A solution of dimethylsulfoxide (0.113 ml) in anhydrous dichloromethane (1 ml) was added to a solution of oxalyl chloride (0.093 ml) in anhydrous dichloromethane (5 ml) at −78° C. under nitrogen. After 5 minutes, a solution of 5-[(3-hydroxypropyl)thio]-3-methyl-1-(2-methylpropyl)-6-(1-naphthalenylmethyl)thieno[2,3-d]pyrimidine-2,4(1H,3H)-dione (0.25 g) in anhydrous dichloromethane (4 ml) was added dropwise. After 10 minutes, triethylamine (0.372 ml) was added and the mixture was warmed to room temperature. Ether (30 ml) was added and the mixture was washed with 2M hydrochloric acid (10 ml) then with saturated sodium hydrogen carbonate solution (10 ml), then dried over anhydrous magnesium sulfate, filtered and evaporated. The residue was dissolved in anhydrous tetrahydrofuran (10 ml), cooled to −78° C. under nitrogen and treated with methyl magnesium chloride (3.0M is solution in tetrahydrofuran, 0.265 ml). The mixture was warmed to room temperature, then added to saturated sodium hydrogen carbonate solution (30 ml) and then extracted with ethyl acetate (2×30 ml). The organic extracts were dried over anhydrous magnesium sulfate, filtered through a small silica pad and evaporated. The residue was purified by preparative normal-phase HPLC with ethyl acetate/isohexane gradient elution followed by recrystallisation from ethyl acetate/isohexane to give the title compound (58 mg).
