AgNO₃-Catalyzed 5-endo-dig Cyclizations to Form Pyrazole N-Oxides and Pyrazoles

Significance

Reported is the silver nitrate catalyzed 5-endo-dig cyclization of N-nitrosaminepropargylamines to form pyrazole N-oxides in excellent yields. Deoxygenation of these N-oxides was achieved, also in excellent yield, using the established protocol of PCl₃ in refluxing, ethanol-free, chloroform. The N-nitrosamines were prepared in moderate to good yield through addition of an alkyne to an imine and subsequent N-nitrosation. A reasonable, yet tentative, mechanism for the cyclization is proposed.