Novel transition metal-free synthetic protocols toward the construction of 2,3-dihydrobenzofurans: a recent update

2,3-Dihydrobenzofurans are noteworthy scaffolds in organic and medicinal chemistry, constituting the structural framework of many of the varied medicinally active organic compounds. Moreover, a diverse variety of biologically potent natural products also contain this heterocyclic nucleus. Reflecting on the wide biological substantiality of dihydrobenzofurans, several innovative and facile synthetic developments are evolving to achieve these heterocycles. This review summarizes the transition-metal-free, efficient, and novel synthetic pathways toward constructing the dihydrobenzofuran nucleus established after 2020.

Intramolecular Palladium(II)-Catalyzed Regioselective 6-endo or 6-exo C-H Benzannulation: An Approach for the Diversity-Oriented Synthesis of Quinolinone Derivatives from Pyridones

Herein, a new intramolecular palladium(II)-catalyzed regioselective 6-endo-trig or 6-exo-trig annulation through direct C-H activation is presented as a method for the diversity-oriented synthesis of highly substituted quinolinones from pyridones. The reaction occurs under mild conditions and exhibits excellent regioselectivity, good functional group tolerance, and broad applications. This innovative approach has been successfully utilized in the synthesis of Glycopentanolone A and an intermediate of (R)-(+)-Tipifarnib.

Diquat Based Dyes: A New Class of Photoredox Catalysts and Their Use in Aerobic Thiocyanation

A series of new organic donor-π-acceptor dyes incorporating a diquat moiety as a novel electron-acceptor unit have been synthesized and characterized. The analytical data were supported by DFT calculations. These dyes were explored in the aerobic thiocyanation of indoles and pyrroles. Here they showed a high photocatalytic activity under visible light, giving isolated yields of up to 97 %. In addition, the photocatalytic activity of standalone diquat and methyl viologen through formation of an electron donor acceptor complex is presented.

Visible-Light-Active Coumarin- and Quinolinone-Based Photocatalysts and Their Applications in Chemical Transformations

Organic dyes have been actively studied as useful photocatalysts because they allow access to versatile structural flexibility and green synthetic applications. The identification of a new class of robust organic chromophores is, therefore, in high demand to increase structural diversity and variability. Although coumarins and quinolinones have long been acknowledged as organic chromophores, their ability to participate in photoinduced transformations is somewhat less familiar. Fascinated by their chromophoric features and adaptable platform, our group is interested in the identification of fluorescent bioactive molecules and in the development of new photoinduced synthetic methods using coumarins and quinolinones as photocatalysts. This account provides an overview of our recent progress in the discovery and application of light-absorbing coumarin and quinolinone derivatives in photochemistry and medicinal chemistry.

Metal‐Free Photochemical Olefin Isomerization of Unsaturated Ketones via 1,5‐Hydrogen Atom Transfer

The photochemical isomerization of α,β‐ to β,γ‐unsaturated ketones through a 1,5‐hydrogen atom transfer mechanism under mild conditions with high efficiency and selectivity is reported. The reaction is carried out in the absence of metal catalysts or other additives, and its stereoselectivity can be tuned by selecting appropriate solvent mixtures. The reaction‘s scope and tolerance towards functional groups, including light‐sensitive halogens, free acids and alcohols, were studied, providing reliable access to a wide variety of β,γ‐unsaturated ketones. This methodology details the deconjugation of a wide range of unsaturated ketones and, when combined with olefin metathesis, provides an efficient process for either dehomologation or one‐carbon double‐bond migration of terminal alkenes.

Visible-light-induced tandem radical addition/cyclization of 2-alkenylphenols and CBr4 for the synthesis of 4-arylcoumarins

A visible-light-induced photoredox-catalyzed tandem radical addition/cyclization of 2-alkenylphenols and CBr4 is developed, providing efficient and practical access to various 4-arylcoumarins in a one-pot fashion.