Protonation of aminopyrone: A four centers problem, CNDO /2 versus MNDO study

The mechanism of protonation of aminopyrones and the nature of the protonated forms are of some interest because of the biological importance of these compounds. The use of CNDO/2 molecular electrostatic potentials contour maps shows that the most reactive centre is the extracyclic oxygen. MNDO protonation energies confirm the protonation of this atom and elucidate the basicity of the nitrogen atom. The effect of a N substitution on the basicity of the N 13 atom has been studied. The calculations are in agreement with preliminary experimental results.