Quinoline synthesis: scope and regiochemistry of photocyclisation of substituted benzylidenecyclopentanone O-alkyl and O-acetyloximes
Austin, Mark and Egan, Oliver J. and Tully, Raymond and Pratt, Albert C. (2007) Quinoline synthesis: scope and regiochemistry of photocyclisation of substituted benzylidenecyclopentanone O-alkyl and O-acetyloximes. Organic and Biomolecular Chemistry, 5 (23). pp. 3778-3786. ISSN 1477-0520
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Irradiation of substituted 2-benzylidenecyclopentanone O-alkyl and O-acetyloximes in methanol provides a convenient synthesis of alkyl, alkoxy, hydroxy, acetoxy, amino, dimethylamino and benzo substituted annulated quinolines. para-Substituents yield 6-substituted-2,3-dihydro-1H-cyclopenta[b]quinolines with 8-substituted products being obtained from ortho-substituted starting materials. Reactions of meta-substituted precursors are highly regioselective, with alkyl substituents leading to 5-substituted 2,3-dihydro-1H-cyclopenta[b]quinolines and more strongly electron-donating substituents generally resulting in 7-substituted products. 2-Furylmethylene and 2-thienylmethylene analogues yield annulated furo- and thieno-[2,3e]pyridines respectively. Sequential E- to Z-benzylidene group isomerisation and six [pi]-electron cyclisation steps result in formation of a short-lived dihydroquinoline intermediate which spontaneously aromatises by elimination of an alcohol or acetic acid. For 2-benzylidenecyclopentanone O-allyloxime, singlet excited states are involved in both steps.
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