Thermal and acid-catalyzed transformations of 3<Emphasis Type="Italic">H</Emphasis>-pyrazoles obtained from diphenyldiazomethane and methyl phenylpropiolate

Reaction of diphenyldiazomethane with methyl phenylpropiolate in diethyl ether alongside the expected methyl triphenyl-3H-pyrazole-4-and-5-carboxylates (I and II) (38 and 24%) gave rise also to 8% of methyl 3,5-diphenyl-1-(1-ethoxyethyl)-1H-pyrazole-4-carboxylate. The main thermolysis product obtained from 4-methoxy-carbonyl derivative I was methyl 1,3,5-triphenyl-1H-pyrazole-4-carboxylate, whereas from regioisomer II formed predominantly methyl 4,4,5-triphenyl-4H-pyrazole-3-carboxylate and 1-methoxycarbonyl-2,3,3-triphenylcyclopropene that was a minor product of 3H-pyrazole I thermolysis. Addition of concn. H₂SO₄ to the solutions of methyl triphenyl-3H-pyrazole-4-and-5-carboxylates in AcOH resulted in fast regioselective isomerization of the 3H-pyrazole derivatives into the corresponding 4H-pyrazoles.