Diversity-Oriented Synthesis of Spirocyclopentadiene Oxindoles and 2-Acyl Cyclopenta[b]indoles via Intramolecular Wittig Reaction

Herein, we report an efficient strategy for the diversity-oriented synthesis of spirocyclopentadiene oxindoles and 2-acyl cyclopenta[b]indoles. A phospha-1,6-addition of PBu₃to α,β,γ,δ-unsaturated oxindoles provides phosphorus zwitterions as key intermediates and a branching point for the synthesis of these diverse cyclic products. Notably, spiro compounds are formed via a unique δ-C-acylation/Wittig reaction sequence, while the proton transfer/Wittig/β-acylation reactions afforded 2-acyl cyclopenta[b]indoles. Significantly, this study discloses the first report of a direct β-acylation reaction of an α,β-unsaturated ester bearing an endocyclic double bond.