Regioselective synthesis of ethyl pyrazolecarboxylates from ethyl 3‐[(dimethylamino)methylidene]pyruvate and diethyl 3‐[(dimethylamino)methylidene]‐2‐oxosuccinate. Isolation of ethyl 4,5‐dihydro‐1‐heteroaryl‐5‐hydroxy‐1H‐pyrazole‐5‐carboxylates as stable intermediates in the pyrazole ring formation

Reactions of ethyl 3‐[(E)‐(dimethylamino)methylidene]pyruvate (3) and 3‐[(dimethylamino)methyl‐idene]‐2‐oxosuccinate (4) with hydrazine monohydrochloride (5a) and (hetero)arylhydrazines (5b‐i) afforded, regioselectively, 1‐substituted ethyl 1H‐pyrazole‐5‐carboxylates 9a‐f and diethyl 1H‐pyrazole‐3,4‐dicarboxylates, 11a‐i, respectively. Upon treatment of 3 with pyridazinylhydrazines 5d‐f, the stable intermediates, 1‐substituted ethyl 4,5‐dihydro‐5‐hydroxy‐1H‐pyrazole‐5‐carboxylates 8d‐f, were isolated. Treatment of compounds 8d‐f in acetic acid under reflux furnished the pyrazoles 9d‐f. On the other hand, reaction of 3 with N,N′‐dimethylhydrazine (51) gave ethyl 1‐methyl‐1H‐pyrazole‐3‐carboxylate (14). The structures of compounds 3, 4, 14 were determined by nmr (noesy and hmbc techniques), while the structures of compounds 8f, 9f, and 11e, f were determined by X‐ray diffraction.