Nazarov Reactions Intercepted by (4 + 3) Cycloadditions with Oxygen-Substituted Dienes

The
oxyallyl cation intermediate from the Lewis acid mediated Nazarov
reaction of an allenyl vinyl ketone was intercepted by acyclic, 2-silyloxy-substituted
butadienes by highly regioselective (4 + 3) cycloadditions. Stereoselectivity
was often modest, but in some instances steric interactions were responsible
for high selectivity. The results are consistent with concerted (4
+ 3) cycloadditions. In many instances, the (4 + 3) products were
susceptible to fragmentation or rearrangement in the presence of the
Lewis acid.