Gold-catalysed cyclic ether formation from diols

Ready Access to Benzannulated [5,5]-Oxaspirolactones Using Au(III)-Catalyzed Cascade Cyclizations

This work showcases an unprecedented Au(III)-catalyzed cascade cyclization of 2-(4-hydroxyalkynyl)benzoates to access benzannulated [5,5]-oxaspirolactones related to biologically active natural products. This reaction proceeds through an initial 5-endo-dig mode of hydroalkoxylation of the alkynol segment to give the oxocarbenium species (via cyclic enol-ether) followed by the addition of carboxylate onto the oxocarbenium that delivers the oxaspirolactone scaffold. While testing this method's scope, we found that the steric and electronic environment of the hydroxyl group could alter the reaction pathway that delivers isochromenone through a competitive 6-endo-dig mode of attack of the carboxylate onto the tethered alkyne.

Diversified Transformations of Tetrahydroindolizines to Construct Chiral 3-Arylindolizines and Dicarbofunctionalized 1,5-Diketones

Enantioselective diverse synthesis of a small-molecule collection with structural and functional similarities or differences in an efficient manner is an appealing but formidable challenge. Asymmetric preparation and branching transformations of tetrahydroindolizines in succession present a useful approach to the construction of N-heterocycle-containing scaffolds with functional group, and stereochemical diversity. Herein, we report a breakthrough toward this end via an initial diastereo- and enantioselective [3 + 2] cycloaddition between pyridinium ylides and enones, following diversified sequential transformations. Chiral N,N'-dioxide-earth metal complexes enable the generation of optically active tetrahydroindolizines in situ, across the strong background reaction for racemate-formation. In connection with deliberate sequential transformations, involving convenient rearomatic oxidation, and light-active aza-Norrish II rearrangement, the tetrahydroindolizine intermediates were converted into the final library including 3-arylindolizine derivatives and dicarbofunctionalized 1,5-dicarbonyl compounds. More importantly, the stereochemistry of four-stereogenic centered tetrahydroindolizine intermediates could be efficiently transferred into axial chirality in 3-arylindolizines and vicinal pyridyl and aryl substituted 1,5-diketones. In addition, densely functionalized cyclopropanes and bridged cyclic compounds were also discovered depending on the nature of the pyridinium ylides. Mechanism studies were involved to explain the stereochemistry during the reaction processes.

Nonclassical Mechanism in the Cyclodehydration of Diols Catalyzed by a Bifunctional Iridium Complex

1,4- and 1,5-diols undergo cyclodehydration upon treatment with cationic N-heterocyclic carbene (NHC)-IrIII complexes to give tetrahydrofurans and tetrahydropyrans, respectively. The mechanism was investigated, and a metal-hydride-driven pathway was proposed for all substrates, except for very electron-rich ones. This contrasts with the well-established classical pathways that involve nucleophilic substitution.

Direct cross-coupling between alkenes and tetrahydrofuran with a platinum-loaded titanium oxide photocatalyst

A Pt-loaded TiO₂ photocatalyst successfully catalyzed the direct cross-coupling between various alkenes and tetrahydrofuran (THF) without any additional oxidizing agent.

Cyclic ether synthesis from diols using trimethyl phosphate

Cyclic ethers are simply synthesized from diols by using trimethyl phosphate at room temperature.

Bifunctional property of Pt nanoparticles deposited on TiO2 for the photocatalytic sp3C–sp3C cross-coupling reactions between THF and alkanes

Photocatalytic direct cross-coupling between THF and alkanes was accelerated by Pt nanoparticles on TiO₂, as a receiver for photoexcited electron and a metal catalyst.