[CH3:1][S:2][C:3]1[N:4]=[CH:5][C:6]2[CH:12]=[CH:11][C:10](=O)[NH:9][C:7]=2[N:8]=1.O=P(Cl)(Cl)[Cl:16]>>[Cl:16][C:10]1[CH:11]=[CH:12][C:6]2[CH:5]=[N:4][C:3]([S:2][CH3:1])=[N:8][C:7]=2[N:9]=1A suspension of 2-(methylthio)pyrido[2,3-d]pyrimidin-7(8H)-one (297 mg, 1.54 mmol) in POCl3 (6 mL) was stirred under argon at 80° C. for 16 hours. The solution was cooled, and the resulting precipitate was filtered and washed with EtOAc to afford the title compound (43 mg, 13%). The filtrate was diluted with EtOAc, washed with NaHCO3 and dried with Na2SO4. After removal of the solvent, a second crop was obtained (154 mg, 48%).
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