Synthesis of Pyrazolyl-2-pyrazolines by Treatment of 3-(3-Aryl-3-oxopropenyl)-chromen-4-ones with Hydrazine and Their Oxidation to Bis(pyrazoles)
The synthesis of several 1-acetyl-3-aryl-5-[3-(2-hydroxyphenyl)pyrazol-4-yl]-2-pyrazolines 3a−3h has been accomplished by treatment of the 3-(3-aryl-3-oxopropenyl)chromen-4-ones 1a−h with hydrazine hydrate in hot acetic acid. The 1-acetyl-3-aryl-5-(3-chromonyl)-2-pyrazolines 2a−2f were also obtained as by-products. Oxidation of the 1-acetyl-4-pyrazolyl-2-pyrazolines 3a−3f with DDQ gave the 3(5)-aryl-5(3)-[3-(2-hydroxyphenyl)pyrazol-4-yl]pyrazoles 5a−5f. The oxidation of the 2-pyrazoline rings was accompanied by N-deacylation. The reaction mechanisms of both transformations are discussed, the first one being supported by experimental results. The structures of all new derivatives were established by NMR and the evidence of prototropic tautomerism is carefully discussed. Theoretical calculations of energies and of the 1H and 13C NMR chemical shifts of the possible tautomeric forms of 5(3)-[3-(2-hydroxyphenyl)pyrazol-4-yl]-3(5)-(4-methoxyphenyl)pyrazole (5c), by B3LYP and GIAO, showed that compounds of this type probably exist as mixtures of two tautomers.