C(Cl)(Cl)=O.O(C)C([S-])=S.[K+].NN.[CH3:13][O:14][C:15]1[S:16][C:17](=[O:20])[NH:18][N:19]=1.Br>C(O)(=O)C>[CH3:13][O:14][C:15]1[S:16][C:17](=[O:20])[NH:18][N:19]=1.[S:16]1[C:17](=[O:20])[N:18]=[N:19][C:15]1=[O:14]2-Methoxy-1,3,4-thiadiazol-5(4H)-one (7) was prepared from phosgene, potassium methyl xanthate, and hydrazine according to the method of K. Rufenacht, Helv. Chim. Acta, 51, 518 (1968). This was hydrolyzed to 1 by the following procedure. A mixture of 50 g of thiadiazole 7 in 300 ml of acetic acid was warmed to 80° to effect solution and then cooled to 70°, whereupon HBr gas was bubbled rapidly into the solution for 25 min. The HBr was shut off, the mixture was held at 70° for 35 min, and then it was cooled to 10°. Ether (200 ml) was added and the mixture was cooled back to 10°. The mixture was filtered through sintered glass and the filter cake was washed with ether, giving 37 g (75%) of 1,3,4-thiadiazoline-2,5-dione or 2-hydroxy-1,3,4-thiadiazol-5-(4H)-one (1). A thirty-gram sample was crystallized from 300 ml of ethanol, affording 18 g of pure 2, mp 175°-255°, infrared maxima (IR), νmax 1700 and 1680 cm-1, ultraviolet maxima (UV), λmax (EtOH) 235 nm (ε 3000) and λmax (EtOH+ NaOH), 258 nm (ε 2730), high resolution mass spectrum (HRMS), m/e calculated for C2H2N2O2S (M+): 117.9837; measured: 117.9825.
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