Cascade Cyclization Research Articles

Photocatalytic Synthesis of Indanone, Pyrone, and Pyridinone Derivatives with Diazo Compounds as Radical Precursors.

We disclose herein a photocatalytic radical cascade cyclization of diazoalkanes for the divergent synthesis of important carbocycles and heterocycles. Under the optimal reaction conditions, various indanone, pyrone, and pyridinone derivatives can be obtained in moderate to good yields. Mechanistic experiments support the formation of carbon-centered radicals from diazoalkanes through the proton-coupled electron transfer process. Scale-up reaction using continuous flow technology and useful downstream application of the formed heterocycles further render the strategy attractive and valuable.

Asymmetric Organocatalyzed Cyclization Cascade Reactions of 3,3-Difluoro-2-aryl-3H-indoles and Enamides.

The direct functionalization of β-C(sp2)-H bonds in enamides has garnered increasing attention within the realm of organic synthesis. However, these remarkable advancements are predominantly dependent on transition metals; limited success has been achieved via organocatalytic catalysis. Herein, we report a CPA-catalyzed β-C(sp2)-H functionalization of enamides cascade intramolecular cyclization to synthesize the chiral dihydropyrimido[1,6-a]indoles bearing gem-difluoromethylene. Moreover, this methodology enables the synthesis of diverse chiral dihydropyrimido[1,6-a]indoles with outstanding enantioselectivities in moderate to high yields.

Tethering Three Radical Cascades for Controlled Termination of Radical Alkyne peri-Annulations: Making Phenalenyl Ketones without Oxidants.

Although Bu3Sn-mediated radical alkyne peri-annulations allow access to phenalenyl ring systems, the oxidative termination of these cascades provides only a limited selection of the possible isomeric phenalenone products with product selectivity controlled by the intrinsic properties of the new cyclic systems. In this work, we report an oxidant-free termination strategy that can overcome this limitation and enable selective access to the full set of isomerically functionalized phenalenones. The key to preferential termination is the preinstallation of a "weak link" that undergoes C-O fragmentation in the final cascade step. Breaking a C-O bond is assisted by entropy, gain of conjugation in the product, and release of stabilized radical fragments. This strategy is expanded to radical exo-dig cyclization cascades of oligoalkynes, which provide access to isomeric π-extended phenalenones. Conveniently, these cascades introduce functionalities (i.e., Bu3Sn and iodide moieties) amenable to further cross-coupling reactions. Consequently, a variety of polyaromatic diones, which could serve as phenalenyl-based open-shell precursors, can be synthesized.

Recent Advances in Cyclization Reactions of 1,6‐Enynes

AbstractElaborated molecular architectures, specifically those bearing one or more carbon stereocenters, stand as an important class of carbocyclic and heterocyclic frameworks because they are frequently occurring core structures in numerous natural products and biologically active pharmaceutical molecules. Over the past few decades, the development of versatile synthetic approaches via cascade cyclization reactions of 1,6‐enynes for the construction of a series of fused and spiro compounds has been the focus of a great deal of research initiatives. These synthesis strategies are peculiarly fascinating in the context of the assembly of a wide array of pharmaceutical molecules, natural products, agrochemicals, and functional materials. In this review, recent developments in the transformations of 1,6‐enynes with diverse coupling reagents are summarized since 2018, which could be divided into five categories: 1) Introduction; 2) Transition metal catalyzed cyclization of 1,6‐enynes; 3) Metal‐free cyclization of 1,6‐enynes; 4) Visible‐light‐induced cyclization of 1,6‐enynes; 5) Electrocatalytic cyclization of 1,6‐enynes.

Hybrid Heterocycles: Ag(I)-Catalyzed C-C/C-N/C-O Coupled Cascade Dual Cyclization to Valuable Indolo-4H-indolones and Indolo-4H-chromenes.

Herein, we report a highly efficient Ag(I)-catalyzed indolyzation with Friedel-Crafts alkylation through a cascade cyclization strategy for accessing valuable hybrid heterocycles for the first time. This general strategy consists of forming four C-C/C-N/C-O bonds toward dual annulation reactions of 2-alkynylanilines with methyl benzoate-2-carboxaldehydes and aromatic amines, as well as with salicylaldehydes and malononitrile. Variably substituted new indolo-4H-phthalimidines and indolo-4H-chromenes were synthesized with excellent yields (85-93%) under mild reaction conditions.

Bioinspired Total Synthesis of Bipolarolides A and B

AbstractWe have achieved the first total synthesis of bipolarolides A and B, which possess an intriguing and complex 5/6/6/6/5 caged pentacyclic skeleton with seven contiguous stereocenters. The synthesis features a lithium‐halogen exchange/intermolecular nucleophilic addition to link two enantioenriched fragments, two ring‐closing metathesis reactions to assemble the five‐ and eight‐membered rings, and a bioinspired Prins reaction/ether formation cascade cyclization to construct the 5/6/6/6/5 caged skeleton.

Bioinspired Total Synthesis of Bipolarolides A and B.

We have achieved the first total synthesis of bipolarolides A and B, which possess an intriguing and complex 5/6/6/6/5 caged pentacyclic skeleton with seven contiguous stereocenters. The synthesis features a lithium-halogen exchange/intermolecular nucleophilic addition to link two enantioenriched fragments, two ring-closing metathesis reactions to assemble the five- and eight-membered rings, and a bioinspired Prins reaction/ether formation cascade cyclization to construct the 5/6/6/6/5 caged skeleton.

Copper Catalyst-Promoted Regioselective Multicomponent Cascade Cyclization of 3-Aza-1,5-enynes with Sulfur Dioxide and Cycloketone Oxime Esters to Access Cyanoalkylsulfonylated 1,2-Dihydropyridines.

Radical cascade cyclization via the cracking of alkenyl C-H has emerged as an attractive and remarkable tool for the rapid construction of ring frameworks with endocyclic double bonds. We developed a cascade reaction of 3-aza-1,5-enynes with sulfur dioxide and cycloketone oxime esters to access cyanoalkylsulfonylated 1,2-dihydropyridines, which can be easily converted to pyridine derivatives. This protocol involves radical addition to the C≡C bond and 6-endo cyclization and features high regioselectivity and a broad substrate scope.

Visible-light-induced radical cascade cyclization: a catalyst-free synthetic approach to trifluoromethylated heterocycles.

A visible-light-promoted research protocol for constructing dihydropyrido[1,2-a]indolone skeletons is herein described proceeding through a cascade cyclization mediated by trifluoromethyl radicals. This method allows the efficient synthesis of various indole derivatives without the need of photocatalysts or transition-metal catalysts. Mechanism experiments indicate that the process involves a radical chain process initiated by the homolysis of Umemoto's reagent. This straightforward method enables a rapid access to heterocycles containing a trifluoromethyl group.

Asymmetric Cascade Cyclization of Enynamides with Photogenerated Ketenes via Relay Gold and N-Oxide Catalysis

Asymmetric Cascade Cyclization of Enynamides with Photogenerated Ketenes via Relay Gold and <i>N</i>-Oxide Catalysis

Photochemical Radical Bicyclization of 1,5-Enynes: Divergent Synthesis of Fluorenes and Azepinones.

A dual nickel- and iridium-photocatalyzed radical cascade bicyclization reaction for the synthesis of highly complex molecular structures in an atom- and step-economic manner has been described. A series of radical precursors are utilized for the divergent synthesis of diversely substituted fluorenes and indenoazepinones bearing quaternary carbons by using cascade cyclization reactions of 1,5-enynes. This reaction is characterized by its mild conditions, broad substrate scope, excellent selectivity, and satisfactory yield including facile scale-up synthesis.

Synthesis of Indenones via Persulfate Promoted Radical Alkylation/Cyclization of Biaryl Ynones with 1,4-Dihydropyridines.

The oxidative radical cascade cyclization of alkynes has emerged as a versatile strategy for the efficient construction of diverse structural units and complex molecules in organic chemistry. This work reports an alkyl radical initiated 5-exo-trig cyclization of biaryl ynones with 1,4-dihydropyridines to selectively synthesize indenones.

Accessing Functionalized Furans from Reacting Enynones and Enynals through Furyl Metal Carbenes

AbstractEnynones and enynals represent donor‐type carbene precursors that efficiently form furyl metal carbenes via a catalytic intramolecular 5‐exo‐dig cyclization and rearrangement cascade, in a 100 % atom‐economical manner, using various metals. These furyl metal carbenes have been successfully utilized in numerous carbene reactions, including metal carbene X−H insertion, cyclopropanations, metathesis, and cross‐coupling, demonstrating a broad reaction scope and efficiency in constructing highly functionalized furans. Recent years have witnessed significant advancements in transformations involving furyl metal carbenes, especially in asymmetric carbene insertion reactions, domino reactions involving zwitterionic intermediates, various rearrangement strategies, and their application in synthesizing functional materials and drug candidates. In this review, we summarize recent progress in furyl metal carbene reactions generated from enynones and enynals, focusing on the synthesis of highly functionalized furans.

Synthesis of the Indolizidine Core of Virosinine A via a Microwave-Promoted Cascade Cyclization Involving Iminyl Radicals.

The indolizidine core of virosinine A was synthesized by means of a microwave-promoted cascade reaction featuring 5-exo-trig iminyl radical cyclization, thiyl radical elimination, and intramolecular imine alkylation. The resulting bicyclic iminium ion underwent stereoselective reduction by Red-Al to deliver the target compound. DFT calculations suggested that both the radical cyclization and thiyl radical elimination steps are reversible at high reaction temperatures.

Copper(I)-Catalyzed Dearomatization of Benzofurans with 2-(Chloromethyl)anilines through Radical Addition and Cyclization Cascade.

Herein, we described a copper(I)-catalyzed dearomatization of benzofurans with 2-(chloromethyl)anilines to prepare various tetrahydrobenzofuro[3,2-b]quinolines and 2-(quinolin-2-yl)phenols in good to excellent yields through radical addition and an intramolecular cyclization process. Mechanistic studies revealed that 2-(chloromethyl)anilines served as radical precursors. The present method features broad substrate scope, good functional group tolerance, quinoline scaffold diversity, and radical addition dearomatization of benzofurans.

K2S2O8‐Mediated Access to Alkylsulfonated Spiro[4,5] Trienones from N‐Arylpropiolamides with the Insertion of Sulfur Dioxide

AbstractA four‐component reaction of N‐arylpropiolamides, Hantzsch esters, Na2S2O5 and water via a radical cascade cyclization process with the insertion of sulfur dioxide mediated by K2S2O8 is reported. A series of alkylsulfonated spiro[4,5] trienones are prepared up to 86% yield with a good functional group tolerance and substrate applicability. Preliminary mechanism experiments indicate that carbonyl oxygen of spiro[4,5] trienones originated from water.

Synthesis of 3,3-disubstituted oxindoles from N-arylacrylamides and unactivated alkyl bromides via nickel-catalyzed cascade cyclization and their inhibitory effect on NO release

A Ni-catalyzed cascade cyclization of N-acrylamides and unactivated alkyl bromides to synthesize 3,3-disubstituted oxindoles was reported.

Expedient radical phosphonylations via ligand to metal charge transfer on bismuth.

Bismuth, in spite of its low cost and low toxicity, has found limited application in organic synthesis. Although the photoactivity of Bi(iii) salts has been well studied, this has not been effectively exploited in photocatalysis. To date, only a single report exists for the Bi-based photocatalysis, wherein carbon centered radicals were generated using ligand to metal charge transfer (LMCT) on bismuth. In this regard, expanding the horizon of bismuth LMCT catalysis for the generation of heteroatom centered radicals, we hereby report an efficient radical phosphonylation using BiCl3 as the LMCT catalyst. Phosphonyl radicals generated via visible-light induced LMCT of BiCl3 were subjected to a variety of transformations like alkylation, amination, alkynylation and cascade cyclizations. The catalytic system tolerated a wide range of substrate classes, delivering excellent yields of the scaffolds. The reactions were scalable and required low catalytic loading of bismuth. Detailed mechanistic studies were carried out to probe the reaction mechanism. Diverse radical phosphonylations leading to the formation of sp3-C-P, sp2-C-P, sp-C-P, and P-N bonds in the current work present the candidacy of bismuth as a versatile photocatalyst for small molecule activation.

Synthetic studies towards naturally occurring aporpinones: asymmetric synthesis of 1'-deshydroxymethyl analogues of aporpinone A, aporpinone B and 4'-hydroxyaporpinone A.

Herein, we disclose the asymmetric total synthesis of 1'-deshydroxymethyl analogues of naturally occurring aporpinone A, aporpinone B, and 4'-hydroxyaporpinone A, featuring a γ-Z-alkylidene butenolide framework. Bimetallic (Pd-Cu) cascade cyclization on a properly functionalized bis-alkyne with Z-2-bromoacrylic acid was employed to construct the butenolide framework with an alkyne appendage. Late-stage enzymatic kinetic resolution (EKR) was adopted for the synthesis of (R)-1'-deshydroxymethyl aporpinone A and (S)-1'-deshydroxymethyl acetyl aporpinone A. The enantiopure bis-alkyne required for the synthesis of (S)-1'-deshydroxymethyl aporpinone B and (R)-1'-deshydroxymethyl 4'-hydroxyaporpinone A was constructed through the Sonogashira cross-coupling reaction.

Facile access to C-substituted piperazin-2-ones and mianserin derivative enabled by chemoselective carbene insertion and cyclization cascade.

The chemoselective N-H insertion of unsymmetrical diamines into carbene is a longstanding challenge. A simple copper-catalyzed strategy for synthesizing C-substituted piperazinones is described, employing easily accessible diazo compounds and 1,2-diamines. The reaction proceeded via chemo-selective carbene insertion at the comparatively less nucleophilic amine, followed by instantaneous cyclization. The protocol was further extended to access NH-free piperazinone, and the synthesis of a Mianserin derivative.