Philippov I, Gatilov Y, Sonina A, Vorob’ev A. Oxidative [3+2]Cycloaddition of Alkynylphosphonates with Heterocyclic
N-Imines: Synthesis of Pyrazolo[1,5-
a]Pyridine-3-phosphonates.
Molecules 2022;
27:molecules27227913. [PMID:
36432015 PMCID:
PMC9694626 DOI:
10.3390/molecules27227913]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2022] [Revised: 11/01/2022] [Accepted: 11/04/2022] [Indexed: 11/19/2022] Open
Abstract
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)3·9H2O. 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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