Gold(I)‐Catalyzed Intermolecular Formal [4+2] Cycloaddition of O‐Aryl Ynol Ethers and Enol Ethers: Synthesis of Chromene Derivatives

Gold(I)‐catalyzed formal [4+2] cycloaddition of O‐aryl ynol ethers 1 and enol ethers 2 is described. This intermolecular reaction between two electron‐rich unsaturated systems takes place, under mild conditions, in the presence of 5 mol% [IPrAu(CH(3)CN)]SbF(6) as catalyst giving chromene derivatives with good yields. The cycloaddition is completely regio‐ and stereoselective, as well as versatile for both reactives. Silyl enol ethers can also react in the same way and under the same reaction conditions with quantitative yields. A plausible mechanism through a selective addition of the enol ether to the alkyne gold activated complex followed by an intramolecular aromatic electrophilic substitution is proposed. Several experimental results support the presence of a cationic oxonium intermediate prior to the aromatic substitution. The reaction represents a new entry to the chromene core.