Electrochemical properties of biologically active heterocyclic naphthoquinones

A series of natural and synthetic heterocyclic naphthoquinones, some of them with anti-plasmodial and trypanocidal activities, were studied by cyclic voltammetry in aprotic media (DMF/TBAP). In this solvent, the voltammograms of the quinones show two pairs of waves, corresponding to reversible or quasi-reversible one-electron transfer processes to form the semiquinone anion radical (Q(radical anion)) and the dianion (Q(2-)). Within the studied series, ortho-quinones undergo reduction easier than isomeric para-quinones. However, para-naphthoquinones fused to an aromatic heterocyclic ring are more easily reduced than the corresponding ortho-naphtoquinones with a non aromatic heterocycle. Substituents effects correlated very well with the first reduction potential (Ep(c1)) and the half wave potential (E-1/2). High yield reductive acetylation was achieved by electrolyses in the presence of acetic anhydride. Comparison of electrochemical data with reported antimalarial activities showed no correlation,.differently from the already reported trend in relation to trypanocidal activities.