Ring-chain isomerism of aldehyde semicarbazones/1,2,4-triazolidin-3-ones was first studied in detail. Aliphatic aldehyde semicarbazones completely cyclized under the action of very strong Brønsted acids (TfOH, HCl) in aprotic solvents at room temperature to give the corresponding salts of the N1-protonated 1,2,4-triazolidin-3-ones. Both the obtained salts and their free bases are unstable in the presence of air oxygen and undergo slow oxidative aromatization. A novel synthesis of 2-alkyl-2,4-dihydro-3H-1,2,4-triazol-3-ones via the TfOH-promoted cyclization of aliphatic aldehyde semicarbazones followed by aromatization of the isolated crude 1,2,4-triazolidin-3-ones or their hydrotriflates with m-CPBA was developed. In contrast to aliphatic aldehyde semicarbazones, the cyclization of benzaldehyde semicarbazones does not proceed in the presence of strong Brønsted acids. Thermodynamic and kinetic characteristics of the aldehyde semicarbazone ring-chain isomerism are discussed based on the DFT B3LYP/6-311++G(d,p) calculations.

首次详细研究了醛缩氨基脲/1,2,4-三唑烷-3-酮的环链异构现象。脂肪醛缩氨基脲在室温下在非质子溶剂中在极强的布朗斯台德酸（TfOH，HCl）的作用下完全环化，得到N 1-质子化的1,2,4-三唑烷-3-酮的相应盐。所获得的盐及其游离碱在空气氧存在下均不稳定并经历缓慢的氧化芳构化。通过TfOH 促进脂肪醛缩氨基脲环化，然后对分离的粗 1,2,4 进行芳构化，合成2-烷基-2,4-二氢-3 H -1,2,4-三唑-3-酮开发了-三唑烷-3-酮或其氢三氟甲磺酸酯与m -CPBA。与脂肪醛缩氨基脲相反，苯甲醛缩氨基脲的环化在强布朗斯台德酸存在下不会进行。基于DFT B3LYP/6-311++G(d,p)计算讨论了醛缩氨基脲环链异构现象的热力学和动力学特征。