Under nitrogen atmosphere, to a solution of 1-(2-fluoro-4-iodophenyl)-5-hydroxy-3-(4-methoxybenzyl)-6,8-dimethylpyrido[2,3-d]pyrimidine-2,4,7(1H,3H,8H)-trione (41 g, 72.8 mmol) (Prepared as per reference WO2005121142) in acetonitrile (300 ml), triethylamine (30.4 ml, 218 mmol) and trimethylamine hydrochloride (3.48 g, 36.4 mmol) were added slowly followed by addition of p-toluensulfonylchloride (27.8 g, 146 mmol) in acetonitrile (300 ml) at 0° C., and the mixture was stirred under ice cooling for 1 hr, and then at room temperature for 24 h. To the reaction mixture was added methanol (220 ml), and the mixture was stirred at room temperature for 1 h. The precipitated crystals were collected by filtration, dried under vacuum to afford the titled compound (40.5 g, 78%)
