THE LACTAM-LACTIM TAUTOMERIZATION OF MONOAMINO-SUBSTITUTED 2-PYRIDINOLS IN TETRAHYDROFURAN

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 π,π∗ 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 π,π∗ singlet states. © 1990 The Chemical Society of Japan.