An
efficient and controlled site-selective annulation of 3,5-diethoxycarbonyl
4-hydrazonyl pyrazoles is described. The relative proportion of the
products is affected by hydrazone intermediate configuration, reaction
temperature, and Lewis acid employed. At a temperature of 110–120
°C, the reaction preferentially afforded 1H-pyrazolo[3,4-d]pyridazin-7(6H)-ones, whereas using Yb(OTf)3 in MeCN reflux, 2H-pyrazolo[3,4-d]pyridazin-7(6H)-ones were favored. Computational
investigations were performed to clarify the mechanism and the origin
of the regiodivergence.