Structural characterization by NMR, determination of regioselectivity. For the assignment of the substitution positions and residual protons, non-decoupled heteronuclear single quantum coherence (HSQC) NMR spectra were taken and the resulting JCH coupling constants determined. Klemm et al. showed that in a fused thiophene ring the 1 JCH coupling constant for the -position (C2-H2, signal at lower field) always is larger than this for the position (C3-H3, signal at higher field) due to the vicinity to the sulfur. Therefore, for comparison HSQC NMR spectra of the basic dithienopyrazines 5 and 6 were recorded and 1 JCH coupling constants of 194/190 Hz were determined for the proton signals at  = 7.89 ppm and 176/177 Hz for the -proton signals at  = 7.57/7.53 ppm. Therefore, we were able to unequivocally distinguish the substitution pattern in various follow-up products which are collected in Table .