O=[C:2]1[CH2:7][CH2:6][CH2:5][S:4][CH:3]1[C:8]([O:10]CC)=O.[OH-].[K+].Br.[CH3:16][S:17][C:18](=[NH:20])[NH2:19]>C(O)(=O)C>[OH:10][C:8]1[C:3]2[S:4][CH2:5][CH2:6][CH2:7][C:2]=2[N:19]=[C:18]([S:17][CH3:16])[N:20]=120 g of ethyl 3-oxotetrahydrothiopyran-2-carboxylate was added to 120 ml of a methanolic solution containing 9 g of potassium hydroxide at room temperature while stirring. Then, a small amount of S-methylisothiourea hydrobromide was added to the mixture and the stirring was continued for 2 hours. The reaction solution was then poured into ice-water, and the mixture was made acidic with acetic acid. The precipitated crystals were separated by filtration and recrystallized from acetic acid to obtain 4-hydroxy-2-methylthio-7,8-dihydro-6H-thiopyrano[3,2-d]pyrimidine as colorless needles having a melting point of 248° C.
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