Studies on 2, 5-Diaryl-2, 4-dihydro-3H-pyrazol-3-ones. II. Synthesis of 4-Cycloalkyl-2, 5-diaryl-2, 4-dihydro-3H-pyrazol-3-ones Using the Tautomeric Properties of 2, 5-Diaryl-2, 4-dihydro-3H-pyrazol-3-ones

The reaction of 2-aryl-5-phenyl-2, 4-dihydro-3H-pyrazol-3-ones (1) with cyclopentanone (5a) in the absence of NEt3 gives 2-aryl-4-cyclopentylidene-5-phenyl-2, 4-dihydro-3H-pyrazol-3-ones (7) in nearly quantitative yield. In the presence of NEt3, the reaction of 1 with 5a gives 2-aryl-4-cyclopentyl-5-phenyl-2, 4-dihydro-3H-pyrazol-3-ones (6a-c) in ca. 40% yields together with epoxides (8a-c). Similarly, various 4-cycloalkyl-2-aryl-5-phenyl-2, 4-dihydro-3H-pyrazol-3-ones (6d-i) were obtained in ca. 40% yields. In these reactions, the expected cycloalkylidene products were reduced to cycloalkyl derivatives. The reaction mechanisms and the synthetic utility of these reactions are discussed.