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U.v. spectra of the pyridinoid title compounds and of models for the corresponding pyridonoid tautomers show that the pyridinoid tautomeric forms are greatly preferred in aqueous solutions. pKa Measurements allow quantitative determinations of tautomeric equilibrium constants which are compared with data for various other systems; the latter data include new results for some enamine–imine equilibria. The aromatic character of the pyridonoid forms of the title compounds is discussed.
Chua et al. (Mon,) studied this question.