Divergent C–H functionalization reactions (arylation, carboxylation, olefination, thiolation, acetoxylation, halogenation, naphthylation) using a pyridazinone moiety as an internal directing group were successfully established. This approach offers a late-stage, ortho-selective diversification of a biologicallyactive pyridazinone scaffold. Seven series of novel pyridazinone analogues were synthesized