Total Synthesis of (±)-Corymine

Asymmetric 1,2-Migration at Vicinal Tetrasubstituted Stereocenters Constructed from α-Keto Imines

A carbonyl-assisted asymmetric 1,2-migratory allylation through in situ generation of vicinal tetrasubstituted stereocenters is reported to access enantiopure α-amino ketones and amino alcohols with excellent yields and diastereoselectivities. In a remarkable divergence, despite higher steric hindrance, the allylation exclusively occurs on ketones over imines in the first step, followed by a face-selective 1,2-allyl transfer, thus highlighting an exciting interplay between two distinct electrophiles. The methodology distinguishes itself through its adaptability to gram-scale synthesis, showcasing broad functional-group tolerance and stereodivergence. Density functional theory (DFT) analysis led to a deeper understanding of its selectivity and mechanistic framework. Highlighting its transformative potential, the method was applied to the total synthesis of hapalindole alkaloids.

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.

Asymmetric Synthesis of Arboduridine

The first asymmetric total synthesis of the monoterpenoid indole alkaloid arboduridine has been accomplished. The tricyclic A/B/D ring system was constructed by an enantioselective Michael reaction followed by intramolecular nucleophilic addition. Intramolecular α-amination of a ketone forged the piperidine ring, while a Horner-Wadsworth-Emmons (HWE) reaction was used to form the pyrrolidine ring. A reduction cyclization cascade led to formation of the tetrahydrofuran ring.

A New Reaction Mode of 3-Halooxindoles: Acting as C-C-O Three-Atom Components for (3+3) Cycloaddition to Access Indolenine-Fused 2H-1,4-Oxathiines

Herein, we report an unprecedented implementation of 3-halooxindoles as C-C-O three-atom components for (3+3) cycloaddition with pyridinium 1,4-zwitterionic thiolates, affording structurally diverse indolenine-fused 2H-1,4-oxathiines in moderate to high yields. A combined experimental and computational mechanistic study suggests that the reaction proceeds through addition of a S conjugate to the o-azaxylylene intermediate, followed by O-Michael addition and a sequential retro-Michael addition/pyridine extrusion pathway.

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.

Catalytic Enantioselective Alkylation of Aldehydes with 3-Bromooxindoles

An asymmetric conjugate addition of aldehydes with o-azaxylylene intermediates (indol-2-ones) from 3-bromooxindoles has been developed. The use of a novel spiro-pyrrolidine (SPD)-derived bifunctional N-sulfonylated amide catalyst is essential for a highly diastereo- and enantioselective transformation to provide a wide array of enantioenriched C3 quaternary oxindoles with structurally diverse β-aldehyde appendages. Further application of this synthetic methodology enables the construction of the tricyclic cores of akuammiline-type alkaloids.

Palladium-Catalyzed Ring-Closing Aminoalkylative Amination of Unactivated Aminoenynes

An efficient strategy for preventing the β-hydride elimination of alkylpalladium species by ligation of the palladium with adjacent amino-group was developed, which enabled a novel palladium-catalyzed ring-closing aminoalkylative amination of unactivated aminoenynes. The reaction is amenable to aminals, as well as aliphatic aldehydes with secondary amines, which provides straightforward access to structurally diverse exocyclic allenic amines bearing 5 to 12-membered N-heterocycles. With chiral phosphoramidite-ligated palladium complex as the catalyst, an enantioselective variant was achieved with up to 93 % ee. Simultaneously, synthetic transformations of the chiral products were also conducted to afford structurally unique spirodiamines including one pharmaceutically active molecule via axial-to-central chirality transfer.

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.

Proton or Carbene Transfer? On the Dark and Light Reaction of Diazoalkanes with Alcohols

The formal alkylation reaction of OH groups with diazoalkanes under catalyst-free reaction conditions finds broad application in organic synthesis. However, even today, this reaction is mainly limited to the use of diazomethane as reaction partner. In this combined experimental and theoretical study, we aim at a fundamental understanding of the reaction of diazoalkanes with alcohols to make this transformation amenable to a generalized approach towards formal alkylation reactions of alcohols with diazoalkanes. Experimental and theoretical studies suggest a direct proton transfer only in exceptional cases. In a more general setting, such O-H functionalization proceed both under dark and photochemical conditions via a key hydrogen-bonded singlet carbene intermediate that undergoes a protonation-addition mechanism. We conclude with applications of this approach in O-H functionalization reactions of alcohols, including simple fluorinated, halogenated and aliphatic alcohols and showcase functional-group tolerance of this method in the reaction of biologically active and pharmaceutically relevant alcohols.

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.

Synthesis of 4-amino-5-allenylisoxazoles via gold(I)-catalysed propargyl aza-Claisen rearrangement

Propargyl aza-Claisen rearrangement of 4-propargylaminoisoxazoles 1 proceeded in the presence of cationic gold(i) catalysts to give 4-amino-5-allenylisoxazoles 2 in good to high yields. The silyl group at the terminal alkyne and a cationic gold(i) catalyst bearing a sterically bulky ligand are essential for the generation of isolable allene intermediates. The N-protection of the generated 4-amino-5-allenylisoxazoles 2 allowed the isolation of 5-allenylisoxazoles 4 that have never been synthesized. N-Propargyl aniline 5 was successfully converted to the corresponding ortho-allenyl aniline 6 under the current reaction conditions.

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.

[1,3]/[1,4]-Sulfur atom migration in β-hydroxyalkylphosphine sulfides

β-Hydroxyalkylphosphine sulfides undergo [1,3]- or [1,4]-sulfur atom phosphorus-to-carbon migration in the presence of Lewis or Brønsted acids. The direction of sulfur atom migration depends on the type of acid used for the reaction. In the presence of a Brønsted acid, mainly [1,3]-rearrangement is observed, whereas a Lewis acid catalyzes the [1,4]-sulfur migration. To gain insight into the mechanism of these transformations, the stereochemistry of these rearrangements have been tested, along with the conduction of some control experiments and DFT calculations.

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 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).

Additive Tuned Selective Synthesis of Bicyclo[3.3.0]octan-1-ols and Bicyclo[3.1.0]hexan-1-ols Mediated by AllylSmBr

The selective construction of bicyclo[3.3.0]octan-1-ols and bicyclo[3.1.0]hexan-1-ols was achieved by using an allylSmBr/additive(s) system. By employing HMPA as the only additive, the momoallylation/ketone-alkene coupling occurred preferably and afforded bicyclo[3.3.0]octan-1-ols in good yields with high diastereoselectivities. While the ester-alkene coupling predominated to generate bicyclo[3.1.0]hexan-1-ols in moderate yields with excellent diastereoselectivities in the presence of a proton source, such as pyrrole as the coadditive with HMPA. The tunable reactivity of allylSmBr by additive(s) would make it a versatile reagent in organic synthesis.

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.

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.

Formal total synthesis of the akuammiline alkaloid (+)-strictamine

An asymmetric formal total synthesis of the akuammiline alkaloid (+)-strictamine is reported.