Two protocols for oxygenation of aromatic C–H bonds ortho-positioned to the phthalazine ring were developed. The transannulation of the phthalazine ring to a naphthalene moiety by an Inverse Electron Demand Diels–Alder (IEDDA) reaction led to the synthesis of naphtho[2,1-c]chromenes, 1-(ortho-hydroxyaryl)naphthalenes and 6,7-dihydrobenzo[b]naphtho[1,2-d]oxepine. This new strategy based on the utilization of transformable nitrogen heterocycles in C–H functionalization chemistry can be potentially applicable to the synthesis of a broad range of biaryl compounds.