Unified Access to Pyrimidines and Quinazolines Enabled by N-N Cleaving Carbon Atom Insertion

Phosphine-Mediated Reductive Insertion of α-Keto Esters and Isatins into Phthalic Anhydride Derivatives

Herein, we report an unprecedented P(NMe2)3-mediated reductive insertion of 1,2-dicarbonyl compounds including α-keto esters and isatins into phthalic anhydride-derived alkenes and phthalic anhydrides, which furnishes the corresponding isochroman-1-ones and isochroman-1,4-diones, respectively, in moderate to excellent yields with high chemo- and regioselectivity. Furthermore, the asymmetric version of the ring expansion reaction could be realized by using a chiral auxiliary strategy. Mechanistically, the nucleophilic attack of the Kukhtin-Ramirez adduct, generated from P(NMe2)3 and 1,2-dicarbonyl compound, to the anhydride derivative, followed by a cascade ring-opening and ring-closure process, affords the ring expansion product. The reaction represents a novel metal-free carbon insertion ring expansion of aliphatic rings and also the first [1 + 5] annulation involving the Kukhtin-Ramirez adducts.

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.

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.

Removing Neighboring Ring Influence in Monocyclic B-OH Diazaborines: Properties and Reactivity as Phenolic Bioisosteres with Dynamic Hydroxy Exchange

The design of small molecules with unique geometric profiles or molecular connectivity represents an intriguing yet neglected challenge in modern organic synthesis. This challenge is compounded when emphasis is placed on the preparation of new chemotypes that have distinct and practical functions. To expand the structural diversity of boron-containing heterocycles, we report herein the preparation of novel monocyclic hemiboronic acids, diazaborines. These compounds have enabled the study of a pseudoaromatic boranol-containing (B-OH) ring free of influence from an appended aromatic system. Synthetic and spectroscopic studies have provided insight into the aromatic character, Lewis acidic nature, chemical reactivity, and unique ability of the exocyclic B-OH unit to participate in hydroxy exchange, suggesting their use in organocatalysis and as reversible covalent inhibitors. Moreover, density functional theory and nucleus-independent chemical shift calculations reveal that the aromatic character of the boroheterocyclic ring is increased significantly in comparison to known bicyclic benzodiazaborines (naphthoid congeners), consequently leading to attenuated Lewis acidity. Direct structural comparison to a well-established biaryl isostere, 2-phenylphenol, through X-ray crystallographic analysis reveals that N-aryl derivatives are strikingly similar in size and conformation, with attenuated logP values underscoring the value of the polar BNN unit. Their potential application as low-molecular-weight scaffolds in drug discovery is demonstrated through orthogonal diversification and preliminary antifungal evaluation (Candida albicans), which unveiled analogs with low micromolar inhibitory concentration.

Electrochemical Skeletal Indole Editing via Nitrogen Atom Insertion by Sustainable Oxygen Reduction Reaction

Skeletal molecular editing gained considerable recent momentum and emerged as a uniquely powerful tool for late-stage diversifications. Thus far, superstoichiometric amounts of costly hypervalent iodine(III) reagents were largely required for skeletal indole editing. In contrast, we herein show that electricity enables sustainable nitrogen atom insertion reactions to give bio-relevant quinazoline scaffolds without stoichiometric chemical redox-waste product. The transition metal-free electro-editing was enabled by the oxygen reduction reaction (ORR) and proved robust on scale, while tolerating a variety of valuable functional groups.

Electrosynthesis of 1,2,3-Benzotriazines through an Iodide-Catalyzed Skeletal Editing of 3-Aminoindazoles

This report describes an environmentally benign synthesis of 1,2,3-benzotriazines through an iodide-catalyzed electro-oxidative N-centered [1,2]-rearrangement of 3-aminoindazoles. The developed method demonstrates the activation of heteroatoms via electrochemically generated reactive iodide species without using any metal catalysts and peroxides. The protocol features practical and mild reaction conditions and displays a wide substrate scope. Various mechanistic experiments and cyclic voltammetric studies have been instrumental in elucidating the reaction mechanism, operating via a skeletal rearrangement of 3-aminoindazoles.

N-(Sulfonio)Sulfilimine Reagents: Non-Oxidizing Sources of Electrophilic Nitrogen Atom for Skeletal Editing

The one-pot synthesis of λ4-dibenzothiophen-5-imino-N-dibenzothiophenium triflate (1) in multigram scale is reported. This compound reacts with Rh2(esp)2 (esp=α,α,α',α'-tetramethyl-1,3-benzenedipropionic acid) generating a Rh-coordinated sulfonitrene species, which is able to transfer the electrophilic nitrene moiety to olefins. When indenes are used as substrates, isoquinolines are obtained in good yields. We assumed that after formation of the corresponding N-sulfonio aziridine, a ring expansion occurs via selective C-C bond cleavage and concomitant elimination of dibenzothiophene. Unexpectedly, a similar protocol transforms 1-arylcyclobutenes into 1-cyano-1-arylcyclopropanes. Our calculations indicate that aziridination is not favored in this case; instead, sulfilimine-substituted cyclobutyl carbocations are initially formed, and these evolve to the isolated cyclopropanes via ring contraction. Both procedures are operationally simple, tolerate a range of functional groups, including oxidation-sensitive alcohols and aldehydes, and enable the convenient preparation of valuable 15N-labelled products. These results demonstrate the potential of 1 to provide alternative pathways for the selective transfer of N-atoms in organic molecules.

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.

N-Oxide-to-Carbon Transmutations of Azaarene N-Oxides

Reactions that change the identity of an atom within a ring system are emerging as valuable tools for the site-selective editing of molecular structures. Herein, we describe the expansion of an underdeveloped transformation that directly converts azaarene-derived N-oxides to all-carbon arenes. This ring transmutation exhibits good functional group tolerance and replaces the N-oxide moiety with either unsubstituted, substituted, or isotopically labeled carbon atoms in a single laboratory operation.

Leveraging Pyrazolium Ylide Reactivity to Access Indolizine and 1,2-Dihydropyrimidine Derivatives

N-Heterocyclic ylides are important synthetic precursors to rapidly build molecular complexity. Pyrazolium ylides have largely been unexplored, and we demonstrate their diverse utility in this report. We show that these readily accessible building blocks can be used to construct different heterocyclic skeletons by varying the coupling partner. Indolizines can be formed via an N-deletion type mechanism when reacting pyrazolium salts with electron deficient alkynes. 1,2-Dihydropyrimidines can be formed via a rearrangement mechanism when reacting pyrazolium ylides with isocyanates. These reactions enable access to valuable heteroarenes without the need for transition metal catalysis, high temperatures, or strong bases.

Cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles

This Feature Article discusses recent advances in the development of cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles. Cascade ring expansion reactions have much potential for use in the synthesis of biologically important medium-sized rings and macrocycles, most notably as they don't require high dilution conditions, which are commonly used in established end-to-end macrocyclisation methods. Operation by cascade ring expansion method can allow large ring products to be accessed via rearrangements that proceed exclusively by normal-sized ring cyclisation steps. Ensuring that there is adequate thermodynamic driving force for ring expansion is a key challenge when designing such methods, especially for the expansion of normal-sized rings into medium-sized rings. This Article is predominantly focused on methods developed in our own laboratory, with selected works by other groups also discussed. Thermodynamic considerations, mechanism, reaction design, route planning and future perspective for this field are all covered.

Skeletal editing of pyridines through atom-pair swap from CN to CC

Skeletal editing is a straightforward synthetic strategy for precise substitution or rearrangement of atoms in core ring structures of complex molecules; it enables quick diversification of compounds that is not possible by applying peripheral editing strategies. Previously reported skeletal editing of common arenes mainly relies on carbene- or nitrene-type insertion reactions or rearrangements. Although powerful, efficient and applicable to late-stage heteroarene core structure modification, these strategies cannot be used for skeletal editing of pyridines. Here we report the direct skeletal editing of pyridines through atom-pair swap from CN to CC to generate benzenes and naphthalenes in a modular fashion. Specifically, we use sequential dearomatization, cycloaddition and rearomatizing retrocycloaddition reactions in a one-pot sequence to transform the parent pyridines into benzenes and naphthalenes bearing diversified substituents at specific sites, as defined by the cycloaddition reaction components. Applications to late-stage skeletal diversification of pyridine cores in several drugs are demonstrated.

Reactivity of α-diazo sulfonium salts: rhodium-catalysed ring expansion of indenes to naphthalenes

In the presence of catalytic amounts of the paddlewheel dirhodium complex Rh2(esp)2, α-diazo dibenzothiophenium salts generate highly electrophilic Rh-coordinated carbenes, which evolve differently depending on their substitution pattern. Keto-moieties directly attached to the azomethinic carbon promote carbene insertion into one of the adjacent C-S bonds, giving rise to highly electrophilic dibenzothiopyrilium salts. This intramolecular pathway is not operative when the carbene carbon bears ester or trifluoromethyl substituents; in fact, these species react with olefins delivering easy to handle cyclopropyl-substituted sulfonium salts. When indenes are the olefins of choice, the initially formed cyclopropyl rings smoothly open with concomitant departure of dibenzothiophene, enabling access to a series of 2-functionalized naphthalenes.

Selective Ring-Opening Amination of Isochromans and Tetrahydroisoquinolines

The molecular structure-editing through selective C-C bond cleavage allows for the precise modification of molecular structures and opens up new possibilities in chemical synthesis. By strategically cleaving C-C bonds and editing the molecular structure, more efficient and versatile pathways for the synthesis of complex compounds could be designed, which brings significant implications for drug development and materials science. o-Aminophenethyl alcohols and amines are the essential key motifs in bioactive and functional material molecules. The traditional synthesis of these compounds usually requires multiple steps which could generate inseparable isomers and induce low efficiencies. By leveraging a molecular editing strategy, we herein reported a selective ring-opening amination of isochromans and tetrahydroisoquinolines for the efficient synthesis of o-aminophenethyl alcohols and amines. This innovative chemistry allows for the precise cleavage of C-C bonds under mild transition metal-free conditions. Notably, further synthetic application demonstrated that our method could provide an efficient approach to essential components of diverse bioactive molecules.

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.

Rhodium-Catalyzed One-Carbon Ring Expansion of Aziridines with Vinyl-N-triftosylhydrazones for the Synthesis of 2-Vinyl Azetidines

Azetidines, being four-membered N-heterocycles, possess significant potential in contemporary medicinal chemistry owing to their favorable pharmacokinetic properties. Regrettably, the incorporation of functionalized azetidines into pharmaceutical lead structures has been impeded by the absence of efficient synthetic methods for their synthesis. In this study, a Rh-catalyzed one-carbon ring expansion of aziridines with vinyl-N-triftosylhydrazones is presented, which facilitates the synthesis of high value-added 2-alkenyl azetidine products. This research represents the first example of ring expansion of aziridines enabled by vinyl carbenes. Additionally, a one-pot two-step protocol, initiated from cinnamaldehyde, was successfully achieved, offering a step-economical and facile approach for the synthesis of these compounds. The pivotal aspect of this successful transformation lies in the in situ formation of an alkenyl aziridinium ylide intermediate. Experimental investigations, coupled with computational studies, suggest that a diradical pathway is involved in the reaction mechanism.

Silver-Catalyzed Dearomative Skeletal Editing of Indazoles by Donor Carbene Insertion

Given the prevalence of heterocyclic scaffolds in drug-related molecules, converting these highly modular heterocyclic scaffolds into structural diversified and dearomatized analogs is an ideal strategy for improving their physicochemical and pharmacokinetic properties. Here, we described an efficient method for silver carbene-mediated dearomative N-N bond cleavage leading to skeletal hopping between indazole and 1,2-dihydroquinazoline via a highly selective single-carbon insertion procedure. Using this methodology, a series of dihydroquinazoline analogues with diarylmethylene-substituted quaternary carbon centers were constructed with excellent yields and good functional group compatibility, which was further illustrated by the late-stage diversification of important pharmaceutically active ingredients. DFT calculations indicated that the silver catalyst not only induces the formation of the silver carbene, but also activates the diazahexatriene intermediate, which plays a crucial role in the formation of the C-N bond.

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.

Methylene Insertion into Nitrogen-Heteroatom Single Bonds of 1,2-Azoles via a Zinc Carbenoid: An Alternative Tool for Skeletal Editing

The nitrogen-heteroatom single bonds of 1,2-azoles and isoxazolines underwent methylene insertion in the presence of CH2 I2 (6 equiv.) and diethylzinc (3 equiv.) to produce a wide variety of the ring-expanded six-membered heterocycles. Density functional theory calculations suggest that the methylene insertion proceeds via cleavage of nitrogen-heteroatom single bonds followed by ring closure.

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.

Skeletal Editing through Molecular Recombination of 2H-Indazoles to Azo-Linked-Quinazolinones

A new protocol by the combinatory use of two equivalent of indazoles starting material with one being the carbon source via its C3-reactivity and the other, the coupling partner has been developed for the selectfluor-mediated single atom skeletal editing of 2H-indazoles. The azo-linked-2,3-disubstituted quinazolin-4-one derivatives were obtained through a carbon atom insertion between the two nitrogens of the indazole ring and simultaneous oxidation at C3 position of both indazole moieties. Mechanistic investigations reveal that the amidic carbonyl oxygen of the product is derived from water and the reaction proceeds through in-situ generated N-centred indazolone radical intermediate.

An Accesss to 4,5,6-Trisubstituted Pyrimidines from 2H-Azirines and α-Isocyanoacetates or α-Isocyanoacetamides Enabled by 1,3-Dipolar [3 + 2] Cycloaddition/Ring-Expanding/Oxidative Aromatization Process

The products containing pyrimidine scaffolds exhibit various important physiological and biological activities. To date, the strategies to generate 4,5,6-trisubstituted pyrimidines were not reported. Here, a copper-catalyzed reaction of 2H-azirines with α-isocyanoacetates or α-isocyanoacetamides has been developed, rapidly preparing 4,5,6-trisubstituted pyrimidines. The mechanistic results reveal that this strategy underwent a formal 1, 3-dipolar [3 + 2] cycloaddition/ring-expanding/oxidative aromatization procedure to construct the desired pyrimidines.

Dearomatization of Pyridines: Photochemical Skeletal Enlargement for the Synthesis of 1,2-Diazepines

In this report, we developed a unified and standardized one-pot sequence that converts pyridine derivatives into 1,2-diazepines by inserting a nitrogen atom. This skeletal transformation capitalizes on the in situ generation of 1-aminopyridinium ylides, which rearrange under UV light irradiation. A thorough evaluation of the key parameters (wavelength, reaction conditions, activating agent) allowed us to elaborate on a simple, mild, and user-friendly protocol. The model reaction was extrapolated to more than 40 examples, including drug derivatives, affording unique 7-membered structures. Mechanistic evidence supports the transient presence of a diazanorcaradiene species. Finally, pertinent transformations of the products, including ring contraction reactions to form pyrazoles, were conducted and paved the way to a broad application of the developed protocol.

Dearomative Insertion of Fluoroalkyl Carbenes into Azoles Leading to Fluoroalkyl Heterocycles with a Quaternary Center

The skeletal ring expansion of heteroarenes through carbene insertion is gaining popularity in synthetic chemistry. Efficient strategies for heterocyclic ring expansion to access heterocycles containing a fluoroalkyl quaternary carbon center through fluoroalkyl carbene insertion are highly desirable because of their broad applications in medicinal chemistry. Herein, we report a general strategy for the dearomative one-carbon insertion of azoles using fluoroalkyl N-triftosylhydrazones as fluoroalkyl carbene precursors, resulting in ring-expanded heterocycles in excellent yields with good functional-group compatibility. The broad generality of this methodology in the late-stage diversification of pharmaceutically interesting bioactive molecules and versatile transformations of the products has been demonstrated.

Molecular medicinal insights into scaffold hopping-based drug discovery success

In both academia and the pharmaceutical industry, innovative hypotheses, methodologies and technologies that can shorten the drug research and development, leading to higher success rates, are vital. In this review, we demonstrate how innovative variations of the scaffold-hopping strategy have been used to create new druggable molecular spaces, drugs, clinical candidates, preclinical candidates, and bioactive agents. We also analyze molecular modulations that enabled improvements of the pharmacodynamic (PD), physiochemical, and pharmacokinetic (PK) properties (P3 properties) of the drugs resulting from these scaffold-hopping strategies.

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.

Six-Membered Aromatic Nitrogen Heterocyclic Anti-Tumor Agents: Synthesis and Applications

Cancer stands as a serious malady, posing substantial risks to human well-being and survival. This underscores the paramount necessity to explore and investigate novel antitumor medications. Nitrogen-containing compounds, especially those derived from natural sources, form a highly significant category of antitumor agents. Among these, antitumor agents with six-membered aromatic nitrogen heterocycles have consistently attracted the attention of chemists and pharmacologists. Accordingly, we present a comprehensive summary of synthetic strategies and clinical implications of these compounds in this review. This entails an in-depth analysis of synthesis pathways for pyridine, quinoline, pyrimidine, and quinazoline. Additionally, we explore the historical progression, targets, mechanisms of action, and clinical effectiveness of small molecule inhibitors possessing these structural features.

Skeletal rearrangement through photocatalytic denitrogenation: access to C-3 aminoquinolin-2(1H)-ones

The addition of an amine group to a heteroaromatic system is a challenging synthetic process, yet it is an essential one in the development of many bioactive molecules. Here, we report an alternative method for the synthesis of 3-amino quinolin-2(1H)-one that overcomes the limitations of traditional methods by editing the molecular skeleton via a cascade C-N bond formation and denitrogenation process. We used TMSN3 as an aminating agent and a wide variety of 3-ylideneoxindoles as synthetic precursors for the quinolin-2(1H)-one backbone, which demonstrates remarkable tolerance of sensitive functional groups. The control experiments showed that the triazoline intermediate plays a significant role in the formation of the product. The spectroscopic investigation further defined the potential reaction pathways.

1,2-Redox Transpositions of Tertiary Amides

Reactions capable of transposing the oxidation levels of adjacent carbon atoms enable rapid and fundamental alteration of a molecule's reactivity. Herein, we report the 1,2-transposition of the carbon atom oxidation level in cyclic and acyclic tertiary amides, resulting in the one-pot synthesis of 1,2- and 1,3-oxygenated tertiary amines. This oxidation level transfer was facilitated by the careful orchestration of an iridium-catalyzed reduction with the functionalization of transiently formed enamine intermediates. A novel 1,2-carbonyl transposition is described, and the breadth of this redox transposition strategy has been further explored by the development of aminoalcohol and enaminone syntheses. The diverse β-functionalized amine products were shown to be multifaceted and valuable synthetic intermediates, accessing challenging biologically relevant motifs.

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.

Recent Advances in the Transition-Metal-Free Synthesis of Quinazolines

Quinazolines are a privileged class of nitrogen-containing heterocycles, widely present in a variety of natural products and synthetic chemicals with a broad spectrum of biological and medicinal activities. Owing to their pharmaceutical applications and promising biological value, a variety of synthetic methodologies have been reported for these scaffolds. From the perspective of green and sustainable chemistry, transition-metal-free synthesis provides an alternative method for accessing several biologically active heterocycles. In this review, we summarize the recent progress achieved in the transition-metal-free synthesis of quinazolines and we cover the literature from 2015 to 2022. This aspect is present alongside the advantages, limitations, mechanistic rationalization, and future perspectives associated with the synthetic methodologies.

Skeletal Editing of Pyrimidines to Pyrazoles by Formal Carbon Deletion

A method for the conversion of pyrimidines into pyrazoles by a formal carbon deletion has been achieved guided by computational analysis. The pyrimidine heterocycle is the most common diazine in FDA-approved drugs, and pyrazoles are the most common diazole. An efficient method to convert pyrimidines into pyrazoles would therefore be valuable by leveraging the chemistries unique to pyrimidines to access diversified pyrazoles. One method for the conversion of pyrimidines into pyrazoles is known, though it proceeds in low yields and requires harsh conditions. The transformation reported here proceeds under milder conditions, tolerates a wide range of functional groups, and enables the simultaneous regioselective introduction of N-substitution on the resulting pyrazole. Key to the success of this formal one-carbon deletion method is a room-temperature triflylation of the pyrimidine core, followed by hydrazine-mediated skeletal remodeling.