Intramolecular regioselective addition of radicals and carbanions to ynol ethers. A strategy for the synthesis of exocyclic enol ethers

Abstract

The scope and limitations of radical and anionic cyclization reactions involving halo ynol ethers have been investigated. 5- exo and 6- exo radical cyclizations of 6-iodo and 7-iodo ynol ethers proceeded well when the oxygen of the ynol ether was bearing an ethyl group. Exocyclic iodoenol ethers resulting from these cyclizations were highly unstable and decomposed rapidly. Li–I exchange of iodo ynol ethers proceeded smoothly at −78 °C. 6-Alkoxy-5-hexynyllithiums underwent regiospecific 5- exo-dig anionic cyclization to produce five-membered rings bearing an exocyclic enol ether moiety. The cyclized vinyllithium intermediate was successfully trapped with electrophiles to afford functionalized cycloalkoxyalkylidene derivatives in modest to good yields. 7-Alkoxy-6-heptynyllithiums did not cyclize via a 6- exo anionic process.