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

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

Synthesis of Azonia Aromatic Heterocycles Bearing 6-6-6-5-6 Pentacyclic Core via Intramolecular [4 + 2]-Cycloaddition and Oxidative Aromatization Reaction Sequence in One Pot

Cationic aza-heterocycle-fused compounds have gained wide applications in materials science, biological applications, and synthetic organic chemistry. In this report, synthesis of benzothiazolochromenopyridinium tetrafluoroborates, a novel molecular scaffold, bearing 6-6-6-5-6 pentacyclic core is described that proceeds via (i) piperidine-catalyzed Knoevenagel condensation between 2-propargyloxyarylaldehydes bearing internal alkynes and 2-benzothiazoleacetonitrile, (ii) intramolecular formal [4 + 2]-cycloaddition, and (iii) crucial molecular oxygen-mediated oxidative aromatization reaction sequence in one pot. These quaternary pyridinium salts are obtained at ambient temperature in good to high yields.

Dearomative Spirocyclization of Tryptamine-Derived Isocyanides via Iron-Catalyzed Carbene Transfer

Tryptamine-derived isocyanides are valuable building blocks in the construction of spirocyclic indolenines and indolines via dearomatization of the indole moiety. We report the Bu4N[Fe(CO)3NO]-catalyzed carbene transfer of α-diazo esters to 3-(2-isocyanoethyl)indoles, leading to ketenimine intermediates that undergo spontaneous dearomative spirocyclization. The utility of this iron-catalyzed carbene transfer/spirocyclization cascade was demonstrated by its use as a key step in the formal total synthesis of monoterpenoid indole alkaloids (±)-aspidofractinine, (±)-limaspermidine, (±)-aspidospermidine, and (±)-17-demethoxy-N-acetylcylindrocarine.

Regiodivergent Asymmetric Pyridinium Additions: Mechanistic Insight and Synthetic Applications

A practical protocol for the first regiodivergent asymmetric addition of aryl and alkenyl organometallic reagents to substituted N-alkyl pyridinium heterocycles is described. The couplings proceed with high regiochemical and stereochemical selectivities, and provide access to chiral 1,2,3- and 1,3,4-trisubstituted dihydropyridine products, controlled by judicious choice of nitrogen activating agent. To this end, a correlation was found between the parameterized size of the activating group and the C2/C4 regioselectivity in the couplings. The utility of the described chemistry was demonstrated in two concise asymmetric syntheses of (+)-N-methylaspidospermidine and (-)-paroxetine.

Total Synthesis of (±)-Aspidospermidine, (±)-Aspidofractinine, (±)-Limaspermidine, and (±)-Vincadifformine via a Cascade and Common Intermediate Strategy

A concise strategy for the total synthesis of several Aspidosperma alkaloids is reported. A Suzuki-Miyaura cross-coupling provides access to a 2-vinyl indole that undergoes a Diels-Alder cascade reaction with butyn-2-one to deliver a pyrroloindoline intermediate. This undergoes cascade amidation, reduction, skeletal rearrangement, and intramolecular Michael addition to provide a common intermediate containing the full framework of the Aspidosperma alkaloids. The utility of this intermediate is shown in the synthesis of four different natural products.

Enantioselective Total Synthesis of (-)-Limaspermidine and (-)-Kopsinine by a Nitroaryl Transfer Cascade Strategy

We report an intramolecular conjugate addition/Truce-Smiles/E1cb cascade of 2-nitrobenzenesulfonamide-functionalized cyclohexenones as a new entry to the core scaffold of monoterpene indole alkaloids. The method was applied to the asymmetric total synthesis of (-)-limaspermidine, (-)-kopsinilam, and (-)-kopsinine, as well as the framework of the kopsifoline alkaloids, thus highlighting its complementarity to existing approaches involving the use of indole-based starting materials or the interrupted Fischer indole synthesis. Furthermore, we show that the cascade tolerates various substituents on the nitroarene, opening the way to other natural products as well as non-natural analogues.

Total Synthesis of (-)-Voacinol and (-)-Voacandimine C

We describe the first total synthesis of complex aspidosperma alkaloids (-)-voacinol and (-)-voacandimine C via a late-stage C7-methylenation strategy inspired by a biogenetic hypothesis. We envisioned rapid access to these natural alkaloids from a common, symmetrical precursor assembled by methylenation of a D-ring-oxidized variant of the structurally related natural product (-)-deoxoapodine. Chemoselective N9-oxidation of a pentacyclic deoxoapodine precursor enabled the synthesis of the corresponding hexacyclic C8-aminonitrile. Stereocontrolled methylenation of a C8-enamine derivative of deoxoapodine, accessed by ionization of the C8-aminonitrile, afforded a symmetrical dodecacyclic bisaminonitrile as a versatile precursor to these bisindole alkaloids. The final-stage, biosynthesis-inspired, controlled reductive opening of the oxolane substructures of this dodecacyclic intermediate provided a unified approach to (-)-voacinol and (-)-voacandimine C, while direct reduction of the same intermediate afforded the structurally related (-)-methylenebisdeoxoapodine.

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.

Aspidosperma and Strychnos alkaloids: Chemistry and biology

Of Nature's nearly 3000 unique monoterpene indole alkaloids derived from tryptophan, those members belonging to the Aspidosperma and Strychnos families continue to impact the fields of natural products (i.e., isolation, structure determination, biosynthesis) and organic chemistry (i.e., chemical synthesis, methodology development) among others. This review covers the biological activity (Section 2), biosynthesis (Section 3), and synthesis of both classical and novel Aspidosperma (Section 4), Strychnos (Section 5), and selected bis-indole (Section 6) alkaloids. Technological advancements in genetic sequencing and bioinformatics have deepened our understanding of how Nature assembles these intriguing molecules. The proliferation of innovative synthetic strategies and tactics for the synthesis of the alkaloids covered in this review, which include contributions from over fifty research groups from around the world, are a testament to the creative power and technical skills of synthetic organic chemists. To be sure, Nature-the Supreme molecular architect and source of a dazzling array of irresistible chemical logic puzzles-continues to inspire scientists across multiple disciplines and will certainly continue to do so for the foreseeable future.

Recent applications of asymmetric organocatalytic annulation reactions in natural product synthesis

The past two decades have witnessed remarkable growth of asymmetric organocatalysis, which is now a firmly established synthetic tool, serving as a powerful platform for the production of chiral molecules. Ring structures are ubiquitous in organic compounds, and, in the context of natural product synthesis, strategic construction of ring motifs is often crucial, fundamentally impacting the eventual fate of the whole synthetic plan. In this review, we provide a comprehensive and updated summary of asymmetric organocatalytic annulation reactions; in particular, the application of these annulation strategies in natural product synthesis will be highlighted.

Total Syntheses of Aspidospermidine, N-Methylaspidospermidine, N-Acetylaspidospermidine, and Aspidospermine via a Tandem Cyclization of Tryptamine-Ynamide

The total syntheses of aspidospermidine, N-methylaspidospermidine, N-acetylaspidospermidine, and aspidospermine were achieved from a common pentacyclic indoline intermediate. The common pentacyclic indoline intermediate was synthesized on a gram scale through a Stork-enamine alkylation of 1H-pyrrolo[2,3-d]carbazole derivatives, which were prepared through a Brønsted acid-catalyzed tandem cyclization of tryptamine-ynamide. The scalable synthesis of 1H-pyrrolo[2,3-d]carbazole afforded facile access and a practical approach to the Aspidosperma indole alkaloid family.

Synthetic Approaches to Tricyclic Aminoketones in the Total Synthesis of Aspidosperma and Kopsia Alkaloids

Aspidosperma and kopsia alkaloids are significant functional molecules because of their potent biological activities. Their intricate structures present an intrinsic synthetic challenge and thus attract significant attention from synthetic organic academic community. Over the past decades, a series of elegant strategies has been developed, in particular, the Stork's original Fischer indolization of tricyclic aminoketones 1. Herein, we report a comprehensive review on various synthetic approaches access to tricyclic aminoketones 1 and provide a practical guidance to readers whose are interested in employing tricyclic aminoketones 1 as versatile building blocks in the realm of total synthesis of aspidosperma, kopsia and structurally related alkaloids.