PIFA-Mediated Oxidative Cyclization of 1-Carbamoyl-1-oximylcycloalkanes: Synthesis of Spiro-Fused Pyrazolin-5-one N-Oxides

A convenient and efficient synthesis of spiro-fused pyrazolin-5-one N-oxides starting from readily available 1-carbamoyl-1-oximylcycloalkanes is developed. This general protocol features a novel and facile way for access to the five-membered azaheterocycles by formation of a new N−N single bond. The key cyclization step utilizes the formation of an N-oxonitrenium intermediate, mediated by the hypervalent iodine reagent PIFA, and its subsequent intramolecular trapping by the amide moiety under rather mild experimental conditions.本研究成功开发了一种从易得的1-羰基-1-氧甲酰基环烷烃出发，高效合成螺环 fused pyrazolin-5-one N-氧化物的便捷方法。该通用合成途径以形成新的N-N单键为特色，提供了一种新颖且简便的途径以获取五元杂环化合物。关键的环化步骤通过利用高氧化态碘试剂PIFA介导的N-亚硝酰基脲中间体的形成，并在相对温和的实验条件下，由酰胺官能团进行分子内捕获来实现。