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A facile method for the preparation of 4,5-diaminopyrazoles from N -tosylhydrazones and isocyanides was developed. The reaction was general for a wide range of substrates, and it demonstrated excellent tolerance to a variety of substituents. More importantly, the protective groups of aminos could be selectively removed by controlling the amount of acid. A novel acid-promoted [3 + 1 + 1] cyclization mechanism was proposed.