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Synthesis of 1,2,4-triazolo[1,5-a]pyridines

Recent Literature

[1,2,4]Triazolo[1,5-a]pyridines have been prepared in good yields from 2-aminopyridines by cyclization of N-(pyrid-2-yl)formamidoximes under mild reaction conditions with trifluoroacetic anhydride.
E. Huntsman, J. Balsells, Eur. J. Org. Chem., 2005, 3761-3765.

A copper-catalyzed reaction under an atmosphere of air provides 1,2,4-triazole derivatives by sequential N-C and N-N bond-forming oxidative coupling reactions. Starting materials and the copper catalyst are readily available and inexpensive. A wide range of functional groups are tolerated.
S. Ueda, H. Nagasawa, J. Am. Chem. Soc., 2009, 131, 15080-15081.

The biologically important 1,2,4-triazolo[1,5-a]pyridines were readily synthesized from N-(pyridin-2-yl)benzimidamides via PIFA-mediated intramolecular annulation via a direct metal-free oxidative N-N bond formation in short reaction times and high yields.
Zheng, S. Ma, L. Tang, D. Zhang-Negrerie, Y. Du, K. Zhao, J. Org. Chem., 2014, 79, 4687-4693.

An I2/KI-mediated oxidative N-N bond formation reaction enables an environmentally benign synthesis of various 1,2,4-triazolo[1,5-a]pyridines and other 1,5-fused 1,2,4-triazoles from readily available N-aryl amidines in an efficient and scalable fashion.
L. Song, X. Tian, Z. Lv, E. Li, J. Wu, Y. Liu, W. Yu, J. Chang, J. Org. Chem., 2015, 80, 7219-7225.

A convenient synthesis of 1,5-fused 1,2,4-triazoles in high yields from readily available N-arylamidines is efficiently promoted by chloramine-T through direct metal-free oxidative N-N bond formation. The mild nature of the synthesis and short reaction time are notable advantages of the developed protocol.
A. S. Singh, A. K. Agrahari, S. K. Singh, M. S. Yadav, V. K. Tiwari, Synthesis, 2019, 51, 3443-3450.