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

Asymmetric Synthesis of 10-Demethoxyvincorine Enabled by Dual Ni/Ti-Catalyzed Reductive Cyclization

Herein, we report the asymmetric synthesis of 10-demethoxyvincorine in 12 steps. The synthesis is highlighted by several key transformations: (1) a Pd-catalyzed Catellani-type reaction for the preparation of C2-alkylated tryptamine, (2) a chiral phosphoric acid (CPA)-catalyzed asymmetric bromocyclization to construct enantioenriched 3a-bromo-hexahydropyrroloindoline, (3) a dual Ni/Ti-catalyzed reductive cyclization to establish the bridged ring system, and (4) a SmI2-promoted reductive cyclization to forge the strained E-ring.

Recent strategy for the synthesis of indole and indoline skeletons in natural products

Indole alkaloids are one of the most important classes of natural products found in nature, particularly in a wide variety of plants. These compounds have compact polycyclic systems with at least one nitrogen atom. Several of these alkaloids are bioactive and have raised hopes for the development of new drugs. Their biosynthesis involves tryptophan as an amino acid precursor, since the indole or indoline moiety is the main heterocycle of these natural products. However, in their quest to synthesize such complex architectures, chemists have developed several different strategies to produce this key heterocycle quickly and in an unnatural way. This review focuses on the recent total synthesis methods used to prepare the indole and indoline core of these important alkaloids. Novel and older methods that allow the rapid formation of this heterocycle are described as key steps in the total synthesis of these fascinating structures designed by Mother Nature.

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.

Gold-catalyzed cyclization and cycloaddition in natural product synthesis

Covering: 2016 to mid 2023Transition metal catalysis, known for its remarkable capacity to expedite the assembly of molecular complexity from readily available starting materials in a single operation, occupies a central position in contemporary chemical synthesis. Within this landscape, gold-catalyzed reactions present a novel and versatile paradigm, offering robust frameworks for accessing diverse structural motifs. In this review, we highlighted a curated selection of publications in the past 8 years, focusing on the deployment of homogeneous gold catalysis in the ring-forming step for the total synthesis of natural products. These investigations are categorized based on the specific ring formations they engender, accentuating the prevailing gold-catalyzed methodologies applied to surmount intricate challenges in natural products synthesis.

Discovery of a Novel Pseudo-Natural Product Aurora Kinase Inhibitor Chemotype through Morphological Profiling

The pseudo-natural product (pseudo-NP) concept aims to combine NP fragments in arrangements that are not accessible through known biosynthetic pathways. The resulting compounds retain the biological relevance of NPs but are not yet linked to bioactivities and may therefore be best evaluated by unbiased screening methods resulting in the identification of unexpected or unprecedented bioactivities. Herein, various NP fragments are combined with a tricyclic core connectivity via interrupted Fischer indole and indole dearomatization reactions to provide a collection of highly three-dimensional pseudo-NPs. Target hypothesis generation by morphological profiling via the cell painting assay guides the identification of an unprecedented chemotype for Aurora kinase inhibition with both its relatively highly 3D structure and its physicochemical properties being very different from known inhibitors. Biochemical and cell biological characterization indicate that the phenotype identified by the cell painting assay corresponds to the inhibition of Aurora kinase B.

Deprotective Functionalization: A Direct Conversion of Nms-Amides to Carboxamides Using Carboxylic Acids

The nature of protecting group chemistry necessitates a deprotection step to restore the initially blocked functionality prior to further transformation. As this aspect of protecting group manipulation inevitably adds to the step count of any synthetic sequence, the development of methods enabling simultaneous deprotection and functionalization ("deprotective functionalization"-distinct from "deprotection followed by functionalization") is appealing, as it has the potential to improve efficiency and streamline synthetic routes. Herein, we report a deprotective functionalization of the newly introduced Nms-amides guided by density functional theory (DFT) analysis, which exploits the inherent Nms reactivity. Mechanistic studies further substantiate and help rationalize the exquisite reactivity of Nms-amides, as other commonly used protecting groups are shown not to exhibit the same reactivity patterns. The practicality of this approach was ultimately demonstrated in selected case studies.

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.

Finding activity through rigidity: syntheses of natural products containing tricyclic bridgehead carbon centers

Covering: up to 2022Tricyclic bridgehead carbon centers (TBCCs) are a synthetically challenging substructure found in many complex natural products. Here we review the syntheses of ten representative families of TBCC-containing isolates, with the goal of outlining the strategies and tactics used to install these centers, including a discussion of the evolution of the successful synthetic design. We provide a summary of common strategies to inform future synthetic endeavors.

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp3-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

Asymmetric Total Synthesis of (+)-Alstonlarsine A

The first asymmetric total synthesis of (+)-alstonlarsine A has been realized. The prominent features of the current synthesis include the following: (i) a Pd/self-adaptable ligand complex-catalyzed asymmetric allylic alkylation of 2-methyl-2-cyclopentenyl carbonate with 2-indolylsubstituted dimethyl malonate to establish the key stereocenter of C15, (ii) an intramolecular nitrile oxide-alkene [3 + 2] cycloaddition (INOC [3 + 2]) to construct the cyclohepta[b]indole backbone with the installment of the requisite stereochemistry of the all-carbon quaternary center of C20, and (iii) a late-stage interrupted Pictet-Spengler reaction (IPSR) to rapidly assemble the core structure of (+)-alstonlarsine A.

Natural product total synthesis using rearrangement reactions

The synthetic utility of rearrangement reactions in total synthesis for the rapid construction of core skeletons, the precise control of stereochemistry, and the identification of suitable synthons has been discussed.

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.

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.

Total Synthesis of (+)-Alsmaphorazine C and Formal Synthesis of (+)-Strictamine: A Photo-Fries Approach

A bioinspired photo-Fries/imine capture cascade reaction was developed in continuous-flow mode, which facilitated the rapid construction of a series of diversely functionalized 2,7-heterocycle-fused tetrahydrocarbazoles, the ubiquitous core structures embedded in strychnos and akuammiline-type monoterpene indole alkaloids. The synthetic utility of this novel method has been preliminarily explored by the first total synthesis of (+)-alsmaphorazine C and formal synthesis of (+)-strictamine in a concise and efficient manner.

Nonbiomimetic total synthesis of indole alkaloids using alkyne-based strategies

Biomimetic natural product synthesis is generally straightforward and efficient because of its established feasibility in nature and utility in comprehensive synthesis, and the cost-effectiveness of naturally derived starting materials. On the other hand, nonbiomimetic strategies can be an important option in natural product synthesis since (1) nonbiomimetic synthesis offers more flexibility and can demonstrate the originality of chemists, and (2) the structures of derivatives accessible by nonbiomimetic synthesis can be considerably different from those that are synthesised in nature. This review summarises nonbiomimetic total syntheses of indole alkaloids using alkyne chemistry for constructing core structures, including ergot alkaloids, monoterpene indole alkaloids (mainly corynanthe, aspidosperma, strychnos, and akuammiline), and pyrroloindole and related alkaloids. To clarify the differences between alkyne-based strategies and biosynthesis, the alkynes in nature and the biosyntheses of indole alkaloids are also outlined.

Regio- and Diastereoselective Copper-Catalyzed Carbomagnesiation for the Synthesis of Penta- and Hexa-Substituted Cyclopropanes

Despite the highly strained nature of cyclopropanes possessing three vicinal quaternary carbon stereocenters, the regio- and diastereoselective copper-catalyzed carbomagnesiation reaction of cyclopropenes provides an easy and efficient access to these novel persubstituted cyclopropyl cores with a complete regio- and diastereoselectivity.

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.

Synthesis of Phenol-Derived cis-Vinyl Ethers Using Ethynyl Benziodoxolone

The stereoselective synthesis of cis-β-phenoxyvinyl benziodoxolones (cis-β-phenol-VBXs) from an ethynyl benziodoxolone-acetonitrile complex (EBX-MeCN) and various phenols is reported herein. The reaction tolerates different phenol derivatives, including complex natural products, and can be conducted under mild conditions. The synthesis was performed in an aqueous solvent in the absence and presence of a catalytic amount of a base. Selectively mono- and di-deuterated cis-β-phenol-VBXs were also prepared. cis-β-Phenol-VBXs were stereospecifically derivatized to cis-alkynylvinyl ethers and cis-iodovinyl ethers without loss of stereoselectivity or reduction in the deuterium/hydrogen ratio.

Scalable Synthesis of Vicinal Quaternary Stereocenters via the Solid-State Photodecarbonylation of a Crystalline Hexasubstituted Ketone

We report the synthesis and stereospecific solid-state photodecarbonylation of a hexasubstituted ketone featuring six distinct α-substituents. The photoproduct of the solid-state transformation features vicinal all-carbon quaternary stereocenters. While reactions carried out in bulk powders and aqueous crystalline suspensions were complicated by secondary photochemistry of the primary photoproduct, optimal conditions provided good yields and recyclable starting material. Subsequent transformations of the α-substituents having orthogonal chemical reactivity illustrate the potential of this transformation toward constructing complex architectures.

Total Syntheses of Dihydroindole Aspidosperma Alkaloids: Reductive Interrupted Fischer Indolization Followed by Redox Diversification

We report a novel reductive interrupted Fischer indolization process for the concise assembly of the 20-oxoaspidospermidine framework. This rapid complexity generating route paves the way toward various dihydroindole Aspidosperma alkaloids with different C-5 side chain redox patterns. The end-game redox modulations were accomplished by modified Wolff-Kishner reaction and photo-Wolff rearrangement, enabling the total synthesis of (-)-aspidospermidine, (-)-limaspermidine, and (+)-17-demethoxy-N-acetylcylindrocarine and the formal total synthesis of (-)-1-acetylaspidoalbidine.

New Synthetic Routes to (+)-Uleine and (-)-Tubifolidine: General Approach to 2-Azabicyclo[3.3.1]nonane Indole Alkaloids

Novel asymmetric synthetic routes to (+)-uleine and (-)-tubifolidine are reported herein. The regioselective formation of enol triflates from 2-azabicyclo[3.3.1]nonane ketones followed by indolizations of the resultant ene-hydrazides allowed the efficient construction of key indole intermediates, facilitating the total synthesis of the target natural alkaloids.

Rhodium-catalyzed reductive carbonylation of aryl iodides to arylaldehydes with syngas

The reductive carbonylation of aryl iodides to aryl aldehydes possesses broad application prospects. We present an efficient and facile Rh-based catalytic system composed of the commercially available Rh salt RhCl3·3H2O, PPh3 as phosphine ligand, and Et3N as the base, for the synthesis of arylaldehydes via the reductive carbonylation of aryl iodides with CO and H2 under relatively mild conditions with a broad substrate range affording the products in good to excellent yields. Systematic investigations were carried out to study the experimental parameters. We explored the optimal ratio of Rh salt and PPh3 ligand, substrate scope, carbonyl source and hydrogen source, and the reaction mechanism. Particularly, a scaled-up experiment indicated that the catalytic method could find valuable applications in industrial productions. The low gas pressure, cheap ligand and low metal dosage could significantly improve the practicability in both chemical researches and industrial applications.

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).

Total Syntheses of (-)-Deoxoapodine, (-)-Kopsifoline D, and (-)-Beninine

The total syntheses of Aspidosperma and Kopsia alkaloids (-)-deoxoapodine, (-)-kopsifoline D, and (-)-beninine are described through a domino deprotection-Michael addition-nucleophilic substitution protocol to assemble the core framework in efficient steps. Corey-Bakshi-Shibata reduction was employed to afford the enantioenriched intermediate for the total syntheses of the aforementioned alkaloids. The chirality was shown to completely transfer to the backbone using Johnson-Claisen rearrangement. The enantioselective total syntheses of (-)-kopsifoline D and (-)-beninine were accomplished for the first time. Our strategy opens up practical avenues for the total synthesis of structurally similar alkaloids.

Indole Alkaloid Synthesis Facilitated by Photoredox Catalytic Radical Cascade Reactions

The monoterpene indole alkaloids, containing over 3000 known members and more than 40 structural types, represent one of the largest natural product families that have proven to be an important drug source. Their complex chemical structures and significant biological activities have rendered these alkaloids attractive targets in the synthetic community for decades. While chemists have developed many synthetic methodologies and tactics toward this end, general strategies allowing divergent access to a large variety of structural types and members of monoterpene indole alkaloids are still limited and highly desirable. Photoredox catalysis has emerged in recent years as a powerful tool to realize chemical transformations via single electron transfer (SET) processes that would otherwise be inaccessible. In particular, when the radical species generated by the visible light photoinduced approach is involved in well-designed cascade reactions, the formation of multiple chemical bonds and the assembly of structurally complex molecules would be secured in a green and economic manner. This protocol might serve to remodel the way of thinking for the preparation of useful pharmaceuticals and complex natural products. Due to a long-standing interest in the synthesis of diverse indole alkaloids, our group previously developed a cyclopropanation strategy ( Qin , Y. Acc. Chem. Res. 2011 , 44 , 447 ) that was versatile to access several intriguing indole alkaloid molecules. With an idea of developing more general synthetic approaches to as many members of various indole alkaloids as possible, we recently disclosed new radical cascade reactions enabled by photoredox catalysis, leading to the collective asymmetric total synthesis of 42 monoterpene indole alkaloids belonging to 7 structural types. Several important discoveries deserve to be highlighted. First, the use of photocatalytic technology allowed us to achieve an unusual reaction pathway that reversed the conventional reactivity between two nucleophilic amine and enamine groups. Second, a crucial nitrogen-centered radical, directly generated from a sulfonamide N-H bond, triggered three types of cascade reactions to deliver indole alkaloid cores with manifold functionalities and controllable diastereoselectivities. Moreover, expansion of this catalytic, scalable, and general methodology permitted the total synthesis of a large collection of indole alkaloids. In this Account, we wish to provide a complete picture of our studies concerning the original synthetic design, method development, and applications in total synthesis. It is anticipated that the visible-light-driven cascade strategy will find further utility in the realm of natural product synthesis.

Total Synthesis of (+)-Arborisidine

The first total synthesis of arborisidine, a unique Kopsia indole alkaloid possessing a fully substituted cyclohexanone ring system with two quaternary carbons, has been achieved in seven steps in racemic format from tryptamine and in nine steps in asymmetric format from d-tryptophan methyl ester. Key elements of the design include a carefully orchestrated decyanation protocol to finalize the asymmetric formation of an aza-quaternary center that is challenging to access in optically active format via direct Pictet-Spengler cyclizations with tryptamine, a metal-promoted 6- endo-dig cyclization of an enyne to establish the second core quaternary center, and regiospecific functionalizations of the resultant complex diene to finalize the target structure. The distinct and efficient nature of the developed solution is highlighted by several unsuccessful approaches and unexpected rearrangements.

Total Syntheses of Pleiocarpamine, Normavacurine, and C-Mavacurine

The total syntheses of C-mavacurine-type indole alkaloids, (±)-pleiocarpamine, (±)-normavacurine, and (±)- C-mavacurine, were accomplished. The key step in the syntheses was the cyclization between the metal carbenoid at C₁₆ and the N₁ position in a Corynanthe-type compound that was equipped with a diazo function. For this cyclization, the N₄ modification of the substrate using an amine-borane complex was indispensable to fix the molecular conformation.

Total Syntheses of Echitamine, Akuammiline, Rhazicine, and Pseudoakuammigine

Echitamine (1) and akuammiline (2) are representative members of a fascinating class of monoterpenoid indole alkaloids. We report the syntheses of 2 and its congener deacetylakuammiline (3). The azabicyclo[3.3.1]nonane motif was assembled through silver-catalyzed internal alkyne cyclization, and one-pot C-O bond cleavage/C-N bond formation furnished the pentacyclic scaffold. Compound 3 then served as a common intermediate for preparing a series of structurally diverse and synthetically challenging congeners including 1. A position-selective Polonovski-Potier reaction followed by formal N-4 migration built the core of N-demethylechitamine (4) and 1. An alternative route featuring Meisenheimer rearrangement gave 4 as well. Oxidation of the alcohol within 3 gave rhazimal (5), which underwent tandem indolenine hydrolysis, hemiaminalization, and hemiketalization to form rhazicine (6). A sequence of N,O-ketalization and reductive amination secured the chemoselectivity of N-methylation, leading to pseudoakuammigine (7).