Late-stage diversification of indole skeletons through nitrogen atom insertion

Skeletal Editing Strategies Driven by Total Synthesis

ConspectusSingle-atom skeletal editing strategies that precisely modify the core frameworks of molecules have the potential to streamline and accelerate organic synthesis by enabling conceptually simple, but otherwise synthetically challenging, retrosynthetic disconnections. In contrast to broader skeletal remodeling and rearrangement strategies, these methodologies more specifically target single-atom changes with high selectivity, even within complex molecules such as natural products or pharmaceuticals. For the past several years, our laboratory has developed several skeletal editing methodologies, including single-atom ring contractions, expansions, and transpositions of both saturated and unsaturated heterocycles, as well as other carbon scaffolds. This Account details the evolution of "skeletal editing logic" within the context of our extensive work on natural product total synthesis.Early work in the Sarpong group leveraged metal-mediated C-C bond cleavage of in situ-generated strained intermediates to accomplish total syntheses of natural products, such as the icetexane diterpenoids and cyathane diterpenes. Continuing our focus on leveraging C-C bond cleavage through "break-it-to-make-it" strategies, we then developed carvone remodeling strategies to access a variety of terpenoids (including longiborneol sesquiterpenoids, phomactins, and xishacorenes) from hydroxylated pinene derivatives. In applying this skeletal remodeling and C-C cleavage framework to alkaloid natural products, such as the preparaherquimides and lycodine-type alkaloids, we recognized that single-atom changes to the saturated nitrogen-containing rings within these natural products would enable the direct conversion between distinct but structurally related natural product families. Thus, we began developing methods that selectively modify the core frameworks of N-heterocycles; this focus led to our work on the deconstructive fluorination and diversification of piperidines and ultimately to our recent body of work on direct, single-atom core framework modifications (single-atom skeletal editing). In the context of saturated heterocycles, we developed photomediated enantioselective ring contractions of α-acylated motifs and reductive ring contractions of cyclic hydroxylamines. For unsaturated heterocycles, we have developed ring contractions of azines (e.g., pyrimidine to pyrazole), 15N isotopic labeling of azines, and phototranspositions of indazoles to benzimidazoles. To direct our focus on reaction development, a cheminformatic analysis of heteroaromatic skeletal edits served to quantitatively inform which transformations would most significantly expand the accessible chemical space. Apart from heterocycles, we also reported single-nitrogen insertion through the reductive amination of carbonyl C-C bonds. Ultimately, the goal of this research is to develop mild and selective skeletal editing methodologies that can be applied to total synthesis and organic synthesis more generally. While recent total syntheses from our group have targeted simplified retrosyntheses through single-atom skeletal editing logic (e.g., daphenylline and harringtonolide), multiple steps were still required to achieve the formal desired "edit". As such, the continued development of truly single-step, mild, and selective reactions that can edit the cores of highly complex molecules remains highly desirable.

Pd-Catalyzed Photoinduced Interceptive Decarboxylative Allylation

Herein, we describe a photoinduced Pd-catalyzed interceptive decarboxylative allylation of allyl esters. Our protocol provides a new gateway to enable atom pair swaps or a series of contractions and elongations, thus offering unconventional disconnections and a modular yet broadly applicable tool for rapidly and reliably accessing sp3 architectures in drug discovery.

Photogeneration and Visualization of a Surface-Stabilized Dinitrene

Nitrenes are known as key intermediates in various chemical reactions. Nitrene transfer reactions are particularly effective for synthesizing nitrogen-containing compounds, where metal catalysts play a crucial role in controlling nitrene reactivity and selectivity. In this study, we demonstrate the formation of a stable surface-supported dinitrene on Au(111) through UV irradiation of its diazide precursor, characterized by scanning probe techniques. The photoreaction mechanism is elucidated with wavelength-dependent experiments and time-dependent density functional theory calculations. Our findings present the first real-space visualization of a metal nitrene adsorbed on a surface, highlighting its potential in catalysis and surface functionalization.

Advancing Total Synthesis Through Skeletal Editing

ConspectusTotal synthesis has long been a proving ground for advancing chemical thought, pushing chemists to develop strategies that not only replicate nature's complexity but often surpass it. The pursuit of efficiency, practicality, and elegance continues to challenge and reshape the guiding principles of total synthesis. In recent years, skeletal editing has emerged as a powerful strategy for reconfiguring skeletal frameworks in ways that were previously difficult to imagine. Unlike conventional chemical synthesis approaches, which primarily rely on the logic of bond construction reactions and functional group manipulations, skeletal editing introduces elements that allow for atom insertion, deletion, and exchange and skeletal rearrangement/reorganization by harnessing the potential energy and reactivity of certain structural motifs and morphing them into new electronic and spatial configurations. The logic of modern skeletal editing has been fueling the development of new editing methods and advancing the fields of total synthesis, medicinal chemistry, materials science, and others.In this Account, we detail our program using skeletal editing-based retrosynthetic logic to facilitate natural product synthesis. We first highlight two one-carbon insertion editing strategies utilizing the Ciamician-Dennstedt rearrangement and the Büchner-Curtius-Schlotterbeck ring expansion to streamline the total syntheses of complanadine and phleghenrine Lycopodium alkaloids. We next present our synthesis of crinipellin and gibberellin diterpenes by leveraging the facile synthesis and intrinsic strain of cyclobutanes as precursors to challenging cyclopentanes via cut-and-insert editing (crinipellins) or C-C bond migratory ring expansion (GA18). Toward the end, we describe our early efforts in orchestrating structural rearrangement and functional group pairing reactions to access seven monoterpene indole alkaloids and highlight the divergent potential of skeletal editing. Each of the five examples follows a build-edit-decorate workflow, inspired by Schreiber's build-couple-pair in diversity-oriented synthesis. In the build stage, key scaffolds are efficiently assembled from starting materials with matched reactivity. The edit stage morphs these scaffolds to the desired but more challenging ones encoded by the target molecules, reminiscent of Corey's application of rearrangement transforms as a topological strategy. The decorate stage introduces additional functional groups and adjusts oxidation states to complete the total synthesis, similar to the oxidase phase of Baran's two-phase synthesis. The essence of skeletal editing-based retrosynthetic analysis is to identify latent structural relationships between the readily assembled key scaffolds constructed in the build stage and the desired ones encoded by the target molecules as well as proper editing methods to transform the former into the latter with precision. The build-edit-decorate approach parallels the dynamism of biosynthesis, enabling rapid building of complexity with great efficiency and step economy, as analyzed by the spacial scores (SPS) of each case. Drawing on these principles, chemists can adopt skeletal editing-based retrosynthetic logic by identifying latent intermediates and employing and developing strategic editing methods to overcome synthetic bottlenecks.

Catalytic difluorocarbene insertion enables access to fluorinated oxetane isosteres

Skeletal editing of heterocyclic building blocks offers an appealing way to expand the accessible chemical space by diversifying molecular scaffolds for drug discovery. Despite the recent boom in this area, catalytic strategies that directly introduce fluorine into the backbone of small-ring heterocycles remain rare owing to the challenges of strain-induced ring cleavage and defluorination. Here we describe a copper-catalysed approach for skeletal expansion of oxygen heterocycles by reaction with a difluorocarbene species generated in situ to induce carbon atom insertion. The α,α-difluoro-oxetane products are potential surrogates of oxetane, β-lactone and carbonyl pharmacophores on the basis of their computed molecular properties and electrostatic potential maps. The utility of this approach is highlighted by synthesis of various drug-like molecules and fluorinated isosteres of biologically active compounds. Experimental and computational investigations provide insight into the mechanism and the unique role of the copper catalyst in promoting both ring-opening and cyclization steps of the reaction.

Visible Light-Induced Single-Atom Insertion of Indenes via Aerobic Ring Scission-Condensation-Rearomatization

In this study, we present a photocatalyzed single-atom insertion of indenes, involving an aerobic ring scission into dicarbonyl intermediates, which subsequently undergo condensation and rearomatization to efficiently synthesize isoquinoline and naphthalene derivatives. The use of an inexpensive organic dye as the photocatalyst under aerobic conditions with cheap ammonium acetate (NH4OAc) as the nitrogen source makes this method very practical and environmentally friendly to access isoquinoline. Alternatively, an intramolecular carbon-atom-insertion process, involving the Aldol reaction of the dicarbonyl intermediates, affords the naphthalenamine and naphthalen-2-ol derivatives. Mechanistic studies support that the superoxide anion radical species mediates the C═C double bond scission of indenes rather than the singlet oxygen intermediate.

Single-atom molecular editing: transformative advances in carbocyclic and heterocyclic frameworks

Single-atom editing has emerged as a transformative strategy in organic synthesis, enabling precise modification of carbocyclic and heterocyclic frameworks by selectively targeting single atoms. These frameworks are crucial backbones of pharmaceuticals, agrochemicals, and advanced materials, making this approach powerful for organic chemists. In drug discovery and natural product synthesis, single-atom editing diversifies molecular scaffolds and tailors molecular properties to enhance pharmacological activity. In heterocyclic synthesis, this approach enables controlled heteroatom substitution, addition or deletion in an unprecedented and highly selective manner compared to traditional methods. Recent advances in transition-metal catalysis, organocatalysis, photoredox catalysis, and heterocycle-to-heterocycle metamorphosis have expanded the versatility of single-atom editing, enabling the synthesis of various carbocyclic and heterocyclic compounds. Principally, this approach has been exploited to design new architectures that are not easily accessible by other methods and to establish major improvements in the synthesis of known scaffolds, providing more efficient and sustainable routes towards large-scale chemical synthesis. This review overviews recent advances, focusing on carbocyclic and heterocyclic frameworks, and is organized by key single-atom editing strategies, such as ring contractions, atom deletions, ring expansions, and atom insertions. The review highlights key transformations like Favorskii and Wolff rearrangements, alongside modern photochemical and transition-metal-catalyzed processes, to provide a broad overview of synthetic applications and inspire further advancements in targeted molecular edits.

Fusion of Four Aromatic Rings via an Atom-Mutual-Embedding Strategy to Form a Tetrahexacyclic System

Skeletal manipulation of aromatic compounds has emerged as a potent tool in synthetic chemistry, but simultaneous multiring manipulation remains largely unexplored due to the inherent complexities of ring and site selectivity. Herein, we report an unprecedented multiring skeletal manipulation that fuses four 5-membered aromatic rings, comprising two organic and two metal-containing aromatic systems, into a novel metal-bridged 6/6/6/6-membered ring scaffold. The sequential ring fusion is accomplished through an atom-mutual-embedding strategy; this strategy entails the stepwise insertion of two nitrogen atoms into separate metal-carbon bonds and simultaneously integrates a metal atom as a bridge across two isoxazole moieties. The presence of a central metal atom is crucial for ensuring precise substrate alignment and enhancing both the ring and site specificity. The resulting tetrahexacyclic products exhibit remarkable stability and superior near-infrared (NIR) functional properties, surpassing those of the precursor compounds. This work not only establishes a conceptual foundation for designing versatile substrate molecules amenable to intricate editing but also contributes to the rational and performance-targeted manipulation of molecular architectures.

Fluorescent pyridine phosphonium salts via transmutation of metallabenzenes

Metallabenzenes are recognized as a unique class of aromatic compounds, not only of structural and theoretical interest but also as platforms to design powerful transformations. Here, we report the successful transmutation of a metallabenzene for pyridine synthesis. This 'metal-to-nitrogen swapping' process utilizes readily available ruthenabenzene phosphonium salts and commercially available 2-aminopyridines under mild conditions. The isolation of ruthena-azepines, containing a planar seven-membered aza-metallacycle, along with DFT calculations, supports the nitrogen insertion/metal deletion cascade driven by aromatization. Additionally, we investigate the tunable photophysical properties of the resulting pyridine phosphonium salts.

Indole-Quinoline Transmutation Enabled by a Formal Rhodium Carbynoid

Skeleton editing is an emerging and powerful tool in organic chemistry because it can simplify synthetic procedures towards complex molecules. Herein, we present an approach for indole-quinoline transmutation through rhodium-catalyzed single-carbon insertion into the C2(sp2)─C3(sp2) bond of an indole with an α-diazotrifluoroethyl sulfonium salt that we developed. This protocol involves a formal trifluoromethyl rhodium carbynoid (CF3C+ = Rh) resembling a trifluoromethyl cationic carbyne (CF3C+:), allowing facile access to an array of quinolines in moderate to high yields. Potential applications in the late-stage skeletal editing of pharmaceutical and natural product derivatives, preparation of adapalene analogs, scaled-up synthesis, and transformations of products are highlighted. Finally, a computational study was conducted to support the envisioned mechanism.

Skeletal Editing of Isoindolines to Tetralins

We present a skeletal editing strategy for transforming isoindolines into tetralins via a cascade N-atom removal deconstruction followed by a Diels-Alder reaction between in situ generated o-quinodimethanes and activated alkenes. This approach features a broad substrate scope, excellent stereoselectivity, and high yields, demonstrating its applicability to complex bioactive compounds and natural products. Notably, case studies showcase the efficient construction of challenging spirocyclic and bridged systems, underscoring the method's versatility and potential for advancing applications in synthetic chemistry.

Reimagining Dearomatization: Arenophile-Mediated Single-Atom Insertions and π-Extensions

ConspectusDearomatization of simple aromatics serves as one of the most direct strategies for converting abundant chemical feedstocks into three-dimensional value-added products. Among such transformations, cycloadditions between arenes and alkenes have historically offered effective means to construct complex polycyclic architectures. However, traditionally harsh conditions, such as high-energy UV light irradiation, have greatly limited the scope of this transformation. Nevertheless, recent progress has led to the development of visible-light-promoted dearomative photocycloadditions with expanded scope capable of preparing complex bicyclic structures.A fundamentally distinct approach to dearomative photocycloadditions involves the visible-light activation of arenophiles, which undergo para-photocycloaddition with various aromatic compounds to produce arene-arenophile cycloadducts. While only transiently stable and subject to retro-cycloaddition, further functionalization of the photocycloadducts has allowed for the development of a wide collection of dearomatization methodologies that access products orthogonal to existing chemical and biological processes. Central to this strategy was the observation that arene-arenophile photocycloaddition reveals a π-system that can be functionalized through traditional olefin chemistry. Coupled with subsequent [4 + 2]-cycloreversion of the arenophile, this process acts to effectively isolate a single π-system from an aromatic ring. We have developed several transformations that bias this methodology to perform dearomative single-atom insertion and π-extension reactions to prepare unique products that cannot be prepared easily through traditional means.Through the application of a dearomative epoxidation, we were able to develop a method for the epoxidation of arenes and pyridines to arene-oxides and pyridine-oxides, respectively. Notably, when this arenophile chemistry is applied to polycyclic arenes, photocycloaddition reveals a π-system transposed from the site of native olefinic reactivity, enabling unique site-selectivity for dearomative functionalization. As a result, we were able to perform a single-atom insertion of oxygen into polycyclic (aza)arenes to prepare 3-benzoxepines. When applying this strategy in the context of cyclopropanations, we were able to accomplish a dearomative cyclopropanation of polycyclic (aza)arenes which yield benzocycloheptatrienes upon cycloreversion. Notably, while the Buchner ring expansion is a powerful method for the direct single-atom insertion of carbon into arenes, the corresponding cyclopropanation of polycyclic arenes does not yield ring-expanded products. Furthermore, this strategy could be utilized for the synthesis of novel nanographenes through the development of an M-region annulative π-extension (M-APEX) reaction. Traditionally, methods for π-extension rely on the native reactivity of polycyclic aromatics at the K- and bay-region. However, photocycloaddition of polycyclic aromatics with arenophiles acts as a strategy to activate the M-region for further reactivity. As a result, arenophile-mediated dearomative diarylation, followed by cycloreversion, could deliver π-extended nanographenes with exclusive M-region site selectivity.

Reaction development: a student's checklist

So you've discovered a reaction. But how do you turn this new discovery into a fully-fledged program that maximises the potential of your novel transformation? Herein, we provide a student's checklist to serve as a helpful guide for synthesis development, allowing you to thoroughly investigate the chemistry in question while ensuring that no key aspect of the project is overlooked. A wide variety of the most illuminating synthetic and spectroscopic techniques will be summarised, in conjunction with literature examples and our own insights, to provide sound justifications for their implementation towards the goal of developing new reactions.

Synthesis of Masked 2-Pyridones from 1,3-Enynyl Esters via Tandem Gold-Catalyzed Cycloisomerization and Oxidative Nitrogen Insertion

An efficient synthetic method to regioselectively assemble masked 2-pyridones that relies on sequential gold-catalyzed cycloisomerization and phenyliodonium diacetate (PIDA)-mediated oxidative nitrogen insertion from 1,3-enynyl esters in a one-pot, two-step manner is described. The synthetic utility of the cascade protocol was further demonstrated by the 2 mmol scale synthesis of one example and its elaboration to other potentially useful building blocks as well as the late-stage modification of a diverse array of structurally complex bioactive molecules.

Cheminformatic Analysis of Core-Atom Transformations in Pharmaceutically Relevant Heteroaromatics

Heteroaromatics are the basis for many pharmaceuticals. The ability to modify these structures through selective core-atom transformations, or "skeletal edits", can dramatically expand the landscape for drug discovery and development. However, despite the importance of core-atom modifications, the quantitative impact of such transformations on accessible chemical space remains undefined. Here, we report a cheminformatic platform to analyze which skeletal edits would most increase access to novel chemical space. This study underscores the significance of emerging single and multiple core-atom transformations of heteroaromatics in enhancing chemical diversity, for example, at a late-stage of a drug discovery campaign. Our findings provide a quantitative framework for prioritizing core-atom modifications in heteroaromatic structural motifs, calling for the development of new methods to achieve these types of transformations.

Regiospecific 2,3-Dialkylindole Synthesis Enabled by Alkylpalladium 1,2-Migration to In Situ Formed Aldimine

2,3-Dialkylindoles play crucial roles in natural products and pharmaceuticals, but the step-efficient and regioselective construction of such privileged structures remains a long-standing challenge. Here, we report a regiospecific non-Fischer indole synthesis through chemoselective 1,2-migratory addition of alkylpalladium to an aldimine intermediate, formed in situ through a palladium hydride-triggered sequential isocyanide and intramolecular olefin insertion. This unprecedented 1,2-migratory addition leads to formal C═C bond cleavage and isocyano carbon insertion between the two sp2 carbons, offering a novel approach to specific 2,3-dialkyl substituted N─H free indoles from readily available alkyl substituted 1-isocyano-2-vinylbenzenes.

Diversity-Generating Skeletal Editing Transformations

ConspectusSkeletal editing, as a synthetic tool, offers the unique potential to selectively and efficiently modify the core skeleton of a target molecule at a late-stage. The main benefit of such transformations is the rapid exploration of the chemical space around lead compounds without necessitating a de novo synthesis for each new molecule. However, many skeletal editing transformations are inherently restricted to generating a single product from a single starting compound, limiting the potential for diversification, a concept central to expediting structure-activity relationship (SAR) investigations. In this Account, we describe our efforts to develop novel skeletal editing transformations in which a modification to the central motif of a molecule is performed simultaneously with the incorporation of additional functionality that can be easily varied through a judicious choice of the reagents. Specifically, we successfully developed an α-iodonium diazo-based carbynyl radical equivalent reagent that, under photoredox conditions, could facilitate the ring-expansion of indene scaffolds while enabling the insertion of over ten different functionalized carbon atoms into the corresponding naphthalene products. This concept was later extended to the design of an atomic carbon equivalent reagent that could promote mild and selective Ciamician-Dennstedt-type indole ring-expansion reactions, while simultaneously installing an oxime ester handle that could undergo further functionalization. Furthermore, we highlight recent work from our group on multiple-atom insertion reactions, namely, the development of a photocatalyzed De Mayo reaction for the ring-expansion of cyclic ketones and a photocatalyzed dearomative ring-expansion of thiophenes via small-ring insertion. In both of these cases, multiple products can be potentially accessed from a single starting material upon variation of the insertion reagent. The diversity-generating skeletal editing strategy could also be applied to single-atom transmutation, as demonstrated by the development of a nitrogen-to-functionalized carbon atom transmutation reaction to convert pyridine to benzene rings. Here, the desired transformation was achieved via a sequence of pyridine ring-opening, Horner-Wadsworth-Emmons (HWE) olefination, and ring-closure, with a judicious choice of the HWE reagent allowing the installation of a wide variety of versatile functional groups. Finally, an energy transfer-mediated quinoline ring-contraction is discussed, specifically with reference to the ways in which it does and does not fit the criteria of a skeletal editing reaction. Although formal atom deletion transformations are typically restricted to single products from each discrete substrate, this [2 + 2] cycloaddition/rearrangement cascade also involves the incorporation of an alkene into the molecule and introduces a point of variation that can be exploited for diversity generation. We hope to not only highlight the transformations reported herein but also inspire further research into this synthetic strategy to access new classes of skeletal editing transformations that, through rapid diversity generation, provide the potential to expedite SAR investigations.

Transition Metal-Catalyzed Nitrogen Atom Insertion into Carbocycles

ConspectusN-Heterocycles are essential in pharmaceutical engineering, materials science, and synthetic chemistry. Recently, skeletal editing, which involves making specific point changes to the core of a molecule through single-atom insertion, deletion, or transmutation, has gained attention for its potential to modify complex substrates. In this context, the insertion of nitrogen atoms into carbocycles to form N-heterocycles has emerged as a significant research focus in modern synthetic chemistry owing to its novel synthetic logic. This distinctive retrosynthetic approach enables late-stage modification of molecular skeletons and provides a different pathway for synthesizing multiply substituted N-heterocycles. Nevertheless, nitrogen atom insertion into carbocycles has proven challenging because of the inherent inertness of carbon-based skeletons and difficulty in cleaving C-C bonds. Therefore, selective insertion of nitrogen atoms for skeletal editing remains a challenging and growing field in synthetic chemistry. This Account primarily highlights the contributions of our laboratory to this active field and acknowledges the key contributions from other researchers. It is organized into two sections based on the type of the carbocycle. The first section explores the insertion of nitrogen atoms into cycloalkenes. Recent Co-catalyzed oxidative azidation strategies have enabled nitrogen atom insertion into cyclobutenes, cyclopentenes, and cyclohexenes, facilitating the synthesis of polysubstituted pyridines, which has been conventionally challenging through pyridine cross-coupling. The subsequent section highlights our discovery in the realm of nitrogen atom insertion into arenes. The site-selective skeletal editing of stable arenes is challenging in synthetic chemistry. We developed a method for the intramolecular insertion of nitrogen atoms into the benzene rings of 2-amino biaryls by suppressing the competing C-H insertion process by using a paddlewheel dirhodium catalyst. In addition, to address the challenging site-selective issues in nitrogen atom insertion, we employed arenols as substrates, which could act as selective controlling elements in site-selective skeletal editing. We reported a Cu-catalyzed nitrogen atom insertion into arenols, which proceeds through a dearomative azidation/aryl migration process, enabling the site-selective incorporation of nitrogen atoms into arenes. Inspired by this result, we recently extended the reaction model by using a Fe-catalyst to facilitate the ring contraction of the nitrogen-inserted product, achieving the carbon-to-nitrogen transmutation of arenols. Various complex polyaromatic arenols could effectively undergo the desired atom's transmutation, presenting considerable potential for various applications in materials chemistry. In this Account, we present an overview of our achievements in nitrogen atom insertion reactions, with a focus on the reaction scopes, mechanistic features, and synthetic applications. We anticipate that this Account will provide valuable insights and propel the development of innovative methodologies in both skeletal editing and N-heterocycle synthesis.

Molecular Ring Remodeling through C-C Bond Cleavage

ConspectusStable and inert C-C bonds form the fundamental framework of organic compounds. Consequently, direct transformations involving C-C bond cleavage present an innovative approach for the rapid modification and remodeling of molecular skeletons. In recent years, the concept of molecular skeletal editing has garnered widespread attention and has been significantly developed, providing new opportunities for the late-stage modification of bioactive molecules, the high-value transformation of bulk chemicals, and a revolution in the traditional fragment coupling strategies of chemical synthesis. Notable advancements in this field have focused on C-C bond cleavage and the remodeling of cyclic molecules, including ring expansion, ring contraction, and ring-opening reactions, thereby enriching the synthetic toolbox available to chemists. However, selective C-C bond transformation remains a formidable challenge, especially in the remodeling of complex molecules, due to the high bond dissociation energy and the difficulty in achieving precise selectivity control. Over the past few years, our group has made efforts to address these challenges. We have demonstrated the potential of cyclic molecule remodeling reactions as an efficient strategy for the synthesis and modification of complex molecules.Herein, we present two major thematic advancements achieved by our group, utilizing cascade activation and entropy-driven reconstruction strategies for molecular ring remodeling via C-C bond cleavage. These strategies are characterized by mild conditions, the accessibility of catalysts and reagents, and exceptional functional group compatibility, thereby emerging as novel approaches for molecular ring remodeling through atom-incorporation reactions mainly on nitrogenation, oxygenation, and halogenation to synthesize pharmaceuticals, natural products, and material molecules. (1) Ring expansion reactions: We developed novel reactions that enable the insertion of C-, N-, and O-containing units into molecular rings. These methodologies offer practical and efficient routes for synthesizing amides, amines, lactones, and nitrogen-containing heterocycles. (2) Ring-opening reactions: C-C bond cleavage in ring-opening reactions enables the efficient construction of distally difunctionalized molecular frameworks. By utilizing a transition metal catalysis and radical-mediated process, we have successfully achieved the cleavage of both C-C single bonds and C═C double bonds within molecular rings. Furthermore, we have tackled the highly challenging arene ring-opening (ARO) reaction, enabling the construction of stereoselective conjugated systems through the unsaturation liberation of aromatic systems. Mechanistic studies and DFT calculations have provided critical insights into these processes. We have also identified key intermediates involved in C-C bond cleavage, including benzyl azide, O-acetyl hydroxylamine, β-azido peroxyl radical, copper bisnitrene, and 2-nitrene indazole. These findings have deepened our understanding of the mechanisms and the entropy-driven reconstruction strategy, which has further promoted the discovery of related C-C bond transformations of acyclic substrates.

Visible-Light-Induced Sulfonylation Cyclization to Generate Indole-Fused Medium-Sized N-Heterocycles in 2-Me-THF

A novel visible-light-induced sulfonylation cyclization to indole-fused medium-sized N-heterocycles was established under room temperature with biomass-derived 2-Me-THF as the solvent. This reaction proceeds in the absence of external photocatalyst, additive, metal salts, and base. Broad substrate scope, good functional group compatibility, and large-scale synthesis and derivatization via iodination, nitration, chlorination, cyanation, and selenylation demonstrate the utility of this protocol. A radical cyclization route was proposed based on radical inhibition experiments, visible-light irradiation on-off test, apparent quantum efficiency calculation, and UV-vis absorption spectroscopic studies.

Photochemical aerobic sulfonylation-cyclization-selenylation to indole-fused medium-sized N-heterocycles in 2-Me-THF

Indole-fused medium-sized diazepinones are privileged structural motifs found in many high-value pharmaceuticals. To construct these challenging molecular skeletons, previous methods are mainly achieved by [5+2] annulation strategies with the aid of transition metal (Pd, Rh, Ru) catalyzed oxidative C-H coupling. Herein, we report a novel visible-light-induced sulfonylation-cyclization-selenylation reaction for the rapid construction of highly functionalized indole-fused medium-sized diazepinones with biomass feedstock 2-Me-THF as the medium. Based on the mechanism studied, an electrophilic radical sulfonylation-cyclization and subsequent aerobic electrophilic C-3 selenylation route is proposed.

Dibenzothiophenium Salts: Practical Alternatives to Hypervalent I(III)-Based Reagents

ConspectusDuring the past few years, the interest among organic synthesis practitioners in the use of sulfonium salts has exponentially growth. This can arguably be attributed to a series of specific factors: (a) The recent development of more direct and efficient protocols for the synthesis of these species, which make sulfonium reagents of a wide structural variety easily available in multigram scale. (b) The recognition that the reactivity of these salts resembles that of hypervalent iodine compounds, and therefore, they can be used as effective replacement of such species in most of their applications. (c) Their intrinsic thermal stability and tolerance to air and moisture, which clearly surpass that of I(III)-reagents of analogue reactivity, and facilitate their purification, isolation as well-defined species, storage, and safely handling on larger scale. (d) Finally, the possibility to further functionalize sulfonium salts once the sulfur-containing platform has been incorporated. Specifically, this last synthetic approach is not trivial when working with hypervalent I(III)-species and facilitates the access to sulfonium salts with no counterpart in the I(III) realm.This renewed interest in sulfonium salts has led to the improvement of already existing transformations as well as to the discovery of unprecedented ones; in particular, by the development of protocols that incorporate sulfonium salts as partners in traditional cross-coupling and C-H activation steps or combine them with more modern technologies such as photocatalysis or electrosynthesis. In this Account, the reactivity of a series of sulfonium salts originally prepared in our laboratory will be outlined and compared to their I(III)-counterparts. Some of these reagents are now commercially available, and their use has started to spread widely across the synthetic chemistry community, helping to speed the process of identification of potentially bioactive products or new functionaliced materials. However, challenges still remain. The development of sulfonium reagents characterized by an optimal balance between reactivity and site-selectivity, or showing broader compatibility toward sensitive functional groups is still a need. In addition, the intrinsic stability of sulfonium salts often makes necessary the use of (sophisticated) catalysts that activate the latent reactivity hidden in their structures. Although a priori one can see this fact as a disadvantage, it might actually be decisive to harvest the full synthetic potential of sulfonium salts because their thermal stability will surely facilitate the preparation of operational reagents with no counterpart in the context of I(III)-chemistry. If this becomes true, sulfonium salts may contribute to the expediting of retrosynthetic disconnections that, to date, are impossible.

Copper-Catalyzed Enantioselective Skeletal Editing through a Formal Nitrogen Insertion into Indoles to Synthesize Atropisomeric Aminoaryl Quinoxalines

Skeletal editing represents an attractive strategy for adding complexity to a given molecular scaffold in chemical synthesis. Isodesmic reactions provide a complementary skeletal editing approach for the redistribution of chemical bonds in chemical synthesis. However, catalytic enantioselective isodesmic reaction is extremely scarce and enantioselective isodesmic reaction to synthesize atropisomeric compounds is unknown. Herein, we report a facile method to synthesize axially chiral aminoaryl quinoxalines through Cu(I)-catalyzed dearomatization and sequential chiral phosphoric acid (CPA) catalyzed enantioselective isodesmic C-N bond formation and cleavage from indoles and 1,2-diaminoarenes under mild reaction conditions. In this process, the five-membered ring of the indole scaffold was broken and a novel quinoxaline skeleton was constructed. This method allows the practical and atom-economical synthesis of valuable axially chiral aminoaryl quinoxalines in high yields (up to 95 %) and generally excellent enantioselectivities (up to 99 % ee). Notably, this novel type of quinoxaline atropisomers has promising applications in developing axially chiral ligand in asymmetric catalysis. This strategy represents the first example of CPA-catalyzed enantioselective isodesmic reaction to form axially chiral compounds.

Unifying principles for the design and evaluation of natural product-inspired compound collections

Natural products play a major role in the discovery of novel bioactive compounds. In this regard, the synthesis of natural product-inspired and -derived analogues is an active field that is further developing. Several strategies and principles for the design of such compounds have been developed to streamline their access and synthesis. This perspective describes how individual strategies or their elements can be combined depending on the project goal. Illustrative examples are shown that demonstrate the blurred lines between approaches and how they can work in concert to discover new biologically active molecules. Lastly, a general set of guidelines for choosing an appropriate strategy combination for the specific purpose is presented.

Micromolar Concentration Affinity Study on a Benchtop NMR Spectrometer with Secondary 13C Labeled Hyperpolarized Ligands

Benchtop NMR is becoming an increasingly important tool, sometimes providing a simple and low-cost alternative to high-field NMR. The Achilles heel of NMR and even more critically of benchtop NMR is its limited sensitivity. However, when combined with hyperpolarization techniques, the sensitivity boost can provide excellent sensitivity that can even make benchtop NMR compatible with affinity studies for drug discovery. Hyperpolarization by dissolution dynamic nuclear polarization (dDNP) provides a route to enhancing 13C nuclear magnetic resonance (NMR) sensitivity by more than 5 orders of magnitude for a wide range of small molecules on a benchtop NMR system. We show here how ligands can be secondarily labeled with 13C tags and hyperpolarized with conventional dDNP methods. These hyperpolarized ligands display long nuclear spin-lattice relaxation time constants and can therefore be used to probe interactions with target proteins in conventional dDNP settings. The boost in sensitivity combined with the simplicity of the 13C spectra (one peak per ligand) enables detection on an 80 MHz benchtop NMR spectrometer at micromolar concentrations, which may ultimately provide a way of improving and accelerating the discovery of new drug candidates.

Oxenoid Reactivity Enabled by Targeted Photoactivation of Periodate

The chemistry of low-valent intermediates continues to inspire new modes of reactivity across synthetic chemistry. But while the generation and reactivity of both carbenes and nitrenes are well-established, difficulties in accessing oxene, their oxygen-based congener, has severely hampered its application in synthesis. Here, we report a conceptually novel approach towards oxenoid reactivity through the violet-light photolysis of tetrabutylammonium periodate. Computational studies reveal an unexpected geometric change upon periodate photoexcitation that facilitates intersystem crossing and near-barrierless dissociation of triplet periodate into oxene. Under these operationally simple conditions, we have demonstrated the epoxidation of a wide range of substituted olefins, revealing unprecedented functional group compatibility. By overcoming the historic challenges associated with employing oxene as an intermediate in organic chemistry, we believe that this platform will inspire the development of new reactive oxygen-based methodologies across industry and academia.

Redox-Tunable Ring Expansion Enabled By A Single-Component Electrophilic Nitrogen Atom Synthon

Controllable installation of a single nitrogen atom is central to many major goals in skeletal editing, with progress often gated by the availability of an appropriate N-atom source. Here we introduce a novel reagent, termed DNIBX, based on dibenzoazabicycloheptadiene (dbabh), which allows the electrophilic installation of dbabh to organic substrates. When indanone β-ketoesters are aminated by DNIBX, the resulting products undergo divergent ring expansions depending on the mode of activation, producing heterocycles in differing oxidation states under thermal and photochemical conditions. The mechanism of each transformation is discussed, and the different reactivity modes of the indanone-dbabh adducts are compared to other nitrogenous precursors.

Controllable Molecular Editing of 2-Amino-N-substituted Benzamides: Site-selective Synthesis of 6-Selenylated N-Substituted 1,2,3-Benzotriazine-4(3H)-ones

We present an efficient silver-catalyzed one-pot controllable molecular editing protocol for the transformation of 2-amino-N-substituted benzamides into 6-selenylated N-substituted 1,2,3-benzotriazine-4(3H)-ones under mild reaction conditions. This three-component reaction strategy is achieved by building N-N/N═N/C-Se bonds, which provides a practical pathway for the preparation of selenylated 1,2,3-benzotriazine-4(3H)-ones with a broad substrate scope and good functional group tolerance, as well as high site-selectivity. Mechanistic experiments suggest that this reaction proceeds via intermolecular site-selective C-H selenylation of 2-amino-N-substituted benzamides with readily available diselenides, followed by annulation of selenylated 2-amino-N-substituted benzamides using AgNO3 as the nitrogen synthon.

Asymmetric dearomative single-atom skeletal editing of indoles and pyrroles

Heterocycle skeletal editing has recently emerged as a powerful tactic for achieving heterocycle-to-heterocycle transmutation without the need for multistep de novo heterocycle synthesis. However, the enantioselective skeletal editing of heteroarenes through single-atom logic remains challenging. Here we report the enantiodivergent dearomative skeletal editing of indoles and pyrroles via an asymmetric carbon-atom insertion, using trifluoromethyl N-triftosylhydrazones as carbene precursors. This strategy provides a straightforward methodology to access enantiomerically enriched six-membered N-heterocycles containing a trifluoromethylated quaternary stereocentre from planar N-heteroarenes. The synthetic utility of this enantiodivergent methodology was demonstrated by a broad evaluation of reaction scope, product derivatization and concise syntheses of drug analogues. Mechanistic studies reveal that the excellent asymmetric induction arises from the initial cyclopropanation step. The asymmetric single-atom insertion strategy is expected to have a broad impact on the field of single-atom skeletal editing and catalytic asymmetric dearomatization of aromatic compounds.

Electrochemical Ring Opening and [3 + 2] Cycloaddition of Aziridines: Access to 1,2-Bifunctionalized Products and Imidazolines

Herein, we report an electricity-driven activation of aziridine via direct anodic oxidation to give N-heterocycles and 1,2-bifunctionalized products by excluding any oxidant/reductant or metal catalyst. Many structurally modified aziridines were employed in the presence of different nitriles. A large variety of nucleophiles were screened to furnish chemoselectively O-alkylated and C-alkylated products. Late-stage derivatization of aziridine with natural and medicinally active compounds has also been done. Remarkably, our strategy was found to be a greener, sustainable, and atom-economical approach (E-factor = ca. 0.8). Azetidine was also found to be compatible with our protocol and generated six-membered N-heterocycles. The detailed mechanistic study highlighted that the reaction is driven via the generation of an aziridine radical cation followed by the SN2 nucleophilic attack.

Substituent-Controlled Regiodivergent Rearrangement of Gramine Ammonium Ylide

The complicated mechanism makes the regiodivergent rearrangement of ammonium ylide seem to be out of reach. Herein, we reported a regiodivergent rearrangement of gramine ammonium ylide well controlled by the substituents. Density functional theory studies reveal that the ammonium ylide with a more steric hindrance substituent 2-diazo-2-arylacetate goes through a stepwise mechanism to yield both a kinetically and thermodynamically preferred [1,2]-rearrangement product. In contrast, the ammonium ylide with a less steric hindrance ethyl diazoacetate goes through a concerted mechanism to generate the [2,3]-rearrangement product, which is kinetically favored as a result of the release of the ring strain in the transition state. This study would open up avenues to grasp the rearrangement of ammonium ylide, which will promote application in the skeletal editing and synthesis of complex natural products.

One-Step Realization of Skeleton Editing, gem-Dinitromethyl Functionalization, and Zwitterionization in a Laser-Sensitive 1,3,4-Oxadiazole Energetic Molecule

The single-atom skeletal editing technology is an efficient method for constructing molecular skeletons, which has broad coverage in synthetic chemistry. However, its potential in the preparation of energetic heterocyclic molecules is grossly underexplored. In this work, an unexpected one-step reaction for the synthesis of novel energetic molecules was discovered which combines single-atom skeletal editing, gem-dinitromethyl functionalization, and zwitterionization in one step. The reaction demonstrates high efficiency while maintaining the characteristics of being mild and facile. The reaction mechanism was verified by experimental evidence and theoretical calculations. This reaction produces a novel energetic molecule (NPX-04) with good laser ignition performance, indicating its promise as a laser-sensitive energetic material.

Synthesis of Functionalized Cycloheptadienones Starting from Phenols and Using a Rhodium/Boron Asymmetric Catalytic System

Skeletal editing offers a unique route to assemble complex architectures from simple feedstocks that are otherwise difficult to obtain. However, the asymmetric version of skeletal editing has not been widely studied. Herein, we present a modular rhodium/boron asymmetric catalytic system that enables ring-expansion of phenols with cyclopropenes to synthesize highly functionalized cycloheptadienones in excellent chemo- and regioselectivities. This unique protocol features with low-catalyst loading, atom and step-economies, and mild neutral reaction conditions. Isotope-labelling experiments and DFT calculations have been conducted to reveal that boron reagent plays a vital role in the whole catalytic cycle.

Polyfluoroalkanoic Acids as Fluoroalkylating Reagents: Strategy for Direct Access to Rf-Embedded Amides

Herein, we have reported the application of bench stable perfluoroalkanoic acids as fluoro-alkylating reagents in combination with DIB and primary amides for sequential one-pot transformation to Rf-embedded functionalized amides under metal-free conditions. The protocol is tolerant to a range of sensitive functional groups (>33 examples and up to 90% yield), and perfluoro acids. Preliminary mechanistic studies, control experiments, in situ 19F-NMR analyses, and the synthesis of intermediate species were performed to understand the reaction pathways.

Interrogation of Enantioselectivity in the Photomediated Ring Contractions of Saturated Heterocycles

We recently reported a chiral phosphoric acid (CPA) catalyzed enantioselective photomediated ring contraction of piperidines and other saturated heterocycles. By extruding a single heteroatom from a ring, this transformation builds desirable C(sp3)-C(sp3) bonds in the ring contracted products; however, the origins of enantioselectivity remain poorly understood. In this work, enantioselectivity of the ring contraction has been explored across an expanded structurally diverse substrate scope, revealing a wide range of enantioselectivities (0-99%) using two distinct CPA catalysts. Mechanistic investigations support rate-determining excitation that generates short-lived achiral intermediates that are intercepted by the CPA in an enantiodetermining ring closure. The effects of competitive uncatalyzed reactivity and light-driven reversibility of the enantiodetermining ring closure on enantioselectivity have been elucidated. Statistical models were built by regressing the range of enantioselectivities from the substrate scope against key structural features of the products for both CPA catalysts. The resultant models suggested distinct factors that influence the enantioselectivity response for each catalyst and enabled rational modification of a pharmaceutically relevant target molecule to improve enantioselectivity. Finally, density functional theory (DFT)-based transition state analysis identified distinct noncovalent interactions with each catalyst that correlated with the unique selectivity-relevant features uncovered through statistical modeling. Our findings not only offer comprehensive insight into the origins of enantioselectivity in this system but should also aid future development of related photomediated CPA-catalyzed reactions.

A Formal 1,2-Stevens Rearrangement of Thioester Ylides as a Single-Atom Molecular Editing Tool

A rhodium-catalyzed reaction of thioesters with diazo reagents was recognized as a powerful and unprecedented tool for single-atom molecular editing by the insertion of a single carbon atom into the C(O)─S thioester bond, thereby leading to various α-thioketones possessing a quaternary carbon atom. A selective and precise defunctionalization of the polyfunctionalized products further demonstrated the synthetic utility of the reaction for the synthesis of more common structural classes of compounds.

Single-Carbon Insertion into Single C-C Bonds with Diazirines

A novel platform for the skeletal editing of single C-C bonds via a single-carbon insertion has been developed using diazirines. This strategy involves the photogeneration of arylchlorocarbenes as carbynoid species that undergo site-selective carbene insertion into tertiary C-H bonds and a subsequent Wagner-Meerwein rearrangement promoted by a silver salt. Our skeletal editing strategy based on a formal selective carbyne C-C bond insertion has been demonstrated in six core-to-core conversions, including linear and cyclic benzylic substrates, alkanes and late-stage functionalizations.

Sulfenylnitrene-mediated nitrogen-atom insertion for late-stage skeletal editing of N-heterocycles

Given the prevalence of nitrogen-containing heterocycles in commercial drugs, selectively incorporating a single nitrogen atom is a promising scaffold hopping approach to enhance chemical diversity in drug discovery libraries. We harness the distinct reactivity of sulfenylnitrenes, which insert a single nitrogen atom to transform readily available pyrroles, indoles, and imidazoles into synthetically challenging pyrimidines, quinazolines, and triazines, respectively. Our additive-free method for skeletal editing employs easily accessible, benchtop-stable sulfenylnitrene precursors over a broad temperature range (-30 to 150°C). This approach is compatible with diverse functional groups, including oxidation-sensitive functionalities such as phenols and thioethers, and has been applied to various natural products, amino acids, and pharmaceuticals. Furthermore, we have conducted mechanistic studies and explored regioselectivity outcomes through density functional theory calculations.

Transforming an azaarene into the spine of fusedbicyclics via cycloaddition-induced scaffold hopping of 5-Hydroxypyrazoles

Skeleton editing for heteroarenes, especially pyrazoles, is challenging and remains scarce because these non-strained aromatics exhibit inert reactivities, making them relatively inactive for performing a dearomatization/cleavage sequence. Here, we disclose a cycloaddition-induced scaffold hopping of 5-hydroxypyrazoles to access the pyrazolopyridopyridazin-6-one skeleton through a single-operation protocol. By converting a five-membered aza-arene into a five-unit spine of a 6/6 fused-bicyclic, this work unlocks a ring-opening reactivity of the pyrazole core that involves a formal C = N bond cleavage while retaining the highly reactive N-N bond in the resulting product. A [4 + 2] cycloaddition of a temporarily dearomatized 5-hydroxypyrrole with an in situ generated aza-1,3-diene, followed by oxidative C-N bond cleavage, constitutes the domino pathway. A library of pyrazolopyridopyridazin-6-ones, which are medicinally relevant nitrogen-atom-rich tricyclics, is obtained efficiently from readily available materials.

Aerobic Reconstruction of Amines to Amides: A C-N/C-C Bond Cleavage Approach

Herein, an aerobic reconstruction of amines to amides via C(sp3)-N bond and C(sp2)-C(sp3) bond cleavage is described. This method features a metal-free reaction, insensitivity to oxygen or moisture, and ambient air as the terminal oxidant. Preliminary mechanistic studies suggest that the reaction pathway of amine oxidation, followed by imine exchange and Beckmann rearrangement, is involved.

Triftosylhydrazone in Single-Atom Skeletal Editing

ConspectusIn the past decade, single-atom skeletal editing, which involves the precise insertion, deletion, or exchange of single atoms in the core skeleton of a molecule, has emerged as a promising synthetic strategy for the rapid construction or diversification of complex molecules without laborious de novo synthetic processes. Among them, carbene-initiated skeletal editing is particularly appealing due to the ready availability and diverse reactivities of carbene species. The initial endeavors to modify the core skeleton of heteroarenes through carbon-atom insertion could date back to 1881, when Ciamician and Denstedt described the conversion of pyrroles to pyridines by trapping haloform-derived free carbene. Despite its potential synthetic value, the general applicability of this one-carbon insertion has seen limited progress due to poor yields and harsh reaction conditions. Significant advances in skeletal editing via carbene insertion were achieved only in the past 3 years by Levin, Ball, Xu, Song, Glorius, and others. The hallmark of these approaches is facile halocyclopropanation followed by regioselective ring opening facilitated by the expulsion of the halide ion. Consequently, only specially designed α-halocarbene precursors, such as haloform derivatives, α-halodiazoacetates, chlorodiazirines, and α-chlorodiazo oxime esters, can be employed to achieve Ciamician-Denstedt-type skeletal editing. This not only limits the types of functional groups installed on the ring expansion products but also prevents their widespread adoption, especially in late-stage contexts. The enduring quest to develop environmentally friendly and versatile carbene precursors, superior functional group compatibility, and potential application in late-stage diversifications and the investigation of mechanistic insights into carbon insertion reactions remain a fundamental objective.In our endeavors over the past 5 years, we have developed o-trifluoromethylbenzenesulfonylhydrazones (named Triftosylhydrazones) as operationally safe and easily decomposable diazo surrogates and explored their application in various challenging catalytic carbene transfer reactions. Recently, our group has put great efforts into expanding the application scope and unlocking the potential of triftosylhydrazones as carbene precursors in single-atom skeletal editing reactions. Since 2018, we have realized a range of skeletal editing of acyclic 1,3-dicarbonyls with silver carbenes to access 1,4-dicarbonyls, proceeding through a cyclopropanation/ring-opening process. Inspired by these results, we recently demonstrated a series of transition-metal-catalyzed highly selective single-atom skeletal editing of medicinally interesting heteroarenes like pyrroles, indoles, and 1,2-diazoles via carbenic carbon insertion. We then achieved the skeletal editing of strained three-membered nitrogen- and oxygen-containing heterocycles through the insertion or exchange of single-carbon atoms. In this Account, we present an overview of our achievements in the single-atom skeletal editing of heterocycles, organized based on three types of in situ-generated key intermediates, such as cyclopropane, N-ylide, and O-ylide from triftosylhydrazones and heterocycles, with a focus on reaction scopes, mechanistic features, and synthetic applications. We hope that this Account will provide valuable insights and contribute to the development of new methodologies in both the skeletal editing and carbene chemistry fields.

Making, Using, and Understanding Molecular Systems: The 57th Bürgenstock Conference

The 57th SCS Conference on Stereochemistry, better known as the Bürgenstock Conference, brought together a diverse range of chemistry expertise in Brunnen, Switzerland.

Skeletal Editing of Aromatic N-Heterocycles via Hydroborative Cleavage of C-N Bonds-Scope, Mechanism, and Property

Skeletal editing of the core structure of heterocycles offers new opportunities for chemical construction and is a promising yet challenging research topic that has recently gained increasing interest. However, several limitations of the reported systems remain to be addressed. For example, the reagents employed are generally in high-energy, such as chlorocarbene precursors, nitrene species, and metal carbenes, which are also associated with low atomic efficiencies. Thus, the development of simple systems for the skeletal editing of heterocycles is still desired. Herein, a straightforward and facile BH3-mediated skeletal editing of readily available indoles, benzimidazoles, and several other aromatic heterocycles is reported. Structurally diverse products were readily obtained, including tetrahydrobenzo azaborinines, diazaboroles, O-anilinophenylethyl alcohols, benzene-1,2-diamines, and more. Density functional theory (DFT) calculations and natural bond orbital (NBO) analysis revealed a BH3-induced C-N bond cleavage reaction pathway. An exciting and counterintuitive indole hydroboration phenomenon of -BH2 shift from C3-position to C2-position was disclosed. Moreover, the photophysical properties of the synthesized diazaboroles were studied, and an interestingly and pronounced aggregation-induced emission (AIE) behavior was disclosed.

Halogencarbene-free Ciamician-Dennstedt single-atom skeletal editing

Single-atom skeletal editing is an increasingly powerful tool for scaffold hopping-based drug discovery. However, the insertion of a functionalized carbon atom into heteroarenes remains rare, especially when performed in complex chemical settings. Despite more than a century of research, Ciamician-Dennstedt (C-D) rearrangement remains limited to halocarbene precursors. Herein, we report a general methodology for the Ciamician-Dennstedt reaction using α-halogen-free carbenes generated in situ from N-triftosylhydrazones. This one-pot, two-step protocol enables the insertion of various carbenes, including those previously unexplored in C-D skeletal editing chemistry, into indoles/pyrroles scaffolds to access 3-functionalized quinolines/pyridines. Mechanistic studies reveal a pathway involving the intermediacy of a 1,4-dihydroquinoline intermediate, which could undergo oxidative aromatization or defluorinative aromatization to form different carbon-atom insertion products.

Hyperconjugation-Driven Isodesmic Reaction of Indoles and Anilines: Reaction Discovery, Mechanism Study, and Antitumor Application

Isodesmic reactions, in which chemical bonds are redistributed between substrates and products, provide a general and powerful strategy for both biological and chemical synthesis. However, most isodesmic reactions involve either metathesis or functional-group transfer. Here, we serendipitously discovered a novel isodesmic reaction of indoles and anilines that proceeds intramolecularly under weakly acidic conditions. In this process, the five-membered ring of the indole motif is broken and a new indole motif is constructed on the aniline side, accompanied by the formation of a new aniline motif. Mechanistic studies revealed the pivotal role of σ→π* hyperconjugation on the nitrogen atom of the indole motif in driving this unusual isodesmic reaction. Furthermore, we successfully synthesized a diverse series of polycyclic indole derivatives; among quinolines, potential antitumor agents were identified using cellular and in vivo experiments, thereby demonstrating the synthetic utility of the developed methodology.

Skeletal Reorganization Emanated via the Course of Heterocyclic N1-N2 Bond Cleavage: Electrosynthetic Approach

A unified method toward the synthesis of functionalized diazepines and quinazolines through reorganization of the molecular skeleton has been devised. The process is indulged by electrical energy via a domino N1-N2 bond cleavage followed by concomitant ring closing, initiating from cinnolines and indazoles as designed precursors. Additionally, an intermolecular ring homologation has also been established to synthesize densely functionalized dihydroquinazolines from 2,3-diaryl-indazoles and acetonitrile involving the same electrochemical strategy.

Skeletal Editing of Mechanically Interlocked Molecules: Nitrogen Atom Deletion from Crown Ether-Dibenzylammonium Rotaxanes

Removing the nitrogen atom from secondary amines while simultaneously linking the remaining fragments is a powerful form of late-stage skeletal editing. Here, we report its use for the deletion of the nitrogen atom of the dibenzylammonium template used to assemble crown ether rotaxanes. The reaction uses an anomeric amide that activates secondary amines to generate a carbon-carbon bond that replaces the amine nitrogen. Despite the potential for dethreading of the intermediate diradical pair, the nitrogen atom was successfully deleted from a series of rotaxane axles as long as the macrocycle could access coconformations that did not inhibit the reaction of the amine group. The skeletally edited interlocked molecules were obtained directly from the parent crown ether-dibenzylammonium rotaxanes in modest yields (23-36%) and characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. One skeletally edited rotaxane shows a network of weak CH···O hydrogen bonds between the crown ether and benzylic methylene groups of the axle in the solid state, in place of the crown ether-ammonium binding motif used to assemble the parent, unedited, rotaxane.

Radical 6-Endo Addition Enables Pyridine Synthesis under Metal-Free Conditions

Metal-free synthesis of heterocycles is highly sought after in the pharmaceutical industry and has garnered widespread attention due to eliminating the need to remove trace metal catalysts from the reaction. We report a radical 6-endo addition method for pyridine synthesis from cyclopropylamides and alkynes under metal-free conditions. Various terminal and substituted alkynes are inserted as C2 units into cyclopropylamides to synthesize versatile pyridines with 57 examples. Mechanistic investigations and computational studies indicate the unprecedented 6-endo-trig addition of vinyl radicals to the imine nitrogen atom rather than the conventional 5-exo-trig addition to the imine carbon atom, in which the hypervalent iodine(III) plays a critical role. This reaction easily scales up with excellent functional group compatibility and suits the late-stage pyridine installation on complex molecules.

Selective Upcycling of Polyolefins into High-Value Nitrogenated Chemicals

The selective upcycling of polyolefins to create products of increased value has emerged as an innovative approach to carbon resource stewardship, drawing significant scientific and industrial interest. Although recent advancements in recycling technology have facilitated the direct conversion of polyolefins to hydrocarbons or oxygenated compounds, the synthesis of nitrogenated compounds from such waste polyolefins has not yet been disclosed. Herein, we demonstrate a novel approach for the upcycling of waste polyolefins by efficiently transforming a range of postconsumer plastic products into nitriles and amides. This process leverages the catalytic properties of manganese dioxide in combination with an inexpensive nitrogen source, urea, to make it both practical and economically viable. Our approach not only opens new avenues for the creation of nitrogenated chemicals from polyolefin waste but also underscores the critical importance of recycling and valorizing carbon resources originally derived from fossil fuels. This study provides a new upcycling strategy for the sustainable conversion of waste polyolefins.

Molecular Editing of Pyrroles to Benzenes/Naphthalenes by N2O Deletion

A molecular editing reaction for converting pyrrole rings into benzene rings through a sequential pathway of Diels-Alder and cheletropic reactions was developed. The nitrogen atom in a N-bridged intermediate is eliminated in the form of N2O by a strain-releasing pathway, ultimately leading to the formation of substituted benzene and naphthalene derivatives.