Control of the Intramolecular [2+2] Photocycloaddition in a Bis-Stilbene Macrocycle

The intramolecular [2+2] photocyclization of a bis-stilbene macrocycle was studied. The reaction can be controlled by the insertion and removal of urea protecting groups. Upon UV-irradiation in both the solid state and DMSO solution, the free urea macrocycle undergoes a cis−trans photoisomerization that is followed by a [2+2] cycloaddition to afford a single product in high yield. The presence of the triazinanone urea protecting groups does not alter the cis−trans photoisomerization but greatly decreases the selectivity of the photocycloaddition step.