Cinnoline is a heterocyclic binuclear base containing two vicinal nitrogen atoms. Over the past two centuries, fused N-heterocyclic compounds have been widely used as valuable entities for expanding pharmacologically important reagents and have been recognized as an advantageous scaffold. Among the numerous fused N-heterocyclic compounds, cinnoline, quinoxaline, and quinazoline are important pharmacological agents, and a great deal of research has been done on this class of compounds. In medicinal chemistry, these N-heterocyclic compounds have a wide range of biological properties and can be used as synthetic intermediates, potential drug candidates and chemical probes.