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

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. Cycloaddi...

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Bibliographic Details
Published in:Organic & biomolecular chemistry 2020-02, Vol.18 (7), p.1436-1448
Main Authors: Motornov, Vladimir A, Tabolin, Andrey A, Nelyubina, Yulia V, Nenajdenko, Valentine G, Ioffe, Sema L
Format: Article
Language:English
Subjects:
Chemical reactions
Copper
Crystallography
Cycloaddition
Imines
NMR
Nuclear magnetic resonance
Organic chemistry
Pyrazines
Pyridazines
Pyridines
Pyridinium
Salts
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Summary: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. Novel 3-fluoro-pyrazolo[1,5- a ]pyridines were obtained via copper( ii )-mediated oxidative [3 + 2]-annulation of in situ generated pyridinium imines and fluoronitroalkenes.
ISSN:1477-0520
1477-0539
DOI:10.1039/c9ob02668a