3,4-Dimethoxycarbonyl-5-pyrrolidin-1-ylthiepins (III) can be synthesized by cycloaddition of (substituted) 3-pyrrolidin-1-ylthiophens (I) and dimethyl acetylenedicarboxylate; the reaction proceeds via 2-thiabicyclo[3,2,0]hepta-3,6-diene intermediates (II).