Gold(I)/(III)-Catalyzed Synthesis of Cyclic Ethers; Valency-Controlled Cyclization Modes

Strategic use of oxophilic (hard) gold­(III) and π-philic (soft) gold­(I) catalysts provides access to two types of cyclic ethers from propargylic alcohols. Thus, heating propargylic alcohols with an oxophilic gold­(III) catalyst (AuBr₃) results in cyclization to afford cyclic ethers bearing an acetylenic moiety, due to coordination of gold­(III) to the oxygen of the propargylic hydroxyl group. On the other hand, propargylic alcohols with a π-philic gold­(I) catalyst (Ph₃PAuNTf₂) induces Meyer–Schuster rearrangement to afford α,β-unsaturated ketones, which undergo gold­(III)-catalyzed intramolecular oxa-Michael addition to afford cyclic ethers bearing a carbonyl group, due to coordination of gold­(III) to the oxygen of the carbonyl group