In this review, we provide a comprehensive overview of recent progress in this rapidly growing field by summarizing the 1,6-conjugate addition and annulation reactions of p-QMs with consideration of their mechanisms and applications.

A catalytic domino spirocyclization of 1,7-enynes with simple cycloalkanes and cyclo-1,3-dicarbonyls has been established via multiple C-C bond formations from alkynyl/alkenyl functions and dual α,α-C(sp(3))-H abstraction/insertion. The reaction involves addition, 6-exo-dig cyclization and radical coupling sequences under convenient catalytic conditions and provides a concise access to spiro cyclopenta[c]quinolines in good to excellent yields.

A metal‐free [2+2] cycloaddition and 1,4‐addition sequence induced by S‐centered radicals has been achieved by treating benzene‐linked allene‐ynes with aryldiazonium tetrafluoroborates and DABCO‐bis(sulfur dioxide) in a one‐pot procedure. The reaction provides a greener and more practical access to functionalized cyclobuta[a]naphthalen‐4‐ols with valuable applications. More than 50 examples are demonstrated with excellent diastereoselectivity and chemical yields. The reaction pathway is proposed to proceed by the following steps:[2+2] cycloaddition, insertion of SO2, 1,4‐addition, diazotization, and tautomerization.

Over the past years, impressive progress has been made on the development of the chemoselective and direct formation of carbon-carbon and carbon-heteroatom bonds involving internal alkynes under catalytic oxidative systems. These transformations heavily rely on the in situ generation of reactive radical intermediates that are initiated by oxidants, visible light photocatalysis, or electrocatalysis and so on. Numerous chemically and biomedically important building blocks have been synthesized, and will continue to be obtained by taking advantage of the radical transformation of internal alkynes in the following years. In this review, we highlight the recent progress in this rapidly growing area by presenting a series of catalytic oxidative reactions, mechanisms and applications which have appeared in the most recent literature.

A new cascade three-component halosulfonylation of 1,7-enynes for efficient synthesis of densely functionalized 3,4-dihydroquinolin-2(1H)-ones has been established from readily accessible arylsulfonyl hydrazides and NIS (or NBS). The reaction pathway involves in situ-generated sulfonyl radical-triggered α,β-conjugated addition/6-exo-dig cyclization/radical coupling sequence, resulting in continuous multiple bond-forming events including C–S, C-C and C-I (or C-Br) bonds to rapidly build up molecular complexity.

A new visible‐light photocatalytic arylsulfonylation and bicyclization of C(sp3)‐tethered 1,7‐enynes with sulfinic acids has been developed, delivering functionalized sulfone‐containing benzo[a]fluoren‐5‐ones with generally good yields. This Eosin Y‐catalyzed approach makes use of visible light as a safe and eco‐friendly energy source to drive cascade cyclization reactions, resulting in continuous multiple bond‐forming events including C–S and C–C bonds to efficiently construct polycyclic‐linked alkyl aryl sulfones.magnified image

A novel four-component bicyclization strategy has been established, allowing a flexible and practical approach to 37 examples of multicyclic pyrazolo[3,4-b]pyridines from low-cost and readily accessible arylglyoxals, pyrazol-5-amines, aromatic amines, 4-hydroxy-6-methyl-2H-pyran-2-one, and cyclohexane-1,3-diones. The polysubstituted cyclopenta[d]pyrazolo[3,4-b]pyridines were stereoselectively synthesized through a microwave-assisted special [3+2+1]/[3+2] bicyclization with good control of the spatial configuration of exocyclic double bonds. The novel [3+2+1]/[2+2+1] bicyclization resulted in 17 examples of unreported pyrazolo[3,4-b]pyrrolo[4,3,2-de]quinolones. Reasonable mechanisms for forming two new types of multicyclic pyrazolo[3,4-b]pyridines are also proposed.

Over the past few years, the development of new radical reactions has gained considerable momentum. Most such radical transformations heavily depend on the in situ generation of reactive radical species, initiated by oxidants, metal salts, visible-light photocatalysis, or electrocatalysis. In this context, numerous radical-enabled bicyclization and annulation/1,n-bifunctionalization reactions have been developed, thereby providing efficient synthetic strategies for the assembly of functionalized cyclic rings that contain both heterocyclic and isocyclic skeletons. In this Focus Review, we highlight recent progress in this rapidly growing area by presenting a series of catalytic oxidative reactions, along with their reaction mechanisms and applications.