Iodospirocyclization of Tryptamine-Derived Isocyanides: Formal Total Synthesis of Aspidofractinine

Shining light on tryptamine-derived isocyanides: access to constrained spirocylic scaffolds

Dearomatization of indoles through a charge transfer complex constitutes a powerful tool for synthesizing three-dimensional constrained structures. However, the implementation of this strategy for the dearomatization of tryptamine-derived isocyanides to generate spirocyclic scaffolds remains underdeveloped. In this work, we have demonstrated the ability of tryptamine-derived isocyanides to form aggregates at higher concentration, enabling a single electron transfer step to generate carbon-based-radical intermediates. Optical, HRMS and computational studies have elucidated key aspects associated with the photophysical properties of tryptamine-derived isocyanides. The developed protocol is operationally simple, robust and demonstrates a novel approach to generate conformationally constrained spirocyclic scaffolds, compounds with high demand in various fields, including drug discovery.

Electrophile-Controlled Regiodivergent Palladium-Catalyzed Imidoylative Spirocyclization of Cyclic Alkenes

An intermolecular controllable Pd-catalyzed spirocyclization of isocyano cycloalkenes has been developed, offering efficient and selective approaches toward spirocyclic hydropyrrole scaffolds. 2-Azaspiro-1,7-dienes could be obtained through a "chain-walking" process with aryl/vinyl iodides as electrophiles, while the normal Heck product 2-azaspiro-1,6-dienes were selectively generated when aryl triflates were used as the coupling partner of isocyanides. Mechanistic studies suggested that the counteranion of the Pd(II) intermediate played a crucial role in the regioselectivity control. Dihydropyrrole-fused 5,6,7-membered spirocycles were switchably accessed under mild conditions with wide functional group tolerance.

Chiral phosphoric acid-catalyzed enantioselective [5+1] cycloaddition reaction of C,N-cyclic azomethine imines with isocyanides

The first catalytic enantioselective [5+1] cycloaddition reactions of C,N-cyclic azomethine imines with isocyanides are reported herein. The method displays a broad substrate scope and atom-economy. A series of chiral tetrahydroisoquinoline containing indole skeletons were obtained in up to 90% yield with 95% ee under mild reaction conditions. A possible catalytic model was also proposed.

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.

Coupling-Spirocyclization Cascade of Tryptamine-Derived Isocyanides with Iodonium Ylides and Despirocyclization Reactions

A cobalt(II)-catalyzed coupling-cyclization cascade reaction between tryptamine-derived isocyanides and iodonium ylides is investigated, which allowed for the synthesis of different types of spiroindoline compounds by variation of substituents at the N1- and C2-positions in the indole skeleton. More interesting is that the spiroindoline products could undergo despirocyclization in the presence of amines, enabling efficient construction of enamine compounds.

Catalyst-Free Synthesis of Polycyclic Spiroindolines by Cascade Reaction of 3-(2-Isocyanoethyl)indoles with 1-Sulfonyl-1,2,3-triazoles

A catalyst-free cascade reaction of 3-(2-isocyanoethyl)indoles and 1-sulfonyl-1,2,3-triazoles was realized. This dearomative spirocyclization provided an efficient protocol to synthesize a series of polycyclic indolines bearing spiro-α-carboline in moderate to high yields in one step under thermal reaction conditions.

Diastereodivergent Synthesis of Pentacyclic Spiroindolines via a Magnesium(II)-Catalyzed Cascade Reaction of N,N'-Cyclic Azomethine Imines with Indolyl-Substituted Isocyanides

Magnesium(II)-catalyzed cascade reactions of N,N'-cyclic azomethine imines with indolyl-substituted isocyanides are reported herein. The method exhibited a high functional group tolerance and broad substrate scope. A series of anti-pentacyclic spiroindolines containing N,N'-fused heterocycle skeletons were obtained in up to 82% yield with 8.5:1 dr under mild reaction conditions. Intriguingly, a sequential HOAc-mediated protonation results in a diastereoenriched epimerization, which gives rise to the syn-pentacyclic spiroindolines as the sole isomers.

1,3-Migrative Ring Expansion of Spiroindolenines to Azepino[3,4-b]indoles

We serendipitously found an unprecedented 5-to-7-membered ring expansion of 2-alkylspiroindolenines to azepinoindoles mediated by n-tetrabutylammonium fluoride. The starting materials can be easily prepared by the hypoiodite-catalyzed oxidative dearomative spirocyclization of indole derivatives. Mildly basic conditions and electron-deficient protecting groups for the amines were found to be crucial to promoting chemoselective reactions. Moreover, the ring expansion of aniline-derived spiroindolenines proceeds smoothly under much milder conditions using only a catalytic amount of cesium carbonate.

Synthesis of Spiro[indole-3,3'-pyrrolidine]-2'-(thi)ones

Spiro[indole-3,3'-pyrrolidine]-2'-ones were synthesized via one-pot chloroformylation-dearomatizing spirocyclization of tryptamine derivatives. Moreover, the "thio" equivalent spiro[indole-3,3'-pyrrolidine]-2'-thiones, for which the synthesis and properties were previously unreported, were synthesized. The procedures are easily implemented, have a broad scope, and are transition-metal-free and cheap. To demonstrate the utility of the synthetic methodology, the spiro[indole-3,3'-pyrrolidine]-2'-ones were converted into heterocyclic scaffolds, such as an optically active spiroindoline and spirooxindole.

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.

Three-Component Coupling of Anilines, Amines, and Difluorocarbene to Access Formamidines

A one-pot three-component reaction of two anilines (or one aniline and one alkylamine) and in situ-generated difluorocarbene is developed herein to enable efficient construction of formamidines. Crucial formimidoyl fluoride intermediate RN═CHF is proposed from the reaction of a primary aniline and difluorocarbene. Ensuing nucleophilic iminyl substitution of this intermediate with a second amine allows cross-condensation of the two amines to produce formamidines. When only one type of primary aniline is used as the substrate, the difluoromethylated homo-condensation products can also be produced under a 1:1 molar ratio of aniline/difluorocarbene. Intramolecular variant of this method allows concise synthesis of benzimidazoquinazolines and nitrogen-fused/spirocyclic compounds, showing the potential of this method in organic synthesis. More interesting reactions are anticipated by exploiting the reactivity of difluorocarbene and primary amines to isocyanides or the formimidoyl fluoride intermediates.

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.

Organocatalytic multicomponent coupling to access a highly functionalised tetracyclic furoindoline: interrupted Passerini/Joullié-Ugi cascade reaction

The interrupted Passerini reaction of 3-(2-isocyanoethyl)-indole catalysed by 3,5,6-trifluoro-2-pyridone is described. The reaction diastereoselectively provided a tetracyclic furolindoline, which proved to be a good substrate for the Joullié-Ugi reaction; therefore, the sequential Passerini/Joullié-Ugi reactions were performed in one-pot to rapidly provide versatile and highly functionalised furoindolines from 3-(2-isocyanoethyl)-indole.

Divergent Synthesis of Chromenoindoles and Spiroindolines via Domino Reaction of Indolyl-Substituted Isocyanides with Quinone Esters

A dearomative spirocyclization of tryptamine-derived isocyanides with quinone esters is developed for the divergent synthesis of structurally complex chromeno[2,3-b]indole and polycyclic spiroindoline scaffolds. This domino reaction features the formation and conversion of the six-membered dihydropyran ring with an amendable N,O-aminal moiety.

Synthesis of Tryptamines from Radical Cyclization of 2-Iodoaryl Allenyl Amines and Coupling with 2-Azallyls

An efficient synthesis of tryptamines is developed. Indole structures were constructed using 2-iodoaryl allenyl amines as electron acceptors and radical cyclization precursors. Radical-radical coupling of indolyl methyl radicals and azaallyl radicals led to the tryptamine derivatives. The utility and versatility of this method are showcased by the synthesis of 22 examples of tryptamines in ≤88% yield. In each case, indole formation is accompanied by in situ removal of the Boc protecting group.

Asymmetric synthesis of polycyclic spiroindolines via the Dy-catalyzed cascade reaction of 3-(2-isocyanoethyl)indoles with aziridines

An asymmetric cascade reaction catalyzed by a chiral N,N′-dioxide–Dy(iii) complex was realized to construct the valuable [6,5,5,6] tetracyclic spiroindolines with good yields and enantioselectivities by a concise and one-step protocol.

Highly diastereoselective cascade dearomatization of 3-(2-isocyanoethyl)indoles with nitrile imines: a facile access to unexpected polycyclic indolines

A novel cascade annulation between 2-isocyanoethylindoles and hydrazonyl chlorides has been realized to assemble a variety of unexpected polycyclic indoline derivatives with excellent diastereoselectivities under simple reaction conditions.

One-pot synthesis of natural-product inspired spiroindolines with anti-cancer activities

A post-Ugi/diastereoselective cascade reaction was developed to construct the spiroindoline scaffold through a 5-exo-dig indole cyclization and the intramolecular trapping of the spiro intermediate forming a quaternary carbon center.

Synthesis of tetracyclic spiroindolines by an interrupted Bischler-Napieralski reaction: total synthesis of akuammicine

Judicious substrate design allows interruption of the classical Bischler–Napieralski reaction, providing access to a range of diversely substituted tetracyclic spiroindolines. These complex polycyclic scaffolds are valuable building blocks for the construction of indole alkaloids, as showcased in a concise total synthesis of (±)-akuammicine.

The interrupted Bischler–Napieralski reaction of β,γ-unsaturated tryptamides affords tetracyclic spiro pyrroloindolines, which can be used in the total synthesis of the Strychnos alkaloid, akuammicine.

Synthesis of polycyclic spiroindolines via the cascade reaction of 3-(2-isocyanoethyl)indoles

Tandem reactions of the yttrium(iii) catalyzed ring-opening reaction of 2,2'-diester aziridines with 3-(2-isocyanoethyl)indoles and the subsequent Friedel-Crafts/Mannich/desulfonylation were reported. A series of polycyclic spiroindolines containing tetrahydro-β-carbolines were obtained in moderate to excellent yields (56-92%) in one step under mild reaction conditions. A possible catalytic mechanism was also proposed.

Palladium-catalysed imidoylative spirocyclization of 3-(2-isocyanoethyl)indoles

A palladium-catalysed construction of spiroindolines through dearomative spirocyclization of 3-(2-isocyanoethyl)indoles has been developed. 2'-Aryl-, vinyl-, and alkyl-substituted spiroindolines could be accessed under mild conditions with excellent functional group tolerance. C1-tethered oxindole- and indole-spiroindoline bisheterocycles were generated in high yields via alkene/allene insertion and an imidoylative spirocyclization cascade. Additionally, a tandem dearomatization of two different indoles was realized with N-(2-bromobenzoyl)indoles as the electrophilic coupling partner of 3-(2-isocyanoethyl)indoles, affording polyindoline - spiroindoline bisheterocyclic scaffolds conveniently. Under the catalysis of Pd(OAc)₂ and a spinol-derived phosphoramidite ligand, chiral spiroindolines were successfully accessed with up to 95% yield and 85% ee.

Cascade Reaction of Tryptamine-Derived Isocyanides with Nitrile Oxides: Construction of Oxazine Fused-Spiroindoline Derivatives

A one-step cascade reaction of tryptamine-derived isocyanides with in situ generated nitrile oxides for the synthesis of fused spiroindolines was described. The desired products could be efficiently synthesized in moderate to good yields (42-87%). The protocol features mild conditions, wide substrate scope, and high efficiency.

Synthesis of Carbazoles by a Diverted Bischler-Napieralski Cascade Reaction

An unforeseen twist in a seemingly trivial Bischler–Napieralski reaction led to the selective formation of an unexpected carbazole product. The reaction proved to be general, providing access to a range of diversely substituted carbazoles from readily available substrates. Judicious variation of substituents revealed a complex cascade mechanism comprising no less than 10 elementary steps, that could be diverted in multiple ways toward various other carbazole derivatives.

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.

Total Syntheses of (-)-Minovincine and (-)-Aspidofractinine through a Sequence of Cascade Reactions

We report 8‐step syntheses of (−)‐minovincine and (−)‐aspidofractinine using easily available and inexpensive reagents and catalyst. A key element of the strategy was the utilization of a sequence of cascade reactions to rapidly construct the penta‐ and hexacyclic frameworks. These cascade transformations included organocatalytic Michael‐aldol condensation, a multistep anionic Michael‐S_N2 cascade reaction, and Mannich reaction interrupted Fischer indolization. To streamline the synthetic routes, we also investigated the deliberate use of steric effect to secure various chemo‐ and regioselective transformations.

Unraveling Plant Natural Chemical Diversity for Drug Discovery Purposes

The screening and testing of extracts against a variety of pharmacological targets in order to benefit from the immense natural chemical diversity is a concern in many laboratories worldwide. And several successes have been recorded in finding new actives in natural products, some of which have become new drugs or new sources of inspiration for drugs. But in view of the vast amount of research on the subject, it is surprising that not more drug candidates were found. In our view, it is fundamental to reflect upon the approaches of such drug discovery programs and the technical processes that are used, along with their inherent difficulties and biases. Based on an extensive survey of recent publications, we discuss the origin and the variety of natural chemical diversity as well as the strategies to having the potential to embrace this diversity. It seemed to us that some of the difficulties of the area could be related with the technical approaches that are used, so the present review begins with synthetizing some of the more used discovery strategies, exemplifying some key points, in order to address some of their limitations. It appears that one of the challenges of natural product-based drug discovery programs should be an easier access to renewable sources of plant-derived products. Maximizing the use of the data together with the exploration of chemical diversity while working on reasonable supply of natural product-based entities could be a way to answer this challenge. We suggested alternative ways to access and explore part of this chemical diversity with in vitro cultures. We also reinforced how important it was organizing and making available this worldwide knowledge in an "inventory" of natural products and their sources. And finally, we focused on strategies based on synthetic biology and syntheses that allow reaching industrial scale supply. Approaches based on the opportunities lying in untapped natural plant chemical diversity are also considered.

Enantioselective Synthesis of Spiroindolines via Cascade Isomerization/Spirocyclization/Dearomatization Reaction

The spiroindoline skeleton featured with 2,7-diazaspiro[4.4]nonane exists in various structurally intricate and biologically active monoterpene indole alkaloids. A catalytic asymmetric cascade enamine isomerization/spirocyclization/dearomatization succession to construct the spiroindoline was developed, which employed the indolyl dihydropyridine as a substrate under catalysis of the chiral phosphoric acid. This cascade reaction provided various spiroindolines in both diastereoselective and enantionselective fashions.

Fused N-Heterocycles with Contiguous Stereogenic Centers Accessed by an Asymmetric Catalytic Cascade Reaction of Tertiary Enamides

We report in this article a cascade reaction strategy for the synthesis of complex N-heterocyclic compounds with contiguous and tetrasubstituted stereogenic carbons. Under the sequential catalysis of a chiral binol-Ti complex and BF₃ , cyclopentanone-derived tertiary enamides undergo an enantioselective enamine addition to ketone carbonyls followed by diastereoselective trapping of the resulting acyliminiums by electron-rich aryl moieties to furnish four- and five-membered ring-fused N-heterocyclic products as the sole diastereomers in high yields with up to 99 % ee.

Mild and Practical Indole C2 Allylation by Allylboration of in situ Generated 3-Chloroindolenines

C2 allylation of indole derivatives is a challenging but important transformation given the biological relevance of the products. Herein we report a selective C2 allylation strategy that proceeds via allylboration of in situ-generated 3-chloroindolenines. The reaction is mild, practical, and compatible with a wide range of C3-substituted indoles. As allylboronates are readily accessible from commercial precursors, various substituted allyl moieties can be introduced using the same protocol. To showcase the utility of this method we applied it to the synthesis of the natural product, tryprostatin B.

Total Synthesis of Aspidofractinine Alkaloid Paucidirinine

The first total synthesis of paucidirinine (1d), a highly congested aspidofractinine alkaloid containing a special contracted five-membered lactam ring, was achieved in 10 steps with 8% overall yield from commercially available materials. Several key maneuvers, including tandem enamination/[4 + 2] cycloaddition reaction and SmI₂-promoted radical cyclization, were featured in this potentially scalable strategy.