5-Endo-trig radical cyclizations: disfavored or favored processes?

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.

Iron-Mediated Hydrogen Atom Transfer Radical Cyclization of Alkenyl Indoles and Pyrroles Gives Their Fused Derivatives: Total Synthesis of Bruceolline E and H

The iron-mediated hydrogen atom transfer (HAT) reaction is efficaciously employed for the synthesis of dihydropyrroloindoles and dihydropyrrolizines via 5-exo-trig radical cyclization where indoles and pyrroles are used as an acceptor. This radical approach has also been extended for the synthesis of tetrahydrocyclopenta[b]indolones via the Baldwin-disfavored 5-endo-trig cyclization pathway. The formal synthesis of bruceolline J and the total synthesis of bruceollines E and H have been expeditiously carried out by employing the former strategy.

300-fold higher neuro- and immunotoxicity from low-redox transformation of carbamazepine

Current challenges in (eco)toxicology are in understanding the transformation of (reactive) substances, and how transformation affects toxic modes of action. Empirical assessment of transformation products of, practically an infinite number of substances, via experimentation, is impossible. Predicting transformation products for (benchmarking) compounds from conditions, facilitates risk analyses. This study applied calculus to predict transformation products of an important environmental and medicinal/toxicological marker, carbamazepine. As radicals are ubiquitous in humans and the environment, we looked into radical-mediated transformations of carbamazepine as a benchmark. We calculated proportions of their speciation states as function of redox conditions, which we took as pH and O2 concentration, describing transformation via covalent and ionic interactions. Formation of ring-contracted products with neuro-immunological activity is thermodynamically favored under anaerobic conditions and at low pH. Experimentally observed product distributions and toxicities reflect that pattern. Our predictive method may support toxicity predictions for other substances and conditions 'similar' to the current case study via interpolation. This paves the way for a more coherent, effective and easier risk assessment of transformation products.

Photocatalytic redox-neutral selective single C(sp3)-F bond activation of perfluoroalkyl iminosulfides with alkenes and water

Visible-light-promoted site-selective and direct C-F bond functionalization of polyfluorinated iminosulfides was accomplished with alkenes and water under redox-neutral conditions, affording a diverse array of γ-lactams with a fluoro- and perfluoroalkyl-substituted carbon centre. A variety of perfluoroalkyl units, including C2F5, C3F7, C4F9, and C5F11 underwent site-selective defluorofunctionalization. This protocol allows high chemoselectivity control and shows excellent functional group tolerance. Mechanistic studies reveal that the remarkable changes of the electron geometries during the defluorination widen the redox window between the substrates and the products and ensure the chemoselectivity of single C(sp3)-F bond cleavage.

Access to cyclopentenones via copper-catalyzed 5-endo trifluoromethylcarbocyclization of ynones

A copper-catalyzed 5-endo-selective trifluoromethylcarbocyclization of ynones is realized for the direct construction of trifluoromethylated cyclopentenones.

Acidic pH-Activatable Visible to Near-Infrared Switchable Ratiometric Fluorescent Probe for Live-Cell Lysosome Targeted Imaging

Here, we have designed and synthesized acidic pH-activatable visible to NIR switchable ratiometric pH-sensitive fluorescent dye. The design consists of a cell-permeable organic probe containing a lysosome targeting morpholine functionality and an acidic pH-activatable oxazolidine moiety. The visible closed oxazolidine form (λ_abs 418 nm) can be switched to the highly conjugated NIR Cy-7 form (λ_abs 780 nm) through ring opening of the oxazolidine moiety at acidic pH. This switching of the ratiometric fluorescent probe is highly reversible and can be controlled by pH. NMR, UV/vis, and fluorescence spectroscopies allowed monitoring of pH switching behavior of the probe. This bioresponsive in situ acidic organelle activatable fluorophore showed reversible pH-switchable ratiometric optical properties, high photostability, huge bathochromic emission shift of 320 nm from basic to acidic pH, off-to-on narrow NIR absorption and emission bands with enhanced molar extinction coefficient at lysosomal pH, good quantum yield, low cytotoxicity, and targeted imaging ability of live cell lysosomes with ideal pK_a. The report demonstrated ratiometric imaging with improved specificity of the acidic lysosome while minimizing signals at the NIR region from nontargeted neutral or basic organelles in human carcinoma HeLa and A549 as well as rat healthy H9c2(2-1) live cells, which is monitored by confocal laser scanning microscopy.

5-Endo Trig Oxidative Radical Cyclizations of Ugi-3CR Products toward 1,4-Imidazolidinones

A 5-endo trig oxidative radical cyclization of benzylamine-derived Ugi three-component reaction products rapidly affords imidazolidinones with three diversity elements. This adaptation of our previously described multicomponent reaction-oxidation methodology further showcases manipulation of the diversity elements in multicomponent reaction products via oxidative radical cyclizations, which generates highly decorated privileged heterocycles.

Sequential One-Pot Synthesis of 3-Arylbenzofurans from N-Tosylhydrazones and Bromophenol Derivatives

A divergent and efficient one-pot sequence allowing direct access to 3-arylbenzofuran derivatives has been developed. The process, involving N-tosylhydrazones and bromophenols, proceeds via a palladium-catalyzed Barluenga-Valdés cross-coupling, followed by an aerobic, copper-catalyzed, radical cyclization to form Csp²-Csp² and O-Csp² bonds. 3-Arylated benzofurans bearing various substituents were obtained with good to excellent yields (up to 90%). Mechanistic investigation strongly supports a radical process for the cyclization step.

Catalytic Cycloisomerization onto a Carbonyl Oxygen

We have found that terminal N-vinylindoles bearing cycloalkanone substituents are excellent hydrogen atom acceptors, generating α-aminyl radicals with a variety of catalysts (Co(II)/H2 or Co(III)Cl precatalysts with silane reductants). These radicals can be converted to internal vinylindoles but eventually add to the oxygen of the cycloalkanone substituents. These cyclizations eventually furnish a densely functionalized dihydrofuran (a net cycloisomerization). The internal vinylindoles are slowly converted to the dihydrofurans, but the final cycloisomerization/isomerization ratio is affected by the size of the cycloalkanone ring (seven- and eight-membered rings give the highest ratio). These results demonstrate how HAT can isomerize substrates in nonintuitive ways, here leading to the first HAT-promoted formation of a C-O bond.

Tandem Anionic oxy-Cope Rearrangement/Oxygenation Reactions as a Versatile Method for Approaching Diverse Scaffolds

Tandem anionic oxy-Cope rearrangement/radical oxygenation reactions provide δ,ϵ-unsaturated α-(aminoxy) carbonyl compounds, which serve as convenient precursors to diverse compound classes. Functionalized carbocycles are accessible by very rare all-carbon 5-endo-trig cyclizations, but also common 5-exo-trig radical cyclizations, based on the persistent radical effect. The tandem reactions can be further extended by highly diastereoselective allylation or reduction steps to give complex scaffolds.

The Fascinating Chemistry of α-Haloamides

The aim of this review is to highlight the rich chemistry of α-haloamides originally mainly used to discover new C-N, C-O and C-S bond forming reactions, and later widely employed in C-C cross-coupling reactions with C(sp3), C(sp2) and C(sp) coupling partners. Radical-mediated transformations of α-haloamides bearing a suitable located unsaturated bond has proven to be a straightforward alternative to access diverse cyclic compounds by means of either radical initiators, transition metal redox catalysis or visible light photoredox catalysis. On the other hand, cycloadditions with α-halohydroxamate-based azaoxyallyl cations have garnered significant attention. Moreover, in view of the important role in life and materials science of difluoroalkylated compounds, a wide range of catalysts has been developed for the efficient incorporation of difluoroacetamido moieties into activated as well as unactivated substrates.

Synthesis of Cyclobutane-Fused Tetracyclic Scaffolds via Visible-Light Photocatalysis for Building Molecular Complexity

We describe the synthesis through visible-light photocatalysis of novel functionalized tetracyclic scaffolds that incorporate a fused azabicyclo[3.2.0]heptan-2-one motif, which are structurally interesting cores with potential in natural product synthesis and drug discovery. The synthetic approach involves an intramolecular [2 + 2] cycloaddition with concomitant dearomatization of the heterocycle via an energy transfer process promoted by an iridium-based photosensitizer, to build a complex molecular architecture with at least three stereogenic centers from relatively simple, achiral precursors. These fused azabicyclo[3.2.0]heptan-2-one-based tetracycles were obtained in high yield (generally >99%) and with excellent diastereoselectivity (>99:1). The late-stage derivatization of a bromine-substituted, tetracyclic indoline derivative with alkyl groups, employing a mild Negishi C-C bond forming protocol as a means of increasing structural diversity, provides additional modularity that will enable the delivery of valuable building blocks for medicinal chemistry. Density functional theory calculations were used to compute the T₁-S₀ free energy gap of the olefin-tethered precursors and also to predict their reactivities based on triplet state energy transfer and transition state energy feasibility.

H· Transfer-Initiated Synthesis of γ-Lactams: Interpretation of Cycloisomerization and Hydrogenation Ratios

A cobaloxime/H2 system used to synthesize valuable γ-lactams from acrylamide molecules is described. In addition to cycloisomerized lactams, linear hydrogenated products were also observed. The amounts of the hydrogenation product were observed to correlate with the bulk of the substituent on the acrylamide nitrogen. Further analysis of the product distributions with experimental and computational studies suggested that while cyclization can occur from one C=C acrylamide rotamer, hydrogenation can occur from both. This observation was further evinced through calculation of the hydrogenation rate constant, which was observed to be ca. 102 faster than previously determined for a related system using n Bu3SnH.

Ground-state dioxygen undergoes metal-free [3 + 2]-annulations with allenes and nitrosoarenes under ambient conditions

The cycloadditions of molecular dioxygen with neutral π-bond motifs rely heavily on singlet-state ¹O₂, whereas ground state ³O₂ is chemically inactive. Here we report novel [3 + 2]-annulations among ground-state ³O₂ (1 bar), allenes, and nitrosoarenes at low temperatures, efficiently yielding dioxygen-containing oxacycles. With less hindered 1-arylallene derivatives, these dioxygen species undergo skeletal rearrangement to 3-hydroxy-1-ketonyl-2-imine oxides. These cycloadditions represent valuable one-pot O,N,O-trifunctionalizations of allenes. Our EPR experiments confirm the presence of 1,4-diradical intermediates from an allene/nitrosoarene mixture, which manifest the hidden diradical properties of nitrosoarenes.

Axially Chiral Enamides: Substituent Effects, Rotation Barriers, and Implications for their Cyclization Reactions

The barrier to rotation around the N-alkenyl bond of 38 N-alkenyl-N-alkylacetamide derivatives was measured (ΔG(⧧) rotation varied between <8.0 and 31.0 kcal mol(-1)). The most important factor in controlling the rate of rotation was the level of alkene substitution, followed by the size of the nitrogen substituent and, finally, the size of the acyl substituent. Tertiary enamides with four alkenyl substituents exhibited half-lives for rotation between 5.5 days and 99 years at 298 K, sufficient to isolate enantiomerically enriched atropisomers. The radical cyclizations of a subset of N-alkenyl-N-benzyl-α-haloacetamides exhibiting relatively high barriers to rotation round the N-alkenyl bond (ΔG(⧧) rotation >20 kcal mol(-1)) were studied to determine the regiochemistry of cyclization. Those with high barriers (>27 kcal mol(-1)) did not lead to cyclization, but those with lower values produced highly functionalized γ-lactams via a 5-endo-trig radical-polar crossover process that was terminated by reduction, an unusual cyclopropanation sequence, or trapping with H2O, depending upon the reaction conditions. Because elevated temperatures were necessary for cyclization, this precluded study of the asymmetric transfer in the reaction of individual atropisomers. However, enantiomerically enriched atropsiomeric enamides should be regarded as potential asymmetric building blocks for reactions that can be accomplished at room temperature.

Copper(I)/Ligand-Catalyzed 5-endo Radical Cyclization-Aromatization of 2,2,2-Trichloroethyl Vinyl Ethers: Synthesis of 2,3-Difunctionalized 4-Chlorofurans

Copper(I)/ligand-catalyzed one pot synthesis of highly substituted 2,3-difunctionalized-4-chlorofurans has been reported. The reaction proceeds via a Cu(I)-catalyzed regioselective 5-endo-trig radical cyclization of 2,2,2-trichloroethyl vinyl ethers followed by the base-promoted dehydrochlorination. The success of the kinetically disfavored 5-endo cyclization was attributed to the formation of captodatively stabilized radical intermediate in the cyclization step and relatively high reaction temperature. Synthetic application of this protocol was also demonstrated in the preparation of alkyl and aryl substituted 4-chlorofuranonapthoquinones.

The Missing C1-C5 Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes

The last missing example of the four archetypical cycloaromatizations of enediynes and enynes was discovered by combining a twisted alkene excited state with a new self-terminating path for intramolecular conversion of diradicals into closed-shell products. Photoexcitation of aromatic enynes to a twisted alkene triplet state creates a unique stereoelectronic situation, which is facilitated by the relief of excited state antiaromaticity of the benzene ring. This enables the usually unfavorable 5-endo-trig cyclization and merges it with 5-exo-dig closure. The 1,4-diradical product of the C1-C5 cyclization undergoes internal H atom transfer that is coupled with the fragmentation of an exocyclic C-C bond. This sequence provides efficient access to benzofulvenes from enynes and expands the utility of self-terminating aromatizing enyne cascades to photochemical reactions. The key feature of this self-terminating reaction is that, despite the involvement of radical species in the key cyclization step, no external radical sources or quenchers are needed to provide the products. In these cascades, both radical centers are formed transiently and converted to the closed-shell products via intramolecular H-transfer and C-C bond fragmentation. Furthermore, incorporating C-C bond cleavage into the photochemical self-terminating cyclizations of enynes opens a new way for the use of alkenes as alkyne equivalents in organic synthesis.

Bond rotation dynamics of enamides: the effect of the acyl group and potential for chirality transfer during 5-endo trig radical cyclizations

Barriers to rotation of the N-alkenyl bond in a series of N-cycloalkenyl-N-benzyl acetamide derivatives have been measured in different solvents by variable-temperature NMR experiments. The barriers range from 9.7 to 14.2 kcal/mol, depending on substituents on the acetamide acyl group. Polar solvents such as chloroform and methanol increase the barrier to rotation compared to nonpolar solvents such as toluene. The barrier to rotation of "mimics" for acetamide-based radicals are estimated. The relative order of the values of k(rot) for different acyl groups parallels their reported Taft E(s) paramaters. For successful chirality transfer in 5-endo trig radical cyclization, it is evident that rotations would need to be significantly slower than those reported here.