A base-promoted tandem reaction of 2-(2-alkynylphenyl)aziridines with 2-isocyanoacetates in the presence of copper(II) triflate gives tetrahydro-3H-indeno[2,1-d]pyrimidines in good yields, with the formation of two new rings and three bonds.

The coupling of aryldiazonium tetrafluoroborates, DABCO⋅(SO₂)₂, and hydrazines under metal-free conditions leads to the formation of aryl N-aminosulfonamides. The reaction proceeds smoothly at room temperature and shows broad functional-group tolerance. A radical process is proposed for this transformation.

The coupling of aryldiazonium tetrafluoroborates, DABCO⋅(SO₂)₂, and hydrazines under metal-free conditions leads to the formation of aryl N-aminosulfonamides. The reaction proceeds smoothly at room temperature and shows broad functional-group tolerance. A radical process is proposed for this transformation.

A palladium-catalyzed reaction of 2-alkynylaniline, isocyanides, and silver acetate is described, leading to 3-amidylindoles in moderate to good yields. During the process, five new bonds are formed with high reaction efficiency in one-pot procedure.

A palladium-catalyzed reaction of 2-alkynylbromobenzenes with 2-(2-alkynylphenyl)malonates gives rise to benzo[a]fluorenes in good yields. This tandem process is efficient with the formation of three bonds from easily available starting materials.

A copper-catalyzed tandem reaction of 1-bromoethynyl-2-(cyclopropylidenemethyl)arenes with N-allylsulfonamide proceeds smoothly, affording functionalized benzoindolines in moderate to good yields. The transformation is a four-step cascade involving Ullmann coupling, aza-Claisen rearrangement, 6π-electrocyclization, and intramolecular rearrangement.

A novel and efficient synthesis of 11-fluoro-11H-indeno[1,2-c]quinolines has been developed via a palladium-catalyzed three-component reaction of 2-alkynylbromobenzenes, 2-alkynylanilines, and N-fluorobenzenesulfonimide. The reaction works well with high selectivity. Additionally, the diversity and complexity could be easily introduced via a simple operation from readily available starting materials. In the meantime, a fluorine atom could be incorporated into the scaffold during the reaction process.

The extensive use of cyclic compounds in many fields encourages the efficient construction of this class of compounds. Tandem reactions provide easy access to cyclic compounds because of their simple procedures, high efficiency, and superior atom economy. 2-Alkynylbenzaldehyde is a versatile and powerful building block for generating functionalized polycyclic compounds, especially heterocycles, by tandem reactions. This Focus Review discusses recent advances in using 2-alkynylbenzaldehydes as starting materials in the preparation of diverse and complex cyclic compounds. The progress that has been made provides important protocols for the design of more complex compounds and total syntheses of natural products.

A novel and facile synthetic protocol for the construction of benzazepine and benzoxepine derivatives through a copper(I)-catalyzed reaction of 2-(2-ethynylphenyl)-1-tosylaziridine or 2-(2-ethynylphenyl)oxirane with sulfonyl azides is disclosed. A ketenimine is the key intermediate during the reaction process.

Polyfluoroarene reacts with N′-(2-alkynylbenzylidene)hydrazide catalyzed by silver triflate in the presence of cesium carbonate, leading to polyfluoroaryl-fused H-pyrazolo[5,1-a]isoquinolines in good yields.

A novel palladium-catalyzed domino reaction of 1-bromo-2-(cyclopropylidenemethyl)benzene and 2-alkynylbenzenamine is reported, which generates 2-(naphthalen-2-yl)benzenamines and 5H-indeno[1,2-c]quinolines via 6-endo and 5-exo cyclization, respectively. The regioselectivity for the final outcome can be affected by phosphine and N-heterocyclic carbene ligands.

A novel and efficient route for the generation of H-pyrazolo[5,1-a]isoquinolines via silver triflate-catalyzed one-pot tandem reaction of 2-alkynylbenzaldehyde, sulfonohydrazide, and ketone or aldehyde is described. This reaction proceeds with good functional group tolerance under mild conditions with high efficiency and excellent selectivity.

The three-component reaction of 2-alkynylbenzaldoximes and α,β-unsaturated carbonyl compounds with bromine or iodine monochloride is described, which generates the unexpected 2-(4-haloisoquinolin-1-yl)ethanol derivatives in good to excellent yields.

2-Alkynylbenzaldoximes react with carbodiimides under mild conditions and silver triflate catalysis in DMF, leading to a diverse range of 1-(isoquinolin-1-yl)ureas in good to excellent yields. This transformation involves tandem 6-endo cyclization, [3+2] cycloaddition, and subsequent rearrangement.

An efficient tandem reaction of 2-alkynylbenzaldoximes with isocyanides co-catalyzed by silver triflate and bismuth triflate has been developed, which gives rise to the unexpected N-(isoquinolin-1-yl)formamides in good to excellent yields. The iodo- and bromo-containing products could be obtained as well by variation of the reaction conditions.

Different outcomes were generated under different conditions for the tandem reactions of N′-(2-alkynylbenzylidene)hydrazides with dimethyl acetylenedicarboxylate (DMAD) catalyzed by silver triflate or in the presence of electrophiles. The unexpected isoquinoline-based azomethine ylides were obtained when the reaction was catalyzed by silver triflate or in the presence of bromine, while the fused 1,2-dihydroisoquinolines were afforded when iodine was employed in the above tandem reactions.

The combination of rhodium(I) and N-hetercyclic carbenes (NHC) was found to be effective as a catalyst for cross-coupling reactions of aryl arenesulfonates with arylboronic acids, which gave rise to the desired biaryl compounds in good yields.

Electrophilic cyclization of 2-alkynylbenzaldoximes with various electrophiles leads to the formation of 4-iodoisoquinoline N-oxides or 4-bromoisoquinoline N-oxides under mild conditions. The reaction can be transferred to highly functionalized isoquinoline N-oxides via palladium-catalyzed cross-coupling reactions.