Highly Regioselective Anaerobic Photocyclization of 3-Styrylpyridines

Abstract

The photochemical behavior of the cis isomers of the three isomeric styrylpyridines and two (aminostyryl)pyridines has been investigated under aerobic and anaerobic conditions. Both 3-styrylpyridine and its 3'-amino derivative 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-azaphenanthrenes in the absence of oxygen is attributed to conversion of the 4a,4b-dihydroazaphenanthrene primary photoproduct to a 1,4-dihydropyridine intermediate by means of a formal 1,7-hydrogen shift. This intermediate is moderately stable in the absence of oxygen and has been characterized by comparison of its (1)H NMR and electronic absorption spectra with calculated spectra. This intermediate is converted to the 2-azaphenanthrene in both the absence and presence of oxygen. The regioselectivity of photocyclization of 3-substituted stilbenes and related diarylethylenes is suggested to depend on the relative rate constants for ring opening and sigmatropic rearrangements of the dihydrophenanthrene intermediates as well as their rates of reaction with oxygen or other oxidants.

Stabilising fleeting intermediates of stilbene photocyclization with amino-borane functionalisation: the rare isolation of persistent dihydrophenanthrenes and their [1,5] H-shift isomers

“…DFT optimized structures show H-bond interactions between the amino nitrogen atom and nearby H atoms such as the inner H b in 1b–4b , which may contribute to the stability of these DHPs. The coexistence of the BMes 2 and NMe 2 group in 1b–4b appears to be essential for their formation and stability, as the previously reported 17 analogue of 1a that lacks the BMes 2 group does not produce a stable DHP, while that which lacks the NMe 2 group ( E -5a ) is photoreactive but does not form a DHP at all (see Fig. S19 in the ESI † ).…”

Photoisomerization and Photocyclization of 5-Styryloxazole

“…Pioneering work on compounds bearing pyridine or pentatomic groups [furan (F), thiophene (T) and pyrrole (Py)] has been carried out in the '60s by various groups. [11][12][13] More recent papers deal particularly with the solvent and oxygen dependent photocyclization pathways and with the competitive photoreactions induced by 1,n-hydrogen shifts in stilbene-like compounds containing the pyridyl group 14 or five-membered heterocycles. [15][16][17][18] In fact, the photocyclization of styryl derivatives of F, T and Py bearing various substituents at the phenyl ring and/or in specific media was described to be accompanied by high chemical yields of competitive photoproducts implying a novel skeletal rearrangement.…”

Azologization of a hetero-stilbene based c-RAF kinase inhibitor: Towards the design of photoswitchable sirtuin inhibitors

“…In high concentrations, (E)stilbene furthermore undergoes photocyclodimerization to cyclobutane derivatives [28]. Photoisomerization and photocyclization are also reported for 3-styrylpyridines, forming two regioisomeric dihydroazaphenanthrenes that are oxidized to 2-and 4azaphenantrene (not shown), respectively [29].…”