Photochemical reaction of bis-aromatic systems: a novel photocycloaddition of pyridine with furan

Abstract

The photochemical reaction of the bis-aromatic system pyridine–furan was investigated. Irradiation of a benzene solution containing 3-cyano-2-methoxypyridines 1 (0.02 M) and furan (0.2 M) resulted in the formation of a 1∶1 adduct, 11-cyano-10-methoxy-8-methyl-4-oxa-9-azapentacyclo[5.4.0.02,6.03,11.05,8]undec-9-ene and 10-cyano-9-methoxy-7-methyl-5-oxa-8-azatricyclo[5.4.0.02,6]undeca-3,8,10-triene in 30 and 16% yield, accompanied by the transpositional pyridine, 5-cyano-2-methoxy-6-methylpyridine, and the pyridine dimer, in 2 and 44% yield, respectively, when the reaction conversion reached 48% yield. The cage and the face-to-face structures were established by X-ray structural analyses. The cage adduct was stable under neutral conditions; however, it easily converted to a face-to-face structure in acidic conditions. On the other hand, though the face-to-face structure was stable at rt, the starting pyridine 1 and furan were easily regenerated quantitatively by heating (>100 °C) or irradiation (>290 nm). The 4 + 4 adduct of pyridine with furan was detected by 1H NMR spectroscopy, and subsequently transformed to a cage structure on irradiation.