Advances in the chemical and biological diversity of heterocyclic systems incorporating pyrimido[1,6- a ]pyrimidine and pyrimido[1,6- c ]pyrimidine scaffolds

]pyrimidin-diones, pyrimidoquinazolines, tricyclic, tetracyclic, and binary systems; (2) the reactivity of the substituents attached to the pyrimidopyrimidine skeleton, including thione and amide groups, nucleophilic substitutions, condensations, ring transformations, and coordination chemistry; (3) compounds of this class of heterocycles containing a significant characteristic scaffold and possessing a wide range of biological characteristics.