Under argon atmosphere, to a suspension of N-{3-[5-(2-fluoro-4-iodophenylamino)-6,8-dimethyl-2,4,7-trioxo-3,4,6,7-tetrahydro-2H-pyrido[4,3-d]pyrimidin-1-yl]-phenyl}-acetamide 89 (75.0 mg), 4-benzyloxybutyl alcohol 90 (25.0 μl) and triphenylphosphine (37.0 mg) in tetrahydrofuran (1.00 ml) was added diisopropyl azodicarboxylate (28.0 μl) with stirring under ice cooling. The mixture was stirred at the same temperature for 2 hrs, and 4-benzyloxybutyl alcohol (13.0 μl), triphenylphosphine (19.0 mg) and diisopropyl azodicarboxylate (14.0 μl) were added. The mixture was stirred at the same temperature for 1 hr, and water and ethyl acetate were added to allow partitioning. The organic layer was washed with saturated aqueous sodium chloride solution, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The residue was purified by column chromatography (chloroform:ethyl acetate=2:1→1:1) and thin layer chromatography (hexane:acetone=1:1) for further purification to give N-{3-[3-(4-benzyloxybutyl)-5-(2-fluoro-4-iodophenylamino)-6,8-dimethyl-2,4,7-trioxo-3,4,6,7-tetrahydro-2H-pyrido[4,3-d]pyrimidin-1-yl]-phenyl}-acetamide 91 (74 mg, yield 77%) as a pale-yellow amorphous form.
