Asymmetric (4+3) Cycloadditions of Enantiomerically Enriched Epoxy Enolsilanes

摘要

The (4+3) cycloaddition reaction is a direct and efficient method to construct seven-membered carbocycles. Isoelec-tronic with the Diels-Alder reaction, the classical mechanistic understanding of this reaction is that an allyl cation dien-ophile, contributing two electrons, undergoes cycloaddition with the diene (Scheme 1). Cyclic dienes are commonly employed, thus resulting in bicyclic adducts which are useful synthetic intermediates. Compared with the (4+2) reaction, however, asymmetric (4+3) cycloadditions remain underdeveloped. Other than two examples of asymmetric catalysis, the majority of asymmetric versions of (4+3) cycloadditions rely on chiral auxiliaries or incorporate chiral elements into the reacting cation or the diene.