Oxidative ring-contraction of 3H-1-benzazepines to quinoline derivatives

Abstract When treated with SeO 2 , 2,4-diphenyl-3 H -1-benzazepine ( 1 ) is oxidized equally at C3 and C5, giving either products of rearrangement or fragmentation; in both cases quinoline derivatives are the primary products. When C3 is oxidized, electrocyclization followed by ring-opening with phenyl migration gives the major product phenyl(3-phenylquinolin-2-yl)methanone ( 6 ), whereas C5 oxidation produces 2,4-diphenylquinoline ( 2 ) and 1,2-bis(2,4-diphenylquinolin-3-yl)diselane ( 8 ). Oxidation of C5 in 1 also results in formation of 2-(3,5-diphenylfuran-2-yl)aniline ( 7 ). On the other hand, 3-methyl-2,4-diphenyl-3 H -1-benzazepine ( 9 ) upon treatment with SeO 2 gives primarily a product of oxidation of C3, 2,3-diphenylquinoline ( 5 ). Oxidation of C5 in ( 9 ) is a minor pathway, and gives both 3-methyl-2,4-diphenylquinoline ( 10 ) and (3-methyl-2-phenylquinolin-4-yl)(phenyl)methanone ( 11 ). CO was detected as a byproduct in both reactions. Although the ring-contraction reaction using SeO 2 has been previously noted, no mechanistic proofs have been firmly established. In this Letter, we provide evidence for the ring-contraction of benzazepines to quinolines through a fragmentation path (loss of CO and acetic acid) or through rearrangement.