In this paper Synthesis new heterocyclic compound from p-phenylenediamine by converting p-diacetanilide (1) and reacting with Vilsmeier-Haack reagent to give ( 2,7-dicloropyrido[2,3-g]quinoline-3,8-dicarbaldehyde) (2). The two carbaldehydes in compound (2) react with two moles 1-tetralone to give chalcone(3). This chalcone is used as the reagent to estimate the pharmaceutical compounds containing aromatic amino groups. Where a sensitive spectrophotometric method has been developed for the determination of mesalazine by diazotization and coupling reaction of mesalazine with chalcone reagent in basic medium to form a colored water-soluble and stable azo-dye. The product shows maximum absorption at 450 nm. Beer's law was obeyed over the concentration range 0.5-27.5 µg/ml with molar absorptivity of 9494.37 l.mol-1.cm-1. The limit of detection and quantitation is 0.161and 0.538 µg.ml-1. The recovery was 101.48% with a relative standard deviation ≤ 3.265%. Mesalazine and Chalcone reagent products were formed in the ratio of 2: 1 . Full-color development was described and the proposed method was successfully applied to determine mesalazine in the pharmaceuticals preparation. The proposed method was applied to pharmaceutical preparations containing mesalazine derivatives.