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1. Pyrimidine-2,4,6-triol (10.0 g, 78.2 mmol) was dissolved with N,N-dimethylformamide (12 mL) in phosphorus trichloride (10 vol. eq.) and heated to reflux for 15 h. 2. Upon completion of the reaction, the excess of phosphorus trichloride was removed by distillation under reduced pressure. 3. The residue of the reaction was slowly poured into crushed ice and precipitated as a solid. 4. The solid product was collected by filtration and washed with cold water. The solid product was collected by filtration and washed with cold water to give 2,4,6-trichloropyrimidine-5-carboxaldehyde (8.0 g, 47% yield) as a yellow solid. 5. The crude product did not need to be further purified, and could be used in the subsequent reaction directly.

[1] Angewandte Chemie - International Edition, 2006, vol. 45, # 43, p. 7262 - 7265
[2] Patent: WO2012/103297, 2012, A1. Location in patent: Page/Page column 84-85
[3] Patent: WO2017/12576, 2017, A1. Location in patent: Page/Page column 197
[4] Patent: WO2011/14535, 2011, A1. Location in patent: Page/Page column 100
[5] Patent: WO2004/18435, 2004, A1. Location in patent: Page 55-56