The Lactam-Lactim Tautomerization of Monoamino-Substituted 2-Pyridinols in Tetrahydrofuran

Article abstract of DOI:10.1246/bcsj.63.2292

MINDO/3 calculations have been performed on 3-amino-, 4-amino-, 5-amino-, and 6-amino-2-pyridinols to estimate their molecular geometries.The lactam-lactim tautomerization from amino-2-pyridone to amino-2-pyridinol was expected for 5-amino- and 6-amino-2-pyridinols from the MINDO/3 calculations.In addition, their dimer formation energies were evaluated by the CNDO/2 method.Among the four amino-2-pyridones, 6-amino-2-pyridone has the largest dimer formation energy and 3-amino-2-pyridone the smallest.Furthermore, to certify the tautomerization of 3-amino-, 5-amino-, and 6-amino-2-pyridinols the UV absorption and fluorescence spectra were measured, and compared with those of their O-methyl and nuclear N-methyl derivatives.From the UV spectral data the equilibrium constants of the lactam-lactim tautomerization were determined for 5-amino and 6-amino derivatives in tetrahydrofuran (THF) at various temperatures.The lactam form is more stable than that of the lactim; the enthalpy changes between two forms of 5-amino and 6-amino derivatives were estimated to be 7.9 and 6.3 kJ mol^-1, respectively.The lactam and lactim dimers of these two derivatives were found to be easily formed in THF and ether.From the fluorescence spectral data the lactim dimer of 6-amino derivative was found to be formed in the lowest excited ?,?^* singlet state.On the other hand, the 3-amino derivative exists predominantly in the lactam monomer form in both the ground and the lowest excited ?,?^* singlet states.