The product from Example 2 (10.73 g; 31 mmol), thymine (7.88 g; 62 mmol) and potassium carbonate (8.64 g; 62 mmol) are suspended in anhydrous N,N-dimethylformamide (325 ml). The suspension is stirred at room temperature for 1 h and then at 80° C. for 6 h. The cooled mixture is codistilled repeatedly with toluene, and the residue is taken up in chloroform (500 ml) and extracted twice with water (150 ml on each occasion). The crude product is purified by chromatography on silica gel (eluent: toluene/ethanol, 27:1). Yield: 5.58 g (48%).
