3.4 ml (0.026 mol) of trimethylsilyl bromide were slowly added dropwise at 0° C., under an argon atmosphere, to a solution of 4.16 g (0.01 mol) of [4-(7-ethoxymethyl-3-methylxanthin-1-yl)butyl]phosphonate in 5 ml of dichloromethane. The solution was stirred at 0° C. for 1 hour and at room temperature for 2 hours. The dichloromethane was then evaporated and the oily residue which remained was dried in a bulb tube in an oil pump vacuum to remove excess trimethylsilyl bromide and ethyl bromide formed. The residue was taken up in a little methanol and treated with 1 ml of concentrated ammonia solution and stirred at room temperature for 2 hours. After evaporating the solvent, the diammonium salt was crystallized from a methanoltdiisopropyl ether mixture.
