Dipyrrolo[1,2-a;2′,1′-c]pyrazines.: 8.: Electrophilic substitution in the series of dipyrrolo[1,2-a;2′1′-c]pyrazine and 5,6-dihydrodipyrrolo[1,2-a;2′,1′-c]pyrazine.: Acylation of dipyrrolo[1,2-a;2′,1′-c]pyrazines