Visible-Light-Induced Cascade Radical Trifluoromethylation/Cyclization/Dearomatization of Isocyanide-Containing Indoles: Synthesis of Trifluoromethylated 3-Spiroindolines

A visible-light-induced cascade radical trifluoromethylation/cyclization/dearomatization reaction between isocyanide-containing indoles and CF3Br has been developed to afford trifluoromethylated spiro[indole-3,3-quinoline] and spiro [indole-3,3-pyrrole] derivatives in good yields. The utility of the process is demonstrated by a scale-up experiment. The mechanism was proposed based on the control experiments. The protocol constitutes a novel and efficient route for the synthesis of trifluoromethylated 3-spiroindolenines with advantages of good generality and practical applicability, broad substrate scope, and green energy source.

Green synthesis and investigation of antioxidant and antibacterial activity of new derivatives of chromenoazepines employing CuO/TiO2@MWCNTs

The synthesis of novel, high-yield derivatives of chromenoazepine was investigated in this work. CuO/TiO2@MWCNTs was used as a nanocatalyst in a multicomponent reaction involving 4-aminocumarine, activated acetylenic chemicals, and alkyl bromide in room temperature water to create these novel compounds. Using MCRs of 4-aminocumarine, isothiocyanate, and alkyl bromide in the presence of CuO/TiO2@MWCNTs as nanocatalysts in room-temperature water, chromenothiazepines were synthesized under comparable conditions. The freshly synthesized azepine exhibits antioxidant activity since its NH group has undergone two evaluation processes. Additionally, using two types of Gram-negative bacteria in a disk distribution procedure, the antibacterial activity of recently developed azepines was evaluated, and these compounds also inhibited the growth of Gram-positive bacteria. This method's benefits include quick reaction times, large product yields, and straightforward catalyst and product separation through easy steps.

Enantioselective Organocatalyzed Cascade Dearomatizing Spirocycloaddition Reactions of Indole-Ynones

An intramolecular organocatalytic cascade dearomatizing spirocycloaddition reaction of indole-ynone compounds containing O-silyl-naphthol substituents has been developed with the use of a chiral bifunctional thiourea. This process was able to provide various structurally diverse polycyclic spiroindolines in high yields (up to 98%) with excellent stereoselectivities (>20:1 dr, up to 98% ee) involving the formation of carbonylvinylidene ortho-quinone methide intermediates.

Dearomative Alkylation-Based Two-Step cis-Diastereoselective Synthesis of Indoline-2,3-Fused Chromans and Tetrahydropyrans

Herein, we describe a two-step, cis-diastereoselective synthesis of indoline-2,3-fused chromans from 3-substituted indoles. The method proceeds without intermediacy of ortho-quinone methides and leverages the dual function of TBS-protected 2-hydroxybenzyl iodides both as highly reactive alkylating agents in a t-BuONa/Et3B-promoted dearomative alkylation step and as a source of masked phenoxide nucleophiles in a subsequent TBAF-induced one-pot deprotection-cyclization step of the resulting indolenines. Importantly, this two-step protocol can also be extended to access indoline-2,3-fused tetrahydropyrans. These syntheses of indoline-2,3-fused chromans and tetrahydropyrans proceed with operational convenience, use easily accessible substrates and reagents, and feature broad substrate scope, high yields and complete diastereoselectivity. Furthermore, the synthesized products have the potential to undergo late-stage functionalization.

Chiral Brønsted Acid-Catalyzed Intramolecular Asymmetric Dearomatization Reaction of Indoles with Cyclobutanones via Cascade Friedel-Crafts/Semipinacol Rearrangement

The highly efficient synthesis of chiral indolines fused with an azabicyclo[2.2.1]heptanone moiety is achieved by an asymmetric dearomatization reaction of indoles with cyclobutanones. A new chiral imidodiphosphorimidate (IDPi) catalyst is synthesized and exhibits extraordinary activity in promoting a cascade Friedel-Crafts/semipinacol rearrangement. Target molecules are prepared in good yields (up to 95%) with excellent enantioselectivity (up to 98% ee) with operational convenience. Combined experimental and computational studies provide detailed mechanistic insights into the energy landscape and origin of the stereochemical induction of the reaction.

Rearrangement of C2-Spirooxindoles: Conversion to the 2-Hydroxyhemi-Indigo and Chromenoindole

This study demonstrates the rearrangement of C2-spirooxindoles to the 2-hydroxyhemi-indigo and chromenoindole. The N-H-spirooxindole exhibits double proton translocation and its conversion to the (Z)-2-hydroxyhemi-indigo photoswitch with trifluoroacetic acid, while the N-methyl-spirooxindole undergoes structural rearrangement to the chromenoindole. The mechanism of the reactions was proposed, and the structure of the products was confirmed by one-dimensional (1D) and two-dimensional (2D) NMR spectra and X-ray structure analysis. The photoswitching performance of (Z)-2-hydroxyhemi-indigo, which allows the stabilization of the E-switched form by intramolecular hydrogen bonding, has been studied in solvents of different polarities. It was found that in the less polar solvents, the E-switched metastable isomer is characterized by high stability.

Silver-Catalyzed Single-Carbon Insertion of Indoles with Acetophenone N-Triftosylhydrazones

Here, we report a silver carbene-enabled single-carbon insertion reaction of indoles via a one-pot, two-step sequence to deliver a dearomative quaternary center quinoline scaffold in a modular fashion. Specifically, we used N-triftosylhydrazones as masked donor-donor carbene precursors that facilitate the insertion of carbon atoms bearing various functional groups to the library of functionalized quinoline. Experimental and DFT evidence support the transient presence of a cyclopropane species and removal of protecting groups.

Photoredox-Catalyzed Strain-Release-Driven Synthesis of Functionalized Spirocyclobutyl Oxindoles

Spirocyclobutyl oxindoles have garnered substantial attention in drug discovery and pharmaceuticals owing to their wide range of biological activities. Strain-release in small-ring compounds is a powerful strategy to enable efficient access to complex molecules. In this study, we successfully realized a photoredox-catalyzed strain-release radical spirocyclization approach to attain functionalized spirocyclobutyl oxindoles. A diverse array of radicals, such as sulfonyl, phosphonyl, and trifluoromethyl, were added efficiently to the strained C-C σ-bond of bicyclobutanes (BCBs) to afford a library of spirocyclobutyl oxindoles. Furthermore, the obtained products could be transformed into valuable building blocks. The observed reactivity and selectivity have been rationalized based on density functional theory calculations.

BBr3-mediated dearomative spirocyclization of biaryl ynones: facile access to spiro[5.5]dienones

This report covers boron tribromide (BBr3) mediated dearomative spirocyclization of biaryl ynones. The direct synthesis of spiro[5.5]dienones with a tri-substituted double bond is described for the first time in this paper. The scope of the reaction is broad and the spirocyclic products were obtained in moderate to good yields.

Diastereoselective synthesis of functionalized spiroindolines via intramolecular ipso-iodocyclization/nucleophile addition cascade reactions of indole-tethered ynones

Herein, we describe a highly diastereoselective approach for synthesizing polyfunctionalized spiroindolines from indolyl-ynones involving an ipso-iodocyclization/nucleophile addition cascade. The developed strategy allows the formation of a spirocyclic core and the installation of two functional groups in a single operation. Also this strategy is accompanied by the generation of two C-C and one C-I bonds and two contiguous stereocenters.

Photo-induced intramolecular dearomative [5 + 4] cycloaddition of arenes for the construction of highly strained medium-sized-rings

Medium-sized-ring compounds have been recognized as challenging synthetic targets in organic chemistry. Especially, the difficulty of synthesis will be augmented if an E-olefin moiety is embedded. Recently, photo-induced dearomative cycloaddition reactions that proceed via energy transfer mechanism have witnessed significant developments and provided powerful methods for the organic transformations that are not easily realized under thermal conditions. Herein, we report an intramolecular dearomative [5 + 4] cycloaddition of naphthalene-derived vinylcyclopropanes under visible-light irradiation and a proper triplet photosensitizer. The reaction affords dearomatized polycyclic molecules possessing a nine-membered-ring with an E-olefin moiety in good yields (up to 86%) and stereoselectivity (up to 8.8/1 E/Z). Detailed computational studies reveal the origin behind the favorable formation of the thermodynamically less stable isomers. Diverse derivations of the dearomatized products have also been demonstrated.

Tunable C-H functionalization and dearomatization enabled by an organic photocatalyst

C-H functionalization and dearomatization constitute fundamental transformations of aromatic compounds, which find wide applications in various research areas. However, achieving both transformations from the same substrates with a single catalyst by operating a distinct mechanism remains challenging. Here, we report a photocatalytic strategy to modulate the reaction pathways that can be directed toward either C-H functionalization or dearomatization under redox-neutral or net-reductive conditions, respectively. Two sets of indoles and indolines bearing tertiary alcohols are divergently furnished with good yields and high selectivity. The key to success is the introduction of isoazatruxene ITN-2 as a novel photocatalyst (PC), which outperforms the commonly used PCs. The ready synthesis and high modulability of isoazatruxene type PCs indicate their great application potential.

Copper-Catalyzed Chemoselective O-Arylation of Oxindoles: Access to Cyclic Aryl Carboxyimidates

We have developed a highly efficient base- and additive-free chemoselective CuO-catalyzed strategy for the O-arylation of 2-oxindoles to synthesize 2-phenoxy-3H-indole and 2-phenoxy-1H-indole derivatives in the presence of diaryl iodonium salts. This method offers a variety of O-arylated oxindoles in good to excellent yields under relatively milder reaction conditions. Furthermore, this methodology was extended for the O-arylation of 2-pyridinone and isoindoline-1-one derivatives as well.

Diastereocontrolled Construction of Spiroindolenines via Hexafluoroisopropanol-Promoted Dearomative Epoxide-Indole Cyclization

Herein, we report the discovery of the ipso-selective, dearomatizing spirocyclization of indole-tethered epoxides as a fundamentally new approach for constructing spiroindolenines equipped with three contiguous stereogenic centers under complete diastereocontrol (dr >99:1) and in high yields. Promoted by hexafluoroisopropanol, the protocol features a broad substrate scope, easy scale-up, and versatile transformations of the synthesized spiroindolenines.

Green preparation of new pyrimidine triazole derivatives via one-pot multicomponent reactions of guanidine

In this research the goal was to produce novel pyrimidine triazole compounds in high yields using triethylamin as an efficient catalyst. These new compounds were synthesized by using multicomponent reaction of aldehydes, guanidine, electron deficient acetylenic compounds, tert-butyl isocyanide and hydrazonoyle chloride in aqueous media. Due to the presence of an NH group, which was assessed using two different methodologies, newly synthesized pyrimidine triazoles have antioxidant properties. Additionally, the antibacterial activity of newly created pyrimidine triazoles was assessed using the disk distribution method with two different types of Gram-positive bacteria and Gram-negative bacteria, demonstrating that the use of these compounds prevented the growth of bacteria. Applied to the preparation of pyrimidine triazole derivatives, this method has short reaction times, high product yields, and the ability to separate catalyst and product using simple procedures.

Design, synthesis, pharmacological evaluation, and in silico studies of the activity of novel spiro pyrrolo[3,4-d]pyrimidine derivatives

In the present study, spiro compounds are shown to have distinctive characteristics because of their interesting conformations and their structural impacts on biological systems. A new family of functionalized spiro pyrrolo[3,4-d]pyrimidines is prepared via the one-pot condensation reaction of amino cyclohexane derivatives with benzaldehyde to prepare fused azaspiroundecanedione and azaspirodecenone/thione derivatives. A series of synthesized spiro compounds were scanned against DPPH and evaluated for their ability to inhibit COX-1 and COX-2. All compounds exhibit significant antiinflammatory activity, and they inhibited both COX-1 and COX-2 enzymes with a selectivity index higher than celecoxib as a reference drug. The most powerful and selective COX-2 inhibitor compounds were 11 and 6, with selectivity indices of 175 and 129.21 in comparison to 31.52 of the standard celecoxib. However, candidate 14 showed a very promising antiinflammatory activity with an IC50 of 6.00, while celecoxib had an IC50 of 14.50. Our findings are promising in the area of medicinal chemistry for further optimization of the newly designed and synthesized compounds regarding the discussed structure-activity relationship study (SAR), in order to obtain a superior antioxidant lead compound in the near future. All chemical structures of the novel synthesized candidates were unequivocally elucidated and confirmed utilizing spectroscopic and elemental investigations.

A New Reaction Mode of 3-Halooxindoles: Acting as C-C-O Three-Atom Components for (3+3) Cycloaddition to Access Indolenine-Fused 2H-1,4-Oxathiines

Herein, we report an unprecedented implementation of 3-halooxindoles as C-C-O three-atom components for (3+3) cycloaddition with pyridinium 1,4-zwitterionic thiolates, affording structurally diverse indolenine-fused 2H-1,4-oxathiines in moderate to high yields. A combined experimental and computational mechanistic study suggests that the reaction proceeds through addition of a S conjugate to the o-azaxylylene intermediate, followed by O-Michael addition and a sequential retro-Michael addition/pyridine extrusion pathway.

A Sequential Transition Metal and Organocatalytic Approach to the Enantioselective Synthesis of C2-Spiroindoline Systems

We report herein an organocatalyzed enantioselective spirocyclization strategy to access valuable C2-spiroindoline scaffolds bearing a quaternary stereocenter via an aza-Michael addition reaction, wherein the acid additive plays the role of dual functionality. The substrates for this key step were put together by an exo-selective, Pd-catalyzed γ-arylation of silyldienol ethers of the corresponding cyclohexenones. A close alliance between a low catalyst loading and a slow reaction rate yields C2-spiroindolines with good enantioselectivity.

Dearomative Spirocyclization of Tryptamine-Derived Isocyanides via Iron-Catalyzed Carbene Transfer

Tryptamine-derived isocyanides are valuable building blocks in the construction of spirocyclic indolenines and indolines via dearomatization of the indole moiety. We report the Bu4N[Fe(CO)3NO]-catalyzed carbene transfer of α-diazo esters to 3-(2-isocyanoethyl)indoles, leading to ketenimine intermediates that undergo spontaneous dearomative spirocyclization. The utility of this iron-catalyzed carbene transfer/spirocyclization cascade was demonstrated by its use as a key step in the formal total synthesis of monoterpenoid indole alkaloids (±)-aspidofractinine, (±)-limaspermidine, (±)-aspidospermidine, and (±)-17-demethoxy-N-acetylcylindrocarine.

Diversity-Oriented Catalytic Asymmetric Dearomatization of Indoles with o-Quinone Diimides

Herein, the first diversity-oriented catalytic asymmetric dearomatization of indoles with o-quinone diimides (o-QDIs) is reported. The catalytic asymmetric dearomatization (CADA) of indoles is one of the research focuses in terms of the structural and biological importance of dearomatized indole derivatives. Although great achievements have been made in target-oriented CADA reactions, diversity-oriented CADA reactions are regarded as more challenging and remain elusive due to the lack of synthons featuring multiple reaction sites and the difficulty in precise control of chemo-, regio-, and enantio-selectivity. In this work, o-QDIs are employed as a versatile building block, enabling the chemo-divergent dearomative arylation and [4 + 2] cycloaddition reactions of indoles. Under the catalysis of chiral phosphoric acid and mild conditions, various indolenines, furoindolines/pyrroloindolines, and six-membered-ring fused indolines are collectively prepared in good yields with excellent enantioselectivities. This diversity-oriented synthesis protocol enriches the o-quinone chemistry and offers new opportunities for CADA reactions.

Base-Promoted Formal (3 + 2) Cycloaddition of α-Halohydroxamates with Electron-Deficient Alkenyl-iminoindolines To Synthesize Spiro-indolinepyrrolidinones

Construction of pyrrolidinyl-spiroindoles with easily available starting materials has attracted considerable attention from the synthesis community and is in great demand. Here, we describe a base-promoted formal (3 + 2) cycloaddition of α-halohydroxamates with alkenyl-iminoindolines. The present methodology features mild reaction conditions and a broad substrate scope with up to 99% yield and excellent diastereoselectivity. The versatility of this approach is demonstrated through valuable synthetic transformations. Preliminary mechanistic studies shed light on the mechanism of this cycloaddition process.

Tandem Oxidative Dearomatizations of Diphenylanthracene Atropisomers

The first examples of tandem oxidative dearomatizations of 9,10-diphenylanthracene atropisomers with ortho,ortho'- formyl substituents are presented. In the presence of KMnO4, their stereoselective tandem double oxidation and spirocyclization mainly afford the syn or anti dearomatized 9,10-diphthalide anthracenes. Using Pinnick's reagent and depending on the conditions, the oxidation can mainly lead to the corresponding syn or anti diacids in good yields or to three oxidation products. An unprecedented further oxidative ring expansion toward dibenzo[b,e]oxepines is also reported.

Palladium-Catalyzed Carbonylative Dearomatization of Indoles to Achieve Carbonyl-Containing Spirocyclic Indolenines Bearing an All-Carbon Quaternary Center

A Pd-catalyzed carbonylative dearomatization via an acyl Pd complex has been developed. Diversified carbonyl-containing spirocyclic indolenines with an all-carbon quaternary center were constructed in an efficient and straightforward way with good to excellent yields. The protocol features a simple catalytic system, operational simplicity, a broad substrate scope, easy scale-up, and versatile transformations. In addition, the asymmetric reaction was initially explored with moderate enantioselectivity.

Electrochemical Promoted Three-Component Trifluoromethylation/Spirocyclization Reaction of N-Arylsulfonylacrylamides to 4-Azaspiro[4.5]decanes

An electrochemical facilitated three-component trifluoromethylation/spirocyclization reaction of N-(arylsulfonyl)acrylamides, CF3SO2Na, and H2O has been developed. Without the requirement of chemical oxidants, a number of unexplored trifluoromethylated 4-azaspiro[4.5]decanes were obtained in satisfactory yields under mild conditions. This work provides a new synthetic strategy for fluorine-containing spirocyclic compounds and shows a new perspective for the reactivity study of N-(arylsulfonyl)acrylamides.

Reactivity of ethyl nitrosoacrylate toward pyrrole, indole and pyrrolo[3,2-c]carbazole: an experimental and theoretical study

Nitrosoalkenes react with 8-methyl-1,6-dihydropyrrolo[3,2-c]carbazole to give both 2- and 3-alkylated products via hetero-Diels-Alder reaction followed by the cycloadduct ring-opening. Quantum chemical calculations, at DFT level of theory, were carried out to investigate the regioselectivity of the cycloaddition of ethyl nitrosoacrylate with 1,6-dihydropyrrolo[3,2-c]carbazoles as well as with pyrrole and indole, allowing a more comprehensive analysis of the reactivity pattern of nitrosoalkenes with five-membered heterocycles. Furthermore, theoretical calculations confirmed that ethyl nitrosoacrylate reacts with these heterocycles via a LUMOheterodiene-HOMOdienophile controlled cycloaddition. The reactivity of one of the oxime-functionalized 1,6-dihydropyrrolo[3,2-c]carbazole was explored and a new hexahydropyrido[4',3':4,5]pyrrolo[3,2-c]carbazole system was obtained in high yield via a one-pot, two-step procedure.

Translating Planar Heterocycles into Three-Dimensional Analogs by Photoinduced Hydrocarboxylation

The rapid preparation of complex three-dimensional (3D) heterocyclic scaffolds is a key challenge in modern medicinal chemistry. Despite the increased probability of clinical success for small molecule therapeutic candidates with increased 3D complexity, new drug targets remain dominated by flat molecules due to the abundance of coupling reactions available for their construction. In principle, heteroarene hydrofunctionalization reactions offer an opportunity to transform readily accessible planar molecules into more three-dimensionally complex analogs through the introduction of a single molecular vector. Unfortunately, dearomative hydrofunctionalization reactions remain limited. Herein, we report a new strategy to enable the dearomative hydrocarboxylation of indoles and related heterocycles. This reaction represents a rare example of a heteroarene hydrofunctionalization that meets the numerous requirements for broad implementation in drug discovery. The transformation is highly chemoselective, broad in scope, operationally simple, and readily amenable to high-throughput experimentation (HTE). Accordingly, this process will allow existing libraries of heteroaromatic compounds to be translated into diverse 3D analogs and enable exploration of new classes of medicinally relevant molecules.

Palladium-Catalyzed Dearomatization of Indoles with Alkynes: Construction of Spirocyclohexaneindolenines

A palladium-catalyzed dearomatization of indoles with alkynes has been developed, providing an efficient route to access a variety of synthetically useful spirocyclohexaneindolenines in moderate to good yields. The current method features a simple catalytic system, operational simplicity, and good functional group compatibility, which will contribute substantially to the development of dearomatization to access spiro compounds. Besides, the ubiquitous existence of spiro molecules, including spirocyclohexaneindolenines, in drugs and biological active molecules suggests the potential application of this methodology in medicinal chemistry.

Gold(I)-Catalyzed Substitution-Controlled Syntheses of Spiro[indoline-3,3'-pyrrolidine] and Spiro[indoline-3,3'-piperidine] Derivatives

Spiro[indoline-3,3'-pyrrolidine] and spiro[indoline-3,3'-piperidine] derivatives were synthesized in a substitution-controlled manner under the catalysis of cationic gold(I) species in the presence of Hantzsch ester (HEH). The optimal reaction condition was determined by screening, and the functional group tolerances of these two pathways were examined by readily synthetic substrates. The endo and exo selectivities of these cyclizations were elucidated by density functional theory calculations, and a plausible mechanism for these transformations was proposed.

Iodine(III)-Mediated C-C Bond Coupling to Construct Spirocyclic Indolenines of Various Ring Sizes

Herein, we report a novel iodine(III)-mediated intramolecular dearomative spirocyclization of indole derivatives to generate highly strained spirocyclobutyl, spirocyclopentyl, and spirocyclohexyl indolenines in moderate to good yields. A set of structurally novel, densely functionalized spiroindolenines with broad functional group compatibility was efficiently constructed in this way under mild reaction conditions. Moreover, the β-enamine ester as a versatile functional group in the product provides great convenience for the synthesis of bioactive compounds and related natural products.

Quantum Mechanical Prediction and Experimental Verification of Au(I)-Catalyzed Substitution-Controlled Syntheses of 1H-Pyrido[4,3-b]indole and Spiro[indoline-3,3'-pyridine] Derivatives

Density functional theory calculations were applied to predict the pathways of gold(I)-catalyzed cycloisomerization of the indole substrates with 1,6-enynes, which were consistent with the ensuing experimental results. The substitution-controlled synthesis led to the formation of 1H-pyrido[4,3-b]indole and spiro[indoline-3,3'-pyridine] derivatives in a tunable way. The reactions had good functional group tolerances, and a possible mechanism was proposed based on the computational and experimental results.

Computational insight into gold(I)-catalyzed intramolecular regioselectivity of tryptamine-ynamide cycloisomerizations

The regioselectivity for gold(I)-catalyzed intramolecular cycloisomerizations of tryptamine-ynamides has long been elusive despite various synthetic examples of similar substrates being available. Computational studies were carried out to provide insight into the mechanisms and the origin of the substrate-dependent regioselectivity of these transformations. Based on the analyses of non-covalent interactions, distortion/interaction, and energy decomposition on the interactions between the terminal substituent of alkynes and the gold(I) catalytic ligand, the electrostatic effect was determined to be the key factor for α-position selectivity while the dispersion effect was determined to be the key factor for β-position selectivity. Our computational results were consistent with the experimental observations. This study provides useful guidance for understanding other similar gold(I)-catalyzed asymmetric alkyne cyclization reactions.

Synthesis, pharmacological evaluation, DFT calculation, and theoretical investigation of spirocyclohexane derivatives

Polycyclic structures fused at a central carbon are of great interest due to their appealing conformational features and their structural implications in biological systems. Although progress in the development of synthetic methodologies toward such structures has been impressive, the stereo selective construction of such quaternary stereo centers remains a significant challenge in the total synthesis of natural products. From the computational calculations by Density Functional Theory along with the B3LYP as basis set, It is obvious that the all studied compounds are soft molecules and η varied from 0.069 for compound (10) to 0.087 for compound (15), while the compound (14) is treated as hard molecule, the value of η is 0.102, also the electronic transition within the soft compounds is easy as indicated from the △E, the compound (10) is absolute soft according to the (σ = 14.49 eV), while the compound (14) is treated as hard compounds (σ = 9.804 eV). The newly formed compounds exhibited both anti-inflammatory and antioxidant activities on HRBC homolytic and membrane stabilization and DPPH scavenging percent, respectively.

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

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

PIFA-mediated selenylative spirocyclization of indolyl ynones: facile access to selenated spiro[cyclopentenone-1,3'-indoles]

A fast selenylative spirocyclization of indolyl ynones mediated by PIFA has been developed. This transformation was enabled by the reactive RSeOCOCF₃ species generated in situ from diselenides with PIFA, involving an electrophilic dearomative cascade cyclization. This protocol provides a facile and efficient method for the synthesis of selenated spiro[cyclopentenone-1,3'-indoles] and tolerates broad functional groups.

Energy-Transfer-Enabled Dearomative Cycloaddition Reactions of Indoles/Pyrroles via Excited-State Aromatics

Exploring the enormous chemical space through an expedient building-up of molecular diversity is an important goal of organic chemistry. The development of synthetic methods toward molecules with unprecedented structural motifs lays the foundation for wide applications ranging from pharmaceutical chemistry to materials science. In this regard, the dearomatization of arenes has been recognized as a unique strategy since it provides novel retrosynthetic disconnections for various spiro or fused polycyclic molecules with increased saturation and stereoisomerism. However, inherent thermodynamic challenges are associated with dearomatization processes. The disruption of the aromaticity of arene substrates usually requires large energy inputs, which makes harsh conditions necessary for many ground-state dearomatization reactions. Therefore, further expansion of the scope of dearomatization reactions remains a major problem not fully solved in organic chemistry.The past decade has witnessed tremendous progress on photocatalytic reactions under visible light. Particularly, reactions via an energy transfer mechanism have unlocked new opportunities for dearomatization reactions. Mediated by appropriately chosen photosensitizers, aromatic substrates can be excited. This kind of precise energy input might make feasible some dearomatization reactions that are otherwise unfavorable under thermal conditions because of the significant energy increases of the substrates. Nevertheless, the lifetimes of key intermediates in energy-transfer-enabled reactions, such as excited-state aromatics and downstream biradical species, are quite short. How to regulate the reactivities of these transient intermediates to achieve exclusive selectivity toward a certain reaction pathway among many possibilities is a crucial issue to be addressed.Since 2019, our group has reported a series of visible-light-induced dearomative cycloaddition reactions for indole and pyrrole derivatives. It was found that the aromatic units in substrates can be excited under the irradiation of visible light in the presence of a suitable photosensitizer. These excited aromatics readily undergo various [m + n] cycloaddition reactions with appropriately tethered unsaturated functionalities including alkenes, alkynes, N-alkoxy oximes, (hetero)arenes, and vinylcyclopropanes. The reactions yield polycyclic indolines and pyrrolines with highly strained small- and/or medium-sized rings embedded, some of which possess unique bridge- or cagelike topologies. Systematic mechanistic studies confirmed the involvement of an energy transfer process. Density functional theory (DFT) calculations revealed the correlation between the substrate structure and the excitation efficiency, which accelerated the optimization of the reaction parameters. Meanwhile, DFT calculations demonstrated the competition between kinetically and thermodynamically controlled pathways for the open-shell singlet biradical intermediates, which allowed the complete switches from [2 + 2] cycloaddition to 1,5-hydrogen atom transfer in reactions with N-alkoxy oximes and to [4 + 2] cycloaddition in reactions with naphthalene. Furthermore, ab initio molecular dynamics (AIMD) simulations uncovered post-spin crossing dynamic effects, which determine the regioselectivity for the open-shell singlet biradical recombination step in the reactions of pyrrole-derived vinylcyclopropanes.An increasing number of scientists have joined in the research on visible-light-induced dearomative cycloaddition reactions and contributed to more elegant examples in this area. The visible-light-induced dearomatization reaction via energy transfer mechanism, although still in its infancy, has exhibited great potential in the synthesis of molecules that can hardly be accessed by other methods. We believe that future development will further push the boundary of organic chemistry and find applications in the synthesis of functional molecules and related disciplines.

Diastereoselective Synthesis of Indoline- and Pyrrole-Embedded Tetracycles via an Unprecedented Dearomative Indole-C3-Alkylation/Aza-Friedel-Crafts Cascade Reaction

Dearomative indole C3-alkylation─intramolecular iminium trapping cascade reaction of indole-C3-tethered nucleophiles is a well-known blueprint for accessing 2,3-fused indolines. In exploring this strategy, synthetic chemists have utilized diverse classes of electrophilic reagents. However, the tethered nucleophiles have mainly been limited to heteronucleophiles and enolates; exploitation of tethered arenes/heteroarenes remains unknown. We herein describe the first examples of pyrrole-intercepted dearomative indole C3-allylation and benzylation of indole-tethered pyrroles toward the synthesis of 2,3-cis-fused tetracyclic indolines featuring a C3 all-carbon quaternary stereocentre. Our methodology capitalizes on the capability of NaO^tBu/Et₃B combination to direct the intermolecular alkylation to take place regioselectively at the indole C3 position over the other reactive sites (indole N and C2 and pyrrole C2 positions) and leverages the high nucleophilicity of the pyrrole template for the concomitant aza-Friedel-Crafts ring closure that traditionally would require an additional acid-catalyzed synthetic step. This cascade reaction is accomplished with broad substrate scope and excellent yields and chemo-, regio-, and diastereoselectivities.

Highly congested spiro-compounds via photoredox-mediated dearomative annulation cascade

Photo-mediated radical dearomatization involving 5-exo-trig cyclizations has proven to be an important route to accessing spirocyclic compounds, whereas 6-exo-trig spirocyclization has been much less explored. In this work, a dearomative annulation cascade is realized through photoredox-mediated C-O bond activation of aromatic carboxylic acids to produce two kinds of spirocyclic frameworks. Mechanistically, the acyl radical is formed through oxidation of triphenylphosphine and subsequent C-O bond cleavage, followed by a 6-exo-trig cyclization/SET/protonation sequence to generate the spiro-chromanone products in an intramolecular manner. Furthermore, the protocol was extended to more challenging intermolecular tandem sequences consisting of C-O bond cleavage, radical addition to an alkene substrate, and 5-exo-trig cyclization to yield complex spirocyclic lactams.

Carbodefluorination of fluoroalkyl ketones via a carbene-initiated rearrangement strategy

The C-F bond cleavage and C-C bond formation (i.e., carbodefluorination) of readily accessible (per)fluoroalkyl groups constitutes an atom-economical and efficient route to partially fluorinated compounds. However, the selective mono-carbodefluorination of trifluoromethyl (CF3) groups remains a challenge, due to the notorious inertness of C-F bond and the risk of over-defluorination arising from C-F bond strength decrease as the defluorination proceeds. Herein, we report a carbene-initiated rearrangement strategy for the carbodefluorination of fluoroalkyl ketones with β,γ-unsaturated alcohols to provide skeletally and functionally diverse α-mono- and α,α-difluoro-γ,δ-unsaturated ketones. The reaction starts with the formation of silver carbenes from fluoroalkyl N-triftosylhydrazones, followed by nucleophilic attack of a β,γ-unsaturated alcohol to form key silver-coordinated oxonium ylide intermediates, which triggers selective C-F bond cleavage by HF elimination and C-C bond formation through Claisen rearrangement of in situ generated difluorovinyl ether. The origin of chemoselectivity and the reaction mechanism are determined by experimental and DFT calculations. Collectively, this strategy by an intramolecular cascade process offers significant advances over existing stepwise strategies in terms of selectivity, efficiency, functional group tolerance, etc.

Unbiased C3-Electrophilic Indoles: Triflic Acid Mediated C3-Regioselective Hydroarylation of N-H Indoles

The direct dearomative addition of arenes to the C3 position of unprotected indoles is reported under operationally simple conditions, using triflic acid at room temperature. The present regioselective hydroarylation is a straightforward manner to generate an electrophilic indole at the C3 position from unbiased indoles in sharp contrast to previous strategies. This atom-economical method delivers biologically relevant 3-arylindolines and 3,3-spiroindolines in high yields and regioselectivities from both intra- and intermolecular processes. DFT computations suggest the stabilization of cationic or dicationic intermediates with H-bonded (TfOH)n clusters.

Dearomatizing Cyclization of 2-Iodoindoles by Oxidative NHC Catalysis to Access Spirocyclic Indolenines and Oxindoles Bearing an All Carbon Quaternary Stereocenter

An intramolecular dearomatizing spirocyclization of indoles by oxidative N-heterocyclic carbene catalysis is reported. C2-iodinated indoles are used as substrates in combination with aroyl azolium ions as acceptors, which provides C2-iodinated indolenines containing an all carbon quaternary stereocenter. The products are readily further C2-functionalized and give access to valuable oxindoles by simple hydrolysis in very good overall yields and excellent enantioselectivities.

Synthesis of spiroindolenine-cyclopentenedione skeletons and their chemical behaviours: the first example of a lactone-type spiroindolenine structure

A manageable, one-pot, and high-yield protocol for synthesising highly reactive spiroindolenine derivatives is reported. Spiroindolenines are furnished by a reaction between DCC (dicyclohexylcarbodiimide) and indole-3-butenoic acid derivatives. The protocol proposed here involves the construction of a carbon-carbon bond through intramolecular domino cyclisation. The reaction mechanism for spirocyclisation is discussed; both NMR and X-ray analysis were used to verify the structure of spiroindolenine. The applied strategy allowed the formation of spiroindolenine with a dione substructure, which is an unknown compound with a spirocyclic nucleus. Further reactions of spiroindolenines with di-amines, a primary amine, and alcohol have been reported, and new types of indole derivatives, such as indoloquinoxalines, where the spirocentre atom undergoes a nucleophilic attack, are yielded.

Sulphur ylide-mediated cyclopropanation and subsequent spirocyclopropane rearrangement reactions

The efficient construction of cyclopropyl spiroindoline skeletons and the exploration of related follow-up synthetic transformations have elicited considerable interest amongst members of the chemistry community. Here, we describe a formal (2 + 1) annulation and three-component (1 + 1 + 1) cascade cyclisation via sulphur ylide cyclopropanation under mild conditions. The spiro-cyclopropyl iminoindoline moiety can be readily transformed into another medicinally interesting pyrrolo[3,4-c]quinoline framework through a novel rearrangement process.

Ag(I)/PPh3-catalyzed diastereoselective syntheses of spiro[indole-3,4'-piperidine] derivatives via cycloisomerizations of tryptamine-ynamides

A Ag(I)/PPh₃-catalyzed chelation-controlled cycloisomerization of tryptamine-ynamide was developed to access the spiro[indole-3,4'-piperidine] scaffold in a racemic and diastereoselective manner. The diastereoselective products were achieved by a chiron approach. Density functional theory (DFT) calculations indicated that strong non-covalent effects between the substrate and catalyst/ligand complex stabilized the spiroindoleninium intermediate via cation-π-π interactions.