Stereodefined ring contraction-rearrangement of thiocoumarins to new fused benzo[b]thiophene derivatives

Mesoionic 1,3-oxazolium-5-olates (münchnones) react with thiocoumarins having an electron-withdrawing group at the 3-position to afford stereodefined fused polycyclic thienopyrroles. The reaction sequence seems to be triggered by a regiospecific dipolar cycloaddition followed by ring opening of the initial 1:1 cycloadduct and intramolecular rearrangement with an unusual ring contraction.