Determination of the pK a values for polycationic species derived from 9-hydroxy and 9-aminothiazolo[5,4- b]quinolines. A problem related to the tautomerism of these systems

Abstract

Protonation constants of polycationic species derived from the tricyclic system thiazolo[5,4- b]quinoline-compounds 1–5 (a-c), 6 (a,b), and 7a have been determined with a limit of error of ±0.5 pK d units, by differential pulse polarography (DPP). The assignment of these values to the charged centres in each one of the species in equilibrium was possible from the calculated values for the enthalpies of the different species in solution, by using the force field MMP2 integrated in the software package PCMODEL. A plot of the calculated enthalpies versus the observed pKa for the compounds belonging to each one of the series, shows a good linear correlation which demonstrates the goodness of the assignment. The IR, 13C, and 15N NMR spectra for these compounds demonstrate their existence in the hydroxy form, this being a not very general modification in systems related to 4-quinolones and that, without doubt, is set by the presence of the fused thiazole ring.