Efficient construction of functionalized pyrroloindolines through cascade radical cyclization/intermolecular coupling

Pyrroloindolines are important structural units in nature and the pharmaceutical industry, however, most approaches to such structures involve transition-metal or photoredox catalysts. Herein, we describe the first tandem SET/radical cyclization/intermolecular coupling between 2-azaallyl anions and indole acetamides. This method enables the transition-metal-free synthesis of C3a-substituted pyrroloindolines under mild and convenient conditions. The synthetic utility of this transformation is demonstrated by the construction of an array of C3a-methylamine pyrroloindolines with good functional group tolerance and yields. Gram-scale sequential one-pot synthesis and hydrolysis reactions demonstrate the potential synthetic utility and scalability of this approach.

Photocatalytic C-N bond construction toward high-value nitrogenous chemicals

The construction of carbon-nitrogen bonds is vital for producing versatile nitrogenous compounds for the chemical and pharmaceutical industries. Among developed synthetic approaches to nitrogenous chemicals, photocatalysis is particularly prominent and has become one of the emerging fields due to its unique advantages of eco-sustainable characteristics, efficient process integration, no need for high-pressure H2, and tunable synthesis methods for developing advanced photocatalytic materials. Here, the review focuses on potential photocatalytic protocols developed for the construction of robust carbon-nitrogen bonds in discrepant activation environments to produce high-value nitrogenous chemicals. The photocatalytic C-N bond construction strategies and involved reaction mechanisms are elucidated.

Synthesis of Pyrroloindolines via N-Heterocyclic Carbene Catalyzed Dearomative Amidoacylation of Indole Derivatives

Pyrroloindoline is a privileged heterocyclic motif that is widely present in many natural products and pharmaceutical compounds. Herein, we report an amidyl radical-mediated dearomatization for synthesizing a series of pyrroloindolines via N-heterocyclic carbene catalysis. In this organocatalytic process, the Breslow enolate served as both a single electron donor and an acyl radical equivalent to assemble C3a-acyl pyrroloindolines with a broad substrate scope. Sequential reduction of the indole derivatives provided the analogues of (±)-desoxyeseroline, which exhibited potential anticancer activity.

Aqueous Flavin Photoredox Catalysis Drives Exclusive C3-Alkylation of Indolyl Radical Cations in Route to Unnatural Tryptophan Mimetics

One way to build chemical diversity into indoles is to oxidize them to indolyl radical cations (Ind•+). These intermediates can accept new functional groups across C2-C3 bonds or independently at C2. Less encountered is selective diversification at C3, a position plagued by competing dearomative side reactions. We disclose an aqueous photoredox-catalyzed method for transforming Ind•+ into C3-substituted tryptophan mimetics that uses water as a transient protecting group to guide site-selective C3 alkylation.

Modern Photocatalytic Strategies in Natural Product Synthesis

Modern photocatalysis has proven its generality for the development and functionalization of native functionalities. To date, the field has found broad applications in diverse research areas, including the total synthesis of natural products. This contribution covers recent reports of total syntheses involving as a key step a photocatalytic reaction. Among the selected examples, the photocatalytic processes proceed in a highly chemo-, regio-, and stereoselective manner, thereby allowing the rapid access to structurally complex architectures under light-driven conditions.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

DIPEA-induced activation of OH- for the synthesis of amides via photocatalysis

The development of green protocols for photocatalysis where water acts as a nucleophile, induced by a weak organic base, is difficult to achieve in organic chemistry. Herein, an efficient light-mediated strategy for the synthesis of amides in which a weak organic base acts as a reductant to induce the formation of OH– from water under metal-free conditions is reported. A mechanistic study reveals that the generation of an N,N-diisopropylethylamine (DIPEA) radical via single electron transfer (SET), with the assistance of photocatalyst, that increases the nucleophilicity of the water molecules with respect to the cyanides is essential. Moreover, the removal rate of nitrile in wastewater can be as high as 83%, indicating that this strategy has excellent potential for nitrile degradation.

Under weak organic base condition DIPEA as a reductant to increase the nucleophilicity of H₂O an excellent potential system for nitrile degradation.

Nitrogen-Centered Radicals in Functionalization of sp2 Systems: Generation, Reactivity, and Applications in Synthesis

The chemistry of nitrogen-centered radicals (NCRs) has plentiful applications in organic synthesis, and they continue to expand as our understanding of these reactive species increases. The utility of these reactive intermediates is demonstrated in the recent advances in C-H amination and the (di)amination of alkenes. Synthesis of previously challenging structures can be achieved by efficient functionalization of sp² moieties without prefunctionalization, allowing for faster and more streamlined synthesis. This Review addresses the generation, reactivity, and application of NCRs, including, but not limited to, iminyl, aminyl, amidyl, and aminium species. Contributions from early discovery up to the most recent examples have been highlighted, covering radical initiation, thermolysis, photolysis, and, more recently, photoredox catalysis. Radical-mediated intermolecular amination of (hetero)arenes can occur with a variety of complex amine precursors, generating aniline derivatives, an important class of structures for drug discovery and development. Functionalization of olefins is achievable in high anti-Markovnikov regioselectivity and allows access to difunctionalized structures when the intermediate carbon radicals are trapped. Additionally, the reactivity of NCRs can be harnessed for the rapid construction of N-heterocycles such as pyrrolidines, phenanthridines, quinoxalines, and quinazolinones.

Visible-light induced dearomatization reactions

Dearomatization reactions provide rapid access to structurally complex three-dimensional molecules from simple aromatic compounds. Plenty of reports have demonstrated their utilities in the synthesis of natural products, medicinal chemistry, and materials science in the last decades. Recently, visible-light mediated photocatalysis has emerged as a powerful tool to promote many kinds of transformations. The dearomatization reactions induced by visible-light have also made significant progress during the past several years. This review provides an overview of visible-light induced dearomatization reactions classified based on the manner in which aromaticity is disrupted.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Visible-light-mediated regioselective ring-opening hydrogenolysis of donor–acceptor cyclopropanes with DIPEA and H2O

A visible-light-mediated regioselective ring-opening hydrogenolysis of donor–acceptor cyclopropanes has been developed for the rapid construction of alkylated aryl ketones in good yields with excellent functional group compatibility.

Tandem elimination-oxidation of tertiary benzylic alcohols with an oxoammonium salt

Tertiary benzylic alcohols react with oxoammonium salts, undergoing a tandem elimination/allylic oxidation to provide an allylic ether product in a single step. This mode of reactivity provides a rapid entry into allylic ethers from certain benzylic tertiary alcohols. The allylic ether may be cleaved under reductive conditions to reveal the allylic alcohol.

Palladium-Catalyzed One-Pot Synthesis of Pyrroloindolines from 2-Alkynyl Arylazides and Thioacetamides

A novel and efficient synthetic method for the preparation of various pyrroloindolines from 2-alkynyl arylazides and thioacetamides was developed. The reaction was carried out in a one-pot process under mild reaction conditions to afford the products in moderate to good yields, which has the potential to be used in organic synthesis.

Strategic Use of Visible-Light Photoredox Catalysis in Natural Product Synthesis

Recent progress in the development of photocatalytic reactions promoted by visible light is leading to a renaissance in the use of photochemistry in the construction of structurally elaborate organic molecules. Because of the rich functionality found in natural products, studies in natural product total synthesis provide useful insights into functional group compatibility of these new photocatalytic methods as well as their impact on synthetic strategy. In this review, we examine total syntheses published through the end of 2020 that employ a visible-light photoredox catalytic step. To assist someone interested in employing the photocatalytic steps discussed, the review is organized largely by the nature of the bond formed in the photocatalytic step.

Multi-Photocatalyst Cascades: Merging Singlet Oxygen Photooxygenations with Photoredox Catalysis for the Synthesis of Alkaloid Frameworks

The development of photocascades that rapidly transform simple and readily accessible furan substrates into polycyclic alkaloid frameworks or erythrina natural products is described. Each of the sequences developed makes use of photocatalyzed energy transfer processes, which generate singlet oxygen, to set up the substrates for the second photocatalyzed reaction, wherein electron transfer generates carbon-centered radicals for the cyclizations that give the final complex frameworks. A chemical switch has been developed that can "switch off" one photocatalyst; thus, allowing a second photocatalyst to take over control of the sequence. As a corollary, this strategy represents the first time it has been possible to use multiple photocatalysts in photocascades, and, as such, it expands significantly the reactions that can be included in such cascades and the order in which they can be initiated.

Organophotocatalytic dearomatization of indoles, pyrroles and benzo(thio)furans via a Giese-type transformation

Accessing fascinating organic and biological significant indolines via dearomatization of indoles represents one of the most efficient approaches. However, it has been difficult for the dearomatization of the electron deficient indoles. Here we report the studies leading to developing a photoredox mediated Giese-type transformation strategy for the dearomatization of the indoles. The reaction has been implemented for chemoselectively breaking indolyl C=C bonds embedded in the aromatic system. The synthetic power of this strategy has been demonstrated by using structurally diverse indoles bearing common electron-withdrawing groups including (thio)ester, amide, ketone, nitrile and even aromatics at either C2 or C3 positions and ubiquitous carboxylic acids as radical coupling partner with high trans-stereoselectivity (>20:1 dr). This manifold can also be applied to other aromatic heterocycles including pyrroles, benzofurans and benzothiophenes. Furthermore, enantioselective dearomatization of indoles has been achieved by a chiral camphorsultam auxiliary with high diastereoselectivity.

Visible-light induced divergent dearomatization of indole derivatives: controlled access to cyclobutane-fused polycycles and 2-substituted indolines

A visible-light-induced catalytic, divergent dearomative functionalization of indole derivatives is achieved, thereby leading to the formation of cyclobutane-fused polycycles and 2-substituted indolines in a controllable fashion.

Visible-Light-Promoted Cascade Radical Cyclization: Synthesis of Chroman-4-ones and Dihydroquinolin-4-ones

A novel visible-light photoredox catalysis protocol for the effective and efficient synthesis of 3-substituted chroman-4-ones and 2,3-dihydroquinolin-4(1H)-ones via tandem radical addition/cyclization of alkenyl aldehydes has been described. This reaction features a broad substrate scope, mild reaction conditions, and good functional group tolerance character.

One-step assembly of alkoxypyrroloindolines via iodine-catalyzed alkoxycyclization of indole derivatives

Herein we report an iodine-catalyzed alkoxycyclization of tryptamine derivatives under mild reaction conditions. This method distinguished itself by providing a catalytic, one-step assembly of diversely functionalized C3a-alkoxypyrroloindolines as well as dihydrofuran and lactone fused indolines. Mechanistic studies suggest that an ionic pathway is operative and this probably accounts for the diastereospecificity of all isolated cycloadducts.

Photoinitiated Cascade for Rapid Access to Pyrroloquinazolinone Core of Vasicinone, Luotonins, and Related Alkaloids

Furylimines of aromatic o-nitro aldehydes undergo a photoinduced cascade transformation offering rapid atom- and step-economical access to complex polyheterocyclic scaffolds possessing a privileged pyrroloquinazolinone core.

Visible light-mediated chemistry of indoles and related heterocycles

The impact of visible light-promoted chemistry on the functionalization of indoles and related heterocycles is reviewed.

Metal-free photocatalytic thiol–ene/thiol–yne reactions

We report a metal-free photocatalytic approach to the synthesis of glycoconjugates, highlighting the mild nature of the reaction conditions.