Formation of hydrindans and tricyclo[4.3.0.0^3,8]nonanes via 6-endo trapping of the Nazarov oxyallyl intermediate

Treatment of trienones 1a-g with BF₃·OEt₂ at -78°C followed by warming effected Nazarov electrocyclization followed by 6-endo cation-olefin cyclization. The resulting tertiary carbocations underwent elimination to give olefinic products 2, intramolecular hydride transfer to furnish enones 3 and/or 3+2 ring-closure providing tricyclic products 4. Use of TiCl₄ in place of BF₃·OEt₂ led in most cases to a mixture of enones 3 and chloride- trapping products 7.