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Due to the innate highly reactive properties and short life‐time, organic free radicals can often serve as promoters or intermediates to engage in various radical transformations, which are often otherwise difficult to access by ionic pathway‐based mechanisms. With the evolvement of radical chemistry, chiral radical catalyzed‐transformations have recently emerged as an attractive and robust platform for synthesis of chiral molecules of interest. Herein, we highlight several creative and strategic advances in chiral radical catalyst design, cyclization reaction achievements, and future challenges. Key Scientists: In the 1980s, some pioneering studies by Feldman and Oshima revealed that the thiyl radical could catalyze cyclization of vinylcyclopropane with alkenes, providing access to racemic cyclopentanes. In 1996, Brian P. Roberts reported an interesting enantioselective hydrosilylation of prochiral alkenes by using thioglucose‐derived chiral thiyl radical catalysts. Since then, chiral radical catalysis has become an emerging and promising catalytic strategy in organic synthesis. However, this field has not been extensively explored further until the Maruoka group in 2014 achieved the asymmetric C—C bond formation by employing a newly designed indanol core‐based chiral thiyl radical catalyst. In this protocol, an enantioselective radical cyclization of vinylcyclopropanes with electron‐rich alkenes was developed. In 2020, Chen and Xiao disclosed a nitrogen radical‐catalyzed version of this reaction. Later, this strategy was further expanded to metalloradical catalyzed asymmetric radical cascade cyclization reactions by Zhang in 2021. Later on, Wang, Fu and Zhang et al. described a novel class of boryl radical‐catalyzed asymmetric radical cycloisomerization reactions, further showcasing enormous synthetic potential of chiral radical catalysis. This Emerging Topic has focused on asymmetric radical cyclization reactions involving diverse chiral radical catalysts. [ABSTRACT FROM AUTHOR]

The 2,3‐dihydrobenzofuran ring systems are a privileged class of oxygen heterocycles, and their synthesis continues to attract considerable effort. Herein, a visible‐light‐driven photoredox‐catalyzed radical multicomponent cyclization of 2‐vinyl phenols, N‐alkoxypyridinium salts, and sulfur ylides is described. The key to the reaction success involves the use of both N‐alkoxypyridinium salts and sulfur ylides as radical precursors. This redox‐neutral protocol features good functional group tolerance, easily available starting materials, simple operation, and mild reaction conditions, enabling the modular synthesis of various 2,3‐disubstituted dihydrobenzofurans. Mechanistic studies have also elucidated some of the aspects associated with the key radical intermediates. [ABSTRACT FROM AUTHOR]

1,3,5-Trisubstituted 1,3,5-triazinanes (hexahydro-1,3,5-triazines), as stable and readily available surrogates for formaldimines, have found extensive applications for the construction of various nitrogen-containing compounds. The formaldimines, formed in situ from this reagent class, can participate in various aminomethylation and cycloaddition- reactions. This short review presents recent advances in this field with emphasis on the conceptual ideas behind the developed methodologies and the reaction mechanisms. 1 Introduction 2 Aminomethylations with 1,3,5-Triazinanes 3 Cycloadditions with 1,3,5-Triazinanes 3.1 Use of 1,3,5-Triazinanes as Two-Atom Synthons 3.2 Use of 1,3,5-Triazinanes as Three-Atom Synthons 3.3 Use of 1,3,5-Triazinanes as Four-Atom Synthons 3.4 Use of 1,3,5-Triazinanes as Six-Atom Synthons 4 Conclusions [ABSTRACT FROM AUTHOR]

A [4+2] cycloaddition of photogenerated aza-ortho-quinone methides with formaldimines is described for the synthesis of perfluoroalkylated tetrahydroquinazolines. Key to this process is the photocatalytic radical-mediated generation of aza-o-QMs from 2-vinylanilines and perfluoroalkyl radical precursors as well as the in situ release of formaldimines from 1,3,5-triazinanes. [ABSTRACT FROM AUTHOR]

Azlactones, also known as oxazolones, possess multiple reactive sites, and are thus widely used as versatile class of reagents in organic synthesis, particularly in cycloaddition reactions for heterocycle synthesis. However, the use of azlactones as three‐member fragments in cycloaddition reactions/decarboxylation cascade were rarely explored. In this context, a metal‐free cycloaddition/decarboxylation reaction between azlactones and aryldiazonium salts has been achieved for the first time, which provides efficient and facile access to privileged 1,3,5‐trisubstituted 1,2,4‐triazoles. Notably, aryldiazonium salts serve as two‐nitrogen unit rather than the sources of aryl radicals in this transformation, which provides an alternative class of reagents for the construction of N‐heterocycles. [ABSTRACT FROM AUTHOR]

A visible light photoredox-catalyzed carbon radical-mediated strategy for in situ formation of ortho-quinone methides from 2-vinyl phenols is described. This strategy enables a multicomponent cyclization reaction of 2-vinyl phenols, Umemoto's reagent, and sulfur ylides, providing access to trifluoromethylated 2,3-dihydrobenzofurans. [ABSTRACT FROM AUTHOR]

On the basis of the strategy of iminyl radical-mediated C–C bond cleavage, a visible light photocatalytic radical addition/cyclization cascade is described, providing an efficient and regioselective access to cyanoalkylated 1,2,3,4-tetrahydrophenanthrenes. [ABSTRACT FROM AUTHOR]

Abstract: A visible light‐induced photocatalytic radical addition/cyclization reaction of o‐vinyl‐N‐alkoxybenzamides has been accomplished for efficient synthesis of diversely functionalized CF3‐containing iminoisobenzofurans. This external oxidant‐free and mild protocol features ready availability of starting materials, excellent regioselectivity, good substrate scope and functional group tolerance. [ABSTRACT FROM AUTHOR]

Abstract: A copper‐catalyzed regio‐reversed asymmetric [3+2] cycloaddition of iminoesters with nitroolefins is disclosed for the first time. This method enables the facile synthesis of polysubstituted chiral pyrrolidines bearing at least one chiral quaternary center in high yields with excellent regio‐, diastereo‐, and enantioselectivity. The application of chiral P,S ligands and the unique effect of α‐aryl groups on the iminoesters are key to the success of this method. The practicality and versatility of the reaction are also demonstrated. [ABSTRACT FROM AUTHOR]

Abstract: In contrast to the plethora of catalytic systems that enable access to any enantiomers of the chiral products by simply choosing between a pair of enantiomeric or pseudoenantiomeric chiral catalysts, few analogously effective protocols exist for the synthesis of compounds bearing multiple stereogenic centers with full control of the absolute and relative stereochemical configurations. Here, we report the application of our previously developed modular phosphoramidite‐thioether ligands for the copper‐catalyzed diastereodivergent asymmetric 1,3‐dipolar cycloaddition of azomethine ylides and nitroalkenes. Our catalytic system enables wide substrate scope, great stereochemical control, and high reaction efficiency. [ABSTRACT FROM AUTHOR]

A visible-light-driven radical-mediated strategy for the in situ generation of aza- ortho-quinone methides from 2-vinyl-substituted anilines and alkyl radical precursors is described. This process enables an efficient multicomponent reaction of 2-vinylanilines, halides, and sulfur ylides, and has a wide substrate scope and good functional group tolerance. Treatment of the cycloaddition products with a base leads to densely functionalized indoles in a single-flask operation. [ABSTRACT FROM AUTHOR]

An efficient visible light photocatalytic and external oxidant-free N-radical 5-exo cyclization/addition/aromatization cascade of β,γ-unsaturated hydrazones is described, which provides a practical route to various biologically interesting dihydropyrazole-fused benzosultams in satisfactory yields. [ABSTRACT FROM AUTHOR]

A convergent and highly stereoselective [4+2] cycloaddition of in situ-generated ortho-Quinone methides ( o-QMs) and azlactone enols has been successfully developed through a triple Brønsted acid catalysis strategy. This protocol provides an efficient and mild access to various densely functionalized dihydrocoumarins bearing adjacent quaternary and tertiary stereogenic centers in high yields with excellent diastereo- and enantioselectivity. [ABSTRACT FROM AUTHOR]

An unprecedented phototandem catalysis based on a single iridium photocatalyst has been successfully developed. This powerful strategy consists of two mechanistically distinct catalytic cycles, namely, photocatalytic energy transfer (ET) and single electron transfer (SET). The novel protocol allows a rapid and efficient construction of biologically and synthetically important 3-ester-3- hydroxy-2-oxindole derivatives from readily available diazoamides through a cyclization/aerobic oxidation sequence under very mild conditions. [ABSTRACT FROM AUTHOR]

A visible-light-induced photocatalytic aerobic oxidation/ [3+2] cycloaddition/aromatization cascade between secondary amines and isocyanides has been successfully developed. The reaction provides a general and efficient access to diversely substituted imidazoles and imidazo[1,5- a]quinoxalin-4(5H)-ones in good yields under mild conditions. [ABSTRACT FROM AUTHOR]

A photocatalytic formal [3+2] cycloaddition of 2 H-azirines with alkynes has been achieved under irradiation by visible light in the presence of organic dye photocatalysts. This transformation provides efficient access to highly functionalized pyrroles in good yields and has been applied to the synthesis of drug analogues. A primary trial of photocascade catalysis merging energy transfer and redox neutral reactions was shown to be successful. [ABSTRACT FROM AUTHOR]

An unprecedented Zn(OTf)2-catalyzed asymmetric Michael addition/cyclization cascade of 3-nitro-2 H-chromenes with 3-isothiocyanato oxindoles has been disclosed. This transformation provides an efficient access to various synthetically important polycyclic spirooxindoles in a highly stereoselective manner under mild conditions (72-99 % yields, up to >95:5 d.r. and >99 % ee). The reaction leads to the formation of three consecutive stereocenters, including 1,3-nonadjacent tetrasubstituted carbon stereocenters, in a single operation. A bifunctional activation model of the chiral Zn(OTf)2/bis(oxazoline) complex was proposed based on control experiments, wherein the ZnII moiety serves as a Lewis acid and the N atom of the free NH group acts as a Lewis base by a hydrogen-bonding interaction. [ABSTRACT FROM AUTHOR]

Carbocyclic and heterocyclic ring systems are important structural architectures in many bioactive natural products, as well as in a variety of fine chemicals. The utilization of visible-light photocatalytic strategies to construct these ring systems has received considerable attention in the past few years. This microreview presents a brief summary of the use of visible-light photoredox catalytic strategies for the synthesis of carbocyclic and heterocyclic targets. [ABSTRACT FROM AUTHOR]

A Ph3P-catalyzed [3+2] cycloaddition of 2-phenyl-4-arylidene-5(4H)-oxazolones with benzyl 2,3-butadienoate has been developed for the efficient synthesis of structurally diverse and conformationally constrained aspartic acid analogues. [ABSTRACT FROM AUTHOR]

An unprecedented [4+3] cycloaddition of in situ generated azoalkenes with C,N-cyclic azomethine imines has been developed without the use of any catalyst, providing an efficient and mild approach to synthesise highly functionalized 1,2,4,5-tetrazepine derivatives in high yields (76–92%). [ABSTRACT FROM AUTHOR]

An efficient and general approach to oxazino[4,3-a]indole architectures is described. The addition-cyclization cascade of (1H-indol-2-yl)methanols with vinyl sulfonium salts affords oxazino[4,3-a]indole derivatives in high yields. [ABSTRACT FROM AUTHOR]

A stereoselective [4+2] cycloaddition of in situ-generated ortho-quinone methides and azlactone enols is developed based on a triple Broensted acid catalysis strategy. [ABSTRACT FROM AUTHOR]

The article presents information on the synthesis of CF3-containing pyrazolines and isoxazolines through photocatalytic radical trifluoromethylation/cyclization cascade

Review: [63 refs. [ABSTRACT FROM AUTHOR]

A chiral copper triflate/bisoxazoline complex catalyzes the reaction of in situ generated 1,2-diaza-1,3-dienes with enol ethers, methoxy styrene or N-vinylcarbazole. [ABSTRACT FROM AUTHOR]

The title cycloaddition reaction affords highly functionalized 1,2,4,5-tetrazepine derivatives such as (III) and (V) in good yields. [ABSTRACT FROM AUTHOR]

Abstract: An asymmetric formal [4+1] cycloaddition of camphor-derived sulfonium salts with aldimines has been described. The reaction allows for the efficient synthesis of trans-2,3-disubstituted-2,3-dihydrobenzofurans in moderate to good yields with >95:5 dr and up to 98% ee under mild reaction conditions. [Copyright &y& Elsevier]

The title synthesis provides an efficient access to highly functionalized pyrroles in good yields in the presence of organic dye photocatalyst. [ABSTRACT FROM AUTHOR]

This is the first example of [3 + 2] cycloaddition/ ring contraction sequence under transition-metal-free conditions yielding the title spiro compounds. [ABSTRACT FROM AUTHOR]

An abstract of the article "Formal [4 + 1] Cycloaddition of Camphor-Derived Sulfonium Salts with Aldimines: Enantioselective Synthesis of 2,3-Dihydrobenzofurans" by Ying Cheng and colleagues is presented.

The title reaction allows the rapid construction of bispirooxindole skeletons bearing three contiguous stereogenic centers with two quaternary stereocenters in almost quantitative yields and with extraordinarily high enantio- and diastereoselectivities. [ABSTRACT FROM AUTHOR]