Bicyclic 5-5 systems: four heteroatoms 2:2

This article focusses on the characteristics and synthesis of a range of bicyclic 5-5 systems bearing four heteroatoms in a 2:2 fashion. Theoretical and structural characteristics of these interesting scaffolds are discussed, before detailing the many methods of synthesis. Most bicycles contain nitrogen, oxygen and sulfur atoms, and the most novel or well-characterized methods are described in terms of the mechanistic rationale, where appropriate. The article is divided into conjugated and nonconjugated rings systems to aid the reader, with subdivisions focusing on specific reaction types. Finally, we highlight how these heterocycles were used in materials science and medicinal chemistry.