Abstract
Our cyclizations and ring transformations via two types of spirocyclic arenium ions are described in this article. A spiro [5.2] octadienyl cation, which is well known as a Cram’s arenium ion, has attracted considerable attention in the aspect from physical organic chemistry. Our effort to utilize the intermediate as a synthetic tool resulted in the development of a lactonization of β-aryltosylate with an internal ester, a lactone ring transformation, and an ether ring transformation via the Cram’s arenium ion. We also developed a 7-endo selective Friedel-Crafts cyclization of various epoxy unsaturated esters promoted by Lewis acids. Detailed experimental data showed that a spiro [5.5] undecadienyl cation is a key intermediate of this reaction. The reaction mechanism seems to involve ipso-cyclization and subsequent skeletal rearrangement of the resulting spirocyclic arenium ion. The Friedel-Crafts cyclization was also applied to the formation of tetrahydro-2-benzazepine.
