The unprecedented ring transformation from thiazoline-spiro-thiophene to thieno[2,3-b]pyrazine involved in the reaction of 2-thiocarbamoyl thiazolium salts with dimethyl acetylenedicarboxylate.

Reaction of 2-thiocarbamoyl thiazolium salts with dimethyl acetylenedicarboxylate proceeded via a tandem [3+2] cycloaddition and a unprecedented ring transformation to produce functionalized thieno[2,3-b]pyrazine derivatives in good to excellent yields.