2-Substituted Pyrazole 1-Oxides. Preparation and Reaction with Electrophilic Reagents.

A Regiospecific Approach toN-Alkylpyrazoles and the DerivedN-Oxides Using 5-endo-dig Cyclisations of Alkynyl Nitrosamines

“…Although there are many potential reagents for effecting this transformation, phosphorus trichloride has been established as one of choice. 18 In our hands, treatment of the N-oxides 9a and 9c with PCl 3 in refluxing, ethanol-free chloroform for two hours, as reported previously, delivered excellent yields of the corresponding pyrazoles 13 (Table 2). …”

Mesomeric Betaines and N-Heterocyclic Carbenes of Pyrazole and Indazole

“…The reactions with electrophilic reagents have been described. 44 An alternative approach was also reported, wherein alkynyl nitrosamines underwent a 5-endo-dig cyclization, catalyzed by silver nitrate on silica gel, to give pyrazol-N-oxides. 45…”

Synthesis of Highly Substituted 4‐Iodopyrazole N‐oxides and Pyrazoles from Propargylamines

“…Pyrazole N ‐oxides are prepared by direct oxidation of pyrazoles with peracids, N ‐alkylation of hydroxypyrazoles, oxidative cyclization of O ‐silylated 3‐oximimines, treatment of 1‐benzylamino‐5‐methylhexan‐3‐one oxime with copper(II) sulfate, reaction of 3‐bromo‐1,5‐di‐(4‐tolyl)‐1,5‐diazapenta‐1,3‐diene with hydroxylamine, and reaction of N ‐aryloxosulfonium ylides with nitriloxides . The drawback of most of the existing methods is low yields, multistep syntheses, and lack of regioselectivity . Knight and coworkers have synthesized pyrazole N ‐oxides by 5‐ endo ‐dig cyclization of alkynyl nitrosamines (Scheme ) .…”