BF3-promoted synthesis of spiroindenyl heterocycles

An easy and straightforward synthesis of spiroindenyl heterocycles by the repeated treatment of boron trifluoride etherate (BF3-OEt2) is reported. The overall transformation from ketones 1 to spiro-fused indenes 3 proceeds via Wittig olefination, deconjugation, Grignard addition, and intramolecular electrophilic cyclization in moderate yields. It presents a novel rearrangement reaction catalyzed by boron trifluoride etherate and broadens the scope of application.