Intramolecular 4+3 cycloadditions. Cycloaddition reactions of cyclic alkoxyallylic and oxyallylic cations

The intramolecular 4 + 3 cycloaddition of oxyallylic and allcoxyallylic cations has been studied. Cyclopentenyl oxyallylic cations tethered to a furan by a three or four carbon chain react with high stereoselectivity via a compact (endo) transition state analogous to that seen in intermolecular cycloadditions. Six- through eight-membered oxyallylic cations react differently and give increasingly larger amounts of cycloadduct derived from extended (exo) transition states. This has been rationalized on the basis of ring strain in the endo transition states in this series. Cyclodecenyl oxyallylic cations are sickle-shaped and stereoselectively undergo cycloaddition via extended transition states to a furan diene tethered by three carbons. Cyclododecenyl oxyallylic cations behave similarly, although the evidence suggests that a small amount of W-shaped cation also is formed. Cyclic alkoxyallylic sulfones give rise to the corresponding cations upon treatment with TiCl4, and the latter give rise to cycloadducts via apparent intramolecular 4 + 3 cycloadditions in variable yields. Side product formation in certain cases provides evidence for the intermediacy of cationic species which may also be important in the formation of the cycloadducts themselves.