Extended mesoionic systems: synthesis and characterization of monocyclic, polycyclic and macrocyclic pyrimidinium-olate derivatives and their photochemical behavior

Mesoionic pyrimidinium-olate derivatives have been known to undergo photoreactions upon irradiation with UV light to form bis(beta-lactame) structures for about two decades. Here, new mono-, poly- and macrocyclic mesoions were prepared and their photochemical rearrangement behavior was investigated. The synthesized compounds were characterized by NMR, IR, UV/Vis spectroscopy, elemental analysis and mass spectrometry. The extension of the aromatic core leads to a significant red-shift of the absorption maximum from ∼380 to ∼430 nm, indicating a strong electronic coupling of the mesoionic base chromophore with the annellated aromatic subunit. The rigidity of the extended aromatic system prevents polycyclic mesoions from shifting to their bis(beta-lactame) isomers, while the alkyl bridge of the 16-membered macrocycle in an ansa-mesoion does not inhibit photochemical rearrangement.