Abstract
This chapter discusses several surveys done on pyrazino pyrimidines or pteridines with some focus on tautomerism. Limited data on other heterocycles is discussed in this chapter. Six–six condensed heterocyclic systems without a tautomeric functional groups do not exhibit prototropic tautomerism. The introduction of an oxo(thioxo)- and/or amino groups into the pteridine system and the appearance of at least one NH group in the ring leads to functional and/or annular tautomerism. Pteridine is formally the parent of three groups of compounds of particular interest because of their biological importance: pterins, lumazines, and flavins.