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A series of 2-(5-aryl-3-styryl-4,5-dihydro-1H-pyrazol-1- yl)-4-(trifluoromethyl)pyrimidines was synthesized by the cyclo- condensation of 5-aryl-1-carboxamidino-3-styryl-4,5-dihydro-1H- pyrazoles with 4-alkoxy-1,1,1-trifluoroalk-3-en-2-ones. Efficient cyclizations catalyzed by Ti(Oi-Pr)4 or BF3·OEt2 gave the desired pyrimidines in good yields. The crystal structure of ethyl 3-(2-(5- phenyl-3-styryl-4,5-dihydro-1H-pyrazol-1-yl)-4-(trifluorometh- yl)pyrimidin-6-yl)propanoate was determined. thesis of 5-aryl-1-carboxamidino-3-styryl-4,5-dihydro- 1H-pyrazole derivatives 2 from (3+2) cyclocondensation of 1,5-diarylpenta-1,4-dien-3-ones 1 and aminoguanidine hydrochloride, and their application in the synthesis of new 2-(5-aryl-3-styryl-4,5-dihydro-1H-pyrazol-1-yl)-4- (trifluoromethyl)pyrimidines 4-15 by (3+3) cycloconden- sation of 5-aryl-1-carboxamidino-3-styryl-4,5-dihydro- 1H-pyrazoles 2 with 4-alkoxy-1,1,1-trifluoroalk-3-en-2- ones 3, in which Ti(Oi-Pr) 4 or BF 3 ·OEt 2 is used as pyrim- idine cyclocondensation catalyst. 1,5-Diarylpenta-1,4-dien-3-ones 1a-f were obtained by condensation of acetone with the appropriate benzalde- hyde (1:3) in ethanol solution with NaOH. 15 The 5-aryl-1- carboxamidino-3-styryl-4,5-dihydro-1H-pyrazoles 2a-f were synthesized by the reactions of 1,5-diarylpenta-1,4- dien-3-ones 1a-f with aminoguanidine hydrochloride in the presence of triethylamine in ethanol, under reflux for 24 hours (Scheme 1). When the mixture had cooled to 25 °C, the unreacted starting material precipitated and was removed by filtration. Compounds 2a-f were ob- tained as hydrochlorides, yellow solids that formed in 60- 85 yield.