A preparação, com bons rendimentos (70-98%), de doze novos ésteres etílicos de 5-triclorometil-4,5-diidro-1H-1-pirazóis, usando-se ambientalmente boas técnicas induzidas por microondas, é descrita. Estes compostos foram sintetizados a partir da reação de ciclocondensação de 1,1,1-tricloro-4-alcoxi-3-alquen-2-onas [CCl 3 C(O)C(R 2 )=C(R 1 )OR, onde R, R 2 = H, alquila; R 1 = H, alquila e arila] com metil carboxilato hidrazina. As vantagens obtidas pelo uso de irradiações de microondas na ausência de solventes, em relação ao método convencional foram demonstradas.Twelve novel 5-trichloromethyl-4,5-dihydro-1H-1-pyrazole ethyl esters have been synthesized in good yields (70-98%) by using environmentally benign microwave induced techniques. The compounds were synthesized from the cyclocondensation of 1,1,1-trichloro-4-alkoxy-3-alken-2-ones [CCl 3 C(O)C(R 2 )=C(R 1 )OR, where R, R 2 = H, alkyl; R 1 = H, alkyl and aryl] with hydrazine methyl carboxylate. The advantages obtained by the using of microwave irradiation under solvent-free conditions, rather than a conventional method, were demonstrated.
Keywords: pyrazoles, enones, microwave irradiations, green chemistry
IntroductionTrihalomethyl substituted pyrazoles belong to an important class of compounds, which possess a wide variety of pharmaceutical and agrochemical properties. 1,2 The main synthetic method used to prepare trihalomethylpyrazoles involves a [3+2] cyclization such as the classical 1,3-diketone with hydrazines. 3 In recent years, we have developed a general synthesis of 1,1,1-trihalo-4-alkoxy-3-alken-2-ones, 4,5 important halogen-containing building blocks and their use in heterocyclic preparations (e.g. isoxazoles, pyrazoles, pyrazolium chlorides, pyrrolidines, pyrimidines, thiazines, diazepines, thiazoles, selenazoles, and quinolines) has been extensively described. 5 In particular, trichloromethyl substituted azoles are important as synthons and reagents in organic synthesis. These compounds have also been used as precursors for the synthesis of carbonyl-azole derivatives 6 in a one-pot procedure. Moreover, the pharmaceutical properties of trichloromethyl substituted pyrazolines have been reported by our research group. 7 Recently, we reported the application of microwave irradiation for the synthesis of halomethylsubstituted azoles. 8 The beneficial effects of microwave irradiation are playing an increasing role in process chemistry, especially in cases where classical methods require forcing conditions or prolonged reaction times. When processes involve sensitive reagents, or there is the possibility of compound decomposition under prolonged reactions conditions, microwaves have also shown an advantage. The use of focused microwave irradiation to decrease reaction times and improve yields has been demonstrated. 9 Microwave irradiation (MW), using commercial domestic ovens, has been recently used to accelerate organic reactions, due to its high heating efficiency, giving remarkable rate enhancement and dramatic reduction in reaction times. In recent...
