Dearomative [2 + 1] Spiroannulation of Bromophenols with Electron-Deficient Alkenes

A base-assisted dearomative [2 + 1] spiroannulation of p/o-bromophenols with activated olefins (methylenemalonates) to construct various cyclopropyl spirocyclohexadienone skeletons is reported. Furthermore, several other halophenols (X = Cl, I) were also tolerated in this process. Control experiments reveal a dearomative Michael addition of phenols at their halogenated positions to methylenemalonates, followed by intramolecular radical-based S_RN1 dehalogenative cyclopropanation. However, according to the density functional theory (DFT) calculations, an S_N2 dehalogenative cyclopropanation with the same low activation energy barrier should not be excluded. The utility of this method is showcased by gram-scale syntheses and transformations of the dearomatized products.