Bicyclic 6-6 systems: Five or more heteroatoms

This chapter covers the diverse range of heterocycles that have two six-membered rings with five or more heteroatoms at positions other than the ring junctions. The 7-azapteridine nucleus (the pyrimido[5,4-e]-1,2,4-triazines) that is present in natural products such as toxoflavin, fervenulin, and reumycin dominates the chapter, although there are significant contributions from the 6-azapteridines (the pyrimido[4,5-e]-triazines), the pyrazinothiadiazines, the pyrimidothiadiazines, and the recently discovered pyridotetrazines. The organization of the chapter follows reaction type and synthesis type and is not arranged according to ring system. Most of the syntheses covered start with one preexisting heterocycle (commonly a triazine) and are categorized according to the way that the remaining heteroatoms are disposed to the ring junction. Spectroscopic, thermodynamic, and theoretical aspects are dealt with, but concentrate necessarily on the more common heterocycles within this category, or upon interesting or unusual results.