Solventless syntheses of pyrazole derivatives

Solventless condensation of a diketone and a hydrazine in the presence of a catalytic amount of sulfuric acid at room temperature afforded pyrazole derivatives 3a-3i and 4g-4i in high yields. The condensation of 2,4-pentanedione and hydrazides gave similar results, while the reaction between 1-phenylbutane-1,3-dione and hydrazides under the same conditions afforded 4,5-dihydro-5-hydroxypyrazole derivatives 6a and 6b, which can be transformed to 4i (for 6b) or a mixture of 1-acylpyrazole and 3i (4i) (for 6a) by thermolysis in the presence of a catalytic amount of sulfuric acid. Similar reaction of ethyl acetoacetate with phenylhydrazine or hydrazine formed 2-pyrazolin-5-one 8 and 3-pyrazolin-5-one 9, respectively. Reactions of 2,4-dinitrophenylhydrazine with 2,4-pentanedione and ethyl acetoacetate yielded hydrazones 10 and 11, respectively.