Enantioselective, Visible Light Mediated Aza Paternò-Büchi Reactions of Quinoxalinones

A General Radical Functionalization of Quinoxalin-2(1H)-ones via a Donor-Acceptor Inversion Strategy

The radical donor-acceptor inversion strategy represents a highly promising approach in the field of radical chemistry. The present study initially describes a metal-free, versatile, and modular approach for the radical functionalization of quinoxalin-2(1H)-ones via a strategy of radical donor-acceptor inversion under simple reaction conditions. More than 66 examples were provided in moderate yields. The mechanistic study has confirmed that the driving force behind this radical reaction is the in situ formation of a salt through the interaction between quinoxalin-2(1H)-ones and acid/HFIP, which exhibits potent oxidation properties. Additionally, it has been observed that the evident hydrogen bonding between quinoxalin-2(1H)-ones and HFIP can effectively mitigate the oxidation potential.

Visible-Light-Mediated [2+2] Photocycloadditions of Alkynes

Visible-light-mediated [2+2] photocycloaddition reaction can be considered an ideal solution due to its green and sustainable properties, and is one of the most efficient methods to synthesize four-membered ring motifs. Although research on the [2+2] photocycloaddition of alkynes is challenging because of the diminished reactivity of alkynes, and the more significant ring strain of the products, remarkable achievements have been made in this field. In this article, we highlight the recent advances in visible-light-mediated [2+2] photocycloaddition reactions of alkynes, with focus on the reaction mechanism and the late-stage synthetic applications. Advances in obtaining cyclobutenes, azetines, and oxetene active intermediates continue to be breakthroughs in this fascinating field of research.

A Paternò-Büchi Reaction of Aromatics with Quinones under Visible Light Irradiation

Reported herein is a Paternò-Büchi reaction of aromatic double bonds with quinones under visible light irradiation. The reactions of aromatics with quinones exposed to blue LED irradiation yielded oxetanes at -78 °C, which was attributed to both the activation of double bonds in aromatics and the stabilization of oxetanes by thiadiazole, oxadiazole, or selenadiazole groups. The addition of Cu(OTf)2 to the reaction system at room temperature resulted in the formation of diaryl ethers via the copper-catalyzed ring opening of oxetanes in situ. Notably, the substrate scope was extended to general aromatics.

Accessing ladder-shape azetidine-fused indoline pentacycles through intermolecular regiodivergent aza-Paternò-Büchi reactions

Small molecules with conformationally rigid, three-dimensional geometry are highly desirable in drug development, toward which a direct, simple-to-complexity synthetic logic is still of considerable challenges. Here, we report intermolecular aza-[2 + 2] photocycloaddition (the aza-Paternò-Büchi reaction) of indole that facilely assembles planar building blocks into ladder-shape azetidine-fused indoline pentacycles with contiguous quaternary carbons, divergent head-to-head/head-to-tail regioselectivity, and absolute exo stereoselectivity. These products exhibit marked three-dimensionality, many of which possess 3D score values distributed in the highest 0.5% region with reference to structures from DrugBank database. Mechanistic studies elucidated the origin of the observed regio- and stereoselectivities, which arise from distortion-controlled C-N coupling scenarios. This study expands the synthetic repertoire of energy transfer catalysis for accessing structurally intriguing architectures with high molecular complexity and underexplored topological chemical space.

Organocatalyzed Enantioselective [2 + 2] Cycloaddition of C,N-Cyclic Ketimines and Allenoates

We report a novel enantioselective and regioselective [2 + 2] cycloaddition of allenoate and C,N-cyclic ketimine catalyzed by a quinidine derivative. The methodology enables the synthesis of fused tricyclic azetidines with a quaternary stereogenic center exhibiting high enantioselectivities. The broad range of substrates demonstrates the generality of the protocol, and the resulting functional products can be easily converted to a variety of valuable synthons. To elucidate the plausible reaction mechanism and how the catalyst affects absolute stereocontrol over the products, we conducted the corresponding density functional theory (DFT) calculations.

The hamburger-shape photocatalyst: thioxanthone-based chiral [2.2]paracyclophane for enantioselective visible-light photocatalysis of 3-methylquinoxalin-2(1H)-one and styrenes

A new thioxanthone-based photocatalyst with a [2.2]paracyclophane skeleton and planar chirality has been developed. The catalyst has been successfully applied in the visible light-mediated enantioselective aza Paternò-Büchi reactions of quinoxalinone and styrenes to produce azetidines. The structures of the catalyst derivatives were unequivocally determined by their single crystal X-ray crystallography analysis.

Enantioselective Synthesis of Cyclobutane Derivatives via Cascade Asymmetric Allylic Etherification/[2 + 2] Photocycloaddition

Chiral cyclobutane presents as a popular motif in natural products and biologically active molecules, and its derivatives have been extensively used as key synthons in organic synthesis. Herein, we report an efficient synthetic method toward enantioenriched cyclobutane derivatives. The reaction proceeds in a cascade fashion involving Ir-catalyzed asymmetric allylic etherification and visible-light induced [2 + 2] cycloaddition. Readily available branched allyl acetates and cinnamyl alcohols are directly used as the substrates under mild reaction conditions, providing a broad range of chiral cyclobutanes in good yields with excellent diastereo- and enantioselectivities (up to 12:1 dr, >99% ee). It is worth noting that all substrates and catalysts were simultaneously added without any separated step in this approach. The gram-scale reaction and diverse transformations of product further enhance the potential utility of this method.

Metal-Free Addition of Alkyl Bromides to Access 3,3-Disubstituted Quinoxalinones Enabled by Visible-Light Photoredox Catalysis

A metal-free addition of unactivated alkyl bromides to quinoxalin-2(1H)-ones is described. This method enables the construction of valuable 3,3-disubstituted dihydroquinoxalin-2(1H)-ones bearing quaternary carbon centers under mild, visible-light photoredox catalysis. High functional group tolerance is observed in both the quinoxalinone and alkyl bromide partners. The ability to scale up this method was demonstrated under photo-flow conditions to enable gram-scale synthesis.

Photosensitized [4+2]- and [2+2]-Cycloaddition Reactions of N-Sulfonylimines

The synthesis of polycyclic compounds is of high interest due to the prevalence of these motifs in drugs and natural products. Herein, we report on the stereoselective construction of 3D bicyclic scaffolds and azetidine derivatives by modulation of N-sulfonylimines to achieve either [4+2]- or [2+2]-cycloaddition reactions. The utility of the method was established by further modulation of the product. Mechanistic studies are also included, which support reaction via Dexter energy transfer.

Mechanisms and Synthetic Strategies in Visible Light-Driven [2+2]-Heterocycloadditions

The synthesis of four membered heterocycles usually requires multi-step procedures and prefunctionalized reactants. A straightforward alternative is the photochemical [2+2]-heterocycloaddition between an alkene and a carbonyl derivative, conventionally based on the photoexcitation of this latter. However, this approach is limited by the absorption profile of the carbonyl, requiring in most of the cases the use of high-energy UV-light, that often results in undesired side reactions and/or the degradation of the reaction components. The development of new and milder visible light-driven [2+2]-heterocycloadditions is, therefore, highly desirable. In this Review, we highlight the most relevant achievements in the development of [2+2]-heterocycloadditions promoted by visible light, with a particular emphasis on the involved reaction mechanisms. The open challenges will also be discussed, suggesting new possible evolutions, and stimulating new methodological developments in the field.

Annulation Reaction of Quinoxalin-2(1H)-ones Initiated by Electrochemical Decarboxylation of N-Arylglycines

A new methodology for the synthesis of tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones has been accomplished through annulation of quinoxalin-2(1H)-ones initiated by electrochemical decarboxylation of N-arylglycines catalyzed by ferrocene. With a pair of oxidative and reductive processes occurring among the substrates and intermediates instead of on the electrodes, the electricity consumption was decreased.

Concerted Enantioselective [2+2] Cycloaddition Reaction of Imines Mediated by a Magnesium Catalyst

Enantioselective [2 + 2] cyclization between an imine and a carbon-carbon double bond is a versatile strategy to build chiral azetidines. However, α-branched allenoates have never been successfully applied in [2 + 2] cyclization reactions with imines, as they always undergo Kwon's [4 + 2] annulation in previous catalytic methods. Herein, a simple in situ generated magnesium catalyst was employed to successfully achieve the enantioselective [2 + 2] cyclization reaction of DPP-imines and α-branched allenoates for the first time. Insightful experiments including KIE experiments, controlled experiments, Hammett plot analysis, and ³¹P NMR studies of initial intermediates indicate that the current [2 + 2] cyclization of imine most likely involves an asynchronous concerted transition state. Further mechanistic investigations by combining kinetic studies, ESI experiments, ³¹P NMR studies of coordination complexes, and controlled experiments on reaction rates under different catalyst loading amounts provided the coordination details for this [2 + 2] cyclization reaction between DPP-imines and α-branched allenoates. This new approach was applied to the synthesis of various chiral aza-heterocycles, including the enantioselective synthesis of the key intermediate of a lipid-lowering agent Ezetimibe.

Synthesis of 10-Phenanthrenols via Photosensitized Triplet Energy Transfer, Photoinduced Electron Transfer, and Cobalt Catalysis

Due to the inert redox activity and high triplet energy, radical chemistry of 1,3-dicarbonyl compounds usually requires prefunctionalization substrates, external oxidant, and high-energy UV light. Here, we report a visible-light-driven photocatalyst/cobaloxime system composed of a photosensitized energy transfer reaction (PEnT) and photoinduced electron transfer reaction (PET) and with an interrupted 6π-photocyclization/dehydrogenative aromatization in one pot to synthesize 10-phenanthrenols. Preliminary mechanistic studies revealed that fac-Ir(ppy)₃ plays the dual roles of energy transfer catalysis for photocycloaddition via 1,2-biradical intermediates of 1,3-dicarbonyl compounds and photoredox/cobaloxime catalysis dehydrogenative aromatization of 1,4-biradical rather than the intermediates via 6π photocyclization in the tandem reaction. In contrast to previous well-established radical chemistry of 1,3-dicarbonyl compounds, we provide a new strategy for the activation of 1,3-dicarbonyl compounds under visible light catalysis, affording a novel cyclization strategy with extremely high atom economy for the synthesis of 10-phenanthrenols.

Photocatalytic Azetidine Synthesis by Aerobic Dehydrogenative [2 + 2] Cycloadditions of Amines with Alkenes

Photocatalytic dehydrogenative [2 + 2] cycloadditions between amines and alkenes were developed that allow for the stereoselective and high-yielding synthesis of functionalized azetidines. The oxidative formal Aza Paternò-Büchi reactions are induced by photoredox-catalyzed aerobic oxidation of dihydroquinoxalinones 1 as the amines, and in the presence of structurally diverse alkenes 3 intermolecular [2 + 2] cyclization to dihydro-1H-azeto[1,2-a]quinoxalin-3(4H)-ones 4 occurs. The utility of the method is illustrated by the selective conversion of amino acid derived dihydroquinoxalinones 1, including oxidation-prone lysine and tryptophan derivatives.

Enantioselective [2 + 2] photocycloaddition of quinolone using a C1-symmetric chiral phosphoric acid as a visible-light photocatalyst

The enantioselective intra- and intermolecular [2 + 2] photocycloaddition of quinolone using a C₁-symmetric chiral phosphoric acid as a visible-light photocatalyst is developed. The thioxanthone chromophore on phosphoric acid plays an important role in both phototransformation and enantioselectivity.

Intramolecular, Visible-Light-Mediated Aza Paternò-Büchi Reactions of Unactivated Alkenes

Azetidines are of particular interest in medicinal chemistry for their favorable properties, including increased resistance to oxidative metabolism and lower lipophilicity. The recent development of [2 + 2] reactions has significantly expanded the limited repertoire of methods for azetidine synthesis, but access to more complex architectures still requires further development. Herein, we report a visible-light-enabled intramolecular [2 + 2] cycloaddition of unactivated alkenes that proved previously unreactive to access tricyclic azetidines with 3D complex structures and high levels of saturation.

Visible Light-Mediated Dearomative Hydrogen Atom Abstraction/ Cyclization Cascade of Indoles

The photochemical synthesis of yet unknown 2‐oxospiro[azetidine‐3,3′‐indolines] (17 examples, 80–95 % yield), 2,4‐dioxospiro[azetidine‐3,3′‐indolines] (eight examples, 87–97 % yield), and 1‐oxo‐1,3‐dihydrospiro[indene‐2,3′‐indolines] (17 examples, 85–97 % yield) is described. Starting from readily accessible 3‐substituted indoles, a dearomatization of the indole core was accomplished upon irradiation at λ=420 nm in the presence of thioxanthen‐9‐one (10 mol%) as the sensitizer. Based on mechanistic evidence (triplet energy determination, deuteration experiments, by‐product analysis) it is proposed that the reaction proceeds by energy transfer via a 1,4‐ or 1,5‐diradical intermediate. The latter intermediates are formed by excited state hydrogen atom transfer from suitable alkyl groups within the C3 substituent to the indole C2 carbon atom. Subsequent ring closure proceeds with pronounced diastereoselectivity to generate a 4‐ or 5‐membered spirocyclic dearomatized product with several options for further functionalization.

Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis

Asymmetric catalysis is a major theme of research in contemporary synthetic organic chemistry. The discovery of general strategies for highly enantioselective photochemical reactions, however, has been a relatively recent development, and the variety of photoreactions that can be conducted in a stereocontrolled manner is consequently somewhat limited. Asymmetric photocatalysis is complicated by the short lifetimes and high reactivities characteristic of photogenerated reactive intermediates; the design of catalyst architectures that can provide effective enantiodifferentiating environments for these intermediates while minimizing the participation of uncontrolled racemic background processes has proven to be a key challenge for progress in this field. This review provides a summary of the chiral catalyst structures that have been studied for solution-phase asymmetric photochemistry, including chiral organic sensitizers, inorganic chromophores, and soluble macromolecules. While some of these photocatalysts are derived from privileged catalyst structures that are effective for both ground-state and photochemical transformations, others are structural designs unique to photocatalysis and offer insight into the logic required for highly effective stereocontrolled photocatalysis.

Construction of Chiral Cyclic Compounds Enabled by Enantioselective Photocatalysis

Chiral cyclic molecules are some of the most important compounds in nature, and are widely used in the fields of drugs, materials, synthesis, etc. Enantioselective photocatalysis has become a powerful tool for organic synthesis of chiral cyclic molecules. Herein, this review summarized the research progress in the synthesis of chiral cyclic compounds by photocatalytic cycloaddition reaction in the past 5 years, and expounded the reaction conditions, characters, and corresponding proposed mechanism, hoping to guide and promote the development of this field.

Photocatalytic synthesis of 10-phenanthrenols via intramolecular cycloaromatization under oxidant-free conditions

A novel tandem photocycloaddition/dehydrogenative aromatization with hydrogen evolution of ortho biaryl-appended 1,3-dicarbonyl compounds for the synthesis of 10-phenanthrenol via cobaloxime catalysis is disclosed.

N,N-Dimethylformamide as Carbon Synthons for the Synthesis of N-Heterocycles: Pyrrolo/Indolo[1,2-a]quinoxalines and Quinazolin-4-ones

N,N-dimethylformamide (DMF) as synthetic precursors contributing especially the methyl, acyl, and amino groups has played a significant role in heterocycle syntheses and functionalization. In this protocol, a wide range of pyrrolo/indolo[1,2-a]quinoxalines and quinazolin-4-ones were obtained in moderate to good yields by using elemental iodine without any metal or peroxides. We considered that N-methyl and N-acyl of DMF participate and complete the reaction separately through different mechanisms, which displayed potential still to be explored of DMF.

Enantioselective synthesis of heterocyclic compounds using photochemical reactions

Different methods for the direct enantioselective photochemical synthesis of heterocycles are presented. Currently, asymmetric catalysis with templates involving hydrogen bonds or metal complexes is intensively investigated. Enzyme catalysis can be simplified under photochemical conditions. For example, in multi enzyme systems, one or more enzyme catalytic steps can be replaced by simple photochemical reactions. Chiral induction in photochemical reactions performed with homochiral crystals is highly efficient. Such reactions can also be carried out with crystalline inclusion complexes. Inclusion of a photochemical substrate and an enantiopure compound in zeolites also leads to enantioselective compounds. In all these methods, the conformational mobility of the photochemical substrates is reduced or controlled. Memory of chirality is a particular case in which a chiral information is temporally lost but the rigid conformations stabilize the molecular structure which leads to the formation of enantiopure compounds. Such studies allows a profound understanding on how particular conformations determine the configuration of the final products.Graphical abstract.

Catalyst-free [2 + 2] photocycloadditions between benzils and olefins under visible light

The catalyst-free [2 + 2] photocycloaddition between benzils and simple olefins is reported. The adoption of visible light proved essential for the transformation, as shorter wavelengths led to uncontrolled decomposition. When cyclic olefins were used, the reaction occurred smoothly to afford the expected oxetanes regio- and stereoselectively after 24 h of irradiation. In contrast, in the case of acyclic olefins, longer reaction times were typically required and small amounts (ca. 20%) of [4 + 2] photocycloadducts and by-products deriving from competitive hydrogen atom abstraction were observed. The selectivity of the transformation could be consistently improved by decreasing the reaction temperature, thus restoring the desired [2 + 2] reactivity. An overall mechanistic picture is also offered based on the chemical and photophysical quenching experiments and the stereochemical output is rationalized based on Griesbeck models.

Recent strategies used in the synthesis of saturated four-membered heterocycles

The importance and prevalance of O-, N-, and S-atom containing saturated four-membered ring motifs in biologically active molecules and potential therapeutics continues to drive efforts in their efficient synthetic preparation. In this review, general and recent strategies for the synthesis of these heterocycles are presented. Due to the limited potential bond disconnections, retrosynthetic strategies are broadly limited to cyclizations and cycloadditions. Nonetheless, diverse approaches for accessing cyclization precursors have been developed, ranging from nucleophilic substitution to C-H functionalization. Innovative methods for substrate activation have been developed for cycloadditions under photochemical and thermal conditions. Advances in accessing oxetanes, azetidines, and thietanes remain active areas of research with continued breakthroughs anticipated to enable future applications.

Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer

For molecules with a singlet ground state, the population of triplet states is mainly possible (a) by direct excitation and subsequent intersystem crossing or (b) by energy transfer from an appropriate sensitizer. The latter scenario enables a catalytic photochemical reaction in which the sensitizer adopts the role of a catalyst undergoing several cycles of photon absorption and subsequent energy transfer to the substrate. If the product molecule of a triplet-sensitized process is chiral, this process can proceed enantioselectively upon judicious choice of a chiral triplet sensitizer. An enantioselective reaction can also occur in a dual catalytic approach in which, apart from an achiral sensitizer, a second chiral catalyst activates the substrate toward sensitization. Although the idea of enantioselective photochemical reactions via triplet intermediates has been pursued for more than 50 years, notable selectivities exceeding 90% enantiomeric excess (ee) have only been realized in the past decade. This review attempts to provide a comprehensive survey on the various photochemical reactions which were rendered enantioselective by triplet sensitization.

Thioxanthone: a powerful photocatalyst for organic reactions

Photoorganocatalysis has been recognised by the organic chemistry community as an important part of photochemistry and catalysis. In general, aromatic ketones constitute key players in this type of catalysis as they are involved in a plethora of examples in the literature. Among the various aromatic ketones, thioxanthone (TX) seems to play a unique role in photochemistry. In comparison with other aromatic ketones, TX has a high triplet energy and a relatively long triplet lifetime, while it has the ability to participate successfully in merger reactions with metal complexes. In this review, we will discuss the photophysical properties of this small organic molecule, as well as the numerous examples of photochemical reactions, where it is employed as a mediator and more specifically in polymerisation reactions, and organic transformations.

Recent Visible Light and Metal Free Strategies in [2+2] and [4+2] Photocycloadditions

When aiming to synthesize molecules with elevated molecular complexity starting from relatively simple starting materials, photochemical transformations represent an open avenue to circumvent analogous multistep procedures. Specifically, light-mediated cycloadditions remain as powerful tools to generate new bonds begotten from non-very intuitive disconnections, that alternative thermal protocols would not offer. In response to the current trend in both industrial and academic research pointing towards green and sustainable processes, several strategies that meet these requirements are currently available in the literature. This Minireview summarizes [2+2] and [4+2] photocycloadditions that do not require the use of metal photocatalysts by means of alternative strategies. It is segmented according to the cycloaddition type in order to give the reader a friendly approach and we primarily focus on the most recent developments in the field carried out using visible light, a general overview of the mechanism in each case is offered as well.

Enantioselective, Visible Light Mediated Aza Paternò-Büchi Reactions of Quinoxalinones

3-Substituted quinoxalin-2(1H)-ones and various aryl-substituted or tethered olefins underwent an enantioselective, inter- or intramolecular aza Paternò-Büchi reaction upon irradiation at λ=420 nm in the presence of a chiral sensitizer (10 mol %). For the intermolecular reaction with 1-arylethenes as olefin components, the scope of the reaction was studied (14 examples, 50-99 % yield, 86-98 % ee). The absolute and relative configuration of the products were elucidated by single-crystal X-ray crystallography. The reaction is suggested to occur by triplet energy transfer in a hydrogen-bonded 1:1 complex between the imine substrate and the catalyst. The intramolecular cycloaddition, consecutive reactions of the product azetidines, and an alternative reaction mode of quinoxalinones were investigated in preliminary experiments.