

Missing enzymes in the biosynthesis of the anticancer drug vinblastine in Madagascar periwinkle

Vinblastine, a potent anticancer drug, is produced by Catharanthus roseus (Madagascar periwinkle) in small quantities, and heterologous reconstitution of vinblastine biosynthesis could provide an additional source of this drug. However, the chemistry underlying vinblastine synthesis makes identification of the biosynthetic genes challenging. Here we identify the two missing enzymes necessary for vinblastine biosynthesis in this plant: an oxidase and a reductase that isomerize stemmadenine acetate into dihydroprecondylocarpine acetate, which is then deacetoxylated and cyclized to either catharanthine or tabersonine via two hydrolases characterized herein. The pathways show how plants create chemical diversity and also enable development of heterologous platforms for generation of stemmadenine-derived bioactive compounds.

Non-monoterpenoid azepinoindole alkaloids

This review discusses various biological and chemical aspects of the non-monoterpenoid azepinoindole class of alkaloids, including their isolation, biosynthesis and total synthesis.

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.

hERG Blockade by Iboga Alkaloids

The iboga alkaloids are a class of naturally occurring and synthetic compounds, some of which modify drug self-administration and withdrawal in humans and preclinical models. Ibogaine, the prototypic iboga alkaloid that is utilized clinically to treat addictions, has been associated with QT prolongation, torsades de pointes and fatalities. hERG blockade as IKr was measured using the whole-cell patch clamp technique in HEK 293 cells. This yielded the following IC50 values: ibogaine manufactured by semisynthesis via voacangine (4.09 ± 0.69 µM) or by extraction from T. iboga (3.53 ± 0.16 µM); ibogaine's principal metabolite noribogaine (2.86 ± 0.68 µM); and voacangine (2.25 ± 0.34 µM). In contrast, the IC50 of 18-methoxycoronaridine, a product of rational synthesis and current focus of drug development was >50 µM. hERG blockade was voltage dependent for all of the compounds, consistent with low-affinity blockade. hERG channel binding affinities (K i) for the entire set of compounds, including 18-MC, ranged from 0.71 to 3.89 µM, suggesting that 18-MC binds to the hERG channel with affinity similar to the other compounds, but the interaction produces substantially less hERG blockade. In view of the extended half-life of noribogaine, these results may relate to observations of persistent QT prolongation and cardiac arrhythmia at delayed intervals of days following ibogaine ingestion. The apparent structure-activity relationships regarding positions of substitutions on the ibogamine skeleton suggest that the iboga alkaloids might provide an informative paradigm for investigation of the structural biology of the hERG channel.

Enantioselective synthesis of Iboga alkaloids and vinblastine via rearrangements of quaternary ammoniums

An efficient and novel strategy for the enantioselective syntheses of various iboga alkaloids has been developed. The salient features include a gold-catalyzed oxidation of a terminal alkyne followed by cyclization, a Stevens rearrangement and a tandem sequence that combines the gold-catalyzed oxidation, cyclization and [1,2]-shift. The catharanthine analogs provided by our approach were further converted to the vinca alkaloid vinblastine and its analogs, which confirmed the remarkable sensitivity of the cytotoxicity to the C20' substituent of vinblastine.

Bioinspired Collective Syntheses of Iboga-Type Indole Alkaloids

We present the application of a bioinspired collective synthesis strategy in the total syntheses of seven iboga-type indole alkaloids: (±)-tabertinggine, (±)-ibogamine, (±)-ibogaine, (±)-ibogaine hydroxyindolenine, (±)-3-oxoibogaine hydroxyindolenine, (±)-iboluteine, and (±)-ervaoffines D. In particular, tabertinggine and its congeners serve as iboga precursors for the subsequent biomimetic transformations into other iboga-type alkaloids.

Enantioselective Synthesis of N-PMP-1,2-dihydropyridines via Formal [4 + 2] Cycloaddition between Aqueous Glutaraldehyde and Imines

A simple and highly practical one-pot formal [4 + 2] cycloaddition approach for the enantioselective synthesis of N-PMP-1,2-dihydropyridines (DHPs) is described. This chemistry involves an amino-catalytic direct Mannich reaction/cyclization followed by IBX-mediated chemo- and regioselective oxidation sequence between readily available aqueous glutaraldehyde and imines under very mild conditions. A series of N-PMP-1,2-DHPs have been prepared in high yields and excellent enantioselectivity. This method also gives access to both enantiomers of 1,2-DHPs in surplus amount by shifting the catalyst configuration.

Constructing Iboga alkaloids via C-H bond functionalization: examination of the direct and catalytic union of heteroarenes and isoquinuclidine alkenes

The iboga alkaloids have attracted considerable attention in both the scientific community and popular media due to their reported ability to reverse or markedly diminish cravings for, and self-administration of, the major drugs of abuse. We have developed three new intramolecular C-H functionalization procedures leading to the core seven-membered ring of the iboga skeleton, a cyclization that proved to be highly challenging. The electrophilic palladium salt Pd(CH3CN)4(BF4)2 was effective for the cyclization of diverse N-(2-arylethyl)isoquinuclidines with yields of 10-35%. A two-step, bromination-reductive Heck reaction protocol was also effective for the synthesis of ibogamine in 42% yield. Finally, a direct Ni(0)-catalyzed C-H functionalization provided the benzofuran analogues of ibogamine (74%) and epi-ibogamine (38%). Although each approach suffers from significant shortcomings, in combination, the methods described provide practical routes to diverse ibogamine analogues.

General synthesis of substituted 1,2-dihydropyridines

A general and practical metal-free protocol for the synthesis of 1,2-dihydropyridines with wide structural/functional diversity at the ring and featuring mono, double, or spiro substitution at the sp(3) position is described. The protocol entails a microwave-assisted domino reaction of a propargyl vinyl ether (secondary or tertiary) and a primary amine (aliphatic or aromatic) in toluene or methanol.

Stereoselective synthesis of isoquinuclidines through an aza-[4 + 2] cycloaddition of chiral cyclic 2-amidodienes

A highly stereoselective aza-[4 + 2] cycloaddition of chiral cyclic 2-amidodienes with N-sulfonyl aldimines is described. While this Lewis acid promoted heterocycloaddition provides an efficient strategy for constructing optically enriched isoquinuclidines, it is mechanistically intriguing. The cycloaddition favored the endo-II pathway in the absence of a viable bidentate coordination. This represents an unexpected switch from the anticipated endo-I selectivity obtained in the all-carbon cycloaddition.

Effect of Iboga alkaloids on µ-opioid receptor-coupled G protein activation

OBJECTIVE

The iboga alkaloids are a class of small molecules defined structurally on the basis of a common ibogamine skeleton, some of which modify opioid withdrawal and drug self-administration in humans and preclinical models. These compounds may represent an innovative approach to neurobiological investigation and development of addiction pharmacotherapy. In particular, the use of the prototypic iboga alkaloid ibogaine for opioid detoxification in humans raises the question of whether its effect is mediated by an opioid agonist action, or if it represents alternative and possibly novel mechanism of action. The aim of this study was to independently replicate and extend evidence regarding the activation of μ-opioid receptor (MOR)-related G proteins by iboga alkaloids.

METHODS

Ibogaine, its major metabolite noribogaine, and 18-methoxycoronaridine (18-MC), a synthetic congener, were evaluated by agonist-stimulated guanosine-5´-O-(γ-thio)-triphosphate ([(35)S]GTPγS) binding in cells overexpressing the recombinant MOR, in rat thalamic membranes, and autoradiography in rat brain slices.

RESULTS AND SIGNIFICANCE

In rat thalamic membranes ibogaine, noribogaine and 18-MC were MOR antagonists with functional Ke values ranging from 3 uM (ibogaine) to 13 uM (noribogaine and 18MC). Noribogaine and 18-MC did not stimulate [(35)S]GTPγS binding in Chinese hamster ovary cells expressing human or rat MORs, and had only limited partial agonist effects in human embryonic kidney cells expressing mouse MORs. Ibogaine did not did not stimulate [(35)S]GTPγS binding in any MOR expressing cells. Noribogaine did not stimulate [(35)S]GTPγS binding in brain slices using autoradiography. An MOR agonist action does not appear to account for the effect of these iboga alkaloids on opioid withdrawal. Taken together with existing evidence that their mechanism of action also differs from that of other non-opioids with clinical effects on opioid tolerance and withdrawal, these findings suggest a novel mechanism of action, and further justify the search for alternative targets of iboga alkaloids.