5-amino-2,4,6-triiodoisophthalic acid dichloride (13.7 kg, 23 mol) was dissolved in dimethylacetamide (17.2 kg) and the mixture was cooled to 15° C. Methoxyacetyl chloride (3.74 kg, 34.5 mol) was added dropwise thereto for 2 hours, and then the mixture was stirred for 15 hours. After confirming the disappearance of the starting material by HPLC analysis for reaction, methylene chloride (45.7 kg) and water (11.5 kg) were subsequently added to the reaction mixture upon being stirred, and then the stirring was stopped and the layers became separated. The obtained organic layer was washed with aqueous sodium bicarbonate solution and concentrated by distillation under reduced pressure. To a solution of the obtained concentrate dissolved in dimethylacetamide (43.1 kg), triethylamine (1.95 kg, 19.32 mol) was added and then a solution of 2,3-dihydroxypropylamine (1.47 kg, 16.13 mol) dissolved in dimethylacetamide (10.78 kg) was added dropwise thereto for 5 hours with maintaining 0 to 5° C. After additional 3 hours with stirring, the reaction mixture was concentrated by distillation under reduced pressure and to the concentrate, methylene dichloride (213.33 kg) was added dropwise for 5 hours to form solid. The solid was filtered and the title compound was obtained as a white solid (10.98 kg, yield 66.1%).
