Transition-Metal-Catalyzed Cycloaddition Reactions to Access Seven-Membered Rings

Base-Promoted (5 + 2) Annulation between 2-(Alkynylaryl)acetonitriles and Arylalkynes for the Synthesis of Benzocycloheptene Derivatives

Herein, we describe a novel approach to the synthesis of benzocycloheptene derivatives via base-promoted (5 + 2) annulation between 2-(alkynylaryl)acetonitriles and arylalkynes. In this chemistry, 2-(alkynylaryl)acetonitriles are employed as a new C5 synthon to construct various benzocycloheptene(s) derivatives by building two C-C bonds in one single step. This method features excellent regioselectivity, the use of readily available starting materials, and good functional group tolerance. The practicality of the strategy was further demonstrated by gram-scale synthesis, late-stage functionalizations, and the post-modification of natural products such as probenecid and tetrahydrofurfuryl alcohol.

Tetraaryl-3H-Azepines: KOBut/DMSO-Catalyzed Assembly from Diaryldiynes and N-Benzylaldimines

Di(het)aryldiynes smoothly react with N-benzylaldimines in a [4 + 3] cycloaddition manner under the action of the KOBut/DMSO system (60 °C, 30 min) to afford pharmaceutically relevant tetra(het)arylsubstituted 3H-azepines in up to 71% yield. The process involves the addition of azaallyl anions to one of the triple bonds of diynes followed by prototropic isomerization and cyclization of anionic intermediates with participation of the second triple bond. The cascade mechanism is consistent with quantum-chemical analysis (B2PLYP-D3/6-311+G**//B3LYP-D3/6-31+G* + PCM).

Oxidative Cascade Iodocyclization of 1,n-Dienes: Synthesis of Iodinated Benzo[b]azepine and Benzo[b]azocine Derivatives

An oxidative cascade iodocyclization of 1,7- or 1,8-dienes has been realized under mild conditions. By employing I2 as an iodine source, this protocol provides a concise and efficient approach to a great deal of biologically significant iodinated benzo[b]azepine and benzo[b]azocine derivatives in moderate to good yields. The gram-scale synthesis and further transformation of products render the approach practical and attractive. Radical trapping and deuterium-labeling experiments help to understand the mechanism.

Synthesis of Seven- and Eight-Membered Rings by a Brønsted Acid Catalyzed Cationic Carbocyclization of Biphenyl Embedded Enynes

A Brønsted acid catalyzed cyclization of o-alkenyl-o'-alkynylbiaryls for the synthesis of biologically relevant dibenzo-fused medium-sized rings has been developed. The outcome of the cyclization is determined by the nature of the substituent at the alkyne, with arenes favoring seven-membered rings and alkyl substituents producing eight-membered rings. These reactions proceed via a vinyl cation, which is captured by water and, notably, by C-nucleophiles, such as electron-rich (hetero)arenes.

Electrochemical oxidative radical cascade cyclization of dienes and diselenides towards the synthesis of seleno-benzazepines

Selenium-containing compounds are important scaffolds owing to their value in medicinal chemistry, biochemistry and material chemistry. Herein, we report an electrochemical approach to access seleno-benzazepines through an oxidative radical cascade cyclization of dienes with diselenides under metal-free, external oxidant-free and base-free conditions. In a simple undivided cell, various dienes and diselenides were suitable for this transformation, generating the desired products in up to 84% yields. This method provides a green and convenient route for the synthesis of valuable selenium-containing seven-membered N-heterocycles from simple starting materials.

Transition-Metal Catalyzed Synthesis of Pyrimidines: Recent Advances, Mechanism, Scope and Future Perspectives

Pyrimidine is a pharmacologically important moiety that exhibits diverse biological activities. This review reflects the growing significance of transition metal-catalyzed reactions for the synthesis of pyrimidines (with no discussion being made on the transition metal-catalyzed functionalization of pyrimidines). The effect of different catalysts on the selectivity/yields of pyrimidines and catalyst recyclability (wherever applicable) are described, together with attempts to illustrate the role of the catalyst through mechanisms. Although several methods have been researched for synthesizing this privileged scaffold, there has been a considerable push to expand transition metal-catalyzed, sustainable, efficient and selective synthetic strategies leading to pyrimidines. The aim of the authors with this update (2017-2023) is to drive the designing of new transition metal-mediated protocols for pyrimidine synthesis.

21-Carba-23-selenaporphyrinoid Dyads-An Azepine Unit as a Merging Motif

The oxidation of 10,15-diaryl-21-carba-23-selenaporphyrinoids resulted in the creation of dyads. The dimerization process follows a [5+2] cycloaddition path with the formation of an azepine unit. The arrays display two direct bonds between the peripheral carbocyclic carbon atoms of one carbaselenaporphyrinic subunit and the central carbon and nitrogen atoms of the second subunit. This results in a unique canted arrangement of two carbaporphyrinoid planes resembling an open seashell-like motif.

Pd-catalyzed oxa-[4 + n] dipolar cycloaddition using 1,4-O/C dipole synthons for the synthesis of O-heterocycles

Transition metal-catalyzed dipolar cycloaddition is one of the most efficient and powerful synthetic strategies to produce diverse heterocycles. In particular, for the construction of oxygen-containing heterocycles, which are valuable structural motifs found in pharmaceuticals and natural compounds, transition metal-catalyzed oxa-dipolar cycloaddition using an oxygen-containing dipole has emerged as a promising method. In recent years, the 1,4-O/C dipole synthons 2-alkylidenetrimethylene carbonate and 2-hydroxymethylallyl carbonate have been developed and successfully applied to palladium-catalyzed oxa-[4 + n] dipolar cycloadditions with diverse dipolarophiles. In this review, we summarize recent advances in palladium-catalyzed oxa-[4 + n] dipolar cycloadditions using 1,4-O/C dipoles including asymmetric catalysis and divergent catalysis toward five- to nine-membered O-heterocycles.

Rh(III) Catalyzed Redox-Neutral C-H Activation/[5 + 2] Annulation of Aroyl Hydrazides and Sulfoxonium Ylides: Synthesis of Benzodiazepinones

We herein report the Rh(III) catalyzed redox-neutral C-H activation/[5 + 2] annulation of aroyl hydrazides with sulfoxonium ylides as safe carbene precursors. The reaction shows excellent functional group tolerance, broad substrate scope, and scalability. We demonstrated the synthetic utility of the protocol via the synthesis of various diazepam drug analogues, late-stage functionalization of probenecid drug, and large scale synthesis. Finally, kinetic studies revealed C-H activation as the rate-determining step.

Construction of Unique Spiro[dibenzo[a,f]azulene-6,2'-indenes] via Unprecedented Annulation of ortho-C-H Bond of Benzylidene Group

The domino reaction of alkyl and aryl isocyanides with two molecules of 2-arylidene-1,3-indanediones in acetonitrile at 80 °C resulted in unique functionalized spiro[dibenzo[a,f]azulene-6,2'-indenes] in good yields, in which the two 2-arylidene-1,3-indanediones acted as different building blocks to construct the polycyclic system. More importantly, the unprecedented anticipation of the ortho-position of benzylidene group to form a novel dibenzo[a,f]azulene ring through a formal [5 + 2] cycloaddition process was first observed. On the other hand, DABCO-promoted reaction of the isocyanides with two molecules of 2-arylidene-1,3-indanediones in acetonitrile at 80 °C afforded functionalized spiro[cyclopenta[a]-indene-2,2'-indene] derivatives.

Chemodivergence in Pd-catalyzed desymmetrization of allenes: enantioselective [4+3] cycloaddition, desymmetric allenylic substitution and enynylation

A class of prochiral allenylic di-electrophiles have been introduced for the first time as three-atom synthons in cycloadditions, and a new type of [4+3] cycloaddition involving transition metal-catalyzed enantioselective sequential allenylic substitution has been successfully developed, enabling challenging seven-membered exocyclic axially chiral allenes to be accessed in good yields with good enantioselectivity. Through the addition of a catalytic amount of ortho-aminoanilines or ortho-aminophenols, the racemization of the [4+3] cycloaddition products is effectively suppressed. Mechanistic studies reveal that elusive Pd-catalyzed enantioselective intramolecular allenylic substitution rather than intermolecular allenylic substitution is the enantio-determining step in this cycloaddition. By tuning the ligands, a Pd-catalyzed enantioselective desymmetric allenylic substitution leading to linear axially chiral tri-substituted allenes or a Pd-catalyzed tandem desymmetric allenylic substitution/β-vinylic hydrogen elimination (formal enynylation) leading to multi-functionalized 1,3-enynes is achieved chemodivergently.

Syntheses and Synthetic Applications of Functionalized Propargylic and Allylic Fluorides

This account presents the synthesis and application of propargylic and allylic fluorides containing hydroxy or carbonyl functional groups. In particular, the Barbier-type reaction of difluoropropargyl bromides with aldehydes or chloroformates provides versatile propargylic fluorides, and the organocatalytic fluorination of dienamine intermediates has been demonstrated as an effective method to obtain allylic fluorides stereoselectively. Additionally, mechanistic insights into such reactions are discussed with the aid of density functional theory calculations. The report also describes the preparation of fluorinated 1,7-diyne or 1,7-enyne derivatives of these compounds. These propargylic and allylic fluorides can be used as building blocks for fluorinated heterocycles, such as fluorinated furans, tetrahydrofurans, and lactams. Additionally, fluorinated bi- or tri-heterocyclic compounds can be synthesized via transition-metal-catalyzed reactions with fluorinated 1,7-diyne or 1,7-enyne derivatives.

Enantioselective Transformations in the Synthesis of Therapeutic Agents

The proportion of approved chiral drugs and drug candidates under medical studies has surged dramatically over the past two decades. As a consequence, the efficient synthesis of enantiopure pharmaceuticals or their synthetic intermediates poses a profound challenge to medicinal and process chemists. The significant advancement in asymmetric catalysis has provided an effective and reliable solution to this challenge. The successful application of transition metal catalysis, organocatalysis, and biocatalysis to the medicinal and pharmaceutical industries has promoted drug discovery by efficient and precise preparation of enantio-enriched therapeutic agents, and facilitated the industrial production of active pharmaceutical ingredient in an economic and environmentally friendly fashion. The present review summarizes the most recent applications (2008-2022) of asymmetric catalysis in the pharmaceutical industry ranging from process scales to pilot and industrial levels. It also showcases the latest achievements and trends in the asymmetric synthesis of therapeutic agents with state of the art technologies of asymmetric catalysis.

Bicyclic N-dihalocyclopropylamide derivatives as precursors of nitrogen-containing fused polycyclic systems

Examples of carbon-carbon bond-forming cyclisation reactions, involving allyl cations generated by the thermal ring-opening of halocyclopropanes, have been scarcely reported. In this contribution, we are describing the results of a study conducted with N-dihalocyclopropylamide substrates, designed as precursors of cyclic iminium intermediates that were aimed at participating in intramolecular reactions with electron-rich aromatic groups. Competitive side-reactions were identified, and access to the desired polycyclic products was carefully evaluated. The results were found to be strongly dependent on the substitution pattern of the nucleophilic aromatic moieties, as well as on the sizes of the rings of the target products. In spite of the rather moderate yields generally obtained, this approach represents a particularly short and inexpensive route to various interesting nitrogen-containing polycyclic systems, namely benzoindolizidine, benzoquinolizidine, piperidinobenzoazepane and azepanoisoquinoline compounds.

Photo- or Electrochemical Cyclization of Dienes with Diselenides to Access Seleno-Benzo[b]azepines

A cascade selenylation/cyclization of dienes with diselenides has been realized under visible-light irradiation or electrolysis conditions. Employing O₂ or electricity as a "green" oxidant, this protocol provides a green and efficient method for an array of biologically important seleno-benzo[b]azepine derivatives in moderate to good yields. The direct sunlight irradiation and gram-scale reaction render the approach practical and attractive.

Recent Progress in Heterocycle Synthesis: Cyclization Reaction with Pyridinium and Quinolinium 1,4-Zwitterions

Heteroarene 1, n-zwitterions are powerful and versatile building blocks in the construction of heterocycles and have received increasing attention in recent years. In particular, pyridinium and quinolinium 1,4-zwitterions have been widely studied and used in a variety of cyclization reactions due to their air stability, ease of use, and high efficiency. Sulfur- and nitrogen-based pyridinium and quinolinium 1,4-zwitterions, types of emerging heteroatom-containing synthons, have attracted much attention from chemists. These 1,4-zwitterions, which contain multiple reaction sites, have been successfully used in the synthesis of three- to eight-membered cyclic compounds over the last decade. In this review, we present the exciting progress made in the field of cyclization reactions of sulfur- and nitrogen-based pyridinium and quinolinium 1,4-zwitterions. Moreover, the mechanistic insights, the transition states, some synthetic applications, and the challenges and opportunities are also discussed. We hope to provide an overview for synthetic chemists who are interested in the heterocycle synthesis from cyclization reaction with pyridinium and quinolinium 1,4-zwitterions pyridinium and quinolinium 1,4-zwitterions.

Strain-Release-Controlled [4 + 2 + 1] Reaction of Cyclopropyl-Capped Diene-ynes/Diene-enes and Carbon Monoxide Catalyzed by Rhodium

Achieving transition-metal-catalyzed reactions of diene-ynes/diene-enes and carbon monoxide (CO) to deliver [4 + 2 + 1] cycloadducts, rather than the kinetically favored [2 + 2 + 1] products, is challenging. Here, we report that this can be solved by adding a cyclopropyl (CP) cap to the diene moiety of the original substrates. The resulting CP-capped diene-ynes/diene-enes can react with CO under Rh catalysis to give [4 + 2 + 1] cycloadducts exclusively without forming [2 + 2 + 1] products. This reaction has a broad scope and can be used to synthesize useful 5/7 bicycles with a CP moiety. Of the same importance, the CP moiety in the [4 + 2 + 1] cycloadducts can act as an intermediate group for further transformations so that other challenging bicyclic 5/7 and tricyclic 5/7/5, 5/7/6, and 5/7/7 skeletons, some of which are widely found in natural products, can be accessed. The mechanism of this [4 + 2 + 1] reaction has been investigated by quantum chemical calculations, and the role of the CP group in avoiding the possible side [2 + 2 + 1] reaction has been identified, showing that the [4 + 2 + 1] is controlled by releasing the ring strain in the methylenecyclopropyl (MCP) group (about 7 kcal/mol) in the CP-capped dienes.

Intramolecular Rhodium-Catalyzed [(3+2+2)] Carbocyclization Reactions with Dienylidenecyclopropanes: A Concise and Stereoselective Total Synthesis of the Sesquiterpene (+)-Zizaene

The development of an intramolecular rhodium-catalyzed [(3+2+2)] carbocyclization reaction of alkylidenecyclopropanes (ACPs) tethered to 1,4- and 1,5-skipped dienes is described. This transformation offers a new approach for the construction of bridged tricyclic compounds with up to three quaternary centers, which are suitable for the synthesis of challenging bioactive natural products. For instance, the synthetic utility of this transformation is illustrated through a concise asymmetric total synthesis of the sesquiterpene (+)-zizaene in ten steps from a commercially available starting material.

Asymmetric Total Synthesis of Twin Bufogargarizins A and B

The first and asymmetric total synthesis of bufogargarizins A and B, two unusual and highly oxygenated twin steroids with rearranged A/B rings, was achieved. The synthetically challenging [7-5-6-5] tetracyclic ring system of bufogargarizin A was efficiently constructed by the first intramolecular Ru-catalyzed [5 + 2] cycloaddition reaction of a vinyl ether cyclopropane-yne. Notably, the interesting [5-7-6-5] tetracyclic skeleton of bufogargarizin B was diastereoselectively reassembled by unique retro-aldol/transannular aldol cascade reactions from the [7-5-6-5] tetracyclic framework.

Recent Progress in Cyclic Aryliodonium Chemistry: Syntheses and Applications

Hypervalent aryliodoumiums are intensively investigated as arylating agents. They are excellent surrogates to aryl halides, and moreover they exhibit better reactivity, which allows the corresponding arylation reactions to be performed under mild conditions. In the past decades, acyclic aryliodoniums are widely explored as arylation agents. However, the unmet need for acyclic aryliodoniums is the improvement of their notoriously low reaction economy because the coproduced aryl iodides during the arylation are often wasted. Cyclic aryliodoniums have their intrinsic advantage in terms of reaction economy, and they have started to receive considerable attention due to their valuable synthetic applications to initiate cascade reactions, which can enable the construction of complex structures, including polycycles with potential pharmaceutical and functional properties. Here, we are summarizing the recent advances made in the research field of cyclic aryliodoniums, including the nascent design of aryliodonium species and their synthetic applications. First, the general preparation of typical diphenyl iodoniums is described, followed by the construction of heterocyclic iodoniums and monoaryl iodoniums. Then, the initiated arylations coupled with subsequent domino reactions are summarized to construct polycycles. Meanwhile, the advances in cyclic aryliodoniums for building biaryls including axial atropisomers are discussed in a systematic manner. Finally, a very recent advance of cyclic aryliodoniums employed as halogen-bonding organocatalysts is described.

Assembly of 5H-dibenzo[a,d]cycloheptenes by a formal [5 + 2] annulation of ortho-aryl alkynyl benzyl alcohols with arenes

A new synthetic methodology for the synthesis of 5H-dibenzo[a,d]cycloheptenes from ortho-aryl alkynyl benzyl alcohols and arenes via a Tf₂O-mediated formal [5 + 2] annulation reaction has been achieved. From this transformation, structurally diverse 5H-dibenzo[a,d]cycloheptenes were achieved in moderate to good yields. This transformation probably involves an intermolecular Friedel-Crafts-type alkylation and a subsequent intramolecular 7-endo-dig cyclization in one pot, highlighting the high efficiency, regioselectivity, and step-economy of this protocol.

Why [4 + 2 + 1] but Not [2 + 2 + 1]? Why Allenes? A Mechanistic Study of the Rhodium-Catalyzed [4 + 2 + 1] Cycloaddition of In Situ Generated Ene-Ene-Allenes and Carbon Monoxide

Transition metal-catalyzed [4 + 2 + 1] cycloaddition of in situ generated ene/yne-ene-allenes (from ene/yne-ene propargyl esters) and carbon monoxide (CO) gives the [4 + 2 + 1] cycloadducts rather than [2 + 2 + 1] cycloadducts. Investigating the mechanism of this [4 + 2 + 1] reaction and understanding why the [2 + 2 + 1] reaction does not compete and the role of the allene moiety in the substrates are important. This is also helpful to guide the future design of new [4 + 2 + 1] cycloadditions. Reported here are the kinetic and computed studies of the [4 + 2 + 1] reactions of ene-ene propargyl esters and CO. A quantum chemical study (at the DLPNO-CCSD(T)//BMK level) revealed that the [4 + 2 + 1] reaction includes four key steps, which are 1,3-acyloxy migration (rate-determining step), oxidative cyclization, CO migratory insertion, and reductive elimination. The allene moiety in the substrates is critical for providing additional coordination to the rhodium center in the final step of the catalytic cycle, which in turn favors the reductive elimination transition state in the [4 + 2 + 1] rather than in the [2 + 2 + 1] pathway. The CO insertion step in the [4 + 2 + 1] reaction, which could occur through either the UP (favored here) or DOWN CO insertion pathway, has also been deeply scrutinized, and some guidance from this analysis has been provided to help the future design of new [4 + 2 + 1] reactions. Quantum chemical calculations have also been applied to explain why [4 + 2] and [4 + 1] cycloadditions do not happen and how trienes as side products for some substrates are generated.

K2S2O8/I2-Promoted Electrophilic Selenylative Cyclization To Access Seleno-Benzo[b]azepines

A novel and simple organoselenium-involved 7-membered cyclization to access diverse seleno-benzo[b]azepines has been developed. This protocol involves an electrophilic cyclization process and is accomplished under mild conditions. Discussion of the mechanism rationalizes the regioselectivity observed in transformation. The studies of further transformation of seleno-benzo[b]azepines and large-scale experiment reveal the promising utility of this methodology.

Iodine-Mediated Pyridine Ring Expansion for the Construction of Azepines

A synthesis of azepines by the expansion of pyridine rings promoted by iodine in air is reported. The scope of the method is demonstrated with 27 examples. Two iodinations are involved in the reaction, in which the second iodination leads to the opening of the six-membered ring.

Migratory cycloisomerization of 1,3-dien-5-ynes conjugated with pseudopeptides in assembly of benzo[7]annulenes

A novel domino cycloisomerization of 1,3-dien-5-ynes for the synthesis of 7H-benzo[7]annulenes is reported. The noticeable feature of this domino reaction involves the assembly of the fused bicyclic motifs through a transamidation/5-exo-trig cyclization/8π-electrocyclization sequence in a single step. Finally, mechanistic investigations were conducted experimentally and supported by DFT calculations.

Stereoselective synthesis of functionalized azepines via gold and palladium relay catalysis

We developed a cycloisomerization/asymmetric [4 + 3] cycloaddition cascade reaction via gold/palladium relay catalysis, furnishing enantioenriched furan-fused azepines efficiently.

Dearomative [4 + 3] cycloaddition of furans with vinyl-N-triftosylhydrazones by silver catalysis: stereoselective access to oxa-bridged seven-membered bicycles

A practical dearomative [4 + 3] cycloaddition of furans with vinylcarbenes to access oxa-bridged seven-membered carbocycles, with complete and predictable stereoselectivity, is achieved by merging silver catalysis and vinyl-N-triftosylhydrazones.

Silver-catalyzed [4 + 3] cycloaddition of 1,3-dienes with alkenyl-N-triftosylhydrazones: a practical approach to 1,4-cycloheptadienes

A formal [4 + 3] cycloaddition of 1,3-dienes with alkenyl-N-triftosylhydrazones was developed using silver catalysis, producing a broad spectrum of complex 1,4-cycloheptadienes with high yields and predictable stereochemistry.

Synthesis of Seven-Membered Cross-Conjugated Cyclic Trienes by 8π Electrocyclic Reaction

A method for the synthesis of 3-methylene-1,4-cycloheptadiene derivatives via an 8π electrocyclization reaction was developed. The triene substrate bearing a phosphate or carbamate group was prepared from γ,δ-unsaturated esters and α,β-unsaturated aldehydes in four steps. Upon treatment with NaHMDS or KHMDS, the substrate formed a heptatrienyl anion, which underwent electrocyclization and subsequent β-elimination of the leaving group. The product could be converted into a tropylium salt in two steps.

Intermolecular [5+2] Annulation between 1-Indanones and Internal Alkynes by Rhodium-Catalyzed C-C Activation

Herein, we report a [5+2] cycloaddition between readily accessible 1-indanones and internal alkynes through Rh-catalyzed activation of less strained C-C bonds. The reaction is enabled by a strongly σ-donating NHC ligand and a carefully modified temporary directing group. A wide range of functional groups is tolerated, and the method provides straightforward access to diverse benzocycloheptenones that are hard to access otherwise. DFT studies of the reaction mechanism imply the migration insertion as the turnover-limiting step and suggest beneficial π-π interactions in the transition states.

Three-Component Bisannulation for the Synthesis of Trifluoromethylated Tetracyclic Aza-Aromatics through Six C(sp3)-F Bond Cleavage and Four C-N Bond Formation

An unprecedented and expeditious tandem bisannulation of polyfluoroalkylated tetralones with benzamidines to access various fluoroalkyl tetracyclic [1,3]-diazepines through multiple C-N bond formation and C(sp³)-F bond cleavage is reported. The process features high regio-/chemoselectivities, broad substrate scope, good functional group tolerance, procedural simplicity, mild reaction conditions, and scale-up synthesis. Mechanistic studies showed that the distinctive fluorine effect of polyfluoroalkyl tetralone plays a vital role for the aza-tetracycle construction.

Pd(0)-Catalyzed Diastereo- and Enantioselective Intermolecular Cycloaddition for Rapid Assembly of 2-Acyl-methylenecyclopentanes

A highly regio-, diastereo-, and enantioselective trimethylenemethane (TMM) cycloaddition reaction for the rapid assembly of 2-acyl-methylenecyclopentane in an atom-economic fashion is described. This intermolecular protocol allows for facile and divergent access to an array of structurally attractive cyclic adducts. The choice of a robust chiral diamidophosphite ligand, developed by our group, proved to be crucial for the success of this transformation.

Recent advances in hydride transfer-involved C(sp3)–H activation reactions

This review summarizes the recent progresses (2016–2020) in the hydride transfer-enabled C(sp³)–H activation according to the reaction types, categorized into the intramolecular/intermolecular C(sp³)–H functionalization, and hydride reduction.