To a 500 mL three-necked flask was added whey acid (15.6 g, 0.10 mol), followed by 300 mL of anhydrous methanol, stirring and cooling the reaction system to 0°C. Thionyl chloride (10.9 mL, 0.15 mol) was added dropwise, slowly, while maintaining the temperature at 0-5°C. The reaction system was then cooled to 0°C. The reaction system was then cooled to 0°C. The reaction system was then cooled to 0°C. After the dropwise addition, the reaction system was warmed up to reflux and the reaction was continued for 6 hours. Upon completion of the reaction, the reaction mixture was slightly cooled and subsequently dried under vacuum to remove the solvent. To the residue, 50 mL of anhydrous methanol was added and removed with thorough stirring. Next, 150 mL of methyl tert-butyl ether was added and stirred at room temperature for 1 hour. Finally, the solid product was collected by filtration and washed with methyl tert-butyl ether to afford 15.8 g of the target compound, methyl 2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylate, in 92.9% yield.
