Additive-Driven Rhodium-Catalyzed [4+1]/[4+2] Annulations of N‑Arylphthalazine-1,4-dione with α‑Diazo Carbonyl Compounds

A Rh­(III)-catalyzed strategy involving the [4+1] annulation of 2-arylphthalazine-1,4-diones with α-diazo carbonyl compounds was developed, accessing a series of unprecedented hydroxy-dihydroindazolo-fused phthalazines in good to excellent yields. By varying the additive, phthalazino-fused cinnolines were synthesized under Rh-catalyzed conditions via [4+2] annulation between the same starting materials. Notably, such two strategies showed a good functional group tolerance and high atom efficiency.