At 0° C., 179 mg of triphenylphosphine and 107 μl of diethyl azodicarboxylate were initially charged in 4.5 ml of tetrahydrofuran, and the mixture was stirred for 15 min. 200 mg of 4-[5-(2-fluorophenoxy)oxazolo[5,4-d]pyrimidin-2-yl]-2,6-dimethylphenol, 95 μl of triethylamine and 141 mg of benzyl 3-hydroxycyclobutanecarboxylate were then added to the reaction, and the reaction was stirred under argon, initially at room temperature for 3 h. Another 179 mg of triphenylphosphine and 107 μl of diethyl azodicarboxylate were then added, and the reaction was stirred for 12 h. For work-up, the reaction was concentrated and the resulting residue was purified by flash chromatography (silica gel, heptane/ethyl acetate). This gave 216 mg (70%) of the title compound.
