Boron insertion into alkyl ether bonds via zinc/nickel tandem catalysis

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.

Synthesis of strained, air-stable boracycles via boron-carbon-centred diradicals

Boracycles are important functional scaffolds, often exhibiting superior or unique performance compared with their carbon analogues. Five-membered oxaboracycles are key pharmacophores in Food and Drug Administration-approved boron drugs. Meanwhile, six-membered boron-doped polycyclic aromatic hydrocarbons enhance the diversification and functionality of molecular materials. However, boron-containing four-membered rings are less studied owing to limited preparative approaches. Their inherent ring strain makes their synthesis thermodynamically unfavourable and hinders the exploration of their properties and applications. Here we report a triplet energy transfer catalysis for crafting air-stable benzoboretenes through intramolecular coupling of boron-carbon-centred diradicals. In addition, by modulating substrate π-conjugation structures and excitation energies, boron-carbon-centred diradicals can undergo formal 1,6- and 1,5-cyclization to deliver dihydroborinine and dihydrocyclopropaborole derivatives, respectively. The metal-free neutral reaction conditions ensure a broad reaction scope, resulting in structurally diverse boracycles that are stable enough to be purified via column chromatography. Further modification of the boracycles enables the facile synthesis of oxaborabicycles and dihydroborinine-fused polycyclic aromatic hydrocarbons with unique optoelectronic properties.

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.

Asymmetric Multi-Atom Insertion of Esters via Rh-Catalyzed Ring Opening of Oxabicyclic Alkenes

Precise skeletal manipulation involving insertion, deletion, and replacement has garnered considerable attention within the synthetic chemistry community. Among these processes, multi-atom insertion reactions in acyclic compounds remain a formidable challenge, primarily due to the low efficiency of fragment recapture after cleavage, which results from the lack of substrate-specific proximity during the reconstruction stage. Here, we report an asymmetric multi-atom insertion reaction of esters via Rh-catalyzed ring opening of oxabicyclic alkenes, achieving excellent regio-, diastereo-, and enantioselectivity. This approach enables the efficient and rapid construction of a molecular library of esters with a chiral hydroxy-dihydronaphthalene scaffold, showcasing an atom-efficient reaction. Detailed density functional theory calculations reveal key mechanistic features and the stereoselectivity-determining model of this transformation.

Direct Conversion of Aromatic Lactones into Bioisosteres by Carbonyl-to-Boranol Exchange

Bioisosteric replacement is an important strategy in drug discovery and is commonly practiced in medicinal chemistry; however, the incorporation of bioisosteres typically requires laborious multistep de novo synthesis. The direct conversion of a functional group into its corresponding bioisostere is of particular significance in evaluating structure-property relationships. Herein, we report a functional-group-exchange strategy that enables the direct conversion of aromatic lactones, a prevalent motif in bioactive molecules, into their corresponding cyclic hemiboronic acid bioisosteres. Scope evaluation and product derivatization experiments demonstrate the synthetic value and broad functional-group compatibility of this strategy, while the application of this methodology to the rapid remodeling of chromenone cores in bioactive molecules highlights its utility.

Site-Selective Carbonylation of Azetidines via Copper-Catalyzed Difluorocarbene Insertion

γ-Lactams are privileged five-membered pharmaco-phores in numerous bioactive compounds, but access to these motifs typically relies on cycloaddition/substitution chemistry involving activated substrates or CO carbonylations under harsh conditions. Here, we report a new route to functionalized γ-lactams through formal carbonylation of azetidines under nonprecious metal catalysis. The method leverages a copper-stabilized difluorocarbene to promote site-selective insertion followed by in situ hydrolysis to unmask the lactam group. In contrast to most difluorocarbene reactions that cause ring cleavage of saturated heterocycles in the presence of heat, the present system operates at a low temperature and retains the integrity of the cyclic structure. Synthesis of various drug-like lactams and a therapeutic agent for diabetes highlights utility.

Skeletal Modification via Activation of Relatively Unstrained C-C Bonds

ConspectusMethods that can directly modify the skeletons of complex molecules have become increasingly attractive for preparing novel analogues without the need for de novo synthesis in drug discovery processes. Among the various skeletal modification approaches, those targeting unstrained C-C bonds are particularly challenging to realize, owing to the relative inertness of these bonds toward common reagents. Compared to C-H or C-X (X: heteroatom) bonds, the activation of unstrained C-C bonds is often not thermodynamically and/or kinetically favorable. As a result, strategies relying on highly strained substrates or oxidative conditions are generally employed, which inevitably limit the scope and applications of C-C bond activation reactions. Hence, the development of redox-neutral catalytic C-C activation methods remains highly sought after for late-stage skeletal modification of complex bioactive compounds.In this Account, we summarize our recent progress in skeletal modifications through the catalytic activation of relatively unstrained C-C bonds. Enabled by transient or removable directing groups (DGs), the scope of C-C bond activation can be greatly expanded, encompassing a wide range of substrates, including ketones, amides, lactams, and biaryls. Consequently, different types of skeletal modification transformations have been developed. The major topics covered include the following: (1) Skeletal rearrangement and "cut-and-sew" transformations of cyclic ketones: we developed an aminopyridine/Rh-N-heterocyclic carbene (NHC) cooperative catalysis system that specifically targets the α-C-C bond of cyclic ketones. For substrates bearing a β-aryl substitution, the rhodacycle formed after the C-C bond activation can undergo an intramolecular C-H activation, resulting in the skeletal rearrangement from cyclopentanones/cyclohexanones to 1-tetralones/1-indanones. Additionally, the "cut-and-sew" transformations between indanones and ethylene or alkynes have been realized to offer a two-carbon ring expansion. (2) Chain homologation of linear amides and downsizing of lactams: the Rh-NHC activation system can be extended to the linear amides and lactams through preinstalling removable DGs. This approach has provided some new tools for precise amide modifications, including tunable homologation of tertiary amides via a "hook-and-slide" strategy and the downsizing transformation of lactams. (3) "Cut-and-sew" transformations of biphenols: using the preinstalled phosphinite DGs, unstrained 2,2'-biphenols can undergo split cross-coupling with various aryl iodides. When diiodide coupling partners are used, an interesting phenylene insertion into the aryl-aryl bond of biphenols can be achieved, which represents another type of "cut-and-sew" transformation.Collectively, these methods provide a reliable means to manipulate inert molecular scaffolds and offer new bond-disconnecting strategies to access useful structural motifs. The applications of these methods in the synthesis of bioactive natural products and complex analogues underscore their practical significance. Mechanistic insights gained from these studies are also discussed, which are expected to inspire future endeavors in this field.

Structure Optimization of Natural Product Catalpol to Obtain Novel and Potent Analogs against Heart Failure

Heart failure (HF) is a major global health threat, characterized by high morbidity and mortality. Targeting cardiac hypertrophy has been identified as a potential therapy for HF, with current treatments showing limited efficacy. Our research aims to address this limitation by exploring new structural classes of therapeutic agents. Starting from the natural product catalpol, we designed a series of novel catalpol analogs to break through the structural limitations of natural analogs, improve the anti-HF efficacy and metabolic properties. Among these, compound JZ19 exhibited remarkable efficacy in both myocardial cell injury assays and in an isoproterenol-induced murine HF model, outperforming catalpol. Our findings indicate that JZ19 potently reversed cardiac function by modulating the PI3K-AKT-GSK3β pathway, a key regulator of hypertrophy and apoptosis. Moreover, JZ19 showed favorable pharmacokinetic properties and safety. Overall, our results provide direct pharmacologic evidence supporting the further development of JZ19 as novel HF therapeutics by inhibiting cardiac hypertrophy and apoptosis.

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.

Reductive coupling of azonaphthalenes for the synthesis of BINAMs via a diboron-enabled [5,5]-sigmatropic rearrangement

The [5,5]-sigmatropic rearrangement is a less-studied reaction and may be strategically utilized to devise unique synthetic processes. Herein, we document a diboron-enabled [5,5]-sigmatropic rearrangement for practical synthesis of BINAM derivatives. Mechanistically, a concerted activation of azonaphthalenes by diboron creates a unique ten-membered transition state, which subsequently triggers a [5,5]-sigmatropic rearrangement. The reaction occurs under mild conditions, and offers operational simplicity, remarkable chemo- and regioselectivities, and good scalability (>10 grams).

Nickel-Catalyzed Atroposelective Carbo-Carboxylation of Alkynes with CO2: En Route to Axially Chiral Carboxylic Acids

Precise synthesis of carboxylic acids via catalytic carboxylation with CO2 is highly appealing. Although considerable advancements have been achieved in difunctionalizing carboxylation of unsaturated hydrocarbons, the asymmetric variants are conspicuously underdeveloped, particularly in addressing axially chiral alkenes. Herein, we report the first catalytic atroposelective carboxylation of alkynes with CO2. A variety of valuable axially chiral carboxylic acids are obtained with good yields and high chemo-, regio-, Z/E and enantio-selectivities. Notably, an unexpected anti-selective carbo-carboxylation is observed in the sp2-hybrid carbo-electrophile-initiated reductive carboxylation of alkynes. Mechanistic studies including DFT calculation elucidate the origin of chiral induction and anti-selectivity in vinyl-carboxylation of alkynes.

Low-Temperature Borylation of C(sp3)-O Bonds of Alkyl Ethers by Gold-Metal Oxide Cooperative Catalysis

Since ether moieties are often found not only in petrochemical products but also in natural organic molecules, the development of methods for manipulating C-O bonds of ethers is important for expanding the range of compound libraries synthesized from biomass resources, which should contribute to the goal of carbon neutrality. We report herein that gold nanoparticles supported on Lewis acidic metal oxides, namely α-Fe2O3, showed excellent catalytic activity for the reaction of dialkyl ethers and diborons, which enables the conversion of unactivated C(sp3)-O bonds to C(sp3)-B bonds at around room temperature. Various acyclic and cyclic ethers as well as a series of diborons participated in the heterogeneous gold-catalyzed borylation of unactivated C(sp3)-O bonds, to give a series of alkylboronates in high yields. Mechanistic studies corroborated that the present borylation of C(sp3)-O bonds of dialkyl ethers proceeded at the interface between gold nanoparticles and Lewis acidic metal oxides. Furthermore, adsorption IR measurements supported the notion that strong Lewis acid sites were generated at the boron atom of diborons adsorbed at the interface between Lewis acidic metal oxides and gold nanoparticles, which enabled us to ensure that the cooperation of gold nanoparticles and Lewis acidic metal oxides was responsible for the efficient transformation of unactivated C(sp3)-O bonds in ethers under mild conditions. This novel reaction technology which is specific to heterogeneous catalysts enables the activation of stable C(sp3)-O bonds of oxygenated chemical feedstock, which is beneficial for the sustainable synthesis of value-added organoboron compounds.

Carbonylation Reactions at Carbon-Centered Radicals with an Adjacent Heteroatom

Heteroatoms are essential to living organisms and present in almost all molecules with medicinal usage. The catalytic functionalization at the carbon-centered radical with an adjacent heteroatom provides an effective way to value added moiety while retaining the unique physicochemical and pharmacological properties of heteroatoms, which can promote the development of pharmaceutical and fine chemical production. Carbonylative transformation was discovered nearly a century ago which is an efficient method for the synthesis of carbonyl-containing molecules with potent applications in both industry and academia. Despite numerous advances in new reaction development, carbonylative transformation involving adjacent heteroatom carbon radical remain a subject that deserves to be discussed. In this minireview, we systematically summarized and discussed the recent advances in carbonylative transformations involving carbon-centered radicals with an adjacent heteroatom, including oxygen (O), nitrogen (N), phosphorus (P), silicon (Si), sulfur (S), boron (B), fluorine (F), and chlorine (Cl). The related reaction mechanism was also discussed.

Treatment of ulcerative colitis via the in situ restoration of local immune and microbial homeostasis by oral administration of Tremella polysaccharide drug-carrying hydrogel

Ulcerative colitis (UC) is a prevalent inflammatory bowel disease, and conventional treatments, such as anti-inflammatory medications and surgery, often prove inadequate due to frequent recurrences and various complications. To alleviate patient suffering, there is an urgent need for a therapeutic system that specifically delivers drugs to the colon for wound healing, inflammation relief, and restoration of microbial homeostasis. In this paper, we developed a Tremella polysaccharide drug-carrying hydrogel that adheres to the inflamed colonic mucosa, forming an effective artificial barrier and releasing the drug in situ to restore local immune and microbial balance. The hydrogel backbone was synthesized through the chemical cross-linking of Tremella polysaccharide with 1,4-butanediol diglycidyl ether in an alkaline environment. During this process, Soluplus® and TPGS-encapsulated ginsenoside compound K adhered to the hydrogel backbone due to electrostatic attraction. The enhanced adhesion following cross-linking enables the hydrogel to stably attach to the inflamed colonic mucosa, releasing mixed micelles that improve drug penetration and absorption by inhibiting the cellular efflux protein P-glycoprotein. This mechanism promotes local immune recovery and eliminates harmful intestinal flora, providing significant relief from UC symptoms. This natural polysaccharide-based hydrogel represents a highly effective oral treatment for UC.

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.

Convergent synthesis of bicyclic boronates via a cascade regioselective Suzuki-Miyaura/cyclisation protocol

Bicyclic boronates have recently emerged as promising candidates to invoke targeted biomolecular interactions, given their selectivity for specific functionalities. Despite this, the general stability of the C-B bond in vivo, for such heterocycles, remains an intractable challenge that can often preclude their utility in drug discovery. To address this challenge, de novo strategies that allow expedient access to strategically substituted boronates, that enable modulation of the C-B bond are urgently required. Herein we disclose an operationally simple, regioselective cross-coupling/cyclisation reaction of easily accessible vicinal boronic esters with 2-halophenols to rapidly forge 3-substituted bicyclic boronates. The utility of the platform was demonstrated via expedient access to Xeruborbactam derivatives, chemoselective manipulation of formed products and the convergent approach to bicyclic boronates with a pendent biomolecular probe.

Direct oxygen insertion into C-C bond of styrenes with air

Skeletal editing of single-atom insertion to basic chemicals has been demonstrated as an efficient strategy for the discovery of structurally diversified compounds. Previous endeavors in skeletal editing have successfully facilitated the insertion of boron, nitrogen, and carbon atoms. Given the prevalence of oxygen atoms in biologically active molecules, the direct oxygenation of C-C bonds through single-oxygen-atom insertion like Baeyer-Villiger reaction is of particular significance. Herein, we present an approach for the skeletal modification of styrenes using O2 via oxygen insertion, resulting in the formation of aryl ether frameworks under mild reaction conditions. The broad functional-group tolerance and the excellent chemo- and regioselectivity are demonstrated in this protocol. A preliminary mechanistic study indicates the potential involvement of 1,2-aryl radical migration in this reaction.

Photocatalytic furan-to-pyrrole conversion

The identity of a heteroatom within an aromatic ring influences the chemical properties of that heterocyclic compound. Systematically evaluating the effect of a single atom, however, poses synthetic challenges, primarily as a result of thermodynamic mismatches in atomic exchange processes. We present a photocatalytic strategy that swaps an oxygen atom of furan with a nitrogen group, directly converting the furan into a pyrrole analog in a single intermolecular reaction. High compatibility was observed with various furan derivatives and nitrogen nucleophiles commonly used in drug discovery, and the late-stage functionalization furnished otherwise difficult-to-access pyrroles from naturally occurring furans of high molecular complexity. Mechanistic analysis suggested that polarity inversion through single electron transfer initiates the redox-neutral atom exchange processes at room temperature.

Single-atom editing with light

A new reaction swaps an oxygen for a nitrogen in structurally complex molecules.

Nitrogen-to-functionalized carbon atom transmutation of pyridine

The targeted and selective replacement of a single atom in an aromatic system represents a powerful strategy for the rapid interconversion of molecular scaffolds. Herein, we report a pyridine-to-benzene transformation via nitrogen-to-carbon skeletal editing. This approach proceeds via a sequence of pyridine ring-opening, imine hydrolysis, olefination, electrocyclization, and aromatization to achieve the desired transmutation. The most notable features of this transformation are the ability to directly install a wide variety of versatile functional groups in the benzene scaffolding, including ester, ketone, amide, nitrile, and phosphate ester fragments, as well as the inclusion of meta-substituted pyridines which have thus far been elusive for related strategies.

Molecular Editing of Ketones through N-Heterocyclic Carbene and Photo Dual Catalysis

The molecular editing of ketones represents an appealing strategy due to its ability to maximize the structural diversity of ketone compounds in a straightforward manner. However, developing efficient methods for the arbitrary modification of ketonic molecules, particularly those integrated within complex skeletons, remains a significant challenge. Herein, we present a unique strategy for ketone recasting that involves radical acylation of pre-functionalized ketones facilitated by N-heterocyclic carbene and photo dual catalysis. This protocol features excellent substrate tolerance and can be applied to the convergent synthesis and late-stage functionalization of structurally complex bioactive ketones. Mechanistic investigations, including experimental studies and density functional theory (DFT) calculations, shed light on the reaction mechanism and elucidate the basis of the regioselectivity.

Selective nitrogen insertion into aryl alkanes

Molecular structure-editing through nitrogen insertion offers more efficient and ingenious pathways for the synthesis of nitrogen-containing compounds, which could benefit the development of synthetic chemistry, pharmaceutical research, and materials science. Substituted amines, especially nitrogen-containing alkyl heterocyclic compounds, are widely found in nature products and drugs. Generally, accessing these compounds requires multiple steps, which could result in low efficiency. In this work, a molecular editing strategy is used to realize the synthesis of nitrogen-containing compounds using aryl alkanes as starting materials. Using derivatives of O-tosylhydroxylamine as the nitrogen source, this method enables precise nitrogen insertion into the Csp2-Csp3 bond of aryl alkanes. Notably, further synthetic applications demonstrate that this method could be used to prepare bioactive molecules with good efficiency and modify the molecular skeleton of drugs. Furthermore, a plausible reaction mechanism involving the transformation of carbocation and imine intermediates has been proposed based on the results of control experiments.

Carbene-Assisted Arene Ring-Opening

Despite the significant achievements in dearomatization and C-H functionalization of arenes, the arene ring-opening remains a largely unmet challenge and is underdeveloped due to the high bond dissociation energy and strong resonance stabilization energy inherent in aromatic compounds. Herein, we demonstrate a novel carbene assisted strategy for arene ring-opening. The understanding of the mechanism by our DFT calculations will stimulate wide application of bulk arene chemicals for the synthesis of value-added polyconjugated chain molecules. Various aryl azide derivatives now can be directly converted into valuable polyconjugated enynes, avoiding traditional synthesis including multistep unsaturated precursors, poor selectivity control, and subsequent transition-metal catalyzed cross-coupling reactions. The simple conditions required were demonstrated in the late-stage modification of complex molecules and fused ring compounds. This chemistry expands the horizons of carbene chemistry and provides a novel pathway for arene ring-opening.

Controllable Skeletal and Peripheral Editing of Pyrroles with Vinylcarbenes

The skeletal editing of azaarenes through insertion, deletion, or swapping of single atoms has recently gained considerable momentum in chemical synthesis. Here, we describe a practical skeletal editing strategy using vinylcarbenes in situ generated from trifluoromethyl vinyl N-triftosylhydrazones, leading to the first dearomative skeletal editing of pyrroles through carbon-atom insertion. Furthermore, depending on the used catalyst and substrate, three types of peripheral editing reactions of pyrroles are also disclosed: α- or γ-selective C-H insertion, and [3+2] cycloaddition. These controllable molecular editing reactions provide a powerful platform for accessing medicinally relevant CF3-containing N-heterocyclic frameworks, such as 2,5-dihydropyridines, piperidines, azabicyclo[3.3.0]octadienes, and allylated pyrroles from readily available pyrroles. Mechanistic insights from experiments and density functional theory (DFT) calculations shed light on the origin of substrate- or catalyst-controlled chemo- and regioselectivity as well as the reaction mechanism.

Tunable molecular editing of indoles with fluoroalkyl carbenes

Building molecular complexity from simple feedstocks through precise peripheral and skeletal modifications is central to modern organic synthesis. Nevertheless, a controllable strategy through which both the core skeleton and the periphery of an aromatic heterocycle can be modified with a common substrate remains elusive, despite its potential to maximize structural diversity and applications. Here we report a carbene-initiated chemodivergent molecular editing of indoles that allows both skeletal and peripheral editing by trapping an electrophilic fluoroalkyl carbene generated in situ from fluoroalkyl N-triftosylhydrazones. A variety of fluorine-containing N-heterocyclic scaffolds have been efficiently achieved through tunable chemoselective editing reactions at the skeleton or periphery of indoles, including one-carbon insertion, C3 gem-difluoroolefination, tandem cyclopropanation and N1 gem-difluoroolefination, and cyclopropanation. The power of this chemodivergent molecular editing strategy has been highlighted through the modification of the skeleton or periphery of natural products in a controllable and chemoselective manner. The reaction mechanism and origins of the chemo- and regioselectivity have been probed by both experimental and theoretical methods.

Photoinduced Site-Selective Aryl C-H Borylation with Electron-Donor-Acceptor Complex Derived from B2Pin2 and Isoquinoline

Due to boron's metalloid properties, aromatic boron reagents are prevalent synthetic intermediates. The direct borylation of aryl C-H bonds for producing aromatic boron compounds offers an appealing, one-step solution. Despite significant advances in this field, achieving regioselective aryl C-H bond borylation using simple and readily available starting materials still remains a challenge. In this work, we attempted to enhance the reactivity of the electron-donor-acceptor (EDA) complex by selecting different bases to replace the organic base (NEt3) used in our previous research. To our delight, when using NH4HCO3 as the base, we have achieved a mild visible-light-mediated aromatic C-H bond borylation reaction with exceptional regioselectivity (rr > 40:1 to single isomers). Compared with our previous borylation methodologies, this protocol provides a more efficient and broader scope for aryl C-H bond borylation through the use of N-Bromosuccinimide. The protocol's good functional-group tolerance and excellent regioselectivity enable the functionalization of a variety of biologically relevant compounds and novel cascade transformations. Mechanistic experiments and theoretical calculations conducted in this study have indicated that, for certain arenes, the aryl C-H bond borylation might proceed through a new reaction mechanism, which involves the formation of a novel transient EDA complex.

Split cross-coupling via Rh-catalysed activation of unstrained aryl-aryl bonds

Constructive functionalization of unstrained aryl-aryl bonds has been a fundamental challenge in organic synthesis due to the inertness of these bonds. Here we report a split cross-coupling strategy that allows two-fold arylation with diverse aryl iodides through cleaving unstrained aryl-aryl bonds of common 2,2'-biphenols. The reaction is catalyzed by a rhodium complex and promoted by a removable phosphinite directing group and an organic reductant. The combined experimental and computational mechanistic studies reveal a turnover-limiting reductive elimination step that can be accelerated by a Lewis acid co-catalyst. The utility of this coupling method has been illustrated in the modular and simplified syntheses of unsymmetrical 2,6-diarylated phenols and skeletal insertion of phenyl units.

Synthesis of Collidine from Dinitrogen via a Tungsten Nitride

The synthesis of pyridines from dinitrogen in homogeneous solution is known to be challenging considering that an N2 cleavage step needs to be combined with two N-C coupling steps. Herein, a tungsten complex bearing a tailor-made 2,2'-(tBu2As)2-substituted tolane ligand scaffold was shown to split N2 to afford the corresponding tungsten nitride, which is not the case for the corresponding (iPr2As)2-substituted derivative. The former nitride was then reacted with 2,4,6-trimethylpyrylium triflate, which led to the formation of a tungsten oxo complex, along with collidine. Over the course of this reaction, the O atom of the pyrylium starting material was replaced with an N atom via a hitherto unprecedented skeletal editing process.

Palladium-catalyzed denitrogenation/vinylation of benzotriazinones with vinylene carbonate

Herein, a novel Pd-catalyzed denitrogenation/vinylation of benzotriazinones using vinylene carbonate as the vinylation reagent is reported. This transformation demonstrates an unprecedented skeletal editing approach, effectively converting NN to CC fragments in situ and synthesizing a collection of isoquinolinones with broad-spectrum functional group tolerance. Moreover, the quite concise reaction system and late-stage modification of bioactive molecules comprehensively underscore the practical potential of this protocol.

Achieving Nickel-Catalyzed Reductive C(sp2)-B Coupling of Bromoboranes via Reversing the Activation Sequence

Transition metal-catalyzed reductive cross-couplings to build C-C/Si bonds have been developed, but the reductive cross-coupling to create the C(sp2)-B bond has not been explored. Herein, we describe a nickel-catalyzed reductive cross-coupling between aryl halides and bromoboranes to construct a C(sp2)-B bond. This protocol offers a convenient approach for the synthesis of a wide range of aryl boronate esters, using readily available starting materials. Mechanistic studies indicate that the key to the success of the reaction is the activation of the B-Br bond of bromoboranes with a Lewis base such as 2-MeO-py. The activation ensures that bromoboranes will react with the active nickel(I) catalyst prior to aryl halides, which is different from the sequence of the general nickel-catalyzed reductive C(sp2)-C/Si cross-coupling, where the oxidative addition of an aryl halide proceeds first. Notably, this approach minimizes the production of undesired homocoupling byproduct without the requirement of excessive quantities of either substrate.

N-Heterocycle-Editing to Access Fused-BN-Heterocycles via Ring-Opening/C-H Borylation/Reductive C-B Bond Formation

Skeletal editing of N-heterocycles has recently received considerable attention, and the introduction of boron atom into heterocycles often results in positive property changes. However, direct enlargement of N-heterocycles through boron atom insertion is rarely reported in the literature. Here, we report a N-heterocyclic editing reaction through the combination boron atom insertion and C-H borylation, accessing the fused-BN-heterocycles. The synthetic potential of this chemistry was demonstrated by substrate scope and late-stage diversification of products.

Ring expansion of indene by photoredox-enabled functionalized carbon-atom insertion

Skeletal editing has received unprecedented attention as an emerging technology for the late-stage manipulation of molecular scaffolds. The direct achievement of functionalized carbon-atom insertion in aromatic rings is challenging. Despite ring-expanding carbon-atom insertion reactions, such as the Ciamician-Dennstedt re-arrangement, being performed for more than 140 years, only a few relevant examples of such transformations have been reported, with these limited to the installation of halogen, ester and phenyl groups. Here we describe a photoredox-enabled functionalized carbon-atom insertion reaction into indene. We disclose the utilization of a radical carbyne precursor that facilitates the insertion of carbon atoms bearing a variety of functional groups, including trifluoromethyl, ester, phosphate ester, sulfonate ester, sulfone, nitrile, amide, aryl ketone and aliphatic ketone fragments to access a library of 2-substituted naphthalenes. The application of this methodology to the skeletal editing of molecules of pharmaceutical relevance highlights its utility.

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

Regiodivergent Ring-Expansion of Oxindoles to Quinolinones

The development of divergent methods to expedite structure-activity relationship studies is crucial to streamline discovery processes. We developed a rare example of regiodivergent ring expansion to access two regioisomers from a common starting material. To enable this regiodivergence, we identified two distinct reaction conditions for transforming oxindoles into quinolinone isomers. The presented methods proved to be compatible with a variety of functional groups, which enabled the late-stage diversification of bioactive oxindoles as well as facilitated the synthesis of quinolinone drugs and their derivatives.

Modular Synthesis of Complex Benzoxaboraheterocycles through Chelation-Assisted Rh-Catalyzed [2 + 2 + 2] Cycloaddition

Benzoxaboraheterocycles (BOBs) are moieties of increasing interest in the pharmaceutical industry; however, the synthesis of these compounds is often difficult or impractical due to the sensitivity of the boron moiety, the requirement for metalation-borylation protocols, and lengthy syntheses. We report a straightforward, modular approach that enables access to complex examples of the BOB framework through a Rh-catalyzed [2 + 2 + 2] cycloaddition using MIDA-protected alkyne boronic acids. The key to the development of this methodology was overcoming the steric barrier to catalysis by leveraging chelation assistance. We show the utility of the method through synthesis of a broad range of BOB scaffolds, mechanistic information on the chelation effect, intramolecular alcohol-assisted BMIDA hydrolysis, and linear/cyclic BOB limits as well as comparative binding affinities of the product BOB frameworks for ribose-derived biomolecules.

Modular synthesis of 1,2-azaborines via ring-opening BN-isostere benzannulation

1,2-Azaborines represent a unique class of benzene isosteres that have attracted interest for developing pharmaceuticals with better potency and bioavailability. However, it remains a long-standing challenge to prepare monocyclic 1,2-azaborines, particularly multi-substituted ones, in an efficient and modular manner. Here we report a straightforward method to directly access diverse multi-substituted 1,2-azaborines from readily available cyclopropyl imines/ketones and dibromoboranes under relatively mild conditions. The reaction is scalable, shows a broad substrate scope, and tolerates a range of functional groups. The utility of this method is demonstrated in the concise syntheses of BN isosteres of a PD-1/PD-L1 inhibitor and pyrethroid insecticide, bifenthrin. Combined experimental and computational mechanistic studies suggest that the reaction pathway involves boron-mediated cyclopropane ring-opening and base-mediated elimination, followed by an unusual low-barrier 6π-electrocyclization accelerated by the BN/CC isomerism. This method is anticipated to find applications for the synthesis of BN-isostere analogues in medicinal chemistry, and the mechanistic insights gained here may guide developing other boron-mediated electrocyclizations.

Exploring Superacid-Promoted Skeletal Reorganization of Aliphatic Nitrogen-Containing Compounds

Here we report a method to reorganize the core structure of aliphatic unsaturated nitrogen-containing substrates exploiting polyprotonation in superacid solutions. The superelectrophilic activation of N-isopropyl systems allows for the selective formal Csp3 -H activation/cyclization or homologation / functionalization of nitrogen-containing substrates. This study also reveals that this skeletal reorganization can be controlled through protonation interplay. The mechanism of this process involves an original sequence of C-N bond cleavage, isopropyl cation generation and subsequent C-N bond and C-C bond formation. This was demonstrated through in situ NMR analysis and labelling experiments, also confirmed by DFT calculations.

One-Carbon Ring Expansion of Indoles and Pyrroles: A Straightforward Access to 3-Fluorinated Quinolines and Pyridines

3-Fluorinated quinolines and pyridines are prevalent pharmacophores, yet their synthesis is often challenging. Herein, we demonstrate that dibromofluoromethane as bromofluorocarbene source enables the one-carbon ring expansion of readily available indoles and pyrroles to structurally diverse 3-fluorinated quinolines and pyridines. This straightforward protocol requires only a short reaction time of ten minutes and can be performed under air atmosphere. Preliminary investigations reveal that this strategy can also be applied to the synthesis of other valuable azines by using different 1,1-dibromoalkanes as bromocarbene sources.

Direct Access to Quinazolines and Pyrimidines from Unprotected Indoles and Pyrroles through Nitrogen Atom Insertion

Recent advances in single-atom insertion reactions have opened up new synthetic approaches for molecular diversification. Developing innovative strategies to directly transform biologically relevant molecules, without any prefunctionalization, is key to further expanding the scope and utility of such transformations. Herein, the direct access to quinazolines and pyrimidines from the corresponding unprotected 1H-indoles and 1H-pyrroles is reported, relying on the implementation of lithium bis(trimethylsilyl)amide (LiHMDS) as a novel nitrogen atom source in combination with commercially available hypervalent iodine reagents. Further application of this strategy in late-stage settings demonstrates its potential in lead structure diversification campaigns.

Stereoselective Unsymmetrical 1,1-Diborylation of Alkynes with a Neutral sp2 -sp3 Diboron Reagent

The incorporation of two distinct boryl groups at the same carbon center in organic molecules has attracted growing research interest due to its potential for facilitating controlled, precise synthesis through stepwise dual carbon-boron bond transformations. Here we report a method to access unsymmetrical 1,1-diborylalkene (UDBA) stereoselectively via the reaction of readily available alkynes with a neutral sp2 -sp3 diboron reagent (NHC)BH2 -Bpin (NHC=N-heterocyclic carbene). Attributing to the chemically easily distinguishable nature of the sp2 and sp3 boryl moieties, controllable stepwise derivatization of the resultant UDBAs is realized. This process leads to various multifunctionalized olefins and organoborons, such as acylboranes, which are difficult to prepare by other methods.

Carbon-to-nitrogen single-atom transmutation of azaarenes

When searching for the ideal molecule to fill a particular functional role (for example, a medicine), the difference between success and failure can often come down to a single atom1. Replacing an aromatic carbon atom with a nitrogen atom would be enabling in the discovery of potential medicines2, but only indirect means exist to make such C-to-N transmutations, typically by parallel synthesis3. Here, we report a transformation that enables the direct conversion of a heteroaromatic carbon atom into a nitrogen atom, turning quinolines into quinazolines. Oxidative restructuring of the parent azaarene gives a ring-opened intermediate bearing electrophilic sites primed for ring reclosure and expulsion of a carbon-based leaving group. Such a 'sticky end' approach subverts existing atom insertion-deletion approaches and as a result avoids skeleton-rotation and substituent-perturbation pitfalls common in stepwise skeletal editing. We show a broad scope of quinolines and related azaarenes, all of which can be converted into the corresponding quinazolines by replacement of the C3 carbon with a nitrogen atom. Mechanistic experiments support the critical role of the activated intermediate and indicate a more general strategy for the development of C-to-N transmutation reactions.

Aromatic nitrogen scanning by ipso-selective nitrene internalization

Nitrogen scanning in aryl fragments is a valuable aspect of the drug discovery process, but current strategies require time-intensive, parallel, bottom-up synthesis of each pyridyl isomer because of a lack of direct carbon-to-nitrogen (C-to-N) replacement reactions. We report a site-directable aryl C-to-N replacement reaction allowing unified access to various pyridine isomers through a nitrene-internalization process. In a two-step, one-pot procedure, aryl azides are first photochemically converted to 3H-azepines, which then undergo an oxidatively triggered C2-selective cheletropic carbon extrusion through a spirocyclic azanorcaradiene intermediate to afford the pyridine products. Because the ipso carbon of the aryl nitrene is excised from the molecule, the reaction proceeds regioselectively without perturbation of the remainder of the substrate. Applications are demonstrated in the abbreviated synthesis of a pyridyl derivative of estrone, as well as in a prototypical nitrogen scan.

Molecular Editing of Pyrroles via a Skeletal Recasting Strategy

Heterocyclic scaffolds are commonly found in numerous biologically active molecules, therapeutic agents, and agrochemicals. To probe chemical space around heterocycles, many powerful molecular editing strategies have been devised. Versatile C-H functionalization strategies allow for peripheral modifications of heterocyclic motifs, often being specific and taking place at multiple sites. The past few years have seen the quick emergence of exciting "single-atom skeletal editing" strategies, through one-atom deletion or addition, enabling ring contraction/expansion and structural diversification, as well as scaffold hopping. The construction of heterocycles via deconstruction of simple heterocycles is unknown. Herein, we disclose a new molecular editing method which we name the skeletal recasting strategy. Specifically, by tapping on the 1,3-dipolar property of azoalkenes, we recast simple pyrroles to fully substituted pyrroles, through a simple phosphoric acid-promoted one-pot reaction consisting of dearomative deconstruction and rearomative reconstruction steps. The reaction allows for easy access to synthetically challenging tetra-substituted pyrroles which are otherwise difficult to synthesize. Furthermore, we construct N-N axial chirality on our pyrrole products, as well as accomplish a facile synthesis of the anticancer drug, Sutent. The potential application of this method to other heterocycles has also been demonstrated.

Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp3)-H to construct C-C bonds

Ether derivatives are widespread as essential building blocks in various drugs, natural products, agrochemicals, and materials. Modern economy requires developing green strategies with improved efficiency and reduction of waste. Due to its atom and step-economy, the cross-dehydrogenative coupling (CDC) reaction has become a major strategy for ether functionalization. This review covers C-H/C-H cross-coupling reactions of ether derivatives with various C-H bond substrates via non-noble metal catalysts (Fe, Cu, Co, Mn, Ni, Zn, Y, Sc, In, Ag). We discuss advances achieved in these CDC reactions and hope to attract interest in developing novel methodologies in this field of organic chemistry.

Skeletal metalation of lactams through a carbonyl-to-nickel-exchange logic

Classical metalation reactions such as the metal-halogen exchange have had a transformative impact on organic synthesis owing to their broad applicability in building carbon-carbon bonds from carbon-halogen bonds. Extending the metal-halogen exchange logic to a metal-carbon exchange would enable the direct modification of carbon frameworks with new implications in retrosynthetic analysis. However, such a transformation requires the selective cleavage of highly inert chemical bonds and formation of stable intermediates amenable to further synthetic elaborations, hence its development has remained considerably challenging. Here we introduce a skeletal metalation strategy that allows lactams, a prevalent motif in bioactive molecules, to be readily converted into well-defined, synthetically useful organonickel reagents. The reaction features a selective activation of unstrained amide C-N bonds mediated by an easily prepared Ni(0) reagent, followed by CO deinsertion and dissociation under mild room temperature conditions in a formal carbonyl-to-nickel-exchange process. The underlying principles of this unique reactivity are rationalized by organometallic and computational studies. The skeletal metalation is further applied to a direct CO excision reaction and a carbon isotope exchange reaction of lactams, underscoring the broad potential of metal-carbon exchange logic in organic synthesis.

A practical preparation of bicyclic boronates via metal-free heteroatom-directed alkenyl sp2-C‒H borylation

Bicyclic boronates play critical roles in the discovery of functional materials and antibacterial agents, especially against deadly bacterial pathogens. Their practical and convenient preparation is in high demand but with great challenge. Herein, we report an efficient strategy for the preparation of bicyclic boronates through metal-free heteroatom-directed alkenyl sp2-C‒H borylation. This synthetic approach exhibits good functional group compatibility, and the corresponding boronates bearing halides, aryls, acyclic and cyclic frameworks are obtained with high yields (43 examples, up to 95% yield). Furthermore, a gram-scale experiment is conducted, and downstream transformations of the bicyclic boronates are pursued to afford natural products, drug scaffolds, and chiral hemiboronic acid catalysts.

Dearomative ring expansion of thiophenes by bicyclobutane insertion

Skeletal ring enlargement is gaining renewed interest in synthetic chemistry and has recently focused on insertion of one or two atoms. Strategies for heterocyclic expansion through small-ring insertion remain elusive, although they would lead to the efficient formation of bicyclic products. Here, we report a photoinduced dearomative ring enlargement of thiophenes by insertion of bicyclo[1.1.0]butanes to produce eight-membered bicyclic rings under mild conditions. The synthetic value, broad functional-group compatibility, and excellent chemo- and regioselectivity were demonstrated by scope evaluation and product derivatization. Experimental and computational studies point toward a photoredox-induced radical pathway.

Selective multifunctionalization of N-heterocyclic carbene boranes via the intermediacy of boron-centered radicals

The selective difunctionalization of N-heterocyclic carbene (NHC) boranes with alkenes has been achieved via decatungstate and thiol synergistic catalysis. The catalytic system also allows stepwise trifunctionalization, leading to complex NHC boranes with three different functional groups which are challenging to prepare by other methods. The strong hydrogen-abstracting ability of the excited decatungstate enables the generation of boryl radicals from mono- and di-substituted boranes for realizing borane multifunctionalization. This proof-of-principle research provides a new chance for fabricating unsymmetrical boranes and developing boron-atom-economic synthesis.