[NH2:1][C:2]1[S:3][C:4]([C:7]2[CH:12]=[CH:11][C:10]([Cl:13])=[CH:9][CH:8]=2)=[N:5][N:6]=1.[CH2:14]([O:16][C:17]([C:19]#[C:20][C:21](OCC)=[O:22])=[O:18])[CH3:15]>C(O)C>[CH2:14]([O:16][C:17]([C:19]1[N:6]2[N:5]=[C:4]([C:7]3[CH:12]=[CH:11][C:10]([Cl:13])=[CH:9][CH:8]=3)[S:3][C:2]2=[N:1][C:21](=[O:22])[CH:20]=1)=[O:18])[CH3:15]A mixture of 5.5 g of 2-amino-5-(p-chlorophenyl)-1,3,4-thiadiazole, 100 ml of absolute ethanol and 4.5 g of acetylene dicarboxylic acid diethyl ester is heated to about 50° C. until a clear solution has formed and then left standing for 2 days. The product is filtered off, washed with ethanol and dried. Thus 2-(p-chlorophenyl)-7H-1,3,4-thiadiazolo-[3,2-a]-pyrimidin-7-one-5-carboxylic acid ethyl ester melting at 220°-222° C. with decomposition is obtained in a yield of 3.5 g (40.2% of the theoretical).
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