INTRAMOLECULAR 4-CYCLOADDITIONS - CYCLOADDITION REACTIONS OF CYCLIC ALKOXYALLYLIC AND OXYALLYLIC CATIONS(3)

The intramolecular 4 + 3 cycloaddition of oxyallylic and allcoxyallyli c cations has been studied. Cyclopentenyl oxyallylic cations tethered to a furan by a three or four carbon chain react with high stereoselec tivity 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 cyc loadduct 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 state s to a furan diene tethered by three carbons. Cyclododecenyl oxyallyli c cations behave similarly, although the evidence suggests that a smal l amount of W-shaped cation also is formed. Cyclic alkoxyallylic sulfo nes 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 cert ain cases provides evidence for the intermediacy of cationic species w hich may also be important in the formation of the cycloadducts themse lves.