Keto-enol tautomery

Bei Imidazolen, die an einem oder zwei Ring-C-Atomen Hydroxy-, Mercapto- oder Amino-Gruppen tragen, sind durch Keto-Enol-Tautomerie mehrere tautomere Formen moglich. Als Beispiel mogen die vier moglichen Tautomeren von 2-Hydroxy-imidazol dienen ... 

In der Literatur wird die Nitroso-Oxim-Umlagerung im allgemeinen als ein Fall von Keto-Enol-Tautomerie behandelt (64). Ein entsprechendes Tautomerie-Gleichgewicht wurde bisher jedoch nur bei bestimmten aro-matischen Nitrosoverbindungen nachgewiesen, z. B. zwischen j>-Nitroso-phenol und p-Chinon-monoxim (2) ... 

UV absorbance at particular wavelengths can be related to the presence of specific chromophores. Absorbance at 254 nm can be mainly correlated to the amount of double bonds in aromatic rings (Pettersson et al. (1994)) and, thus, to the aromaticity of a sample, whereas absorbance at 436 nm is more related to the concentration of unsaturated C=C, C=0, and N=0 units, chinones, keto-enol-tautomerie and complexed iron. Absorbance was observed to be highly pH dependent (Abbt-Braun et al. (1990)). Eaton (1995) subsequently developed a standard method to measure UV absorbance of natural waters. Korshin et al (1997b) correlated absorbance characteristics to the reactivit) of aromatic moieties with chlorine. Also, absorbance decreases after chlorination (Korshin et al (1996)). 

Scheme 4. Schiff-base superoxide dismutase models surriving biological chelators. Upper section Dibenzoy Imethane does not readily react with 2-(2-aminoethyl)-pyridine due to a keto-enol tautomery. Therefore only poorly active and instable complexes are obtained. In contrast, 2-pyridine-aldehyde and putrescine yield a highly active and stable copper

X,Y=0,S,Se,Te], has been undertaken. The stabilities of different tautomerie forms of 4-hydroxycoumarins have been evaluated by MNDO ealeulations, and the four lowest-energy oxo-hydroxy tautomers of 5-fluorouracil have been studied using density functional methods. Semiempirical calculations have been carried out on the keto-enol tautomerism of triazolopyrimidines. A base-catalysed keto-enol tautomer-ism has been proposed to be responsible for the observed deuterium exchange of the hydrogens at the 3-position of diazepam when the molecule is treated with alkaline deuteriated methanol. 

As a general problem, early studies were always obscured in terms of interpretation by the simultaneous presence of both enol and keto tautomers. Only recently, a direct study of the chemistry of enol radical cations was undertaken using stable simple enols of the Fuson type [63,64]. Extensive work originally by Fuson [115-117] and later by Rappoport [118] showed for a series of sterically hindered, mesityl-substituted systems that both the kinetically stabilized enols and the corresponding ketones can be prepared in tautomeri-cally pure form. 

---