Facile Stereoselective Approach to Diverse Spiroheterocyclic Tetrahydropyrans: Concise Synthesis of (+)‐Broussonetine G and H

A highly diastereoselective copper‐catalyzed multicomponent cyclization of exocyclic enol ethers/enamines with methylene malonate and aldehydes has been developed to furnish spiroheterocyclic tetrahydropyrans in high yields with greater than 95:5 d.r. This method is practical, in that 36 examples, including a range of aldehydes and exo‐vinyl heterocycles, are presented. By applying the newly developed method, the total synthesis of (+)‐broussonetine G and formal synthesis of (+)‐broussonetine H were achieved in a concise way. Ring assembly: An effective diastereoselective spiroketalization has been realized for a broad range of aldehydes and proceeds in the presence of a copper catalyst. This method was applied to the total synthesis of (+)‐broussonetine G and formal synthesis of (+)‐broussonetine H.