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

The photochemical reaction of the bis-aromatic system pyridine-furan was in vestigated. Irradiation of a benzene solution containing 3-cyano-2-methoxyp yridines 1 (0.02 M) and furan (0.2 M) resulted in the formation of a 1:1 ad duct, 11-cyano-10-methoxy-8-methyl-4-oxa-9-azapentacyclo[5.4.0.0(2,6).0(3,1 1).0(5,8)]undec-9-ene and 10-cyano-9-methoxy-7-methyl-5-oxa-8-azatricyclo[5 .4.0.0(2,6)]undeca-3,8,10-triene in 30 and 16% yield, accompanied by the tr anspositional pyridine, 5-cyano-2-methoxy-6-methylpyridine, and the pyridin e dimer, in 2 and 44% yield, respectively, when the reaction conversion rea ched 48% yield. The cage and the face-to-face structures were established b y X-ray structural analyses. The cage adduct was stable under neutral condi tions; however, it easily converted to a face-to-face structure in acidic c onditions. On the other hand, though the face-to-face structure was stable at rt, the starting pyridine 1 and furan were easily regenerated quantitati vely by heating (>100 degrees C) or irradiation (>290 nm). The 4 + 4 adduct of pyridine with furan was detected by H-1 NMR spectroscopy, and subsequen tly transformed to a cage structure on irradiation.