Synthesis of Polynitrogen-Containing [6-5-6] Tricyclic Derivatives by N–N Bond Formation and Their Fluorescent Properties

Polynitrogen-containing scaffolds are of major interest for many applications. To optimize and improve the potential of these scaffolds, it is important to be able to easily introduce and modulate the substituents, regardless of the number and position of the nitrogen atoms in the structure. Therefore, the synthetic approach ideally requires mild experimental conditions and an expanded scope of application. To this end, a versatile and efficient synthesis of [6-5-6] tricyclic derivatives of pyridopyrazolopyrazine, dipyridopyrazole, and pyridopyrazolopyrimidine types was undertaken at room temperature. Against all odds, the key oxidative cyclization step was successfully applied to two electron-poor heteroaromatic partners, and theoretical calculations were performed to rationalize the proposed mechanism for the N–N bond formation. Measurements of the fluorescence properties showed the strong impact of the number and position of nitrogen in the tricyclic scaffold. Among the seven families studied, pyridopyrazolopyrazine offers the best fluorescence properties in terms of brightness.