Efficient and regioselective synthesis of isoxazoles was demonstrated through cyclocondensation of β-enamino diketones with hydroxylamine: four of the six possible regioisomers were obtained.
This study reports the results of ab initio and density functional theory (DFT) electronic structure calculations as well as (3)J(HH) experimental and calculated coupling constant data obtained in the investigation of the conformational equilibrium of 3-halo-derivatives of 1-methylpyrrolidin-2-one. The five-membered ring assumes an envelope conformation owing to the plane of formation of the O═C-N-R bond, with C4 forming the "envelope lid". When the conformation changes, the "lid" alternates between positions above and below the amide plane. The α-carbonyl halogen assumes two positions: a pseudo-axial and a pseudo-equatorial. In the gaseous phase, the calculations indicate that the pseudo-axial conformer is more stable and preferable going down the halogen family. Natural bond orbital analysis showed that electronic delocalization is significant only for the iodo derivative. In the other derivatives, the electrostatic repulsion between oxygen and the halogen determines the conformational equilibrium. When the solvated molecule was taken into account, the pseudo-equatorial conformer population increased with the relative permittivity of the solvent. This variation was strong in the fluoro derivative, and the preference was inverted. In the chlorine derivative, the two populations became closer in methanol and acetonitrile. In the bromine and iodine derivatives, the percentage of pseudo-equatorial conformer increased only slightly owing to the dipole moment of the conformation: the pseudo-equatorial conformation has a greater dipole moment and thus is stable in media with high relative permittivity.
A simple method for the regiocontrolled synthesis of the 4,5‐disubstituted N‐phenylpyrazole regioisomers 2 and 3 from β‐enamino diketones and phenylhydrazine is described. Pyrazole 2 was prepared regioselectively by carrying out the reaction in a protic solvent, whereas pyrazole 3 was obtained as the main product by using an aprotic solvent. Steric factors regarding the β‐enamino diketones also influenced the regiochemistry of 2 and 3. The influence of solvent on the reaction outcome can be rationalized by using Fukui indices. The structures of 2 and 3 were unambiguously elucidated by 1H and 13C NMR spectroscopy, two‐dimensional HMBC experiments, high resolution mass spectrometry, and X‐ray crystallographic analysis.
A highly efficient regioselective synthesis of 1,4,5-trisubstituted pyrazoles and pyrazolo[3,4-d]pyridazinones from enamino diketone along with a theoretical and experimental study of the chemical reactivity enamino diketone.
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