Rearrangement of 2-(benzofuran-2-yl)-3-phenylpyridines via photoinduced 6-electrocyclization

By photoinduced 6 pi-electrocyclization of 2-(benzofuran-2-yl)-3-phenylpyridine derivatives 1, a method for the synthesis of trans-dihydrobenzo[f]quinolines 2, cis-dihydrobenzo[ f]quinolines 3 and 8b-methyl1,8b-dihydrobenzo[f]quinolines 4 was developed. Irradiation of 2-(benzofuran-2-yl)-3-phenylpyridine 1 in acetone-H2O (5 : 1, v/v) with a 313 nm UV lamp under an argon atmosphere at room temperature successfully yielded 2, which was further converted into 3 at elevated temperature (200 degrees C) in glycerol. However, irradiating 2-(3-methylbenzofuran-2-yl)-3-phenylpyridines 1 in CH2Cl2 with a 254 nm UV lamp gave 4 in good yields. The syntheses of 2, 3 and 4 via the 6 pi-electrocyclization rearrangement of 1 not only offer high atom efficiency but also do not require transition metal catalysts or additives.