Ring-Chain Tautomerism of 3-Unsubstituted Trifluoromethyl-Containing n -Acyl-5-Hydroxy-2-Pyrazolines

Reaction of the hydrazides of aromatic and pyridine-3(4)-carboxylic acids with 4-ethoxy-1,1,1-trifluorobut-3-en-2-one leads to the formation of derivatives having a 5-hydroxy-2-pyrazoline structure. In solution in DMSO-d6 they are capable of exhibiting ring-chain equilibrium involving the 5-hydroxy2-pyrazoline and enehydrazine tautomers. The latter form is represented by a mixture of Z- and E-isomers with a significant preponderance of the latter. The product from the reaction of 4-aminobenzohydrazide with 4-ethoxy-1,1,1-trifluorobut-3-en-2-one in a ratio of 1:2 has a structure containing 5-hydroxy-2-pyrazoline and enamine fragments.