Calcium carbide as the acetylide source: transition-metal-free synthesis of substituted pyrazoles via [1,5]-sigmatropic rearrangements

Under transition-metal-free conditions, calcium carbide was used as the acetylide source to react with a wide range of N-tosylhydrazones derived from aldehydes or ketones, affording various substituted pyrazoles in good yields with high regio-selectivities. The transformations go through [3 + 2] cycloadditions followed by [1,5]-sigmatropic rearrangements, which are supported by deuterium-labeling experiments.