Direct Access to Bridged Polycyclic Skeletons by Merging Oxidative C–H Annulation and Cascade [4 + 2] Cycloaddition

We report a step-economic strategy for the direct synthesis of bridged polycyclic skeletons by merging oxidative C–H annulation and cascade cycloaddition. In the protocol, spiro[cyclopentane-1,3′-indoline]-2,4-dien-2′-ones were first synthesized by oxidative C–H annulation of ethylideneoxindoles with alkynes. Subsequent cascade [4 + 2] cycloaddition with dienophiles gave the bridged bicyclo[2.2.1]quinolin-2(1 H )-ones and enabled the one-pot construction of two quaternary carbon centers and three C–C bonds. Mechanistic investigations of the latter suggest a cascade ring-opening, 1,5-sigmatropic rearrangement, and [4 + 2] cycloaddition process.