Condensation of isatoic anhydride (I) with DL-phenylalanine ethyl ester (II) affords anthranilamide (III). Subsequent acylation of amino amide (III) with indole-2-carbonyl chloride (IV) provides diamide (V). The ethyl ester group of (V) is finally hydrolyzed to the target carboxylic acid with methanolic KOH