Tautomeric transformation

Both ionic forms have the same set of characteristic bands in their IR spectra and thus cannot be identified in solution. The 3IP NMR spectrum contains only one averaged signal. The IR study revealed tautomeric transformations in solution. The characteristic absorption band of the P—H bond and carbonyl group appeared in solution spectra that is possible only on dissociation of the a-hydroxyalkyl fragment, present only in the second tautomeric form. An X-ray single-crystal study showed the compound (128) to be in a cyclic form. 

Ovchinnikov, V.V., Lapteva, L.I., Sagadeev, E.V., and Konovalova, A.I., Thermochemistry of heteroaromatic compounds. Part 9. Enthalpies of tautomeric transformation of hydrophosphorylic compounds, Thermockim. Acta, 288,105, 1996. 

Because of the multiplicity of possible forms, the tautomerism of purines offers a challenging field of investigation for physical and quantum chemists. The following principal types of tautomeric transformation liable to occur in the most significant group of purines, namely, those of biological interest, can be considered. 

Tertiary nitro compounds, of course, do not undergo tautomeric transformation, and they might be expected to be resistant to alkalis. Nevertheless aromatic nitro compounds, and polynitro-ones in particular, are very sensitive to alkalis, and undergo transformation when treated with them. For example, sym-trinitrobcnzcnc and also a- trinitrotoluene, when reacted with potassium hydroxide in methyl alcohol solution, form dark addition products (see also p. 202). Under certain conditions the nitro group can break off to form high molecular compounds. 

Nitroazoles possess a very broad array of practical applications. They can be used as anticancer preparations, antiseptics, radiosensitizers, herbicides, fungicides, dyes, ionic liquids, etc. The significant number of applications of nitroazoles makes them rather promising for research and requires deep understanding of their peculiar electronic structure, spectral properties, and chemical and tautomeric transformations [56, 57],... 

Chemistry of Small Carbohydrates - Equilibrium Solution Properties, Pure Appl. Chem. 59, 1189 (1987) and references dted therein. [160] L. A. Fedorov, D. N. Kravtsov, and A. S. Peregudov Metallotropic Tautomeric Transformations of the a, a-Type in Organometallic and Complex Compounds, Usp. Khim. 50, 1304 (1981) Russ. Chem. Rev. 50, 682 (1981). 

Thus the acceptance of the Bloomstrand-Strecker-Erlenmeyer formula for the diazonium base and salts, and of the Hantzsch modification of the Kekule formula for the isomeric diazotates and the acid diazo hydroxide, together with the tautomeric transformations which occur, makes possible the explanation of all the facts which we have considered in the preceding discussion. These may be stated again briefly for the sake of emphasis and review, (i) The reaction of diazotization (2) the diazonium base and salts, (3) the diazotates and acid diazo hydroxide, (4) the isomerism of the diazotates, (5) the diazonium sulphites and diazo sulphonic acidsj (6) the relationship of diazo compounds to nitroso amines (7) to hydrazines (8) the coupling with amino and hydroxy compounds to form amino azo and hydroxy azo compounds, (9) the reactions of decomposition. Most of these last reactions we have not considered, but will do so presently and we shall find that they all may be likewise satisfactorily explained. The tautomeric constitution of the diazo compounds, therefore, meets every test and is generally accepted. 

I, 2,4-triazoline intermediate (12) (Scheme 10) (82CJC285). The reaction mechanism involves the tautomeric transformation of 9 to the amidrazone 9a (65JCS3528 82CJC285) the one-carbon fragment is provided by imine... 

The tautomeric transformations of phenols can be subdivided into two groups (i) keto-enol tautomerism which is accompanied by loss of the aromatic character of the ring, and (ii) tautomeric equilibrium involving participation of substituents where the aromatic phenol nucleus is conserved. 

It should be noted, however, that most of the ring-chain tautomeric transformations of phenols proceed without loss of aromaticity of the arene cycle. The metal [Cu(II), Fe(II), Fe(ni)] catalyzed oxidation of flavonols 76 gives the 2-(hydroxybenzoyl)-2-hydroxybenzofuran-3(277)-ones 78 which are in an equilibrium with the initially formed 2-(hydroxyphenyl)-2-hydroxybenzopyran-3,4-diones 77 (equation 32). 

Many examples of tautomeric transformations as weU as rearrangements in the phenol series were considered in detail in a book . 

The mushroom tyrosinase-catalyzed oxidative decarboxylation of 3,4-dihydroxyphenyl mandelic acid (111, R = H) and a-(3,4-dihydroxyphenyl) lactic acid (111, R = Me) proceeds via the quinone methide intermediate 112. The coupled dienone-phenol rearrangement and keto-enol tautomerism transforms the quinone methide 112 into 1-acyl-3,4-dihydroxyphenyl compounds 113 (equation 48) . ... 

Various isomerizations were reported, including the tautomeric transformations of 2,6-disubstituted phenols which involve participation of phenoxy radicals and cation... 

In general, the tautomeric transformation between the two forms A and B is slower in the more polar solvents. Thus separate signals for the two tautomers can readily be observed even in unpurified HMPA, whereas in CDC13 it can be seen only in purified solvent and very dilute solutions. The proton-transfer process is hindered by the formation of the hydrogen-bonded complexes with the polar solvents. 

Using the charges on the appropriate hydrogens and Wiberg indexes W 6 from the ETSA of the active, weakly active, and inactive compound sets obtained above, we can suggest the following regression equation for the inhibitory activity. These relate to the ease of the above tautomeric transformation. 

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