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Synthesis of indazolones

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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.

B2(OH)4-mediated reductive N-N bond formation enables an efficient approach to 2-substituted indazolones. This method offers mild conditions, no request for a metal catalyst, and a wide scope for both aliphatic and aromatic amines.
Y. Bao, Z. Deng, J. Feng, W. Zhu, J. Li, J. Wan, G. Liu, Org. Lett., 2020, 22, 6277-6282.

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.

A facile synthesis of hydrazides from N-tosylhydrazones under metal-free conditions was suitable for different substrates and tolerated various substituents. In a subsequent step, a series of indazolones could be prepared from corresponding o-bromobenzohydrazides under mild conditions.
S. Zhang, Q. Zhang, M. Tang, J. Org. Chem., 2022, 87, 3845-3850.

An efficient oxo-sulfonylation protocol enables the synthesis of N-sulfonylated indazolones from 2H-indazoles employing sulfinic acid as a sulfonylating agent in the presence of tert-butyl hydroperoxide (TBHP) under ambient air. A series of structurally diverse 1-sulfonylindazol-3(2H)-one derivatives were obtained in good yields.
P. Ghosh, S. Mondal, A. Hajra, Org. Lett., 2020, 22, 1086-1090.