For comparison to the syn-isomer 6, the reactivity of anti-dithienopyrazine 5 in electrophilic substitution reactions was investigated and the optimized reactions transferred. The highly effective bromination protocol gave corresponding dibrominated and tetrabrominated products 16 and 17 in 88% and 76% yield, respectively (Scheme 4). Also in this case, NBS-bromination was not effective and only led to marginal formation of bromination products. Therefore, the claimed results of the patent could not be reproduced (vide supra). Scheme 4: Bromination of anti-dithienopyrazine 5.