The oxidative cyclization of some aldehyde semicarbazones 10 with four different oxidizing agents has been effected. The structure of the semicarbazones and the nature of cyclizing agent affected the rate and yield of cyclization but they did not show any influence on the regiochemistry of reaction. In fact, 1,2,4‐triazoline 20 was the only heterocyclic ring obtained by the cyclization reaction.
The rates of the mononuclear heterocyclic rearrangement of the phenylhydrazone of 3‐benzoyl‐5‐phenyl‐1,2,4‐oxadiazole (1) into 2,5‐diphenyl‐4‐benzoylamino‐1,2,3‐triazole (II) have heen measured in dioxane/water (50:50, v:v) in the range of pS+ 3.8–12.2 at various temperatures and the activation parameters determined. On the basis of the results obtained, we present evidence for the occurrence of two different types of reaction: the first, base‐catalyzed; the second, pS+ ‐independent. In the base‐catalyzed range the catalysis is of the general type.
The first example of mononuclear isoheterocyclic rearrangement is reported. The 3‐benzoylamino‐5‐methyl‐1,2,4‐oxadiazole (5) furnishes through a reversible process (slowly at room temperature in methanol, acetone or dioxane, fast in DMSO or in methanol in the presence of strong bases) a mixture of 5 and 3‐acethylamino‐5‐phenyl‐1,2,4‐oxadiazole (6). The equilibrium process can be achieved also by heating 5 at 181° and the same reaction mixture can be obtained using 6 as the starting material. 3‐Trichloroacetylamino‐5‐methyl‐1,2,4‐oxadiazole (7) was unaffected by similar treatment. The results obtained are discussed.
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