Methyltriphenylphosphonium bromide (3.52 g) was dissolved in tetrahydrofuran (20 ml), potassium tert-butoxide (948 mg) was added to the solution, and the mixture was stirred at room temperature for 1 hour. A solution of 4-[1,3-bis(2,2-dimethylpropionyloxymethyl)-7-(2-formylphenyl)-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-8-yl]piperazine-1-carboxylic acid tert-butyl ester (1.94 g) in tetrahydrofuran (20 ml) was added to the obtained reaction mixture at room temperature, and the mixture was stirred at room temperature for 3 hours and 50 minutes. The reaction mixture was diluted with ethyl acetate and washed with water. The organic layer was dried over anhydrous magnesium sulfate and filtered. The filtrate was concentrated under reduced pressure and the residue was purified by silica gel column chromatography to give 704 mg of the title compound from the elution fraction of hexane-ethyl acetate (2:1).
