(4+3) Cycloadditions of Allylic and Related Cations

The reaction of allylic cations with 1,3‐dienes is formally equivalent to the Diels–Alder reaction but leads to seven‐membered rings. An allylic cation contains 2 pi electrons, and is thus precisely analogous to an alkene and, like the latter, can function as a dienophile. The substituent at the central carbon of the allylic cation serves to terminate the reaction. Most often, this substituent is an oxyanion or a silyl ether, but other groups have been used. These (4+3) cycloadditions can proceed via mechanisms that range from stepwise to concerted. Diastereoselectivity, regioselectivity, and enantioselectivity have all been achieved, though there are still opportunities for the development of new reactions in this area. Asymmetric catalysis and enzymatic catalysis of such reactions is in its infancy. More recent advances center on cations that are related to benzylic cations but involve electron rich heterocycles such as furans, indoles, thiophenes and related structures. This chapter covers advances in (4+3) cycloadditions of allylic and related cations from 1997 to 2017, with supplemental references added to bring the work up to date through 2023. It illustrates both the growth in technical progress and theoretical understanding of this reaction during that time.