[C:1]([O:11]CC)(=O)[CH2:2][C:3]([C:5]([O:7]CC)=[O:6])=O.[OH-].[Na+].S(O)(O)(=O)=O.[CH3:21][S:22][C:23](=[NH:25])[NH2:24].Cl>O>[CH3:21][S:22][C:23]1[NH:25][C:1](=[O:11])[CH:2]=[C:3]([C:5]([OH:7])=[O:6])[N:24]=1Neat diethyl oxalacetate (23.5 g, 125 mmol) was added to a solution of sodium hydroxide (15 g, 0.375 mol, 3 eq) in water (150 mL), giving a yellow solution. Solid S-methylisothiourea sulfate (17.4 g, 62.5 mmol, 0.5 eq) was added to the stirred solution. The mixture was stirred 15 hours. Concentrated HCl (40 mL, 0.48 mol) was added bringing the pH to 1, the resultant pale orange-pink suspension was stirred vigorously for 2 hours then filtered. The filter cake was washed with water (100 mL), and dried under high vacuum to afford 2-methylsulfanyl-6-oxo-1,6-dihydro-pyrimidine-4-carboxylic acid as an off-white solid. 1H NMR (500 MHz, DMSO-d6) □ 13.22 (br s, 1H), 6.60 (s, 1H), 3.37 (br s, 1H), 2.53 (s, 3H); ESI-MS m/z 186.9 (M+H)+.
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