Syntheses of Seven- and Five-Membered Rings from Allyl Cations

The cycloaddition of allyl cations to conjugated dienes offers an efficient and general tool for the construction of seven-membered carbocycles. The silver salt route is highly suitable for generating allyl cations and can be used even in most difficult cases, i. e. for elusive and intractable allyl cations such as 2-methylallyl, 2-methoxyallyl, and the parent allyl cation. Cycloadditions to the 2-methoxyallyl cation come very close to the prototype of a 6π, 7C reaction, since the carbonyl group introduced into the new ring can either be used for further transformations or be removed altogether. The combination of an allyl cation with π reactants, especially conjugated dienes, is a powerful and strongly convergent synthetic method since it permits one-step assembly, from a C₃ and a C₄ unit, of a cluster of up to four chiral centers in a ring which is larger than six-membered. Allyl cations combine with electron-rich 2π systems to form novel five-membered rings. The nature of the organometallic intermediates has been elucidated in many cases.