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Synthesis of pyrazolines

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A one-pot synthesis of nitrogen-containing heterocycles from alkyl dihalides and primary amines and hydrazines occurs under microwave irradiation via a simple and efficient cyclocondensation in an alkaline aqueous medium.
Y. Ju, R. S. Varma, J. Org. Chem., 2006, 71, 135-141.


One-pot condensations of ketones, aldehydes and hydrazine monohydrochloride readily formed pyrazoline intermediates under mild conditions. In situ oxidation employing bromine afforded a wide variety of pyrazoles in very good yields. Alternatively, a more benign oxidation protocol affords 3,5-disubstituted or 3,4,5-trisubstituted pyrazoles by simply heating pyrazolines in DMSO under oxygen.
V. Lellek, C.-y. Chen, W. Yang, J. Liu, X. Ji, R. Faessler, Synlett, 2018, 29, 1071-1075.


Low loadings of a combination of Ru3(CO)12 and a NHC-diphosphine ligand catalyze acceptorless dehydrogenative coupling reactions of 1,3-diols with arylhydrazines to provide pyrazoles and 2-pyrazolines in good yields. The reactions offer high selectivity and wide substrate scope with only water and hydrogen gas as the byproducts.
Y. Zheng, Y. Long, H. Gong, J. Xu, C. Zhang, H. Fu, X. Zheng, H. Chen, R. Li, Org. Lett., 2022, 24, 3878-3883.


Arylhydrazines regioselectively react with 3-butynol in the presence of a catalytic amount of zinc triflate to give aryl-substituted pyrazolines. The resulting products are easily oxidized in a one-pot procedure to the corresponding pyrazoles.
K. Alex, A. Tillack, N. Schwarz, M. Beller, Org. Lett., 2008, 10, 2377-2379.


A ligand-free, palladium-catalyzed aminoarylation of unactivated alkenes in β,γ-unsaturated hydrazones provides diversely substituted dihydropyrazoles in good yields via an efficient and simultaneous formation of C(sp3)-N and C(sp3)-C(sp2) bonds under mild conditions.
M.-N. Yang, D.-M. Yan, Q.-Q. Zhao, J.-R. Chen, W.-J. Xiao, Org. Lett., 2017, 19, 5208-5211.


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.


In situ formed 1,2-diaza-1,3-dienes were used in formal [4 + 1]-annulation reactions with fluorinated sulfur ylides to provide 5-(trifluoromethyl)pyrazolines in good yields.
Z. Wang, Y. Yang, F. Gao, Z. Wang, Q. Luo, L. Fang, Org. Lett., 2018, 20, 934-937.


A novel, efficient, and general domino reaction of 2-acylaziridines with the Huisgen zwitterions furnishes 2-pyrazolines. A possible mechanism for the domino sequence is proposed.
S.-L. Cui, J. Wang, Y.-G. Wang, Org. Lett., 2008, 10, 13-16.


A convenient copper-catalyzed intra-/intermolecular diamination of β,γ-unsaturated hydrazones with simple amines enables an efficient access to various nitrogen-containing pyrazolines under mild reaction conditions.
M. Chen, L.-J. Wang, P.-X. Ren, X.-Y. Hou, Z. Fang, M.-N. Han, W. Li, Org. Lett., 2018, 20, 510-513.