Highly regioselective anaerobic photocyclization of 3-styrylpyridines

The photochemical behavior of the cis isomers of the three isomeric styrylp yridines and two (caminostyryl)pyridines has been investigated under aerobi c and anaerobic conditions. Both 3-styrylpyridine and its 3 ' -amino deriva tive undergo highly regioselective formation of 2-azaphenanthrene products under anaerobic conditions. In the presence of oxygen, mixtures of 4- and 2 -azaphenanthrene products are obtained. The formation of 2-azaphenanthnnes in the absence of oxygen is attributed to conversion of the 4a,4b-dihydroaz aphenanthrene primary photoproduct to a 1,4-dihydropyridine intermediate by means of a formal 1,7-hydrogen shift. This intermediate is moderately stab le in the absence of oxygen and has been characterized by comparison of its H-1 NMR and electronic absorption spectra with calculated spectra. This in termediate is converted to the 2-azaphenanthrene in both the absence and pr esence of oxygen. The regioselectivity of photocyclization of 3-substituted stilbenes and related diarylethylenes is suggested to depend on the relati ve rate constants for ring opening and sigmatropic rearrangements of the di hydrophenanthrene intermediates as well as their rates of reaction with oxy gen or other oxidants.