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Copper-mediated oxidative [3 + 2]-annulation of nitroalkenes and pyridinium imines: efficient synthesis of 3-fluoro- and 3-nitro-pyrazolo[1,5-a]pyridines. 2020

Vladimir A Motornov, and Andrey A Tabolin, and Yulia V Nelyubina, and Valentine G Nenajdenko, and Sema L Ioffe
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, Moscow, 119991, Russia. tabolin87@mail.ru and Higher Chemical College, D. I. Mendeleev University of Chemical Technology of Russia, Miusskaya sq. 9, Moscow, 125047, Russia.

An efficient route to pyrazolo[1,5-a]pyridines by Cu(OAc)2-promoted oxidative [3 + 2]-annulation of nitroalkenes with in situ generated pyridinium imines is developed. The reaction with α-fluoronitroalkenes enables the first preparative synthesis of 3-fluoro-pyrazolo[1,5-a]pyridines. Cycloaddition with α-unsubstituted nitroalkenes provides access to 3-nitro-pyrazolo[1,5-a]pyridines in excellent yields. A broad transformation scope was demonstrated. Both electron-rich and electron-deficient nitroalkenes as well as different aminopyridinium salts can be used for the assembly of the target pyrazolo[1,5-a]pyridines. The related aza-heterocycles, namely, pyrazolo[1,5-a]pyrazines and pyrazolo[1,5-b]pyridazines, were successfully prepared via the present methodology. The possible mechanism of the reaction is discussed.

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