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).