Synthesis of fluorinated pyrazole derivatives from β-alkoxyvinyl trifluoroketones

1,1,1-Trifluoro-4-ethoxy-3-butene-2-one, 3-trifluoroacetyl-3, 4-dihydro-2H-pyran or furan reacted readily with pentafluorophenylhydrazine or per(poly)fluoroacectylhydrazine RfCONHNH2 (Rf: BrCF2, C3F7) to give N-substituted-5-hydroxy-5-trifluoromethyl heterocycles YNNCHCH(R)C(OH)CF3 (Y: H, Arf or RfCO), which were dehydrated by treatment with P2O5 or SOCl2 to form N-substituted 5-trifluoromethyl pyrazoles YNNCHC(R)CCF3 (Y: H, Arf or RfCO) in good yields.