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.
Divergent synthesis of fully substituted isoxazoles and spiro-fused pyrazolin-5-ones from cyclopropyl oximes
Efficient and divergent synthesis of fully substituted isoxazoles and spiro-fused pyrazolin-5-ones is developed from cyclopropyl oximes based on selection of reaction conditions. Under Appel conditions (PPh3/CBr4), substituted isoxazoles were synthesized from cyclopropyl oximes via a ring-opening and intramolecular cyclization process, whereas by treatment of cyclopropyl oximes with p-toluenesulfonyl chloride in the presence of potassium hydroxide, spiro-fused pyrazolin-5-ones were obtained via tandem ketoxime tosylation and intramolecular cyclization. (C) 2011 Elsevier Ltd. All rights reserved.