The [3]Dendralene Motif as an Entry into Nazarov Cyclizations by Silylium-Ion Initiation.

Geminal alkenes bearing an aryl and an allenyl group contain the motif of [3]dendralenes. The central alkene double bond in these cross-conjugated polyenes can be reacted with a silylium ion, thereby initiating a Nazarov cyclization. The cationic intermediate emerging from the electrocyclic ring closure is captured by hydride in the presence of excess hydrosilane. The resulting benzannulated methylenecyclopentene derivatives bearing a silylalkyl group then engage in silylium-ion regeneration followed by an unusual endo -selective intramolecular hydrosilylation. This cascade eventually leads to the formation of a silicon-containing bicyclo[3.2.1]octane skeleton.