Formal (3 + 1 + 1) Carboannulation of Morita–Baylis–Hillman Carbonates with Pyridinium Ylides: Access to Spiro-Cyclopentadiene Oxindoles

An efficient formal (3 + 1 + 1) carboannulation strategy of Morita–Baylis–Hillman (MBH) carbonates with pyridinium ylides was developed for constructing diversely functionalized spiro-cyclopentadiene oxindoles. The reaction initiates with an SN2′ olefination of MBH carbonates with pyridinium ylides. The in situ generated dienes then engage in a challenging (4 + 1) ylide carboannulation, which has been rarely reported before. The reaction features broad substrate scope as well as high chemo- and regioselectivity. (3 + 1 + 1) carboannulation products could be easily transformed into interesting spiro-cyclopenta­[c]­furan oxindoles.