부산대학교 도서관

Fused five-membered nitrogen heterocycles comprise a very important group of compounds frequently utilized in pharmaceutical applications. In this study, we report the first systematic synthesis of substituted pyrazolo[4,3- c]pyrazoles and three regioisomers of their N-methyl derivatives. All compounds were fully characterized by NMR spectroscopy in solution and selected compounds also were studied by X-ray diffraction in the solid state. 1H, 13C, and 15N NMR spectroscopic data for all isomers were interpreted by DFT calculations of nuclear shielding constants and indirect spin-spin coupling constants. The N-methyl isomers were used in the following steps as model compounds to investigate a potential N1-H/ N4-H, N2-H/ N4-H, and N1-H/ N5-H tautomerism of 3,6-substituted pyrazolo[4,3- c]pyrazoles by using low-temperature NMR spectroscopy. All bases were shown to occur predominantly in the N1-H/ N4-H tautomeric form and the structure of minor form was governed by the substituents at positions 3 and 6. Stabilities of individual tautomeric forms are calculated by DFT methods and discussed. A relationship between the tautomeric populations and the ratios among N-methyl isomers obtained upon methylation of selected bases in solution are investigated. [ABSTRACT FROM AUTHOR]