Similar structures have been prepared naturally as a hydrolytically stable pigments. Recently, several natural products containing a rigid aminoiminoquinone structure have been isolated and identified: macrophilone A, the makaluvamine, isobatzelline, prianosin, epinardin  and discorhabdin  families, and waykayin. These alkaloid secondary metabolites from marine organisms were found to possess cytotoxic antitumor properties. It is reported that the aminoiminoquinone system may contribute to the cytotoxic activity. 
Although the structure depicted in Figure 1 was thought to be the most stable, there was evidence for multiple equilibria of these compounds in solution (Fig. 2).  For example, in the case of the methyl derivative (R = Me), NMR evidence at room temperature shows a mixture of tautomers. This equilibrium, especially the possibility of tautomers, is important since the biological activity of these compounds depends on which tautomer is more stable.