2-Pyridone, lactam/lactim tautomerism

Among examples of lactam-lactim tautomerism pyridine derivatives 98a, 98b, and 100 have been proven to be pyridones on the basis of their IR spectra, whereas UV and IR spectra suggest the aminopyridine structure 101 (63BCJ633). Spectral data similarly confirm the existence of substance 31a (Scheme 57) in the 4-hydroxy-2-pyridone form rather than the 2,4-dihydroxypyridine form 234 (74BCJ1750). Consequently, its acetylation by acetic anhydride/pyridine at 130 °C or methylation yield N,0-disubstituted derivatives 235a and 235b, respectively. Acetylation at 100 °C, however, leads to 0,0 -disubstitution (236). 

The tautomeric 6-hydroxy-7-azaindole shows a shift to longer wavelength, which is characteristic of 2-hydroxypyridine as the neutral molecule or of l-methyl-2-pyridone. The degree of tauto-merization is solvent-dependent, and Yakhontov et have determined the lactam/lactim R = Me) ratio from the position of... 

AHOMO and ALUMO are not very different from each other, the same procedure can be applied to such compounds in order to gain information about the protonation site, for example, in the various aza analogues of indolizine (10), or about the tautomerism in heterocycles as exemplified by the lactam-lactim isomerism in 2-pyridone (11) and about positional isomerism such as in the tautomeric 3,5- and 3,6-diazaindoles (12, 13). This is possible since the protonation of a nitrogen or the replacement of a carbon by a heteroatom can be viewed as a special case of substitution. 

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