Photoredox Decarboxylative Alkylation/(2+2+1) Cycloaddition of 1,7‐Enynes: A Cascade Approach Towards Polycyclic Heterocycles Using N ‐(Acyloxy)phthalimides as Radical Source

Formation of cyclopenta[c]quinolines through visible-light-induced photoredox cascade bis-annulations of 1,7-enynes with sulfoxonium ylides

A novel visible-light-driven photoredox-catalyzed cascade bicyclization of 1,7-enynes with aqueous sulfoxonium ylides is reported. The reaction is highly chemoselective with three new C-C bonds, two new rings, and an all-carbon quaternary stereocenter constructed in a one-pot fashion. This mild protocol features a remarkably broad substrate scope with good functional group tolerance, providing a general and practical approach to access various cyclopenta[c]quinolines.

Recent advances of 5-endo-trig radical cyclization: promoting strategies and applications

5-endo-trig radical cyclization has long been recognized as one of the most straightforward ways for the construction of densely functionalized five-membered rings. Nevertheless, according to Baldwin's rules, the 5-endo-trig radical cyclization is kinetically disfavored due to stereoelectronic effects and thus usually proceeds via a slow rate, which renders its application a challenging task. In recent years, with the emergence of efficient radical generation methods and effective cyclization strategies, 5-endo-trig radical cyclization has been successfully accelerated to a synthetically useful rate and has been utilized in the access of diverse five-membered carbo- and heterocyclic compounds. This review comprehensively summarizes the methodologies involving the 5-endo-trig radical cyclization process, with particular emphasis on the elucidation of the promoting strategies, which include the polar effect, geometrical constraints, spin delocalization effect, and persistent radical effect. Each of these strategies is discussed in detail, with illustrative examples from recent literature studies to highlight their practical applications and effectiveness. It is anticipated that the in-depth understanding of the 5-endo-trig radical cyclization provided by this review would inspire further innovation of this privileged reaction mode and expand its applications. Moreover, the potent ring-closure-promoting strategies revealed herein would also contribute to achieving other challenges of cyclizations with particular significance for organic synthesis.

Photocatalytic, α-Aminoalkyl Radical-Mediated, Methylene-Extrusive Ring-Closing Transformation of o-Alkynyl and o-Cyano Acrylamides

Herein we report a visible-light-induced, α-aminoalkyl radical-mediated cascade reaction of 1,7-enynes that establishes a unique ring-closing enyne transformation pathway which occurs with concomitant loss of a methylene moiety. The α-aminoalkyl radical derived from N,N-dimethylaniline was demonstrated to be a traceless promoter of enyne reorganization leading to 4-alkylquinolinones. The reaction can also be extended to nitrile-substituted acrylamide systems, leading to carbostyrils. Experiments with deuterated N,N-dimethylaniline-d6 (PhN(CD3)2) established the involvement of 1,5-H atom transfer in the mechanism.

Synthesis of Acyl Cyclopentaquinolinones through Simultaneous Construction of the Heterocyclic Scaffold and Introduction of the Acyl Group

Presented herein is an effective and concise synthesis of acyl cyclopentaquinolinone derivatives via the cascade reactions of N-(o-ethynylaryl)acrylamides with α-diazo carbonyl compounds. The formation of product involves a visible light-induced radical formation from α-diazo carbonyl compound followed by its addition onto the acrylamide moiety to trigger double radical annulation, single-electron oxidation, and β-elimination. To our knowledge, this is the first example in which the cyclopentaquinolinone scaffold was constructed along with the introduction of an acyl group under visible light irradiation conditions. Compared with literature methods for similar purpose, this newly developed protocol has advantages such as readily accessible substrates, mild reaction conditions, valuable products, concise synthetic procedure, and high sustainability. With all these merits, this method is expected to find wide applications in the construction of related acyl heterocyclic skeletons.

Synthesis of O-Heterocycle Spiro-Fused Cyclopentaquinolinones and Cyclopentaindenes through Visible Light-Induced Radical Cyclization Reactions

Presented herein is an effective and sustainable synthesis of O-heterocycle spiro-fused cyclopentaquinolinone and cyclopentaindene derivatives through light-driven cascade reactions of N-(o-ethynylaryl)acrylamides or 2-(2-(phenylethynyl)benzyl)acrylate with various O-heterocycles. Experimental mechanistic studies revealed that these reactions are initiated by visible light-induced radical formation from O-heterocycle and its regioselective addition onto the acrylamide or acrylate moiety followed by 6-exo-dig and 5-endo-trig cascade radical annulation, which is terminated by single electron oxidation and proton elimination. Compared with previously reported synthetic methods for similar purposes, this newly developed protocol has advantages such as a broad substrate scope, extremely mild reaction conditions, excellent atom-economy, high efficiency, and good compatibility with diverse functional groups. With all of these merits, this method is expected to find wide applications in the related research arena.

A modular scaffold for triggerable and tunable nitroxyl (HNO) generation with a fluorescence reporter

Nitroxyl (HNO) is a short-lived mediator of cell signalling and can enhance the sulfane sulfur pool, a cellular antioxidant reservoir, by reacting with hydrogen sulfide (H₂S). Here, we report esterase-activated HNO-generators that are suitable for tunable HNO release and the design of these donors allows for real-time monitoring of HNO release. These tools will help gain a better understanding of the cross-talk among short-lived gaseous signalling molecules that have emerged as major players in health and disease.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

An umpolung strategy for intermolecular [2 + 2 + 1] cycloaddition of aryl aldehydes and nitriles: a facile access to 2,4,5-trisubstituted oxazoles

We have described the first example of an umpolung strategy for intermolecular [2 + 2 + 1] cycloaddition between two aryl aldehydes and a nitrile under the influence of TMSOTf that proceeds through the formation of N–C, O–C and C–C bonds providing a simple synthetic protocol for obtaining 2,4,5-trisubstituted oxazoles.

An unprecedented intermolecular [2 + 2 + 1] cycloaddition strategy between two aryl aldehydes and a nitrile, wherein one of the aryl aldehydes serves as a carbanion (or equivalent) in the presence of TMSOTf for obtaining oxazole framework is presented.

Electron donor–acceptor complex-catalyzed photoredox reactions mediated by DIPEA and inorganic carbonates

A DIPEA–NHPI ester–inorganic carbonate catalytic EDA complex is reported as an efficient and sustainable radical generation platform for developing photocatalytic reactions.

Recent Advances on Photoinduced Cascade Strategies for the Synthesis of N-Heterocycles

Over the last decade, visible-light photocatalysis has proved to be a powerful tool for the construction of N-heterocyclic frameworks, important constituents of natural products, insecticides, pharmacologically relevant therapeutic agents and catalysts. This account highlights recent developments and established methods towards the photocatalytic cascades for preparation of different classes of N-heterocycles, giving emphasis on our contribution to the field.

Cascade Reactions Assisted by Microwave Irradiation: Ultrafast Construction of 2-Quinolinone-Fused γ-Lactones from N-(o-Ethynylaryl)acrylamides and Formamide

An ultrafast (10 s) methodology to construct novel highly functionalized 2-quinolinones from N-(o-ethynylaryl)acrylamides (1,7-enynes) is described for the first time. Microwave irradiation enabled the ultrafast synthesis of 2-quinolinone-fused γ-lactones from Fenton's reagents in formamide. After six key consecutive reactions, including a diastereoselective step, 2-quinolinone-fused γ-lactones were obtained in good overall yield (up to 46%; 10 s).

Photocatalyzed Intramolecular [2+2] Cycloaddition of N-Alkyl-N-(2-(1-arylvinyl)aryl)cinnamamides

N-Alkyl-N-(2-(1-arylvinyl)aryl)cinnamamides are converted into natural product inspired scaffolds via iridium photocatalyzed intramolecular [2+2] photocycloaddition. The protocol has a broad substrate scope, whilst operating under mild reaction conditions. Tethering four components forming a trisubstituted cyclobutane core builds rapidly high molecular complexity. Our approach allows the design and synthesis of a variety of tetrahydrocyclobuta[c]quinolin-3(1H)-ones, in yields ranging between 20-99 %, and with excellent regio- and diastereoselectivity. Moreover, it was demonstrated that the intramolecular [2+2]-cycloaddition of 1,7-enynes-after fragmentation of the cyclobutane ring-leads to enyne-metathesis-like products.

Copper and manganese co-mediated cascade aza-Michael addition/cyclization and azidation of 1,3-enynes: regioselective synthesis of fully substituted azido pyrroles

A Cu and Mn co-mediated aerobic oxidative cyclization and azidation reaction of 1,3-enynes with amines and trimethylsilyl azide has been developed to synthesize fully substituted azido pyrroles.