A mixture of 2.39 g (16.8 mmol) phosphorus pentoxide, 2.31 g (16.8 mmol) triethylamine hydrochloride, and 1.82 ml (16.8 mmol) m-toluidine is heated and stirred in a nitrogen atmosphere at 200° C. until a homogeneous melt has formed. To the melt, 1 g (4.2 mmol) 4-(4-pyridylmethyl)-1(2H)-phthalazinone is added and the reaction mixture stirred for a further 3.5 h at 205° C. After cooling to about 170° C., 4 ml tetramethylurea is added to the reaction mixture, followed by 2 ml water at about 110° C. The mixture is stirred for a further 30 min while cooling to RT and then distributed between dichloromethane and a mixture of 20 ml water and 5 ml conc. ammonia. The organic phase dried over sodium sulfate is concentrated by evaporation and the residue purified on silica gel by flash chromatography using acetate and acetate/methanol (99:1 and 19:1). Title compound is obtained after crystallization of the product-containing fractions from acetonitrile and subsequent recrystallization from acetate; m.p. 141-143° C.; ESI-MS: (M+H)+=327.
