Gold-catalyzed synthesis of oxygen- and nitrogen-containing heterocycles from alkynyl ethers: application to the total synthesis of andrachcinidine.

In this paper we report that homopropargylic ethers containing pendent oxygen or nitrogen nucleophiles react with electrophilic gold catalysts in the presence of water to form saturated heterocyclic ketones. Mechanistic studies demonstrated that the reactions proceed through a sequence of alkyne hydration, alkoxy group elimination, and intramolecular conjugate addition. Diastereoselectivities for tetrahydropyran and piperidine formation are very good to excellent. This method has been applied to an efficient total synthesis of the natural product andrachcinidine. Utilizing propargylic ether substrates rather than homopropargylic ethers promotes regioselective hydration of internal alkynes, thereby expanding the scope of products that can be accessed through this protocol.