Anhydrous p-toluene sulfonic acid (20.99 g, 110.04 mmol) was melted at 120° C. and 3-amino-4-methoxy benzoic acid methyl ester (10 g, 55.20 mmol) obtained in step 1 of Preparation Example 1 and 2,4-dichlorobenzonitrile (18.99 g, 110.04 mol) were added thereto and stirred at 180° C. for 6 hours. The resulting solution was cooled to room temperature and the reaction was stopped by adding NaHCO3 thereto. The resulting mixture was extracted with ethyl acetate, the extract was dried over MgSO4 and concentrated under a reduced pressure. The concentrate was dissolved in 50% methanol and 5% NaOCl (30 ml, 20.64 mmol) was added dropwise thereto. After 5 min, the resulting mixture was extracted with ethyl acetate, the extract was dried over MgSO4 and concentrated under a reduced pressure. The resulting residue was purified by silica gel column chromatography (eluent—MeOH:CDCl3=5:95, Merck, Silicagel 60) to obtain the title compound (18 g, 10.32 mmol) in a yield of 84%.
