Photochemistry of o-Pyrrolylstilbenes and Formation of Spiro-2H-pyrroles and Their Rearrangement to Dihydroindoles

Excited states of stilbenylpyrroles 1a − 1c deactivate by two photochemical processes: cis−trans-isomerization and hydrogen transfer of NH to the stilbene double bond. NH-transfer results in the formation of two quinone dimethane intermediates, 10 and 11, and biradicals 12. Intramolecular cyclization of intermediates 10 − 12 gives rise to polycyclic compounds spiro-2H-pyrroles 7, pyrroloisoindoles 3, and pyrroloisoquinolines 8. Spiro-2H-pyrroles 7 rearrange on silica gel, giving dihydroindoles 2.