n-Butyllithium (2.0M solution in hexanes, 1.90 ml) was added dropwise to a solution of diisopropylamine (0.069 ml) in anhydrous tetrahydrofuran (30 ml) at 0° C., under nitrogen. The solution was stirred for 5 minutes then cooled to −78° C. and a solution of 3-methyl-1-(2-methylpropyl)-6-(1-naphthalenylmethyl)thieno[2,3-d]pyrimidine-2,4(1H,3H)-dione (1.50 g) in anhydrous tetrahydrofuran (15 ml) was added dropwise. After 15 minutes, the flask was transferred to a sealed bomb containing carbon dioxide pellets (2 g) and heated to 50° C. for 18 hours. The reaction mixture was cooled to room temperature then added to aqueous sodium hydroxide solution (0.25M, 75 ml) and washed with ether (2×75 ml). The aqueous layer was acidified with concentrated hydrochloric acid and extracted with ether (2×75 ml). The organic extracts were dried over anhydrous magnesium sulfate, filtered and evaporated under reduced pressure. The residue was purified by column chromatography over silica, eluting with isohexane:ethyl acetate (3:1) containing 1% acetic acid, to give the subtitle compound (0.337 g) as a solid.
