Intramolecular 4 + 3 Cycloadditions. Cycloaddition Reactions of Cyclic Alkoxyallylic and Oxyallylic Cations

The intramolecular 4 + 3 cycloaddition of oxyallylic and
alkoxyallylic 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.