Gold-catalyzed Bicyclization of Diaryl Alkynes: Synthesis of Polycyclic Fused Indole and Spirooxindole Derivatives

5-Endo-dig cyclizations in organic syntheses

The favorability of ring closure reactions as per Baldwin rules has gained immense importance recently. This is evident from the current literature such as research articles, reviews, and books that have been published in this area. This review covers the recent applications of 5-endo-dig cyclization in organic synthesis focusing in the last two decades. A variety of 5-membered heterocycles as well as carbocycles could be synthesized via 5-endo-dig cyclization reactions. The important applications of 5-endo-dig cyclization in organic synthesis covering different aspects have been summarized in this review.

Silver-catalyzed decarboxylative radical cascade cyclization toward benzimidazo[2,1-a]isoquinolin-6(5H)-ones

A simple and efficient decarboxylative radical addition/cyclization strategy was developed, by which a wide range of benzimidazo[2,1-a]isoquinoline-6(5H)-ones were prepared in one-pot via reaction of functionalized 2-arylbenzoimidazoles and carboxylic acids in the presence of K2S2O8/AgNO3 under mild reaction conditions.

1-Iodobuta-1,3-diynes in Copper-Catalyzed Azide-Alkyne Cycloaddition: A One-Step Route to 4-Ethynyl-5-iodo-1,2,3-triazoles

Cu-catalyzed 1,3-dipolar cycloaddition of iododiacetylenes with organic azides using iodotris(triphenylphosphine)copper(I) as a catalyst was found to be an efficient one-step synthetic route to 5-iodo-4-ethynyltriazoles. The reaction is tolerant to various functional groups in both butadiyne and azide moieties. The synthetic application of 5-iodo-4-ethynyl triazoles obtained was also evaluated: the Sonogashira coupling with alkynes resulted in unsymmetrically substituted triazole-fused enediyne systems, while the Suzuki reaction yielded the corresponding 5-aryl-4-ethynyl triazoles.

Gold-catalyzed dual annulation of azide-tethered alkynes with nitriles: expeditious synthesis of oxazolo[4,5-c]quinolines

A gold-catalyzed dual annulation of azide-tethered internal alkynes, which provides convenient access to quinoline derivatives, has been reported.

Gold-catalyzed cyclization of 1-(2′-azidoaryl) propynols: synthesis of polysubstituted 4-quinolones

α-Imino gold carbene intermediate is produced from 1-(2′-azidoaryl) propynols, which triggers a 1,2-carbon migration and is converted to 2,3-disubstituted 4-quinolones. The reaction works under mild conditions and tolerates various functional groups.

Gold-catalyzed heterocyclic syntheses through α-imino gold carbene complexes as intermediates

This review reports a comprehensive compilation of gold-catalyzed heterocyclic synthesis by using α-imino gold carbene complexes as the proposed intermediates.

Copper-catalyzed carbene/alkyne metathesis terminated with the Buchner reaction: synthesis of dihydrocyclohepta[b]indoles

A copper-catalyzed carbene/alkyne metathesis reaction of alkyne-tethered diazoacetates is reported, which provides dihydrocyclohepta[b]indole skeletons under mild reaction conditions.

Palladium-Catalyzed Double Carbonylative Cyclization of Benzoins: Synthesis and Photoluminescence of Bis-Ester-Bridged Stilbenes

A palladium-catalyzed double carbonylative cyclization of benzoins has been developed, which realizes the synthesis of bis-ester-bridged stilbenes just in two steps from aldehydes. Thus, the obtained fully fused tetracyclic π-systems have a pyrano[3,2- b]pyran-2,6-dione (PPD) core on their center, showing two reversible reductions at low potentials. In addition, their photoluminescence properties are strikingly affected by the aromatic rings fused to the PPD core; bis- thieno-fused PPDs are found to be excellent fluorophores with quantum yields up to 0.98.

Direct synthesis of aryl-annulated [c]carbazoles by gold(i)-catalysed cascade reaction of azide-diynes and arenes

The gold-catalysed annulation of conjugated alkynes bearing an azido group with arenes gave annulated [c]carbazoles. Using benzene, pyrrole, and indole derivatives as the nucleophiles, benzo[c]-, pyrrolo[2,3-c]-, and indolo[2,3-c]carbazoles were produced, respectively. The reaction proceeded through pyrrole and benzene ring construction accompanied by the formation of two carbon-carbon and one carbon-nitrogen bond and the cleavage of two aromatic C-H bonds. The mechanism of the reaction with pyrrole was investigated by density functional theory calculations. N,N'-dimethylated indolo[2,3-c]carbazole showed dual ultraviolet-visible-near-infrared and fluorescence spectral changes upon electrolysis.

Gold(I)-Catalyzed and H2O-Mediated Carbene Cascade Reaction of Propargyl Diazoacetates: Furan Synthesis and Mechanistic Insights

A novel gold-catalyzed water-mediated carbene cascade reaction of propargyl diazoacetates has been developed. Mechanistic investigation indicates that this reaction is initiated by gold-catalyzed gold-carbene formation followed by an unprecedented 6- endo-dig carbocyclization with tethered alkyne through an oxonium ylide intermediate, terminated by a β-H elimination/protodeauration process to give the aromatized furan products in good to high yields with broad substrate generality. Notably, the proposed gold-carbene intermediates are verified by interception experiments.

Synthesis of indole-fused heteroacenes by cascade cyclisation involving rhodium(ii)-catalysed intramolecular C–H amination

Indole-fused heteroacenes have been synthesised from cascade cyclisation of 2-[(2-azidophenyl)ethynyl]anilines and -phenols in a catalytic manner under redox-neutral conditions.

Pd-Catalyzed cascade cyclization of o-alkynylanilines via C–H/C–N bond cleavage leading to dibenzo[a,c]carbazoles

A new and efficient Pd-catalyzed cascade cyclization of biaryl-tethered o-alkynylanilines for the formation of dibenzo[a,c]carbazole derivatives has been reported.