Preparation and Diels‐Alder Reaction of a Reactive, Electron‐Deficient Heterocyclic Azadiene: Dimethyl 1,2,4,5‐Tetrazine‐3,6‐Dicarboxylate. 1,2‐Diazine (Dimethyl 4‐Phenyl‐1,2‐Diazine‐3,6‐Dicarboxylate) and Pyrrole (Dimethyl 3‐Phenylpyrrole‐2,5‐Dicarboxylate) Introduction