Heterociclização entre 4-alcóxi-1,1,1-triflúor-3-alquen-2-onas e hidrazonas: metodologias seletivas para a obtenção de heterociclos trifluormetilados

Abstract

Several methods for the regioselective synthesis of trifluoromethylated heterocycles from 4-alcoxy-1,1,1-trifluoro-3-alken-2-ones and hydrazones are described. In the first part of this work, a series of (E)-2-[N -benzyl(1-phenylethyl)idenehydrazino]-4-trifluoromethyl-pyrimidines were obtained in a convergent manner from the cyclocondensation reaction between Nguanidinobenzylimines and 4-alkoxy-1,1,1-trifluoro-3-alken-2-ones in excellent yields (68-99%). Most reactions were highly diastereoselective and the benzyliminic bound presented preferably the E configuration which was determined by single-crystal X-ray analysis of two diastereoisomeric pure benzylidenepyrimidines derivatives. Afterwards, the synthetic versatility of these pyrimidines was demonstrated through the synthesis of 2-(N -enzylhydrazino)-4-trifluoromethyl-pyrimidines and 3-aryl-7-trifluoromethyl[1,2,4]triazolo[4,3- a]pyrimidines. The synthesis of benzylhydrazinopyrimidines were achieved from the selective reduction of the benzyliminic moiety of the benzylidenepyrimidines employing NaBH3CN as reduction agent in an acidic media (pH 3-5). Yields were in the 33-74% range. The 1,2,4-triazolo[4,3-a]pyrimidines were isolated after oxidative heterocyclisation promoted by a copper II catalyst. Those reactions proceeded under mild conditions (DMF, 1.5h, 50-90 °C), were highly chemoselective and yields from 33 and 70%. In the second part of this work, the synthesis of 1-phenyl-3-trifluoromethyl-1H-pyrazoles from 4-alkoxyvinyl trifluoromethyl ketones and phenylhydrazine hydrazones are reported. Those heterocyclisations were highly selective, presented good yields (40-93%) and furnished a series of new enaminones as reaction intermediates. Those reactions that furnished a mixture isomeric pyrazoles, the major isomer was easily isolated through column chromatography.