Asymmetric Cycloaddition and Cyclization Reactions Catalyzed by Chiral N,N'-Dioxide-Metal Complexes

Ce(OTf)3-Catalyzed Asymmetric 6π Cyclization of Triaryldivinyl Ketones

The rare-earth Ce(OTf)3-catalyzed asymmetric 6π cyclization of triaryldivinyl ketones is realized, affording methyl (R,E)-1-aryl-2-arylmethylidene-3-hydroxy-1,4-dihydronaphthalene-2-carboxylates in moderate to good yields with good to excellent enantioselectivities. The reaction provides a new strategy for the construction of optically active 3-hydroxy-1,4-dihydronaphthalene-2-carboxylic acid derivatives.

Nickel(II)-Catalyzed Unexpected [3 + 2] Cycloaddition/[3,3]-Rearrangement of N-Vinyl α,β-Unsaturated Nitrones with 2-Alkynyl Quinazolinones

A nickel(II)-catalyzed unexpected [3 + 2] cycloaddition/[3,3]-rearrangement cascade reaction was developed for the preparation of various polysubstituted 1-pyrroline-tethered quinazolinones containing three contiguous stereocenters in moderate to good yields with high diastereoselectivity from N-vinyl cinnamaldehyde nitrones and 2-alkynyl quinazolinones. Polysubstituted pyrrolizine-tethered quinazolinones were obtained in good yields when N-vinyl cinnamaldehyde nitrones were replaced by N-vinyl chalcone nitrones. The present method features a broad substrate scope, high [3,3]-rearrangement selectivity and diastereoselectivity, and two substituent bifurcated types of N-heterocycles.

Solvent-Controlled Enantioselective Allylic C-H Alkylation of 2,5-Dihydrofuran via Synergistic Palladium/Nickel Catalysis

Enantioenriched, substituted tetrahydrofuran skeletons extensively occur in natural products, bioactive targets, and organic frameworks. The rapid and diverse synthesis of these tetrahydrofuran molecules is highly desired yet challenging. Herein, we present a practical synthetic strategy for asymmetric allylic C-H bond functionalization of oxyheterocyclic alkenes by making use of the synergistic catalysis of achiral Pd complex and chiral N,N'-dioxide-Ni(II) catalyst. Notably, the chemodivergent synthesis of allylic C-H alkylated products and hydroalkylated products was readily achieved in good outcomes via the regulation of solvents. Furthermore, the post-transformation of these functionalized 2,5-dihydrofurans provides an innovative synthetic route to access tetrahydrofuran skeleton compounds containing multiple stereocenters.

Construction of Chiral Spiro-Bridged Rings with Four Consecutive Stereocenters via Dearomative Diels-Alder Reactions of Anthracenes

A highly diastereo- and enantioselective dearomative Diels-Alder reaction was accomplished by chiral N,N'-dioxide/Mg(II) complex catalyst. Various anthracene derivatives and methyleneindolinones efficiently transformed into the corresponding chiral spiro-bridged cyclic products with four consecutive stereocenters in good yields, excellent dr and er values under mild conditions (46 examples, up to 99% yield, >19:1 dr, >99:1 er). Gram-scale synthesis of chiral products and their further transformations were feasible. On the basis of theoretical calculation, possible working modes were provided to understand the origin of stereoselectivity of this transformation.

Catalytic asymmetric construction of 1,5-remote Si- and C-stereocenters via desymmetrizing ene reaction of bis(methallyl)silanes

The catalytic enantioselective synthesis of chiral silanes has long been a challenging pursuit. Achieving simultaneous construction of remote Si- and C-stereogenic centers in an acyclic molecule via desymmetrization is particularly difficult. Herein, we realized an example of a chiral nickel(ii) complex-catalyzed desymmetrizing carbonyl-ene reaction of bis(methallyl)silanes with α-keto aldehyde monohydrates, enabling the highly chemo-, diastereo- and enantioselective synthesis of chiral δ-hydroxy silanes featuring 1,5-remote Si- and C-stereocenters. This protocol demonstrated good functional group tolerance and a broad substrate scope. A bioactivity study revealed its potential applications in the synthesis of bioactive molecules.

Metal-Controlled Enantiodivergent Tandem Rearrangement to Synthesize 2H-Azirines

A metal-controlled enantiodivergent rearrangement of α-diazo-β-keto oxime ethers has been developed. Switching the metal catalyst from Co(II) to Fe(II) with an identical ligand completely reverses the asymmetric induction. Both enantiomers of chiral 2H-azirines and its derivatives are accessible under thermal or photolytic conditions. The cobalt-based reaction is highly efficient even at 0.05 mol% catalyst loading. Mechanistic studies, including kinetic experiments and density functional theory calculations, reveal that distinct electronic properties of cobalt and iron modulate intermediate conformations, thereby altering the enantioselectivity.

Development of Eagle-Shaped BPE/Phe-2NO Ligands: Switch of Enantioselectivity in Friedel-Crafts Alkylation

The development of single chiral source-derived ligands to fine-switch enantioselectivity has been a key aspect in asymmetric catalysis. Herein, in this study, using the same chiral source l-prolinamide as the starting material, we synthesize 14 new diphenyl ether bridged C2-symmetric rigid chiral tertiary amine-derived dioxide ligands (abberviated as BPE-2NO) and 9 new m-phenylene bridged C2-symmetric rigid chiral tertiary amine-derived dioxide ligands (abberviated as Phe-2NO); their effectiveness was demonstrated in the first switch of enantioselectivity in palladium(II)-catalyzed Friedel-Crafts alkylation. In the presence of palladium acetate as the Lewis acid, both enantiomers of indole derivatives can be prepared in good-to-excellent yields and enantioselectivities by using the single chiral source-derived Eagle-shaped BPE/Phe-2NO ligands. Control experiments and density functional theory calculations provide a rational explanation for the above observations. This study was the first switch of enantioselectivity in palladium(II)-catalyzed Friedel-Crafts alkylation by using the single chiral source-derived ligands.

Regiodivergent and Enantioselective Allylic C-H Alkylation of Allyl Ethers: Optimization, Scope, Mechanism and Application

Vinyl ethers and allyl ethers are important motifs in natural products and pharmaceuticals. Among various methods toward their synthesis, direct allylic C-H functionalization of allyl ethers is one of the most efficient approaches. In this study, one of two regioisomers, a vinyl ether or an allyl ether, could be obtained, depending on whether a Lewis acid co-catalyst was present. Furthermore, branched allyl ethers were smoothly prepared in excellent regio- and enantioselectivity (up to 20 : 1 b/l, 99 % ee) by synergistic catalysis with an achiral Pd(0) complex and a chiral Lewis acid catalyst.

Reductant-Free Enantioselective Aza-Reformatsky Reaction Enabled by Synergistic Visible Light Photocatalysis and Lewis Acid Catalysis

Classical aza-Reformatsky reaction generally involves excess reductants. Herein, we developed a visible light-induced catalytic asymmetric aza-Reformatsky reaction via a chiral Lewis acid-assisted direct excitation of imines without additional reductants, enabling the carbon-iodine bond cleavage of iododifluoromethyl ketones and the subsequent enantioselective radical coupling. This protocol provided an ingenious access to chiral β-amino ketones containing a gem-difluorine moiety. The mechanistic studies including radical trapping experiment, NMR experiment, electron paramagnetic resonance experiment, cyclic voltammetry experiment and spectroscopic analysis rationalized the reaction process.

Applications of innovative synthetic strategies in anticancer drug discovery: The driving force of new chemical reactions

The discovery of novel anticancer agents remains a critical goal in medicinal chemistry, with innovative synthetic methodologies playing a pivotal role in advancing this field. Recent breakthroughs in CH activation reactions, cyclization reactions, multicomponent reactions, cross-coupling reactions, and photo- and electro-catalytic reactions have enabled the efficient synthesis of new molecular scaffolds exhibiting potent biological activities, including anticancer properties. These methodologies have facilitated the functionalization of natural products, the modification of bioactive molecules, and the generation of entirely new compounds, many of which demonstrate strong antitumor activity. This review summarizes the latest synthetic strategies employed over the past five years for discovering anticancer agents, focusing on their influence on drug design. Additionally, the role of new chemical reactions in expanding chemical space and overcoming challenges, such as drug resistance and selectivity, is highlighted, further emphasizing the importance of discovering novel reactions as a key trend in future drug development.

Domino 1,3-dipolar cycloaddition/ring-opening/ring-cleavage: synthesis of trisubstituted pyrrole and chiral dihydropyrrole-3-carbaldehydes

A unique approach has been developed to synthesize trisubstituted 1H-pyrrole-3-carbaldehydes using 4-methyl thiazolium salts, α,β-unsaturated aldehydes, and organocatalysts via a domino 1,3-dipolar cycloaddition/ring-opening/C-S and C-N bond cleavage reaction sequence. This methodology has been successfully extended for the asymmetric synthesis of enantioenriched trisubstituted-4,5-dihydro-1H-pyrrole-3-carbaldehydes employing chiral amine organocatalysts with high efficiency (up to 98% ee, >20 : 1 d.r.).

Pyridine-N-oxide catalyzed asymmetric N-acylative desymmetrization of sulfonimidamides

A highly efficient enantioselective N-acylative desymmetrization of sulfonimidamides with chloroformates was reported using chiral 4-arylpyridine-N-oxide as the catalyst, affording N-acylative sulfonimidamides with sulfur(vi)-stereocenters in high yields and excellent enantioselectivities. Experiments and DFT calculations support an acyl transfer mechanism, and the nucleophilic substitution of sulfonimidamide by the O-acyloxypyridinium cation intermediate is the enantio-determining step of the reaction. The reaction features variability for acyloxy groups and compatibility with moisture.

Asymmetric Catalytic Aziridination to Synthesize Spiro-aziridine Oxindoles

Asymmetric catalytic aza-Michael-initiated ring closure of methyleneindolinones with N-tosyloxycarbamates has been established. The reaction using a chiral nickel complex catalyst enabled the formation of a series of spiro-aziridine oxindoles in good yields (up to 99 %) with high stereoselectivity (up to 97 % ee, >19 : 1 dr) under mild reaction conditions. Ring-opening of spiro-aziridine oxindole leads to formation of glycinate-bearing oxindoles with retention of configuration.

Catalytic Asymmetric Rearrangement of Azoalkene-Derived Sulfonium Ylides via Remote Chirality Control

The [2,3]-sigmatropic rearrangement has been widely utilized to construct C-S bonds. Herein, we report an enantioselective, intermolecular, and noncarbenoid [2,3]-sigmatropic rearrangement of sulfonium ylides using azoalkenes. This process features a broad substrate scope, high efficiency, and excellent enantioselectivity, achieving yields of up to 99% and an enantiomeric excess (ee) of up to 96%. Furthermore, the protocol demonstrated good scalability.

Halogenated Donor-Acceptor Cyclopropanes as Donor-Acceptor Cyclopropene Surrogates

Herein we report (3+2)-cycloaddition reactions of halogenated donor-acceptor cyclopropanes (HDACs) as surrogates for donor-acceptor cyclopropenes. Upon reaction with a variety of 2π-components, cycloaddition followed by elimination generates the unsaturated five-membered ring products. A series of HDACs were shown to be effective in reactions with thioketones. Five further cycloaddition reactions were used to showcase the broad applicability of these donor-acceptor cyclopropene surrogates. Kinetic studies allowed the proposal of a plausible mechanism for this reaction.

Asymmetric Catalytic (3 + 2) Cyclization and Sequential Reaction to Construct Dihydrofuran- and Azepine-Based Spirooxindoles

The enantioselective formal (3 + 2) cyclization and sequential reaction of 2-malononitrile-substituted oxindoles with benzaldehydes and ortho-aminobenzaldehydes were achieved by chiral N,N'-dioxide/metal complex Lewis acid catalysts. This protocol supplies facile and efficient access to highly functionalized chiral dihydrofuran- and azepine-based spirooxindoles. Based on the control experiments and the deuterium labeling studies, the interconversion of (3 + 2) diastereomeric intermediates under the reaction conditions and reversible 1,5-H transfer step were disclosed.

Stereodivergent Construction of 3,3'-Disubstituted Oxindoles via One-Pot Sequential Allylation/Alkylation and Its Application to the Total Synthesis of Trigolute B and D

The absolute and relative configurations of bioactive chiral molecules are typically relevant to their biological properties. It is thus highly important and desirable to construct all possible stereoisomers of a lead candidate or a given bioactive natural compound. Synergistic dual catalysis has been recognized as a reliable synthetic strategy for a variety of predictable stereodivergent transformations. Despite the impressive progress made in this field, stereodivergent carbon-carbon bond-formation reactions involving stabilized nucleophiles remain elusive. Herein, we report an iridium- and magnesium-catalyzed one-pot sequential allylic alkylation/nucleophilic alkylation cascade process for the stereodivergent synthesis of all four stereoisomers of 3,3'-disubstituted oxindoles through a three-component reaction. A diverse array of products is readily prepared with high functional group compatibility in good yields with excellent diastereo- and enantioselectivities. Subsequently, the stereodivergent total synthesis of four stereoisomers of the spirooxindole alkaloid trigolutes B and D has been accomplished through a concise and unified synthetic route using the same set of starting materials.

Asymmetric Relay Catalysis Enables Unreactive Allylic Alcohols to Participate in 1,3-Dipolar Cycloaddition of Azomethine Ylides

Current synthetic transformations occur readily with starting materials that possess both innate reactivity and steric accessibility or functional-group-oriented reactivity. However, achieving reactions with inactive feedstock substrates remains significantly challenging and normally requires cumbersome prior functional group manipulations. Herein, we report an unprecedented example of catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylides with nonactivated alkenes enabled by copper/ruthenium relay catalysis. Key to the success is the temporary activation strategy initiated by oxidative dehydrogenation of inert allylic alcohols into electron-demanding reversed highly reactive enones, which triggers the ensuing Cu-catalyzed asymmetric 1,3-dipolar cycloaddition followed by reductive hydrogenation to deliver highly functionalized chiral pyrrolidines with the construction of two C-C bonds and four well-defined stereogenic centers in an atom-/step-economical and redox-neutral manner. This method features mild reaction conditions, operational simplicity, and broad substrate scope and is also characterized by formal dynamic kinetic resolution. Mechanistic studies and control experiments supported a typical borrowing-hydrogen cascade orthogonally merged with 1,3-dipolar cycloaddition and revealed that the superiority and reliability of relay catalysis are enabled by the controlled release of highly reactive but unstable enones to impede the undesired polymerization. It should be noted that up to four stereoisomers of the challenging and otherwise inaccessible pyrrolidines and cyclobutanes could be readily prepared through concise late-stage elaborations.

Sc-Catalyzed Asymmetric [2 + 2] Annulation of 2-Alkynylnaphthols with Dienes to Access Cyclobutene Frameworks

Herein, we introduce a scandium-catalyzed synthetic strategy that provides access to a diverse and functionalized array of cyclobutene frameworks adorned with a quaternary carbon center. This approach broadens the synthetic repertoire of 2-alkynylnaphthols with alkenes, offering a versatile platform for the construction of complex molecular architectures. The asymmetric catalytic [2 + 2] cycloaddition reaction demonstrates a wide substrate scope and an impressive functional group tolerance, yielding products with high efficiency, up to 97% yield, and excellent enantiomeric excess of up to 97%. The simplicity of scaling up this process, coupled with the ease of converting these cyclobutene frameworks into a variety of substituted products, significantly enhances the synthetic utility of this method.

Asymmetric Three-Component Radical Cascade Reactions Enabled by Synergistic Photoredox/Brønsted Acid Catalysis: Access to α-Amino Acid Derivatives

Multicomponent reactions (MCRs), highly sought-after methods to produce atom-, step-, and energy-economic organic syntheses, have been developed extensively. However, catalytic asymmetric MCRs, especially those involving radical species, remain largely unexplored owing to the difficulty in stereoselectively regulating the extraordinarily high reactivity of open-shell radical species. Herein, we report a conceptually novel catalytic asymmetric three-component radical cascade reaction of readily accessible glycine esters, α-bromo carbonyl compounds and 2-vinylcyclopropyl ketones via synergistic photoredox/Brønsted acid catalysis, in which three sequential C-C (σ/π/σ) bond-forming events occurred through a radical addition/ring-opening/radical-radical coupling protocol, affording an array of valuable enantioenriched unnatural α-amino acid derivatives bearing two contiguous stereogenic centers and an alkene moiety in moderate to good yield with high diastereoselectivity, excellent enantioselectivity and good E-dominated geometry under mild reaction conditions. The radical relay cascade process, especially a unique proton-coupled electron transfer (PCET)-promoted radical-radical coupling, is supported by mechanistic investigations and quantum mechanics calculations and should garner broad interest and further inspire the development of asymmetric multicomponent radical reactions.

An unusual chiral-at-metal mechanism for BINOL-metal asymmetric catalysis

Chiral binaphthols (BINOL)-metal combinations serve as powerful catalysts in asymmetric synthesis. Their chiral induction mode, however, typically relies on multifarious non-covalent interactions between the substrate and the BINOL ligand. In this work, we demonstrate that the chiral-at-metal stereoinduction mode could serve as an alternative mechanism for BINOL-metal catalysis, based on mechanistic studies of BINOL-aluminum-catalyzed asymmetric hydroboration of heteroaryl ketones. Theoretical calculations reveal that an octahedral stereogenic-at-metal aluminum alkoxide species is the most stable species within the reaction system, and also is the catalytic relevant intermediate, promoting the stereo-determining hydroboration reaction through a ligand-assisted hydride transfer mechanism rather than the conventional hydroalumination mechanism. These computations reproduce the experimental selectivities and also rationalize the stereoinduction mechanism, which arises from the aluminum-centered chirality induced by chiral BINOL ligands during diastereoselective assembly. The reliability of the proposed mechanism could be verified by the single-crystal X-ray diffraction characterization of the octahedral aluminum alkoxide complex. Additional NMR and Electronic Circular Dichroism (ECD) experiments elucidated the behavior of the hexacoordinated aluminum alkoxide in the solution phase. We anticipate that these findings will extend the applicability of BINOL-metal catalysis to a broader range of reactions.

Asymmetric Synthesis of Optically Active Pyrazolidines or Pyrazoline Derivatives via Ni(II)-Bipyridine-N,N'-dioxide Complexes

Easily obtainable and efficient chiral C2-symmetric bipyridine-N,N'-dioxide ligands with Ni(OTf)2 were developed for application in catalyzing [3 + 2] cycloaddition reactions to synthesize optically active fused pyrazolidines or pyrazoline derivatives featuring three contiguous stereogenic centers by employing azomethine imines and α,β-unsaturated 2-acyl imidazoles, affording the corresponding adducts with the opposite configuration compared to previous synthetic products in 80-98% yields with 28-99% ee and >20:1 dr. In addition, subsequent amplification experiments and derivative transformations of the product further demonstrated the efficient catalytic performance of the catalyst Ni(II)-bipyridine-N,N'-dioxide complexes and the practicality of this synthesis methodology.

Aza-[4 + 2]-cycloaddition of benzocyclobutenones into isoquinolinone derivatives enabled by photoinduced regio-specific C-C bond cleavage

The activation of C-C bond of benzocyclobutenones under mild reaction conditions remains a challenge. We herein report a photoinduced catalyst-free regio-specific C1-C8 bond cleavage of benzocyclobutenones, enabling the generation of versatile ortho-quinoid ketene methides for aza-[4 + 2]-cycloaddition with imines, which offers a facile route to isoquinolinone derivatives, including seven family members of protoberberine alkaloids, gusanlung A, B, D, 8-oxotetrahydroplamatine, tetrahydrothalifendine, tetrahydropalmatine, and xylopinine. Furthermore, the catalytic enantioselective version of this strategy is also realized by merging synergistic photocatalysis and chiral Lewis acid catalysis. Mechanistic studies provide compelling evidence to rationalize the photoisomerization/cycloaddition cascade process.

Asymmetric Synthesis of Polycyclic Heterocyclic Compounds via Friedel-Crafts Reaction/Cyclization Reaction Catalyzed by Nickel Catalyst

The first highly enantioselective asymmetric Friedel-Crafts reaction/cyclization reaction of 5-aminopyrazoles with 3-alkenyloxindoles to afford polycyclic heterocyclic compounds bearing an all-carbon quaternary stereocenter catalyzed by a complex of NiII with the C2-symmetric bipyridine-N,N'-dioxide ligand L12 has been developed. The relevant products with a wide range of substrates and good functional tolerance were obtained in 89-98% yield with 56-99% ee in the presence of 10 mol % Ni(OTf)2 and 11 mol % L12 in DCM at 25 °C. Moreover, an experiment on scaling up the process along with the transformations of the cycloadducts further emphasized the practical application of the synthesis.

Enantioselective organocatalyzed one-pot synthesis of N-phenyl thioether-tethered tetrasubstituted dihydropyrrole-3-carbaldehydes

An efficient method for the asymmetric one-pot synthesis of N-phenyl thioether-tethered tetrasubstituted chiral 4,5-dihydropyrrole-3-carbaldehydes have been developed using readily available benzothiazolium salts and α,β-unsaturated aldehydes as starting materials in the presence of the chiral organocatalyst (R)-diphenylprolinol trimethylsilyl ether. The protocol afforded various functionally enantioenriched chiral tetrasubstituted 4,5-dihydropyrrole-3-carbaldehydes in high yields, with excellent enantio- and diastereoselectivity (≤90% yield, ≤98% ee, and >20 : 1 d.r.). This asymmetric one-pot reaction starts with the reaction of azomethine ylides with dipolarophiles to yield pyrrolo-thiazine-2-carbaldehydes as intermediates. Subsequently, the electrophile alkyl halide is attacked by the sulfur atom of the intermediate, followed by C-S bond cleavage (ring-opening), which furnishes the desired chiral tetrasubstituted 4,5-dihydropyrroles.

Chemoselectivity Switch between Enantioselective [2,3]-Wittig Rearrangement and Conia-Ene-Type Reactions of Propargyloxyoxindoles

Despite the existence of three competing reactions for propargyloxyoxindoles, we report a chemoselectivity switch between enantioselective propargyl [2,3]-Wittig rearrangement and Conia-ene-type reactions, with suppression of the [1,2]-Wittig-type rearrangement. Using C1-symmetric imidazolidine-pyrroloimidazolone pyridine as the ligand and Ni(acac)2 as the Lewis acid, diverse 3-hydroxy 3-substituted oxindoles containing allenyl groups were obtained in up to 98 % yield and 99 % ee via asymmetric propargyl [2,3]-Wittig rearrangement. In the presence of AgOTf-Duanphos, chiral spiro dihydrofuran oxindoles were given in up to 98 % yield and 91 % ee through a Conia-ene-type reaction.

Atroposelective Synthesis of Biaryl N-Oxides via Cu-Catalyzed De Novo Heteroaromatic N-Oxide Ring Formation

Heteroaromatic N-oxides, renowned for their highly polar N─O bond and robust structure, exhibit significant bioactivities and have played a pivotal role in various drug development projects since the discovery of Minoxidil. Moreover, heteroaromatic N-oxides, featuring axially chiral biaryl frameworks, are indispensable as Lewis base catalysts and ligands in organic synthesis. Despite their importance, synthesizing these chiral compounds is challenging, necessitating chiral starting materials or resolution processes. Catalytic strategies rely on the functionalization of heteroaromatic N-oxide compounds, leading to products with a relatively limited skeletal diversity. This study introduces a Cu-catalyzed atroposelective method for synthesizing biaryl N-oxides via de novo heteroaromatic N-oxide ring formation. This mild and efficient approach achieves excellent stereoselectivities (up to 99:1 er), enabling the production of a wide array of N-oxides with novel heteroaromatic scaffolds. The axially chiral N-oxide product 3f demonstrates high stereoselectivity and recyclability as a Lewis base catalyst. Additionally, product 3e exhibits promising therapeutic efficacy against triple-negative breast cancer, with IC50 values of 4.8 and 5.2 µm in MDA-MB-231 and MDA-MB-468 cells, respectively. This research not only advances the synthesis of challenging chiral heteroaromatic N-oxides but also encourages further exploration of N-oxide entities in the discovery of bioactive small molecules.

Asymmetric catalytic concise synthesis of 3-(3-indolomethyl)-oxindoles for the construction of trigolute analogs

Asymmetric synthesis of 3-(3-indolomethyl)oxindoles through the addition of indole-substituted enolized ketoesters to 3-bromo-3-substituted oxindoles has been achieved using a N,N'-dioxide/Ho(III) complex. A number of 3-(3-indolomethyl)oxindoles, which may possess biological activity, were obtained in good yields with high diastereo- and enantioselectivities (up to 97% yield, >19 : 1 dr, 98% ee). Furthermore, time-dependent reversal of diastereoselectivity enabled access to optically active diastereomers. The product followed by facile transformations gave a new route into trigolute analogs.

Formal High-Order Cycloadditions of Donor-Acceptor Cyclopropanes with Cycloheptatrienes

The design of various cycloaddition/annulation processes is one of the most intriguing challenges in the development of donor-acceptor (D-A) cyclopropane chemistry. In this work, a new class of formal high-order [6+n]-cycloaddition and annulation processes of D-A cyclopropanes with cycloheptatriene systems has been designed and reported, to fill a significant gap in the chemistry of D-A cyclopropanes. The reactivity of methylated cycloheptatrienes from Me1 to Me5 as well as unsubstituted cycloheptatriene was studied in detail under GaCl3 activation conditions, which makes it possible to efficiently generate gallium 1,2-zwitterionic complexes or 1,3-zwitterionic intermediates starting from D-A cyclopropanes, while other Lewis acids are ineffective and non-selective. New examples of formal [6+2]-, [6+3]-, [6+4]-, [6+1]-, and [4+2]-cycloaddition and annulation reactions with cycloheptatrienes along with more complex processes were discovered. Cycloheptatriene itself can also successfully act as a hydride anion donor, which allows the ionic hydrogenation of D-A cyclopropanes to be performed under mild conditions. As a result, a number of efficient and highly diastereoselective protocols for the synthesis of seven-membered carbocycles has been developed.

Tandem catalytic allylic C-H amination and asymmetric [2,3]-rearrangement via bimetallic relay catalysis

A bimetallic relay catalysis protocol for tandem allylic C-H amination and asymmetric [2,3]-sigmatropic rearrangement has been developed with the use of an achiral Pd0 catalyst and a chiral N,N'-dioxide-MgII complex in a one-pot operation. A series of anti-α-amino derivatives containing two stereogenic centers were prepared from readily available allylbenzenes and glycine pyrazolamide with good yields and high stereoselectivities. Moreover, the synthetic potential of this protocol was further demonstrated by the product transformations, and a catalytic cycle was proposed to illustrate the reaction process.

Catalytic Asymmetric Dearomative [2 + 2] Photocycloaddition/Ring-Expansion Sequence of Indoles with Diversified Alkenes

Developing novel strategies for catalytic asymmetric dearomatization (CADA) reactions is highly valuable. Visible light-mediated photocatalysis is demonstrated to be a powerful tool to activate aromatic compounds for further synthetic transformations. Herein, a catalytic asymmetric dearomative [2 + 2] photocycloaddition/ring-expansion sequence of indoles with simple alkenes was reported, providing a facile access to enantioenriched cyclopenta[b]indoles with good to high yields and enantioselectivities by means of chiral lanthanide photocatalysis. This protocol exhibited a broad substrate scope and good functional group tolerance, as well as potential applications in the synthesis of bioactive molecules. Mechanistic studies, including control experiments, UV-vis absorption spectroscopy, emission spectroscopy, and DFT calculations, were carried out, shedding insights into the reaction mechanism and the origin of enantioselectivity.

Enantioselective Synthesis of Spiro[cyclopentane-1,3'-oxindole] Scaffolds with Five Consecutive Stereocenters

A highly diastereo- and enantioselective cascade annulation reaction of Morita-Baylis-Hillman (MBH) maleimides of isatins with ortho-hydroxychalcones was achieved by a chiral N,N'-dioxide/Mg(II) complex Lewis acid catalyst. This strategy provides a concise and efficient route to densely functionalized spiro[cyclopentane-1,3'-oxindole] compounds with five consecutive stereocenters. The reaction itself features mild conditions, good functional group compatibility, and broad substrate scope (62 examples, up to 99% yield, up to >20:1 dr, 97% ee). In addition, an obvious ligand acceleration effect and chiral amplification effect were observed. DFT calculations were performed to elucidate the stereoselectivity observed. The gram-scale synthesis and the inhibitory effect of two products on the viability of A549 cells demonstrate the potential utility of the current method.

Ionic Hydrogen Bond-Assisted Catalytic Construction of Nitrogen Stereogenic Center via Formal Desymmetrization of Remote Diols

The control of noncarbon stereogenic centers is of profound importance owing to their enormous interest in bioactive compounds and chiral catalyst or ligand design for enantioselective synthesis. Despite various elegant approaches have been achieved for construction of S-, P-, Si- and B-stereocenters over the past decades, the catalyst-controlled strategies to govern the formation of N-stereogenic compounds have garnered less attention. Here, we disclose the first organocatalytic approach for efficient access to a wide range of nitrogen-stereogenic compounds through a desymmetrization approach. Intriguingly, the pro-chiral remote diols, which are previously not well addressed with enantiocontrol, are well differentiated by potent chiral carbene-bound acyl azolium intermediates. Preliminary studies shed insights on the critical importance of the ionic hydrogen bond (IHB) formed between the dimer aggregate of diols to afford the chiral N-oxide products that feature a tetrahedral nitrogen as the sole stereogenic element with good yields and excellent enantioselectivities. Notably, the chiral N-oxide products could offer an attractive strategy for chiral ligand design and discovery of potential antibacterial agrochemicals.

Concise synthesis of chiral γ-butenolides via an allylation/lactonization cascade reaction

A highly enantioselective allylation/lactonization cascade was developed, which provides a concise and efficient route to chiral γ-butenolides under mild conditions. Most of the reaction examples can be completed in 10 minutes with high selectivity.

Asymmetric catalytic [1,3]- or [3,3]-sigmatropic rearrangement of 3-allyloxy-4H-chromenones and their analogues

A highly efficient asymmetric [1,3]- and [3,3]-O-to-C sigmatropic rearrangement of 3-allyloxy-4H-chromenones and their analogues was developed. Chiral N,N'-dioxide complexes of 3d late transition metal complexes enabled two mechanistically different processes, giving a series of optically active 2,2-disubstituted chromane-3,4-diones and 2-allyl-3-hydroxy-4H-chromen-4-ones as well as their related compounds in excellent yield and enantioselectivity. Systemic mechanistic studies and DFT calculation revealed the nature of the vinyl ether unit of the substrate, which biased regioselectivity via a stepwise tight ion pair pathway and a concerted pericyclic pathway, respectively. The enantioselectivity of the two processes is also disclosed.

Iron-Catalyzed Asymmetric Imidation of Sulfides via Sterically Biased Nitrene Transfer

Transition-metal-catalyzed enantioselective nitrene transfer to sulfides has emerged as one of the most powerful strategies for rapid construction of enantioenriched sulfimides. However, achieving stereocontrol over highly active earth-abundant transition-metal nitrenoid intermediates remains a formidable challenge compared with precious metals. Herein, we disclose a chiral iron(II)/N,N'-dioxide-catalyzed enantioselective imidation of dialkyl and alkyl aryl sulfides using iminoiodinanes as nitrene precursors. A series of chiral sulfimides were obtained in moderate-to-good yields with high enantioselectivities (56 examples, up to 99% yield, 98:2 e.r.). The utility of this methodology was demonstrated by late-stage modification of complex molecules and synthesis of the chiral insecticide sulfoxaflor and the intermediates of related bioactive compounds. Based on experimental studies and theoretical calculations, a water-bonded high-spin iron nitrenoid species was identified as the key intermediate. The observed stereoselectivity was original from the steric repulsion between the amide unit of the ligand in the chiral cave and the bulky substituent of sulfides. Additionally, dioxazolones proved to be suitable acylnitrene precursors in the presence of an iron(III)/N,N'-dioxide complex, resulting in the formation of enantioselectivity-reversed sulfimides (14 examples, up to 81% yield, 97:3 e.r.).

Cooperative Rh/Achiral Phosphoric Acid-Enabled [3+3] Cycloannulation of Carbonyl Ylides with Quinone Monoimines: Synthesis of Benzofused Dioxabicyclo[3.2.1]octane Scaffolds

A cooperative Rh/achiral phosphoric acid-enabled [3+3] cycloaddition of in situ-generated carbonyl ylides with quinone monoimines has been developed. With the ability to build up the molecular complexity rapidly and efficiently, this method furnishes highly functionalized oxa-bridged benzofused dioxabicyclo[3.2.1]octane scaffolds bearing two quaternary centers in good to excellent yields under mild conditions. Moreover, the utility of the current method was demonstrated by gram-scale synthesis and elaboration of the products into various functionalized oxa-bridged heterocycles.

Chiral Brønsted Base Activation of Donor-Acceptor Cyclopropanes toward Diastereo- and Enantioselective [3 + 2] Cycloaddition with Isatin-Derived Ketimines

Organocatalyzed diastereo- and enantioselective [3 + 2] cycloaddition reactions of donor-acceptor (D-A) cyclopropanes with isatin-derived ketimines are presented. Different from well-developed Lewis acid activation protocols which promote the reactivity of D-A cyclopropanes through coordinating to the acceptor group, in this reaction, dicyanocyclopropylmethyl ketones can be activated through nucleophilic activation of the donor group by using dihydroquinine-derived squaramide as Brønsted base catalyst. The reaction affords functionalized spiro[oxindole-3,2'-pyrrolidines] with two nonadjacent tetra- and tri-substituted stereocenters in 83-99% yields, moderate to excellent diastereoselectivities (up to >20:1 diastereomeric ratio (dr)), and excellent enantioselectivities (up to >99% enantiomeric excess (ee)) under mild conditions.

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.

Brønsted acid catalyzed [4 + 2] cycloaddition for the synthesis of bisbenzannulated spiroketals with antifungal activities

The intermolecular [4 + 2] cycloaddition of o-hydroxy benzyl alcohols with isochroman ketals was realized by CF3CO2H catalysis. A broad range of bisbenzannulated [6,6]-spiroketals were formed under the metal-free mild conditions in moderate to excellent yields (45-98%) with mostly excellent diastereoselectivities (up to >20 : 1 dr). Furthermore, the enantioselective version was also preliminarily investigated and the bisbenzannulated [6,6]-spiroketal was obtained with 61% ee in the presence of Sc(OTf)3/Feng's chiral N,N'-dioxide ligand. Some of the bisbenzannulated [6,6]-spiroketal products showed good in vitro antifungal activities against Sclerotinia sclerotiorum and Rhizoctonia solani.

Photoredox-catalyzed C(sp3)─H radical functionalization to enable asymmetric synthesis of α-chiral alkyl phosphine

α-Chiral alkyl phosphines are privileged structural motifs with a wide application in organic and medical synthesis. It is highly desirable to develop stereoselective methods to prepare these enantioenriched molecules. The incorporation of C(sp3)─H functionalization and chiral phosphine chemistry is much less explored, probably because of the weak reactivity of C(sp3)─H bonds and/or the challenging site- and stereoselectivity issues. Herein, we disclose a synergistic catalysis system to enable an enantioselective radical addition process of α-substituted vinylphosphine oxides. An array of diverse α-chiral alkyl phosphors compounds is smoothly accessed by using the readily available chemicals as the inert C(sp3)─H bond reagent, such as sulfides, amines, alkenes, and toluene derivatives, exerting remarkable chemo-, site-, and enantioselectivity. On the basis of the mechanistic studies, both the C(sp3)─H bond activation and the stereochemistry-determining step are proposed to involve a single-electron transfer/proton transfer process.

Asymmetric Three-Component Radical Alkene Carboazidation by Direct Activation of Aliphatic C-H Bonds

Azide compounds are widely present in natural products and drug molecules, and their easy-to-transform characteristics make them widely used in the field of organic synthesis. The merging of transition-metal catalysis with radical chemistry offers a versatile platform for radical carboazidation of alkenes, allowing the rapid assembly of highly functionalized organic azides. However, the direct use of readily available hydrocarbon feedstocks as sp3-hybridized carbon radical precursors to participate in catalytic enantioselective carboazidation of alkenes remains a significant challenge that has yet to be addressed. Herein, we describe an iron-catalyzed asymmetric three-component radical carboazidation of electron-deficient alkenes by direct activation of aliphatic C-H bonds. This approach involves intermolecular hydrogen atom transfer between a hydrocarbon and an alkoxy/aryl carboxyl radical, leading to the formation of a carbon-centered radical. The resulting radical then reacts with electron-deficient alkenes to generate a new radical species that undergoes chiral iron-complex-mediated C-N3 bond coupling. An array of valuable chiral azides bearing a quaternary stereocenter were directly accessed from widely available chemical feedstocks, and their synthetic potential is further demonstrated through more facile transformations to give other valuable enantioenriched building blocks.

Bipyridine-N,N'-dioxides Catalysts: Design, Synthesis, and Application in Asymmetric Synthesis of 1H-Pyrazolo[3,4-b]pyridine Analogues

A novel type of highly efficient chiral C2-symmetric bipyridine-N,N'-dioxides ligand application in catalyzing Michael addition/Cyclization of 5-aminopyrazoles with α,β-unsaturated 2-acyl imidazoles has been developed, affording the corresponding adducts in 85-97% yield with up to 99% enantioselectivity under mild conditions with a lower catalyst loading and broad scope. Remarkably, this protocol exhibits advantages in terms of reactivity and enantioselectivity, giving the fact that as low as 2.2 mol % of L1 and 2.0 mol % of Ni(OTf)2 can promote the title reaction on gram scale to afford the desired product with excellent enantioselectivity.

Organocatalyzed Enantio- and Diastereoselective Formal Domino 1,3-Dipolar Cycloaddition/Rearrangement: Synthesis of Chiral Pyrrolo-thiazine-2-carbaldehydes

An efficient approach for the synthesis of chiral pyrrolo[1,2-d][1,4]thiazine-2-carbaldehydes is achieved via formal 1,3-dipolar cycloaddition/rearrangement reactions of benzothiazolium salt and α,β-unsaturated aldehydes, utilizing an asymmetric organocatalyst. This process results in the formation of fluorescent, highly enantioenriched chiral molecules with three contiguous stereogenic centers, one of which is a chiral quaternary center, with excellent yields and enantio- and diastereoselectivity. A computational study demonstrated the understanding of the reaction mechanism. The synthetic utility of this protocol was successfully employed for gram scale synthesis. Fluorescent and in silico studies showed the application of the present methodology.

Enantioselective sulfonylation to construct 3-sulfonylated oxindoles

Asymmetric synthesis of 3-sulfonylated 3-substituted oxindoles through the addition of sodium sulfinate salts to 3-bromo-3-substituted oxindoles has been achieved using chiral nickel complexes of N,N'-dioxides. This method facilitates the creation of diverse chiral sulfonyl oxindoles, several of which display promising anticancer properties. Notably, the catalyst demonstrates remarkable tolerance to water, crucial for maintaining enantioselectivity. Furthermore, the utilization of topographic steric maps of the catalysts offers valuable insights into the mechanism underlying enantioselection reversal.

Iron-Catalyzed Asymmetric α-Alkylation of 2-Acylimidazoles via Dehydrogenative Radical Cross-Coupling with Alkanes

The direct α-alkylation of acyclic carbonyls with nonactivated hydrocarbons through C(sp3 )-H functionalization is both extremely promising and notably challenging, especially when attempting to achieve enantioselectivity using iron-based catalysts. We have identified a robust chiral iron complex for the oxidative cross-coupling of 2-acylimidazoles with benzylic and allylic hydrocarbons, as well as nonactivated alkanes. The readily available and tunable N,N'-dioxide catalysts of iron in connection with oxidants exhibit precise asymmetric induction (up to 99 % ee) with good compatibility in moderate to good yields (up to 88 % yield). This protocol provides an elegant and straightforward access to optically active acyclic carbonyl derivatives starting from simple alkanes without prefunctionalization. Density functional theory (DFT) calculations and control experiments were made to gain insight into the nature of C-C bond formation and the origin of enantioselectivity. We propose a radical-radical cross-coupling process enabled by the immediate interconversion between chiral ferric species and ferrous species.

Modular access to chiral bridged piperidine-γ-butyrolactones via catalytic asymmetric allylation/aza-Prins cyclization/lactonization sequences

Chiral functionalized piperidine and lactone heterocycles are widely spread in natural products and drug candidates with promising pharmacological properties. However, there remains no general asymmetric methodologies that enable rapid assemble both critical biologically important units into one three-dimensional chiral molecule. Herein, we describe a straightforward relay strategy for the construction of enantioenriched bridged piperidine-γ-butyrolactone skeletons incorporating three skipped stereocenters via asymmetric allylic alkylation and aza-Prins cyclization/lactonization sequences. The excellent enantioselectivity control in asymmetric allylation with the simplest allylic precursor is enabled by the synergistic Cu/Ir-catalyzed protocol; the success of aza-Prins cyclization/lactonization can be attributed to the pivotal role of the ester substituent, which acts as a preferential intramolecular nucleophile to terminate the aza-Prins intermediacy of piperid-4-yl cation species. The resulting chiral piperidine-γ-butyrolactone bridged-heterocyclic products show impressive preliminary biological activities against a panel of cancer cell lines.

Catalytic Enantioselective Nucleophilic Addition to Arynes by a New Quaternary Guanidinium Salt-Based Phase-Transfer Catalyst

Owing to the mild generation methods, arynes have been widely used in synthetic chemistry. However, achieving asymmetric organocatalytic reactions with arynes remains a formidable and infrequent challenge, primarily because these neutral and transient species tend to spontaneously quench. To address this issue, a solid-liquid phase-transfer strategy is devised in which the generation speed of arynes could be controlled by the in situ generated fluoride-based chiral phase-transfer catalyst. In this study, we present a catalytic enantioselective nucleophilic addition reaction involving arynes, utilizing an amino amide-based guanidinium salt QG•X. Furthermore, we demonstrate the broad compatibility of this reaction with various arynes and cyclic/acyclic β-keto amides, leading to the creation of diverse α-aryl quaternary stereocenters with good stereoselectivity. Mechanistic investigations have uncovered the potential involvement of a chiral intramolecular cationic-anionic pair and HF during the ion exchange between QG•X and CsF for nucleophile activation and aryne generation. Additionally, DFT calculations suggested that the observed high levels of enantioselectivity can be attributed to steric repulsion and the cumulative noncovalent interactions between the catalysts and substrates.

Catalytic Asymmetric Synthesis of Axially and Centrally Chiral 1,2-Dihydrobenzofuro[3,2-b]pyridines through a [2 + 2] Cycloaddition/Retroelectrocyclization/Re-Cycloaddition Cascade

A catalytic asymmetric tandem cyclization of azadienes and ortho-alkynylnaphthols accelerated by the chiral N,N'-dioxide-gadolinium(III) complex is disclosed. This method allows the synthesis of a range of 1,2-dihydrobenzofuro[3,2-b]pyridines containing both axially and centrally chiral elements in high yields and excellent stereoselectivities (up to >99% yield, 91:9 dr, 98% ee). A control experiment revealed that this process proceeded through a multistep [2 + 2] cycloaddition/retroelectrocyclization/tautomerism/1,6-conjugate addition cascade.

New Binaphthyl-Proline-Based Chiral Ligands Bearing Imidazoline Groups: Design, Synthesis, and Their Application in Enantioselective Conjugate Addition of 4-Hydroxycoumarin and Related Nucleophiles to β,γ-Unsaturated α-Ketoesters

This paper describes the design and application of new binaphthyl-proline-based chiral ligands bearing imidazoline functional groups. These chiral ligands incorporate the advantages of both the binaphthyl and proline skeletons, they are featured with regulatable electronic and steric properties for the imidazoline functional groups, and form chiral complexes with different metal salts such as cuprous acetate. In the presence of an appropriate amount of a chiral catalyst, enantioselective conjugate addition of 4-hydroxycoumarin or related nucleophiles to different β,γ-unsaturated α-ketoesters proceeded readily, giving the desired products in high yield (up to 99%) and excellent enantiomeric excess (up to 99%).