Double-Diels–Alder Approach to Maoecrystal V. Unexpected C–C Bond-Forming Fragmentations of the [2.2.2]-Bicyclic Core

Synthetic studies toward maoecrystal V are reported. An oxidative dearomatization/Diels–Alder cascade to assemble the natural product carbocyclic core in one step is proposed. A facile electrocyclization is shown to suppress the intramolecular allene Diels–Alder pathway. This obstacle is alleviated via a stepwise approach with an allene equivalent to access the key cyclopentadiene-fused [2.2.2]-bicyclic core. Upon treatment with Lewis acid, the proposed intramolecular hetero-Diels–Alder reaction is cleanly and unexpectedly diverted either via C–C bond-forming fragmentation to the spiro-indene product (when R = OMe) or via elimination (when R = H).