Oxazolochlorins. 2. Intramolecular Cannizzaro Reaction of meso-Tetraphenylsecochlorin Bisaldehyde

Using mildly basic reaction conditions, the periodate-induced diol cleavage of meso-tetraphenyl-2,3-diolchlorin allows for the generation and isolation of the corresponding hitherto elusive free base secochlorin bisaldehyde. An intramolecular Cannizzaro reaction of this porphyrinoid generates three pyrrole-modified, oxazole-based porphyrins: the known porpholactol (2-oxa-3-hydroxychlorin) as the major product, known porpholactone (2-oxa-3-oxoporphyrin), and a novel porpholactol dimer that is linked through all acetal functionality. The structure of the dimer was confirmed by H-1 NMR spectroscopy, X-ray diffractometry, and ESI(+) collision-induced fragmentation mass spectrometry. The chromophores in the dimer are coupled electronically only to a minor extent. A mechanism to rationalize the formation of all products is advanced.