Development of Synthetic Methodologies via Catalytic Enantioselective Synthesis of 3,3-Disubstituted Oxindoles

Modular synthesis of 3,3-disubstituted oxindoles from nitrones and acrylic acids

We developed a modular synthesis for 3,3-disubstituted oxindoles, utilising readily accessible nitrones and acrylic acids. This approach facilitates the preparation of a diverse array of oxindoles through the variation of the starting materials. We demonstrated the applicability of this method through a gram-scale reaction and a synthesis of esermethole.

Copper-Catalyzed Tunable Oxygenative Rearrangement of Tetrahydrocarbazoles

Herein, we report a general copper-catalyzed method for the tunable oxygenative rearrangement of tetrahydrocarbazoles to cyclopentyl-bearing spiroindolin-2-ones and spiroindolin-3-ones. The method demonstrates excellent chemoselectivity, regioselectivity, and product control simply by using the H2O and O2 as oxygen source, respectively. This open-flask method is safe and simple to operate, and no other chemical oxidants are required. Besides, inspired from the unique pathway of 1, 2-migration rearrangement, a highly controllable hydroxylation of indoles for the construction of C3a-hydroxyl iminium indolines was also developed. Mechanistic experiments suggest that a single-electron transfer-induced oxidation process is responsible for the tunable selectivity control.

Metal-Free Switchable Chemo- and Regioselective Alkylation of Oxindoles Using Secondary Alcohols

In this study, we have disclosed N-alkylation and C-alkylation reactions of 2-oxindoles with secondary alcohols. Interestingly, these chemoselective reactions are tunable by changing the reaction conditions. Utilization of protic solvent and Brønsted acid catalyst afforded C-alkylation, whereas, aprotic solvent and Lewis acid catalyst afforded N-alkylation of 2-oxindoles in good to excellent yields. Regioselectivity is achieved by protecting the N-center of the oxindole and C5 alkylated product is furnished exclusively. This protocol is notable because it demonstrates functionalization at the C7 position of oxindole without the need for any directing group at the N-center. Further, a new protocol has been reported for C-H oxygenation at the benzylic position of one of the C5 alkylated derivative.

Organocatalyzed Asymmetric Michael Addition of 3-Fluorooxindole to Vinylidene Bisphosphonates

Both the 3-fluorooxindole and germinal bisphosphonate structural motifs are prevalent in bioactive molecules because of their associated biological activities. We describe an approach to accessing 3,3-disubstituted 3-fluorooxindoles bearing a geminal bisphosphate fragment through a highly enantioselective Michael addition reaction between 3-fluorooxindoles and vinylidene bisphosphonates. These reactions are catalyzed by a commercially available cinchona alkaloid catalyst, have a broad substrate scope concerning 3-fluorooxindoles, and provide the corresponding addition products in a yield of up to 95% with an enantiomeric excess of up to 95%. A reasonable reaction pathway to explain the observed stereochemistry is also proposed.

Asymmetric magnesium catalysis for important chiral scaffold synthesis

Magnesium catalysts are widely used in catalytic asymmetric reactions, and a series of catalytic strategies have been developed in recent years. Herein, in this review, we have tried to summarize asymmetric magnesium catalysis for the synthesis of important chiral scaffolds. Several important optically active motifs that are present in classic chiral ligands or natural products synthesized by Mg(II) catalytic methods are briefly discussed. Moreover, the representative mechanisms for different magnesium catalytic strategies, including in situ generated magnesium catalysts, are also shown in relation to synthetic routes for obtaining these important chiral scaffolds.

Cu(II)-Catalyzed Enantioselective Aza-Friedel-Crafts Reaction of 1-Naphthols and Electron-Rich Phenols with Isatin-Derived Ketimines

New chiral ligands could be obtained by introducing proline moieties and imidazoline moieties to binaphthyl skeletons. The chiral ligands exhibited balanced rigidity and flexibility which could allow the change of the conformations during the reactions on one hand, and could provide sufficient asymmetric induction on the other. The proline moiety could act as a linker connecting the binaphthyl skeletons and the imidazoline moieties as well as a coordinating group for the central metal, and the electronic and steric properties of the imidazoline groups could be carefully fine-tuned by the use of different substituents. In the presence of Cu(II) catalyst bearing such chiral ligands, aza-Friedel-Crafts reaction of 1-naphthols and electron-rich phenols with isatin-derived ketimines provided the desired products with good to excellent yields and up to 99 % ee. The reactions showed good scalability, and excellent ee could still be obtained when the reaction was carried out in gram-scale.

Nucleophilic Organocatalyst for Photochemical Carbon Radical Generation via SN2 Substitution

Photochemical generation of radicals is a powerful way to construct various molecules. But most of these methods rely on initiators or the redox properties of radical precursors. Herein, we report a photochemical organic catalyst that reacts with benzyl halide to generate carbon radical via an SN2 pathway. This nucleophilic catalyst can be easily prepared and is bench-stable. The SN2 process does not rely on the redox properties of halides, showing potential synthetic utility. Control experiments and UV-vis spectroscopic analysis indicate that the SN2 substitution adduct is the key intermediate.

Visible-light-enabled stereoselective synthesis of functionalized cyclohexylamine derivatives via [4 + 2] cycloadditions

An unprecedented intermolecular [4 + 2] cycloaddition of benzocyclobutylamines with α-substituted vinylketones, enabled by photoredox catalysis, has been developed. The current method enables facile access to highly functionalized cyclohexylamine derivatives that were otherwise inaccessible, in moderate to good yields with excellent diastereoselectivities. This protocol has some excellent features, such as full atom economy, good functional-group compatibility, mild reaction conditions, and an overall redox-neutral process. Additionally, an asymmetric version of this cycloaddition was preliminarily investigated via the incorporation of a chiral phosphoric acid (CPA), and moderate to good enantioselectivity could be effectively realized with excellent diastereoselectivity. Synthetic applications were demonstrated via a scale-up experiment and elaborations to access amino alcohol and cyclobutene derivatives. Based on the results of control experiments, a reasonable reaction mechanism was proposed to elucidate the reaction pathway.

Tandem Vinylogous Aldol and Intramolecular [2 + 2] Cycloaddition toward Benzocyclobutenes by UV Light Photocatalysis

A facile and efficient radical tandem vinylogous aldol and intramolecular [2 + 2] cycloaddition reaction for direct synthesis of cyclobutane-containing benzocyclobutenes (BCBs) under extremely mild conditions without using any photocatalysts is reported. This approach exhibited definite compatibility with functional groups and afforded new BCBs with excellent regioselectivity and high yields. Moreover, detailed mechanism studies were carried out both experimentally and theoretically. The readily accessible, low-cost, and ecofriendly nature of the developed strategy will endow it with attractive applications in organic and medicinal chemistry.

Asymmetric Tandem Michael Addition/Interrupted Nef Reactions of Nitromethane with Oxindole-Derived Alkenes: Enantioselective Synthesis of Spiro-polycyclic Oxindoles

Chiral spiro-polycyclic oxindoles are valuable heterocyclic ring systems that are widely distributed in natural alkaloids and biologically active compounds. Herein, we reported an asymmetric tandem Michael addition/interrupted Nef reaction of nitromethane with oxindole-derived alkenes catalyzed by a chiral 2-aminobenzimidazole bifunctional organocatalyst. A series of novel enantiomerically enriched spiro-polycyclic oxindole derivatives bearing an oxime group were synthesized in moderate to excellent isolated yields (up to 99%) with an excellent level of enantioselectivities (up to 99% ee). Furthermore, the antiproliferation activity of the resulting oxindoles derivatives were evaluated, and compound 2d demonstrated promising anticancer properties against HCT116 (IC50 = 14.08 μM) and HT29 (IC50 = 15.46 μM) cell lines.

Asymmetric Synthesis of Hydroindoles via Desymmetrizing [3+2] Annulation of p-Quinamines and Arylalkylketenes

The asymmetric desymmetrizing [3+2] annulation reaction of p-quinamines and arylalkylketenes to synthesize hydroindoles was realized. Catalyzed by chiral bisguanidinium hemisalt via multiple hydrogen bond interactions, enantiomerically enriched products with reversal of diastereoselectivity in comparison with the racemic version were afforded in good yields under mild reaction conditions. Diaryl-substituted hydroindoles could also perform the Friedel-Crafts type of addition to give more complicated multicycles. Density functional theory calculations revealed that the enantio- and diastereoselectivity stem from varied hydrogen-bonding manners.

Enantioselective Phase-Transfer-Catalyzed Synthesis of Spirocyclic Azetidine Oxindoles

Spiro-3,2'-azetidine oxindoles combine two independently important pharmacophores in an understudied spirocyclic motif that is attractive for medicinal chemistry. Here, the enantioselective synthesis of these structures is achieved in up to 2:98 er through intramolecular C-C bond formation, involving activation of the substrate with a novel SF5-containing chiral cation phase-transfer (PT) catalyst. The products are readily elaborated/deprotected to afford medicinally relevant enantioenriched compounds. Control experiments suggest an interfacial PT mechanism, whereby catalytic asymmetric induction is achieved through the activation of the chloride leaving group.

Synthesis of novel organophosphorus compounds via reaction of substituted 2-oxoindoline-3-ylidene with acetylenic diesters and triphenylphosphine or triphenyl phosphite

An efficient reaction between triphenylphosphine or triphenyl phosphite and 2-oxoindoline-3-ylidene derivatives in the presence of acetylenic esters leads to functionalized 2-oxoindoline-3-ylidene containing phosphorus ylieds or phosphonate esters. All compounds obtained in these reactions are stable and have good yields.

Pd-Catalyzed Enantioselective Creation of All-Carbon Quaternary Center with 2,3-Allenylic Carbonates

Enantioselective construction of all-carbon quaternary centers has been achieved via the palladium-catalyzed highly enantioselective allenylation of oxindoles with 2,3-allenylic carbonates to afford a variety of optically active allene products, which contain oxindole units with different functional groups, in high ee. The corresponding synthetic applications have also been demonstrated.

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.

Cu-Catalyzed Direct Enantioselective Vinylogous Mannich Reaction between β,γ-Unsaturated Pyrazoleamides and Ketimines

A catalytic asymmetric vinylogous Mannich-type reaction between β,γ-unsaturated amides and ketimines has been developed in excellent regio-, diastereo-, and enantioselectivities. The methodology provides an efficient approach to construct chiral homoallylic amines with a 3-amino-2-oxindole scaffold. Moreover, the transformations of the chiral products, including the removal of the pyrazole group or Boc group, the reduction of the C-C double bond, and Suzuki coupling, have been investigated.

Dinuclear Zn-Catalytic System as Brønsted Base and Lewis Acid for Enantioselectivity in Same Chiral Environment

Zinc (Zn) is a crucial element with remarkable significance in organic transformations. The profusion of harmless zinc salts in the Earth's outer layer qualifies zinc as a noteworthy contender for inexpensive and eco-friendly reagents and catalysts. Recently, widely recognized uses of organo-Zn compounds in the field of organic synthesis have undergone extensive expansion toward asymmetric transformations. The ProPhenol ligand, a member of the chiral nitrogenous-crown family, exhibits the spontaneous formation of a dual-metal complex when reacted with alkyl metal (R-M) reagents, e.g., ZnEt2. The afforded Zn complex possesses two active sites, one Lewis acid and the other Brønsted base, thereby facilitating the activation of nucleophiles and electrophiles simultaneously within the same chiral pocket. In this comprehensive analysis, we provide a thorough account of the advancement and synthetic potential of these diverse catalysts in organic synthesis, while emphasizing the reactivity and selectivities, i.e., dr and ee due to the design/structure of the ligands employed.

Formation of bis-spiropyrrolidines from isatin, secondary amines, and alkylidene Meldrum's acids

A catalyst-free synthesis of stable bis-spiropyrrolidines from isatin, secondary amines, and alkylidene Meldrum's acids in MeCN in 75-95% yield is described. The antioxidant and antimicrobial properties of the synthesized compounds are investigated. For this purpose, the radical scavenging activities of four derivatives were studied by radical trapping of diphenylpicrylhydrazine and ferric reduction power experiments. Disk diffusion test on Gram-positive and Gram-negative bacteria was employed to investigate antibacterial activities of five derivatives.

Copper/Iodine Co-catalyzed Oxygenative Transannulation of Tryptamines Enables Direct Synthesis of Donaxaridine and Its Derivatives

We report a general copper/iodine co-catalyzed oxygenative transannulation strategy using readily available tryptamines. Molecular oxygen and water are used as oxygen sources and provide direct access to the donaxaridine scaffold and its derivatives. This methodology is applied to the efficient synthesis of the natural products donaxaridine, chimonamidine, donaxanine, donaxarine, and aline in just one or two steps. The tryptamines, albeit with oxy-sensitive dialkyl N-H groups, are selectively oxidized through a single-electron transfer dioxygenation process.

K2S2O8-mediated direct C-H heteroarylation/hydroxylation of indolin-2-ones with quinoxalin-2(1H)-ones

Herein, a K2S2O8-mediated direct heteroarylation and hydroxylation reaction between quinoxalin-2(1H)-ones with a C(sp2)-H bond and indolin-2-ones with a C(sp3)-H bond via an oxidative cross-coupling reaction has been reported. We have successfully established a feasible and concise reaction system that represents the first example of free-radical-promoted heteroarylation and hydroxylation reaction on the C-3 position of oxindole. A series of 3-substituted 3-hydroxyoxindoles are obtained in 0-83% yield using this methodology.

Enantio- and Diastereodivergent N-Heterocyclic Carbene/Nickel Dual-Catalyzed Umpolung Propargylic Substitutions of Enals

The creation of full stereoisomers of an organic compound comprising multiple contiguous stereocenters with simultaneous control over both relative and absolute configurations remains a significant challenge in synthetic chemistry. Using a cooperative catalysis strategy, we established an N-heterocyclic carbene/nickel-catalyzed enantio- and diastereodivergent propargylation reaction to access 3,3'-disubstituted oxindoles, enabling the incorporation of internal alkyne functionality and the introduction of a single quaternary or vicinal quaternary/tertiary stereogenic center. By selecting the appropriate combination of catalyst chirality, all four potential stereoisomers of α-quaternary propargylated oxindoles were synthesized in a predictable and precise way with remarkable yields, diastereoselectivities, and enantioselectivities from identical starting materials. The synthetic utility of this method was demonstrated in the concise asymmetric total synthesis of (-)-debromoflustramine B and (-)-C(β-Me)-debromoflustramine B.

Synthesis of Spirooxindole-1,2-oxazinan-5-ones through 2,2,2-Trifluoroethanol Promoted [3 + 3] Cycloaddition of N-Vinyl Oxindole Nitrones and Oxyallyl Cations

A variety of spirooxindole-1,2-oxazinan-5-one derivatives were prepared in moderate to excellent yields through 2,2,2-trifluoroethanol (TFE)-promoted [3 + 3] cycloaddition of N-vinyl oxindole nitrones with oxyallyl cations generated from α-tosyloxy ketones under mild reaction conditions. Mechanistic studies revealed that [3 + 3] cycloaddition might involve two possible reaction pathways, including direct [3 + 3] cycloaddition of N-vinyl oxindole ntirones with oxyallyl cations, or the addition of TFE to N-vinyl oxindole nitrones, sequential addition to oxyallyl cations, elimination, and cyclization. The present method features mild reaction conditions, broad substrate scope, good functional group tolerance, easy gram scalable preparation, and new applications of TFE.

Late-Stage Chemo- and Enantioselective Oxidation of Indoles to C3-Monosubstituted Oxindoles

Catalytic asymmetric preparation of chiral 3-monosubstituted oxindoles represents a significant challenge in synthetic chemistry due to the ease of racemization of the tertiary stereocenter through enolization. Here, we describe a general titanium-catalyzed chemo- and enantioselective indole oxidation to produce a diverse set of chiral 3-monosubstituted oxindoles with up to 96% yield, 99% ee, and with a substrate/catalyst ratio of 10,000 by using the combination of a simple titanium(salan) catalyst with green and atom-economic terminal oxidant H2O2. The mild approach tolerates a broad range of functional groups, enabling late-stage asymmetric diversification of a series of commercial drugs and natural products together with late-stage asymmetric construction of a wide set of enzyme antagonists, all of which are difficult to achieve through existing methods.

Bifunctionality of dirhodium tetracarboxylates in metallaphotocatalysis

Metallaphotocatalysis has been recognized as a pivotal catalysis enabling new reactivities. Traditional metallaphotocatalysis often requires two or more separate catalysts and exhibits flaw in cost and substrate-tolerance, thus representing an await-to-solve issue in catalysis. We herein realize metallaphotocatalysis with a bifunctional dirhodium tetracarboxylate ([Rh2]) alone. The [Rh2] shows an photocatalytic activity of promoting singlet oxygen (1O2) oxidation. By harnessing its photocatalytic activity, the [Rh2] catalyzes a photochemical cascade reaction (PCR) via combination of carbenoid chemistry and 1O2 chemistry. The PCR is characterized by high atom-efficiency, excellent stereoselectivities, mild conditions, scalable synthesis, and pharmaceutically interesting products. DFT calculations-aided mechanistic study rationalizes the reaction pathway and interprets the origin of stereoselectivities of the PCR. The products show inhibitory activity against PTP1B, being promising in the treatment of type II diabetes and cancers. Overall, here we show the bifunctional [Rh2] merges Rh-carbenoid chemistry and 1O2 chemistry.

Construction of Enantioenriched Quaternary C-Cl Oxindoles through Palladium-Catalyzed Asymmetric Allylic Substitution with Chloroenolates

A palladium-catalyzed asymmetric chloroenolate allylation with vinyl benzoxazinanones under mild reaction conditions has been developed, affording a series of optically active 3,3-disubstituted oxindoles exhibiting a chloro-group and a linear aryl amino side chain in good yields with up to 96% ee. Versatile functional group tolerance on the benzene ring has been demonstrated, and the utility of this method was probed by a scale-up synthesis and highlighted by product derivatizations.

Bifunctional Thiourea-Catalyzed Enantioselective Aza-Friedel-Crafts Reaction of 3-Aminobenzofurans with Isatin-Derived Ketimines

A quinine-derived thiourea-promoted enantioselective aza-Friedel-Crafts reaction of 3-aminobenzofurans with isatin-derived ketimines is developed, providing a variety of 3-benzofuran-3-amino-2-oxindoles bearing a quaternary stereocenter with good to excellent yields (72-95%) and moderate to excellent enantioselectivities (48-97%). The synthetic potential of this concise and efficient protocol is revealed by gram-scale preparation and further transformation of the adduct to an optically pure spirocyclic oxindole.

Electrochemical Fe-catalysed radical cyclization for the synthesis of oxindoles

We report an efficient and sustainable electrocatalytic approach for the synthesis of 3,3-disubstituted 2-oxindoles bearing ester groups from readily accessible N-arylacrylamides and carbazates. The reaction proceeds through an electrochemical iron-catalyzed radical addition/cyclization sequence with a commercially available iron catalyst and carbazates as alkoxycarbonyl radical precursors. This mild and operationally simple method transforms a wide range of structurally diverse N-arylacrylamides into oxindole derivatives in good yields and can be smoothly scaled up for the preparation of synthetically valuable oxindoles that are key intermediates for the synthesis of natural products.

Enantioselective synthesis of 3-hydroxy- and 3-amino-3-alkynyl-2-oxindoles by the dimethylzinc-mediated addition of terminal alkynes to isatins and isatin-derived ketimines

A common protocol for enantioselective alkynylation of isatins and isatin-derived ketimines using terminal alkynes and Me2Zn in the presence of a catalytic amount of a chiral perhydro-1,3-benzoxazine with moderate to excellent enantioselectivity under mild reaction conditions is described. The additions to ketimines present a novel approach to chiral amines being derivatives of oxindoles. The reaction is broad in scope with respect to aryl- and alkyl-substituted terminal alkynes and isatin derivatives. In isatins, the alkynylation occurs at the Si face of the carbonyl group, whereas in the ketimine derivatives it occurs at the Re face of the imine.

Palladium-catalyzed Heck/aminocarbonylation of alkene-tethered carbamoyl chlorides with nitro compounds for the synthesis of carbamoyl-substituted oxindoles

A straightforward and efficient approach for the synthesis of carbamoyl-substituted oxindoles has been developed via a palladium-catalyzed Heck cyclization and reductive aminocarbonylation reaction of alkene-tethered carbamoyl chlorides with nitro compounds. The reaction showed good compatibility toward versatile functional groups, and both nitroarenes and nitroalkanes were well tolerated. Using Mo(CO)₆ as a solid CO source, without external reductants, a broad range of carbamoyl-substituted oxindoles were obtained in moderate to high yields.

Palladium-Catalyzed Annulative Coupling of Spirovinylcyclopropyl Oxindoles with p-Quinone Methides

Pd-catalyzed annulative coupling of spirovinylcyclopropyl oxindoles with p-quinone methides has been accomplished via cascade carbon-carbon bond formation to afford bis-spirooxindole scaffolds. The mild reaction conditions, diastereoselectivity, functional group diversity, post-synthetic transformations, and mechanistic studies using DFT calculations are the important practical features.

Catalyst-Assisted Selective Vinylation and Methylallylation of a Quaternary Carbon Center Using tert-Butyl Acetate

The In(OTf)₃-catalyzed α-vinylation of various hydroxy-functionalized quaternary carbon centers using in situ generated isobutylene from tert-butyl acetate is presented as a novel synthetic methodology. Moreover, tert-butyl acetate is a nonflammable feed stock and is a readily available source for the in situ production of vinyl substituents, as demonstrated by the vinylation reaction with quaternary hydroxy/methoxy compounds. Moreover, an excellent selectivity for methylallylation over vinylation was obtained with Ni(OTf)₂ as a catalyst. In the case of peroxyoxindole, methylallyl-functionalized 1,4-benzoxazin-3-one derivatives were formed through the sequential rearrangement of peroxyoxindole followed by the nucleophilic attack by isobutylene. The detailed mechanism for this reaction and rationalization for the selectivity are provided using kinetics and density functional theory studies.

Construction of Spiro[benzo[a]acridine-12,4'-imidazolidine]-2',5'-dione Derivatives via Ring-Opening and Recyclization of Isatins and C-OH Cleavage of 2-Naphthol

An efficient three-component reaction to access spiro[benzo[a]acridine-12,4'-imidazolidine]-2',5'-dione derivatives has been developed through the ring-opening and recyclization process of isatins and dehydroxylation of 2-naphthol, which is different from their conventional reaction modes. Experimental observations suggest that p-toluenesulfonic acid is the key factor that promotes the success of this synthetic strategy. The research provided a novel approach for the construction of spiro compounds from isatins and 2-naphthol in organic synthesis.

Substrate-Controlled Diastereo- and Enantiodivergent Synthesis of Bis-Spirocyclopropyloxindoles from Available Isatin as a Single Starting Material

The first diastereo- and enantiodivergent asymmetric synthesis of new bis-spirocyclopropyloxindole scaffolds has been accomplished from the readily available isatin as a single starting material. Four rel-(1R,2R,3R), rel-(1S,2S,3R), rel-(1R,2R,3S), and rel-(1S,2S,3S) configurations of desired products were constructed in excellent enantiopurity via a simple switch in substrates using the chiral auxiliary-controlled method. The absolute configuration of cycloadducts with three contiguous quaternary/tertiary stereogenic centers was confirmed through X-ray diffraction analysis. A facile synthesis of versatile precursor 3-chlorooxindoles was also introduced.

SO2ClF: A Reagent for Controllable Chlorination and Chlorooxidation of Simple Unprotected Indoles

Sulfuryl chlorofluoride was first employed as a versatile reagent for controllable chlorination and chlorooxidation of simple unprotected indoles. Three types of products including 3-chloro-indoles, 3-chloro-2-oxindoles, and 3,3-dichloro-2-oxindoles could be selectively obtained in good to excellent yields by switching the reaction solvents. The present method features easy-to-operate, broad substrate scope, and mild reaction conditions.

Asymmetric Rubottom-Type Oxidation Catalyzed by Chiral Calcium Phosphates

A highly efficient catalytic asymmetric Rubottom-type oxidation is described. Using meta-chloroperbenzoic acid (m-CPBA) as the oxidant and chiral calcium phosphate as the catalyst, the facile transformation enables direct hydroxylation of N-Boc oxindoles and β-ketoesters in high yields (up to 99 %) and in a highly enantioenriched fashion (up to >99 % ee). The application of the established method was demonstrated by the synthesis of a pharmaceutically important 3-hydroxyoxindole with excellent enantiocontrol.

NaI/PPh3-catalyzed visible-light-mediated decarboxylative radical cascade cyclization of N-arylacrylamides for the efficient synthesis of quaternary oxindoles

A practical NaI/PPh3-catalyzed decarboxylative radical cascade cyclization of N-arylacrylamides with redox-active esters is described, which is mediated by visible light irradiation. A wide range of substrates bearing different substituents and derived from ubiquitous carboxylic acids, including α-amino acids, were synthesized and examined under this very mild, efficient, and cost effective transition-metal-free synthetic method. These afforded various functionalized oxindoles featuring a C3 quaternary stereogenic center. Mechanistic experiments suggest a radical mechanism.

Pd-Catalyzed Cascade Intramolecular Heck Cyclization/Dearomatization of Indoles for the Construction of Two All-Carbon Quaternary Centers

A facile Pd-catalyzed cascade of intramolecular Heck cyclization/alkylpalladium activated dearomatization of aryl alkyne-tethered indole is described. In this single step two nonadjacent all-carbon quaternary centers, two nitrogen-containing heterocycles, and three C(sp²)-C(sp³) bonds are efficiently furnished. These products could also undergo 5-to-6 ring migration-expansion reaction under Brønsted-acid conditions to transform into the benzo[c]carbazole skeletons.

Diastereoselective Addition of PhSCF2SiMe3 to Chiral N-tert-Butanesulfinyl Ketimines Derived from Isatins: Synthesis of Enantioenriched gem-Difluoromethylenated Spiro-pyrrolidinyl and Spiro-piperidinyl Oxindoles

A convenient and efficient synthetic strategy to prepare enantioenriched gem-difluoromethylenated spiro-pyrrolidinyl and spiro-piperidinyl oxindoles is described. Fluoride-mediated diastereoselective nucleophilic addition of PhSCF₂SiMe₃ to chiral N-tert-butanesulfinyl ketimines derived from isatins was a key step and provided diastereomeric adducts, which were readily separable. Removal of the chiral sulfinyl group followed by structural manipulation afforded chiral gem-difluoromethylenated spiro-pyrrolidinyl and spiro-piperidinyl oxindoles.

Enantioselective Synthesis of Quaternary Oxindoles: Desymmetrizing Staudinger-Aza-Wittig Reaction Enabled by a Bespoke HypPhos Oxide Catalyst

This paper describes a catalytic asymmetric Staudinger-aza-Wittig reaction of (o-azidoaryl)malonates, allowing access to chiral quaternary oxindoles through phosphine oxide catalysis. We designed a novel HypPhos oxide catalyst to enable the desymmetrizing Staudinger-aza-Wittig reaction through the PIII/PV═O redox cycle in the presence of a silane reductant and an IrI-based Lewis acid. The reaction occurs under mild conditions, with good functional group tolerance, a wide substrate scope, and excellent enantioselectivity. Density functional theory revealed that the enantioselectivity in the desymmetrizing reaction arose from the cooperative effects of the IrI species and the HypPhos catalyst. The utility of this methodology is demonstrated by the (formal) syntheses of seven alkaloid targets: (-)-gliocladin C, (-)-coerulescine, (-)-horsfiline, (+)-deoxyeseroline, (+)-esermethole, (+)-physostigmine, and (+)-physovenine.

Pd-catalyzed Aerobic Cross-Dehydrogenative Coupling of Catechols with 2-Oxindoles and Benzofuranones: Reactivity Difference Between Monomer and Dimer

Persistent radicals, which are generated from 2-oxindole or benzofuranone dimers, are useful tools for designing the radical-based cross-coupling reaction to provide molecules containing a quaternary carbon. The persistent radical is accessible from both the dimer and monomer; however, the reactivity difference between these substrates for the oxidative cross-coupling reaction is not fully understood, most likely because of the mechanistic complexity. Here, we present details of an aerobic cross-dehydrogenative coupling (CDC) reaction using various monomers and catechols. UV-Vis analysis and mechanistic control experiments showed that the monomer is less reactive than the dimer under aerobic conditions. Our Pd(II)-BINAP-μ-hydroxo complex significantly improved the reactivity of the monomers for the aerobic CDC reaction with catechols, yielding results comparable to those of the corresponding dimer. The procedure, which enables the generation of the persistent radical in situ, is particularly useful when employing the monomer that is not readily converted to the corresponding dimer.

Synthesis of Carbonyl-Containing Oxindoles via Ni-Catalyzed Reductive Aryl-Acylation and Aryl-Esterification of Alkenes

Carbonyl-containing oxindoles are ubiquitous core structures present in many biologically active natural products and pharmaceutical molecules. Nickel-catalyzed reductive aryl-acylation of alkenes using aryl anhydrides or alkanoyl chlorides as acyl sources is developed, providing 3,3-disubstituted oxindoles bearing ketone functionality at the 3-position. Moreover, nickel-catalyzed reductive aryl-esterification of alkenes using chloroformate as ester sources is further developed, affording 3,3-disubstituted oxindoles bearing ester functionality at the 3-position. This strategy has the advantages of good yields and high functional group compatibility.

N-Heterocyclic Carbene-Catalyzed [3 + 2] Annulation of 3,3'-Bisoxindoles with α-Bromoenals: Enantioselective Construction of Contiguous Quaternary Stereocenters

An NHC-catalyzed enantio- and diastereoselective [3 + 2] annulation of α-bromoenals with bisoxindoles is developed, affording efficient access to various spirocyclic bisoxindole alkaloids. This protocol tolerates a broad substrates scope, with various spirocyclic bisoxindoles obtained in generally excellent enantioselectivities. More importantly, two contiguous sterically congested all-carbon quaternary stereocenters are successfully created during this process.

N-Heterocyclic Carbene-Catalyzed Umpolung of Cyclopent-4-ene-1,3-diones for Activated Olefin-Isatin Cross-Coupling

N-Heterocyclic carbene (NHC)-catalyzed umpolung of cyclopent-4-ene-1,3-diones that proceeds via the generation of nucleophilic deoxy-Breslow intermediates is presented. The carbene generated from a commercially available thiazolium salt catalyzed the cross-coupling between cyclopent-4-ene-1,3-diones and isatins to furnish the functionalized oxindoles in moderate to good yields with good functional group compatibility. The key deoxy-Breslow intermediates were isolated and characterized by X-ray analysis. Detailed mechanistic studies are also presented.

Cobalt-promoted synthesis of sulfurated oxindoles via radical annulation of N-arylacrylamides with disulfides

An efficient radical annulation of N-arylacrylamides with disulfides is developed for the synthesis of sulfurated oxindoles. The reaction occurs in a facile manner using CoBr₂ as both an initiator and a promoter for the first time and (NH₄)₂S₂O₈ as the oxidant. By controlling the CoBr₂/(NH₄)₂S₂O₈ ratio, a wide range of sulfurated and brominated/sulfurated oxindoles are selectively prepared in good to excellent yields. The present protocol is simple and highly atom economical, and can tolerate a broad range of substrates.

Asymmetric Synthesis of Structurally Sophisticated Spirocyclic Pyrano[2,3-c]pyrazole Derivatives Bearing a Chiral Quaternary Carbon Center

A carbene-catalyzed enantio- and diastereoselective [2 + 4] cycloaddition reaction is developed for quick and efficient access to structurally complex multicyclic pyrano[2,3-c]pyrazole molecules. The reaction tolerates a broad scope of substrates bearing various substitution patterns, with the multicyclic pyrano[2,3-c]pyrazole products afforded in generally good to excellent yields and optical purities. The chiral molecules obtained from this approach has found promising applications in the development of novel bacteriacides for plant protection.

Organocatalytic Asymmetric Construction of Tetrasubstituted Carbon Stereocenters Bearing Three Heteroatoms via Intramolecular Cyclization of Vinylidene ortho-Quinone Methide with Imidates

We report herein an organocatalytic asymmetric protocol for the construction of tetrasubstituted carbon stereocenters bearing three heteroatoms. The reaction proceeded via the enantioselective intramolecular cyclization reaction of vinylidene ortho-quinone methide (VQM) with imidates to form pentacyclic heterocycles. The formed tetrasubstituted carbon center was stable under a high temperature and the conditions for further transformations.