Formation and ring-opening of 8-oxabicyclo[3.2.1]oct-6-en-3-ones from (4+3)-cycloaddition of furan and chlorocyclopentanone derivatives

During the course of studying an ene reaction of an oxyallylic cation derived from a 2-chlorocyclopentanone system, the reactive species was found to undergo quite facile (4+3)-cycloadditions with furans. Moreover, the (4+3)-cycloadducts derived from furans were prone to ring-opening, which resulted in the formation of 2-furanyl cyclopentanones in excellent yields. An acid-catalyzed mechanism was proposed for the ring-opening process. Several examples of both reactions were identified.