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

[摘要]：A convenient and efficient synthesis of spiro-fused pyrazolin-5-one N-oxides, e.g., I, starting from readily available 1-carbamoyl-1-oximylcycloalkanes, e.g., II, is developed. This general protocol features a novel and facile way to access 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 intramol. trapping by the amide moiety under rather mild exptl. conditions.

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