Journal
MOLECULES
Volume 27, Issue 22, Pages -Publisher
MDPI
DOI: 10.3390/molecules27227913
Keywords
alkynes; cycloaddition; heterocycles; oxidation; iron
Funding
- Ministry of Science and Education of Russian Federation [122040800262-9]
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A method for the synthesis of pyrazolo[1,5-a]pyridine-3-ylphosphonates by oxidative [3+2]cycloaddition reaction was reported. Different substituents have an effect on the reaction activity, with 4-halogenated substituents showing lower reactivity.
A series of pyrazolo[1,5-a]pyridine-3-ylphosphonates were prepared with moderate to good yields by the oxidative [3+2]cycloaddition of 2-subtituted ethynylphosphonates with in situ generated pyridinium-N-imines and their annulated analogs. 2-Aliphatic and 2-Ph acetylenes demonstrate low activity, and the corresponding pyrazolopyridines were achieved with a moderate yield in the presence of 10 mol% Fe(NO3)(3)center dot 9H(2)O. At the same time, tetraethyl ethynylbisphosphonate, diethyl 2-TMS- and 2-OPh-ethynylphosphonates possess much greater reactivity and the corresponding pyrazolo[1,5-a]pyridines, and their annulated derivatives were obtained with good to excellent yields without any catalyst. 2-Halogenated ethynylphosphonates also readily reacted with pyridinium-N-imines, forming complex mixtures containing poor amounts of 2-halogenated pyrazolopyridines.
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