N,N-Dimethylaniline (3.2 g, 26.6 mmol) and phosphorus oxychloride (32.6 g, 213 mmol) were added to the solution of 7-benzyl-5,6,7,8-tetrahydro-2-(methylthio)pyrido[3,4-d]pyrimidin-4(4aH)-one (7.6 g, 26.6 mmol) in 1,2-dichloroethane (20 mL, anhydrous) and were stirred in a preheated oil bath at 90° C. for 1 hr. Reaction mixture was poured into ice, neutralized by solid sodium carbonate, extracted by ethyl acetate and dried over sodium sulfate. Solvent was removed in vacuo and light brown oil residue was directly gone to next step.
