9a-Azaphenalene

Heterocycles structurally based on the phenalene ring system frequently possess distinctive colors. With nitrogen as the central atom we have the unstable 9a-azaphenalene (20), (76JCS(Pl)34i). The cyclazine nomenclature is commonly applied to this and related compounds thus, (20) is (3.3.3)cyclazine. Further aza substitution is possible, e.g. as in the heptaazaphenalene (21). Another well-known and important representative of this family is yellow colored perimidine. 

A more subtle case is that of 9a-azaphenalene ([3,3,3]cyclazine) 35, which is also unstable. Here the periphery of the molecule has 12 -electrons (4n) in cyclic conjugation, which is antiaromatic. Resonance hybrids of the type 35b, in which the peripheral electrons are increased to 13 (4n +1), probably make a significant contribution to the structure. Aza substitution, as in the heptaazaphenalene 36, leads to some stabilization. 

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