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Synthesis of pyrazoles and indazoles

Recent Literature

1,3-Diketones, which were synthesized in situ from ketones and acid chlorides, were converted into pyrazoles by the addition of hydrazine. This method allows a fast and general synthesis of previously inaccessible pyrazoles and synthetically demanding pyrazole-containing fused rings.
S. T. Heller, S. R. Natarajan, Org. Lett., 2006, 8, 2675-2678.

A highly regioselective synthesis of 1-aryl-3,4,5-substituted pyrazoles based on the condensation of 1,3-diketones with arylhydrazines proceeds at room temperature in N,N-dimethylacetamide and furnishes pyrazoles in good yields.
F. Gosselin, P. D. O'Shea, R. A. Webster, R. A. Reamer, R. D. Tillyer, E. J. J. Grabowski, Synlett, 2006, 3267-3270.

Pyrazole or isoxazole derivatives are prepared by a palladium-catalyzed four-component coupling of a terminal alkyne, hydrazine (hydroxylamine), carbon monoxide under ambient pressure, and an aryl iodide.
M. S. M. Ahmed, K. Kobayashi, A. Mori, Org. Lett., 2005, 7, 4487-4489.

M. S. M. Ahmed, K. Kobayashi, A. Mori, Org. Lett., 2005, 7, 4487-4489.

A regioselective synthesis of tri- or tetrasubstituted pyrazoles by the reaction of hydrazones with nitroolefins mediated with strong bases such as t-BuOK exhibits a reversed, exclusive 1,3,4-regioselectivity. Subsequent quenching with strong acids such as TFA is essential to achieve good yields. A stepwise cycloaddition reaction mechanism is proposed.
X. Deng, N. S. Mani, Org. Lett., 2008, 10, 1307-1310.

Two general protocols for the reaction of electron-deficient N-arylhydrazones with nitroolefins allow a regioselective synthesis of 1,3,5-tri- and 1,3,4,5-tetrasubstituted pyrazoles. Studies on the stereochemistry of the key pyrazolidine intermediate suggest a stepwise cycloaddition mechanism.
X. Deng, N. S. Mani, J. Org. Chem., 2008, 73, 2412-2415.

A regioselective one-pot synthesis of substituted pyrazoles from N-monosubstituted hydrazones and nitroolefins gives products in good yields. A key nitropyrazolidine intermediate is characterized and a plausible mechanism is proposed.
X. Deng, N. S. Mani, Org. Lett., 2006, 8, 3505-3508.

A general, highly flexible Cu-catalyzed domino C-N coupling/hydroamination reaction constitutes a straightforward alternative to existing methodology for the preparation of pyrroles and pyrazoles.
R. Martin, M. R. Rivero, S. L. Buchwald, Angew. Chem. Int. Ed., 2006, 45, 7079-7082.

Various 1-acyl-5-hydroxy-4,5-dihydro-1H-pyrazoles have been prepared in good yields from the corresponding 2-alkyn-1-ones. The resulting dihydropyrazoles undergo dehydration and iodination in the presence of ICl and Li₂CO₃ at room temperature to provide 1-acyl-4-iodo-1H-pyrazoles.
J. P. Waldo, S. Mehta, R. C. Larock, J. Org. Chem., 2008, 73, 6666-6670.

A series of 4-substituted 1H-pyrazole-5-carboxylates was prepared from the cyclocondensation reaction of unsymmetrical enaminodiketones with tert-butylhydrazine hydrochloride or carboxymethylhydrazine. The compounds were obtained regiospecifically and in very good yields.
F. A. Rosa, P. Machado, P. S. Vargas, H. G. Bonacorso, N. Zanatta, M. A. P. Martins, Synlett, 2008, 1673-1678.

The reaction of diazo(trimethylsilyl)methylmagnesium bromide with aldehydes or ketones gave 2-diazo-2-(trimethylsilyl)ethanols, which were applied to the synthesis of di- and trisubstituted pyrazoles via [3+2] cycloaddition reaction with ethyl propiolate or dimethyl acetylenedicarboxylate.
Y. Hari, S. Tsuchida, R. Sone, T. Aoyama, Synthesis, 2007, 3371-3375.

In the presence of activated carbon, Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines were aromatized with molecular oxygen to the corresponding pyridines and pyrazoles in excellent yields.
N. Nakamichi, Y. Kawashita, M. Hayashi, Synthesis, 2004, 1015-1020.

The [3+2] cycloaddition of a variety of diazo compounds with o-(trimethylsilyl)aryl triflates in the presence of CsF or TBAF at room temperature provides a very direct, efficient approach to a wide range of potentially biologically and pharmaceutically interesting substituted indazoles in good to excellent yields under mild reaction conditions.
Z. Liu, F. Shi, P. D. G. Martinze, C. Raminelli, R. C. Larock, J. Org. Chem., 2008, 73, 219-226.