Br[C:2]1[S:3][C:4]([C:10]([O:12][CH2:13][CH3:14])=[O:11])=[CH:5][C:6]=1[N+:7]([O-:9])=[O:8].[N-:15]=[N+]=[N-].[Na+]>CO.C(O)C>[N+:7]1([O-:9])[O:8][N:15]=[C:2]2[S:3][C:4]([C:10]([O:12][CH2:13][CH3:14])=[O:11])=[CH:5][C:6]=12A suspension of 25.0 g (90 mmol) of ethyl 2-bromo-3-nitro-5-thiophenecarboxylate (Tetrahedron 21 (1965) 1061) and 29.0 g (450 mmol) of sodium azide in 300 ml of methanol is stirred at room temperature for 4 hours in 300 ml of ethanol. The mixture is poured into 4 l of ice-water, the precipitate is filtered off and the residue is heated at 70° C. for 3 hours in 700 ml of toluene. Subsequent purification by flash chromatography yields 6.5 g (34%) of ethyl thieno[2,3-c][1,2,5]oxadiazole-5-carboxylate-1-oxide and -3-oxide, m.p. 72°-74° C.
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