Plants possess arrays of functionally diverse specialized metabolites, many of which are distributed taxonomically. Here, we describe the evolution of a class of substituted alpha-pyrone metabolites in Arabidopsis, which we have named arabidopyrones. The biosynthesis of arabidopyrones requires a cytochrome P450 enzyme (CYP84A4) to generate the catechol-substituted substrate for an extradiol ring-cleavage dioxygenase (AtLigB). Unlike other ring-cleavage-derived plant metabolites made from tyrosine, arabidopyrones are instead derived from phenylalanine through the early steps of phenylpropanoid metabolism. Whereas CYP84A4, an Arabidopsis-specific paralog of the lignin-biosynthetic enzyme CYP84A1, has neofunctionalized relative to its ancestor, AtLigB homologs are widespread among land plants and many bacteria. This study exemplifies the rapid evolution of a biochemical pathway formed by the addition of a new biological activity into an existing metabolic infrastructure.