Tetrazine is an unstable compound that consists of a six-membered aromatic ring containing four nitrogen atoms with the molecular formula C2H2N4. 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.
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-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 
- 6-membered rings with one nitrogen atom: pyridine
- 6-membered rings with two nitrogen atoms: diazines
- 6-membered rings with three nitrogen atoms: triazines
- 6-membered rings with six nitrogen atoms: hexazine
- (see United States Patent 6645325)
- Datasheet: Link
- π-Bond Screening in Benzonorbornadienes: The Role of 7-Substituents in Governing the Facial Selectivity for the Diels-Alder Reaction of Benzonorbornadienes with 3,6-Di(2-pyridyl)-s-Tetrazine. Ronald N. Warrener and Peter A. Harrison Molecules 2001, 6, 353–369 Online Article