[4+2] Cycloaddition of Dimethyl 1,2,4,5-Tetrazine-3,6-dicarboxylate with EWG-Substituted Primary Ketene N,O-Acetals : Synthesis of Tetrafunctional Pyridazines and Pyrroles - Müller, Jörg; Troschütz, Reinhard (Synthesis, 2006)