A suspension of triphenylphosphine (59.3 mg, 0.226 mmol) and 1,3,5-trimethyl-1H-pyrazole-4-carboxylic acid (SALOR) (26.8 mg, 0.174 mmol) in dichloromethane was cooled to 0° C. and N-chlorosuccinimide added. The mixture was stirred at 0° C. for 0.5 h then ambient temperature for an additional 0.5 h. 3-Cyclopropylmethyl-8-[4-(ethyl-amino)-benzyl]-1-(2-fluorobenzyl)-3,7-dihydro-purine-2,6-dione (93.4 mg, 0.209 mmol) was added followed by triethylamine (30.2 mg, 0.226 mmol) and 4-dimethylaminopyridine (a few crystals). The reaction was left to stir at ambient temperature overnight before washing with 1M aqueous hydrochloric acid, saturated aqueous sodium bicarbonate, drying the organic extract (sodium sulfate) and concentrating in vacuo. Purification by chromatography using silica eluted with 5:9 methanol/dichloromethane gave the product as a colorless solid (83.5 mg, 69%). MS, m/z(M+H)=584.2787.
