Tetrazine
Tetrazine is a compound that consists of a six-membered aromatic ring containing four nitrogen atoms with the molecular formula CHN. The name "tetrazine" is used in the nomenclature of derivatives of this compound. Three core-ring isomers exist: 1,2,3,4-tetrazines, 1,2,3,5-tetrazines, and 1,2,4,5-tetrazines, also known as v-tetrazines, as-tetrazines and s-tetrazines respectively. 1,2,3,4-Tetrazines. 1,2,3,4-Tetrazines are often isolated fused to an aromatic ring system and are stabilized as the dioxide derivatives. 1,2,4,5-Tetrazine. 1,2,4,5-Tetrazines are very well known and myriad 3,6-disubstituted 1,2,4,5-tetrazines are known. These materials are of use in  the area of energetic chemistry. The compound 3,6-di-2-pyridyl-1,2,4,5-tetrazine' has two pyridine substituents and is of importance as a reagent in Diels-Alder reactions. It reacts with norbornadiene in a sequence of one DA reaction and two retro-DA reactions to cyclopentadiene and a pyridazine with exchange of an acetylene unit: With norbornadiene fused to an arene the reaction stops at an intermediary stage