Cobalt-Catalyzed Annulation of Salicylaldehydes and Alkynes to Form Chromones and 4-Chromanones

A transition-metal-free fast track to flavones and 3-arylcoumarins

A highly regioselective and transition-metal free one-pot arylation of chromenones with arylboronic acids has been achieved employing K₂S₂O₈. The procedure consists of a sequence of some reactions including an arylation/decarboxylation cascade and proceeds well in aqueous media to afford biologically interesting flavones and 3-arylcoumarins. This method exhibited excellent selectivity and functional group tolerance under mild conditions. The reaction also showed perfect efficacy for the preparation of styryl coumarins.

A Rh(iii)-catalyzed cascade C–H functionalization/cyclization reaction of salicylaldehydes with diazomalonates for the synthesis of 4-hydroxycoumarin derivatives

A cascade C–H functionalization/cyclization reaction of salicylaldehydes with α-diazomalonates has been developed for the synthesis of 4-hydroxycoumarin derivatives under rhodium(iii) catalysis.

Unprecedented formation of 3-(tetrahydrofuran-2-yl)-4H-chromen-4-one in a reaction between 3,3a-dihydro-9H-furo[3,4-b]chromen-9-one and malononitrile

Chromone skeletons are widespread among natural products as well as bioactive molecules. Here, we describe an unprecedented reaction of furo[3,4-b]chromen-9-one with malononitrile to afford 3-(tetrahydrofuran-2-yl)-4H-chromen-4-ones. Experimental data suggest that a sequence of Michael/retro-Michael/nucleophilic addition is involved in this unprecedented transformation.

Metal- and Solvent-Free Approach to Access 3-Se/S-Chromones from the Cyclization of Enaminones in the Presence of Dichalcogenides Catalyzed by KIO3

Herein, we describe a greener protocol for the one-pot synthesis of 3-Se/S-4H-chromen-4-ones. The desired products were obtained in good to excellent yields using 2-hydroxyphenyl enaminones and half equivalents of various odorless diorganyl dichalcogenides (S/Se) in the presence of glycerol (5 molar equiv) and KIO3 (15 mol %) as the catalyst under solvent-free conditions.

Synthesis of Chromones from 1,1-Diacylcyclopropanes: Toward the Synthesis of Bromophycoic Acid E

A tandem deprotection-cyclization reaction of 1,1-diacylcyclopropanes is described which allows rapid access to structurally diverse 2,3-disubstituted chromones in good yields, and with straightforward purification. The utility of this reaction is showcased by the construction of the potent antibacterial marine natural product bromophycoic acid E scaffold.

Iridium-Catalyzed and Ligand-Controlled Carbonylative Synthesis of Flavones from Simple Phenols and Internal Alkynes

Flavones are important natural products with diverse biological activities. In this study, a novel procedure for the carbonylative synthesis of flavones has been developed by using simple phenols and internal alkynes as the substrates. Various flavones were isolated in moderate to good yields with excellent regioselectivity and functional group tolerance by using an iridium catalyst system. Notably, this is the first example of direct carbonylative annulation of non-preactivated phenols and alkynes to produce flavones, with the choice of ligand proving to be critical for the success of this transformation.

Annulation of 2,3-diphenyl-4H-chromen-4-ones via photo-induced hydrogen evolution

An efficient photo-induced direct intermolecular hydrogen evolution and annulation of 2,3-diphenyl-4H-chromen-4-ones in EtOH–H₂O (19 : 1, v/v) at room temperature is described, which eliminates the use of any catalysts, oxidants and additives.

Mechanisms for nickel(0)/N-heterocyclic carbene-catalyzed intramolecular alkene hydroacylation: insights from a DFT study

Density functional calculations have been applied to study and elucidate nickel(0)/N-heterocyclic carbene-catalyzed intramolecular alkene hydroacylation. The calculations showed that nickel(0)-catalyzed intramolecular alkene hydroacylation involved four potential reaction channels (I, II, III, and IV), and pathway IV was predicted to be more favorable than the other three. Two pathways, I and II, had three steps (oxidative addition, hydrogen migration, reductive elimination), and the rate-determining step was hydrogen migration. Pathway III proceeded through oxidative cyclization, β-hydride elimination, and hydrogen migration, and the rate-determining step was β-hydride elimination. Pathway IV included four steps (oxidative cyclization, dimerization, β-hydride elimination, hydrogen migration), and the rate-determining step was again β-hydride elimination. Oxidative cyclization was easy and led to rapid dimerization, greatly reducing the free energy of β-hydride elimination. The binuclear nickelacycle intermediate was observed in Ogoshi's experiments, and it was identified by nuclear magnetic resonance (NMR). The dominant product was the five-membered benzocyclic ketone p1. All results agreed with Ogoshi's experiments. Graphical Abstract Nickel(0)-catalyzed intramolecular alkene hydroacylation involved four potential reaction channels. The binuclear nickelacycle intermediate was important, and the dimerization greatly reduced the free energy of the β-hydride elimination.

Bidentate Directing-Enabled, Traceless Heterocycle Synthesis: Cobalt-Catalyzed Access to Isoquinolines

Traceless heterocycle synthesis based on transition-metal-catalyzed C-H functionalization is synthetically appealing but has been realized only in monodentate directing systems. Bidentate directing systems allow for the achievement of high catalytic reactivity without the need for a high-cost privileged ligand. The first bidentate directing-enabled, traceless heterocycle synthesis is demonstrated in the cobalt-catalyzed synthesis of isoquinolines via 2-hydrazinylpyridine-directed C-H coupling/cyclization with alkynes. Convenient directing group installation through a ubiquitously present ketone group allows synthetic elaboration for complex molecules.

Ru(ii)-Catalyzed C–H activation and annulation of salicylaldehydes with monosubstituted and disubstituted alkynes

An Ru(ii) catalyst was used for the annulation reaction of terminal/disubstituted alkynes with salicylaldehydes to afford chromones.

A catalyst-free approach to 3-thiocyanato-4H-chromen-4-ones

A facile and efficient approach to 3-thiocyanato-4H-chromen-4-ones from enaminones and KSCN was realized at room temperature. In addition, one-pot synthesis of 3-thiocyanatochromenones from 2-hydroxylacetophenones was also developed.

A facile strategy for accessing 3-alkynylchromones through gold-catalyzed alkynylation/cyclization of o-hydroxyarylenaminones

A strategy based on tandem alkynylation ofo-hydroxyarylenaminones followed by intramolecular cyclization has been developed to generate a diverse array of 3-alkynyl chromones.