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DMSO as a C1 Source for [2 + 2 + 1] Pyrazole Ring Construction via Metal-Free Annulation with Enaminones and Hydrazines

Haijin Guo, Lihong Tian, Yunyun Liu and Jie-Ping Wan*

*College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, P. R. China, Email: wanjiepingjxnu.edu.cn

H. Guo, L. Tian, Y. Liu, J.-P. Wan, Org. Lett., 2022, 24, 228-233.

DOI: 10.1021/acs.orglett.1c03879


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Abstract

Iodine catalyzes a cascade reaction between enaminones, hydrazines, and DMSO in the presence of Selectfluor to provide 1,4-disubstituted pyrazoles. DMSO plays a dual role as the C1 source and the reaction medium. In addition, the synthesis of 1,3,4-trisubstituted pyrazoles using aldehydes as alternative C1 building blocks has also been achieved.


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proposed mechanism



General procedure

To a 25 mL round-bottom flask were added enaminone (0.2 mmol, 1.0 equiv), hydrazine hydrochloride (0.4 mmol, 2.0 equiv), I2 (0.02 mmol, 10 mol%), Selectfluor (1 mmol, 5.0 equiv) and DMSO (2 mL). Then, the mixture was stirred at 90C with oil bath heating for 10 min. After being cooled down to room temperature, 5 mL of water was added, and the resulting mixture was extracted with ethyl acetate (3 10 mL). The organic phases were collected and washed with a small amount of water for three times. After being dried with anhydrous Na2SO4, the solid was removed by filtration. The solvent in the resulting solution was removed at reduced pressure. The residue obtained therein was subjected to flash silica gel column chromatography to provide pure products with the elution of mixed petroleum ether and ethyl acetate (v/v = 3:1~15:1).


Key Words

pyrazoles, Selectfluor, multicomponent reactions


ID: J54-Y2022