As a general result, we found that reactivity and regioselectivity of the two isomeric dithienopyrazines 5 and 6 selectively occur at the -positions for electrophilic bromination reactions, whereby metallations are directed to the -positions. Structural characterization by single crystal X-ray structure analysis. Single crystals were obtained for three different structures by recrystallization and analyzed by X-ray structure analysis: syn-dithienopyrazine 10, anti-dithienopyrazine 5, and syn-dithienopyrazine-4-oxide 25. In Figure , individual molecules with atom numbering scheme are shown from top and side view. Whereas syn-dithienopyrazines 10 and 25 crystallized in orthorhombic space groups (P21212 and Pbca, resp.) with 4 and 8 molecules in the unit cell, anti-configured dithienopyrazine 5 crystallized in the monoclinic space group P21/n with 2 molecules in the unit cell. The heterocyclic triacene -systems are as expected fully planar. We shortly discuss the most obvious general trends for bond lengths and angles compared to the basic heterocycles thiophene, pyrazine, and pyrazine-N-oxide. More details can be found under the corresponding CCDC-numbers. We note that due to the fusion of the heterocycles, the central part significantly stretches perpendicular to the long axis compared with the basic heterocycles: on one side thiophene S1-C1 and S1-C4 bonds (correpondingly S1'-C1' and S1' -C4') are elongated (1.74-1.76 Å) compared to thiophene (1.71 Å). The pyrazine N1-C4 (N1'-C4') bonds are as well elongated (1.34-1.36 Å) compared to pyrazine (1.31 Å). On the other side, the C3-C4 (and C3'-C4') bond lengths (1.41-1.43 Å), which belong to both heterocycles in the triacenes and represent the fusing bonds, were substantially longer than in the underlying thiophene or pyrazine (1.37 Å), therefore indicating less double bond character. Concomitantly, the C4-N1-C4' angles in the central pyrazine unit of the syn-derivatives 10 and 25 are compressed to 112° compared to pyrazine with 115°. The angle reduction is, nevertheless, less pronounced for anti-derivative 5 (114°) and for the opposite C3-N2-C3' angle (114°) in 10. In contrast, this angle is widened to 117° in N-oxide 25 due to the increased sp 2 -character of the oxidized unit N2 + -O1 -which is rather typical for pyrazine-N-oxide (118°). All other bond lengths and angles are not significantly different compared to the parent heterocycles.