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Synthesis of indazolones

Recent Literature

The use of blocked (masked) N-isocyanate precursors enables the formation of indazolones by Friedel-Crafts cyclization. Substrate synthesis has also been improved using a copper-catalyzed arylation of phenyl carbazates with pentavalent bismuth reagents that is compatible with the hemilabile OPh blocking group.
E. B. Elkaeed, J. An, A. M. Beauchemin, J. Org. Chem., 2017, 82, 9890-9897.

The key step in a concise, one-step route to indazolones from primary alkyl amines and o-nitrobenzyl alcohols involves base-mediated in situ o-nitrobenzyl alcohol → o-nitrosobenzaldehyde conversion.
J. S. Zhu, N. Kraemer, M. E. Shatskikh, C. J. Li, J.-H. Son, M. H. Haddadin, D. J. Tantillo, M.  J. Kurth, Org. Lett., 2018, 20, 4736-4739.

o-Nitrosobenzaldehyde is a useful reactive intermediate for the synthesis of nitrogen heterocycles. This intermediate can be photochemically generated in situ from o-nitrobenzyl alcohols in an efficient manner for the construction of 1,2-dihydro-3H-indazol-3-ones using an aqueous solvent at room temperature.
J. S. Zhu, N. Kraemer, C. J. Li, M. J. Haddadin, M. J. Kurth, J. Org. Chem., 2018, 83, 15493-15498.

One-Pot, Multistep Reactions for the Modular Synthesis of N,N'-Diarylindazol-3-ones
S. Liu, L. Xu, Y. Wei, J. Org. Chem., 2019, 84, 1596-1604.