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Issue 7, 2020
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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

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Abstract

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.

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

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Article information


Submitted
16 Dec 2019
Accepted
24 Jan 2020
First published
24 Jan 2020

Org. Biomol. Chem., 2020,18, 1436-1448
Article type
Paper

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

V. A. Motornov, A. A. Tabolin, Y. V. Nelyubina, V. G. Nenajdenko and S. L. Ioffe, Org. Biomol. Chem., 2020, 18, 1436
DOI: 10.1039/C9OB02668A

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