Dipyrrolo[1,2-a;2':,1'-c]pyrazines. 7. Electrophilic Substitution of Dipyrrolo[1,2-a;2',1'-c]pyrazines and 5,6-Dihydrodipyrrolo[1,2-a;2',1'-c]pyrazines via Formylation of Dipyrrolo[1,2-a;2':,1'-c]pyrazines