Ciclocondensação entre epoxichalconas e hidrazinas promovida por ultrassom

Abstract

Pyrazoles are heterocyclic compounds which occupy a prominent place as active substances in biological systems and, consequently, the synthesis of these substances has resulted in their industrial scale production as well as the worldwide utilization as pharmaceuticals. The main synthetic routes for obtaining pyrazoles involves cycloaddition reactions between dipoles and dipolarophiles or cyclocondensation reactions between hydrazines and 1,3- dicarbonyl compounds or their 1,3-dielectrophilic analogues. Among these routes, the cyclocondensation between hydrazines and α-enones stands for simplicity, structural diversity of starting materials and great regioselectivity. However, these reactions produce pairs of enantiomeric pyrazolines that require an oxidative aromatization step to be converted into the corresponding pyrazoles. The objective of this work is to study the cyclocondensation reaction between hydrazine and α-epoxychalcones derived from chalcones using ultrasonic irradiation, aiming to pyrazole synthesis. After optimization of the reaction conditions, it was found that the cyclocondensation reaction between hydrazine hydrate and 3-aryl-2,3-epoxy-1-phenyl-1-propanones form 4-(5-aryl)-3-phenyl-4,5-dihydro- 1H-pyrazoles. The reactions were performed in ethanol without catalysts in 40 minutes under ultrasonic irradiation. The products were obtained with high degrees of purity in 89-97% of yields. The structure of the products was confirmed by mass spectrometry analysis, infrared spectroscopy and by comparing the melting points to those reported in the literature. Therefore, a rapid and practical methodology for the preparation of pyrazoles using readily accessible starting materials was developed.