1.3 g (9.44 mmol) of potassium carbonate were added at 60° C. to a solution of 2.4 g (5.9 mmol) of 3-methyl-7-tritylxanthine (prepared according to Example 17a) in 50 ml of N-methylpyrrolidone and the mixture was stirred at this temperature for one hour. 1.57 g (7.67 mmol) of 2-(4-chlorobenzyloxy)ethyl chloride were then added dropwise and the mixture was stirred at 80° C. for one hour. A further 1.0 g (4.9 mmol) of 2-(4-chlorobenzyloxy)ethyl chloride was then added and the mixture was stirred again for one hour. 50 ml of water were then added, the mixture was extracted three times using 60 ml of methyl tert-butyl ether each time, and the combined organic phases were washed twice with 30 ml of water each time, dried using magnesium sulfate and concentrated under reduced pressure. The crude product was purified by flash chromatography on a silica gel column, heptane/ethyl acetate (3/2).
