Methanol → (8E)-8-[(2Z)-2-(3-hydroxy-4-oxo-1-cyclohexa-2,5-dienylidene)ethylidene ]-7-methyl-1,3-dipropyl-9H-purine-2,6-dione

Compound 1 (770 mg, 1.87 mmol) obtained in Reference Example 1 was dissolved in 15 ml of methylene chloride. To the solution was added 5.6 ml (5.6 mmol) of boron tribromide (1.0M methylene chloride solution) under ice cooling in argon atmosphere, and the mixture was stirred overnight at room temperature. Methanol was added thereto and the mixture was separated with chloroform-an aqueous solution of sodium bicarbonate. The organic layer was washed with a saturated aqueous solution of sodium chloride and dried over anhydrous sodium sulfate, followed by evaporation under reduced pressure. The residue was purified by silica gel column chromatography to give 550 mg (yield 77%) of Compound 53 as a yellow solid, which was then triturated with ether to give a yellow powder.