Copper(II)-Catalyzed Tandem Propargylic Nucleophilic Substitution/Regioselective Tetradehydro-Diels-Alder/Meyer-Schuster Rearrangement or Elimination Reaction of Eneynols.

In this study, we report a novel catalytic system comprising a Lewis acid, copper(II) triflate, and an oxalamide ligand for the cascade reaction of eneynols. Based on control experiments, a plausible reaction pathway is proposed. This cascade process appears to involve a Cu(II)-catalyzed tandem sequence: propargylic nucleophilic substitution of methanol, regioselective tetradehydro-Diels-Alder (TDDA) cycloaddition, and either a Meyer-Schuster rearrangement or an elimination reaction. Furthermore, deuterium-labeling experiments indicate that a strained cyclic allene intermediate is formed during the tetradehydro-Diels-Alder cycloaddition. This strained allene intermediate then undergoes isomerization to a benzene ring via proton transfer from the hydroxyl hydrogen of methanol, rather than through intramolecular hydrogen transfer at the vinyl position. Under mild conditions, a diverse range of valuable fluorenylidene derivatives can be efficiently synthesized.