Marie-Thérèse Chenon

6-Azapurines. Part 3. Covalent σ-adducts of the imidazo[4,5-e]-as-triazine ring system

7-Methylimidazo[4.5-e]-as-triazin-3(2H)-one, imidazo[4,5-e]-as-triazin-3(2H)-one, and 5-methyl-imidazo[4,5-e]-as-triazin-3(2H)-one have been synthesized from their respective 5,6-diamino-as-triazin-3-ones and selected one-carbon delivering reagents. These 6-azapurines undergo covalent hydration across the azomethine bond located in the imidazole portion of the ring. In water, the σ-adducts ring open to certain as-triazin-3-ones whose structures were identified by NMR (1H and 13C) spectroscopy. These as-triazin-3-ones provided evidence for the described mode of cleavage.

1H and 13C dynamic nuclear magnetic resonance study of hindered internal rotation in (N,N-dimethylamino)pyrimidines

The free energies of activation ΔG‡ for hindered rotation around the C–N exocyclic bond in a series of 2- and 4-(N,N-dimethylamino)pyrimidines have been determined by 1H and 13C n.m.r. line-shape analysis. Good linear correlations have been obtained between the coupling constant 1J(C,H) for the dimethylamino group and the ΔG‡ parameter. Such correlations have been used to estimate ΔG‡ values of symmetrical or unsymmetrical pyrimidines for which an experimental determination of this thermodynamic parameter is either impossible or extremely difficult. In addition, substituent effects on the barrier heights indicate that: (i) the difference between the ΔG‡4 and ΔG‡2 values decreases with increasing electron-withdrawing power of the substituent, and (ii), substituent effects are larger through a ring nitrogen than a ring carbon atom. Correlation of the ΔG‡ values with Hammett constants is discussed.

6-Azapurines. Part 1. Determination of the tautomeric populations in 3-methylthioimidazo[4,5-e]-as-triazine by 13C and 15N nuclear magnetic resonance spectroscopy

3-Methylthioimidazo[4,5-e]-as-triazine (1) was synthesized via ring closure of 5,6-diamino-3-methylthio-as-triazine. By using carbon-13 and nitrogen-15 chemical shifts and three-bond 13C–1H spin–spin coupling constants, the prototropic tautomerism in the imidazole moiety of this 6-azapurine was investigated. The data from these spectral techniques indicated that the 5H-tautomeric form predominates (69%). A critical evaluation of the three methods is provided.