Oxidation reactions. Since reductions of our dithienopyrazines worked well and selective, we investigated their oxidation behaviour. Regarding the subunits of the triacene system, thiophenes typically can be oxidized to rather labile thiophene 1oxides and further to stable thiophene 1,1-or S,Sdioxides, whereby nitrogen-containing heterocycles such as pyridine or pyrazine are oxidized to N-oxides. Thus, oxidation of structurally related 2-phenyl thieno [2,3-b]pyrazine with 30% hydrogen peroxide (H2O2) solution in acetic acid showed that depending on temperature a N-oxide, N,N-dioxide, and a combined thiophene 1,1-dioxide N-oxide are formed. On the other hand, it has been shown that depending on the oxidant thienopyridines can be selectively oxidized at the nitrogen or at the sulfur. Thus, oxidation with H2O2 in acetic acid or m-chloroperbenzoic acid in chloroform led to the corresponding pyridine N-oxid, oxidation with sodium hypochlorite in diluted hydrochloric acid to the 1,1-dioxide. Structural proofs came from 1 H-NMR and IR data. In this respect, thiophene 1,1-dioxide units show much larger coupling constants (7-7.5 Hz) for the thiophene -protons than thiophene units itself (5.6-5.9 Hz). Furthermore, in IR spectra of sulfones a characteristic symmetric stretching band arise in the regime of 1135-1170 cm -1 , an asymmetric one between 1300-1335 cm -1 . In contrast, for aromatic N-oxides a stretching band in the regime of 1200-1300 cm -1 is expected. In analogy, we tested oxidation of syn-dithienopyrazine 6 with 30% H2O2 in acetic acid at 60 °C and a mixture of pyrazine-4-N-oxide 25, 1,1-dioxide 26, and pyrazine-4-oxide-1,1-dioxide 27 was identified. After chromatographic separation, the three oxidation products were isolated in rather moderate yields from 10% to 23%. Replacement of acetic acid by the stronger trifluoroacetic acid as solvent and reaction at 100 °C gave after work-up as well pyrazine-4-oxide-1,1-dioxide 27 and the higher oxidized pyrazine-4,8-dioxide-1,1-dioxide 28 in quite moderate yields (Scheme 7). The structures and assignments of the various oxidation products comprising positions of the oxygens were discerned and determined by high resolution mass (HR-MS), 1 H-NMR and IR spectra, and as well single crystal X-ray structure analysis. In that respect, thiophene-1,1-dioxide units are clearly identified by the 3 J coupling constants of the thiophene protons which are increased to 7.8-7.9 Hz compared to 6.2 Hz of the non-oxidized thiophene units. Furthermore, absorption bands at 1155-1163 cm -1 and 1311-1325 cm -1 in the IRspectrum can be assigned to a sulfone (1,1-dioxide) unit. In contrast, oxidation of nitrogens to Noxides in the triacene systems can be discerned in the IR-spectrum by absorption bands at lower energies (1241-1260 cm -1 ). The undertaken experiments showed that syn-dithienopyrazine 6 can only be oxidized under tightened conditions and non-selective mixtures of mono to fourfold oxidized products were formed.