Akuammiline Alkaloids Research Articles

Construction of the Akuammiline Alkaloid Core Structure via Stereoselective E-Ring Formation.

Construction of the core structure of akuammiline alkaloids with three-dimensional cage-like structures for their diversity-oriented synthesis was investigated. Extensive exploration centered around the introduction of nitrogen functional groups and construction of the E-ring in an intramolecular or intermolecular manner revealed that a Claisen rearrangement approach involving intramolecular amination provided a common precursor, potentially facilitating divergent access to various types of akuammiline alkaloids.

Regioselective Synthesis of the Tetrahydrocarbazole Core of Akuammiline Alkaloids via Palladium-Catalyzed Intramolecular Arylation Reaction.

Tetrahydrocarbazole is the central core for several biologically active alkaloids, and regioselective synthesis of this core is a challenging task. Herein, we report an efficient strategy for the synthesis of this core involving palladium-catalyzed intramolecular arylation reaction with excellent regioselectivity (>99%) starting from N-phenyl-bromoalkene without having any relocation of double bonds via competitive palladium-catalyzed isomerization reaction. Broad functional group tolerance and exclusive regioselectivity have been observed for meta-substituted halide substrates. Furthermore, this reaction can be scalable on the gram scale.

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.

Akuammiline alkaloid derivatives: divergent synthesis and effect on the proliferation of rheumatoid arthritis fibroblast-like synoviocytes.

During the past decades, rheumatoid arthritis had become a serious problem, torturing millions of patients because of unclear pathogenesis and no ideal therapies. Natural products remain an important source of medicines to treat various major diseases such as rheumatoid arthritis (RA) given their excellent biocompatibility and structural diversity. Herein, we have developed a versatile synthetic method for constructing various skeletons of akuammiline alkaloid analogs based on our previous research on the total synthesis of the related indole alkaloids. We have also evaluated the effect of these analogs on the proliferation of RA fibroblast-like synoviocytes (FLSs) in vitro and analyzed the corresponding structure-activity relationship (SAR). Among these analogs, compounds 9 and 17c have demonstrated a promising inhibitory effect on the proliferation of RA-FLSs, with IC50 values of 3.22 ± 0.29 μM and 3.21 ± 0.31μM, respectively. Our findings provide a solid foundation for future pharmacological studies on akuammiline alkaloid derivatives and inspiration for the development of anti-RA small molecule drugs derived from natural products.

Total Synthesis of (-)-Picrinine, (-)-Scholarisine C, and (+)-5-β-Methoxyaspidophylline.

The first asymmetric total synthesis of three picrinine-type akuammiline alkaloids, (-)-picrinine, (-)-scholarisine C, and (+)-5-β-methoxyaspidophylline, has been accomplished. The synthesis features an efficient acid-promoted oxo-bridge ring-opening and further carbonyl O-cyclization to assemble the furoindoline scaffold, an unusual Dauben-Michno oxidation to construct the requisite α,β-unsaturated aldehyde functionality, and a nickel-mediated reductive Heck reaction to forge the [3.3.1]-azabicyclic core.

Construction of Indoline/Indolenine Ring Systems by a Palladium-Catalyzed Intramolecular Dearomative Heck Reaction and the Subsequent Aza-semipinacol Rearrangement.

The palladium-catalyzed intramolecular dearomative Heck reaction of 2,3-disubstituted indoles serves as an access to spiro-indoline products. Herein, we report an efficient construction of indoline/indolenine core stuctures via a dearomative Heck reaction of simple 2,3-disubstituted indoles with all-carbon tethers and the subsequent aza-semipinacol rearrangement. The Heck reaction features a high C2-selectivity, and the stereospecific aryl/alkyl migration selectivity has been investigated by DFT calculations. Using this method, we accomplished the formal total synthesis of akuammiline alkaloids vincorine.

Intramolecular Sakurai Allylation of Geminal Bis(silyl) Enamide with Indolenine. A Diastereoselective Cyclization To Form Functionalized Hexahydropyrido[3,4-b]Indole.

A fluoride-promoted intramolecular Sakurai allylation of geminal bis(silyl) enamide with indolenine has been developed. The reaction facilitates an efficient cyclization to give hexahydropyrido[3,4-b]indoles in good yields with high diastereoselectivity. The resulted cis, trans-stereochemistry further enables the ring-closing metathesis (RCM) reaction of two alkene moieties, giving a tetracyclic N-hetereocycle widely found as the core structure in akuammiline alkaloids.

Monoterpenoid indole alkaloids from Kopsia hainanensis Tsiang

A phytochemical investigation on the twigs and leaves of Kopsia hainanensis Tsiang resulted in the isolation and identification of 18 alkaloids, including two sarpagine type alkaloids (1 and 2), five eburnane type alkaloids (3–7), three aspidofractinine type alkaloids (8–10), one vincadine type alkaloid (11), three akuammiline type alkaloids (12–13 and 15), one corynanthean type alkaloid (14), two ajmalicine-like type alkaloids (16 and 17), and one aspidospermine type alkaloid (18). The new structure of compound 1 was elucidated by means of spectroscopic analysis, including HRESIMS, and 1D and 2D NMR experiments. Compounds 1–2, 4–5, 7, and 10–17 are herein reported for the first time from this plant, while the compounds 1, 2, 7, and 12–17 have not been previously recorded in the Kopsia genus. The chemotaxonomic significance and distribution of these monoterpenoid indole alkaloids in Kopsia genus are discussed.

Synthesis of polycyclic scaffolds via a gold-catalysed dearomative cyclisation cascade

Polycyclic scaffolds found in akuammiline alkaloid natural products can be synthesised from ynone-tethered indoles, via a gold(I)-catalysed dearomative cyclisation cascade sequence. The ynone starting materials are themselves made via a two-step modular synthesis from simple tryptamine and tryptophol derivatives.

Enantioselective Total Synthesis of (+)-Corymine and (-)-Deformylcorymine.

We report herein the first enantioselective total synthesis of akuammiline alkaloids (+)-corymine and (-)-deformylcorymine. Starting from commercially available N-nosyltryptamine, the target molecules are both achieved in 11 steps. Key elements of the design include (a) a copper-catalyzed enantioselective addition of dimethyl malonate to a 3-bromooxindole to secure the C7 all-carbon quaternary stereocenter, (b) a one-step construction of cyclohexyl and pyrrolidinyl rings via intramolecular nucleophilic C- and N-addition, and (c) a nickel-promoted 7-endo cyclization of alkenyl bromide to furnish the azepanyl ring. The strategy is further extended to the synthesis of another three members of the akuammiline family, namely, (-)-10-demethoxyvincorine, (-)-2(S)-cathafoline, and (-)-3-epi-dihydrocorymine 17-acetate.

Modular Synthesis of Polycyclic Alkaloid Scaffolds via an Enantioselective Dearomative Cascade.

The polycyclic core of the akuammiline alkaloids can be synthesized from simple tryptamine and tryptophol derivatives via a Ag(I)-catalyzed enantioselective dearomative cyclization cascade sequence. The complex tetracyclic scaffolds are prepared via a rapid, versatile, three-step modular synthesis from simple commercially available indole derivatives in high yields and enantiomeric excess (up to 99% yield and >99% ee).

Asymmetric Total Syntheses of the Akuammiline Alkaloids (-)-Strictamine and (-)-Rhazinoline.

Strictamine and rhazinoline are representative methanoquinolizidine-containing akuammiline alkaloids that possess different stereochemistry at the C16 position. A unified approach to the enantioselective total syntheses of these two molecules is described. The key steps in this synthesis include a photocatalytic intra/intermolecular type II radical cascade reaction, a Tsuji-Trost allylation, a palladium- or nickel-mediated cyclization, and a late-stage intramolecular N-alkylation reaction.

A Photocatalyzed Cascade Approach Toward the Tetracyclic Core of Akuammiline Alkaloids.

A cascade approach toward the tetracyclic core of akuammiline alkaloids which features high modularity and convergence is reported. Distinct substitution pattern can be readily introduced to the tetracyclic core by varying three building blocks with similar complexity. The critical event in the cascade is a regio- and stereoselective 1,2-shift enabled by a carbocation that sets up the core.

Enantioselective Total Syntheses of Methanoquinolizidine-Containing Akuammiline Alkaloids and Related Studies.

The akuammiline alkaloids are a structurally diverse class of bioactive natural products isolated from plants found in various parts of the world. A particularly challenging subset of akuammiline alkaloids are those that contain a methanoquinolizidine core. We describe a synthetic approach to these compounds that has enabled the first total syntheses of (+)-strictamine, (-)-2( S)-cathafoline, (+)-akuammiline, and (-)-Ψ-akuammigine. Our strategy relies on the development of the reductive interrupted Fischer indolization reaction to construct a common pentacyclic intermediate bearing five contiguous stereocenters, in addition to late-stage formation of the methanoquinolizidine framework using a deprotection-cyclization cascade. The total syntheses of (-)-Ψ-akuammigine and (+)-akuammiline mark the first preparations of akuammiline alkaloids containing both a methanoquinolizidine core and vicinal quaternary centers. Lastly, we describe the bioinspired reductive rearrangements of (+)-strictamine and (+)-akuammiline to ultimately provide (-)-10-demethoxyvincorine and a new analogue thereof.

Iodine-Catalyzed C-H Amidation and Imination at the 2α-Position of 2,3-Disubstituted Indoles with Chloramine Salts.

A novel iodine-catalyzed amidation and imination at the 2α-position of 2,3-disubstituted indoles in the presence of chloramine salts with high regioselectivity has been achieved. The protocol is applicable to a wide range of substrates to deliver the corresponding 2α-nitrogen-containing indole derivatives. Furthermore, to demonstrate the synthetic value of this established transformation, a concise assembly of the bridged tetracyclic framework of akuammiline alkaloids from the 2α-amidated product has been accomplished in five steps.

Total synthesis of akuammiline alkaloid (+)-strictamine

An asymmetric total synthesis of the akuammiline alkaloid (+)-strictamine is described. The key steps of the synthetic route involve an improved Johnson-Claisen rearrangement to facilitate the formation of the challenging C15–C20 linkage and the (E)-alkene side chain, intramolecular nucleophilic substitution cyclization to establish the E ring and diastereoselective Brown hydroboration of the terminal alkene to introduce the C16 stereocenter.

Unified Total Syntheses of Structurally Diverse Akuammiline Alkaloids.

The unified total syntheses of structurally diverse akuammiline alkaloids deformylcorymine (1), strictamine (2), and calophyline A (3) are reported. The strategy mimics the biosynthesis in nature at a strategic level, which allows for structural diversification from a common synthetic precursor by late-stage ring migrations.

Asymmetric Total Synthesis of (+)-Minfiensine by an Asymmetric Cascade Cyclization Strategy

The Strychnos alkaloid minfiensine and a series of akuammiline alkaloids, such as vincorine, aspidophylline A, and picrinine, possess a common core skeleton, a 4a,9a-heterocycle-fused tetrahydrocarbazole. Efficient construction of this core structure in a highly enantioselective manner would facilitate the total synthesis of these alkaloids. In this article, we briefly summarize the established strategies for obtaining this core structure, together with the corresponding total-synthesis routes, and we describe our own effort on the development of a new strategy, the asymmetric cascade dearomative cyclization, for the efficient total synthesis of (+)-minfiensine.

Formal Total Synthesis of (±)-Strictamine by [2,3]-Sigmatropic Stevens Rearrangements.

To date, more than 100 congeners of the akuammiline alkaloid family have been isolated. Their signature structural element is a methanoquinolizidine moiety, a cage-like scaffold structurally related to adamantane. The structural variations of the family members originate from oxidative processes that mostly trigger rearrangements of the methanoquinolizidine motif. The family of the akuammiline alkaloids is best represented by strictamine. It bears the least functionalized carbon skeleton of all family members without lacking the signature structural motifs. Herein, we report the formal synthesis of strictamine through a Stevens [2,3]-sigmatropic rearrangement as a key step and the synthetic pitfalls related with its synthesis.

A 7-Step Formal Asymmetric Total Synthesis of Strictamine via an Asymmetric Propargylation and Metal-Mediated Cyclization.

Herein is shown how a novel catalytic asymmetric propargylation of 3,4-dihydro-β-carboline, followed by a designed Au(I)/Ag(I)-mediated 6-endo-dig cyclization, can directly deliver the indolenine-fused methanoquinolizidine core of the akuammiline alkaloid strictamine in its native oxidation state, ultimately achieving a 7-step formal asymmetric total synthesis. Also demonstrated are how the cyclization products can rearrange into vincorine-type skeletons and a further use for the developed propargylation with the first catalytic asymmetric total synthesis of decarbomethoxydihydrogambirtannine.