Recent advances in the synthesis of biologically active cinnoline, phthalazine and quinoxaline derivatives

In the recent decades, N-heterocycles are widely used as versatile scaffolds for the development of biologically active compounds and considered as one of the privileged structures. Among the various N-heterocycles, benzodiazines, a series of bicyclic heterocycles in which benzene rings are fused with 6-membered heterocycles containing two nitrogens are especially interesting, due to their structural conciseness and usefulness as synthetic intermediates, and suitable physicochemical properties as potential drug candidates and chemical probes. For this reason, a series of synthetic methodologies for benzodiazines has been developed and generally used in organic and medicinal chemistry fields. Interestingly, synthesis of quinazoline, a kind of benzodiazine in which benzene is fused with pyrimidine, has been extensively reviewed, while cinnoline, phthalazine and quinoxaline, benzene fused with pyridazine or pyrazine, have been relatively less investigated. In this review, recent advances in the synthesis of biologically active benzodiazines, benzene ring-fused 6-membered heterocycles with two nitrogen atoms, cinnoline, phthalazine and quinoxaline, will be addressed. In each section, biological activities of various benzodiazines, recently served as novel scaffolds for drug discovery, are briefly summarized for comprehensive understanding of biological and medicinal significance in advance. Moreover, intensive investigation of the synthetic approaches to various benzodiazines such as cinnoline, phthalazine and quinoxaline is followed. It would hopefully be helpful for the readers who have interests in developing novel drug candidates and related useful chemical probes.