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