To a suspension of 6-amino-2-butoxy-9-{3-[(3-morpholin-4-ylpropyl)amino]propyl}-8-oxo-7,9-dihydro-8H-purine dimaleate (15.00 g, 23.45 mmol) in NMP (54.08 g) was added triethyamine (7.12 g, 70.35 mmol) at 25° C. Then sodium acetoxyborohydride (8.45 g, 39.87 mmol) was added thereto, followed by stirring at 25° C. for 15 minutes. After adding methyl (3-formylphenyl)acetate (6.27 g, 35.18 mol) and stirring at 25° C. for 8 hours, the mixture was cooled to 5° C. and then diluted with cold water (240.00). After adjusting pH to 7 with 10% sodium carbonate solution, a seed crystal was added thereto. After stirring for additional 15 minutes, the mixture was adjusted to pH 8 with 10% sodium carbonate solution and thereto was added water (15 g). After keeping at 5° C. for 1 hour, the resulting white solid was filtered, washed and dried to give the subject compound (12.10 g, 90.6%). The seed crystal used in the above was obtained by the method of the above example 5 without adding a seed crystal as the method of the above Example 5. Purity: 94.81% (HPLC)
