title: Intermolecular photocyclizations of N-(omega-hydroxyalkyl)tetrachlorophthalimide with alkenes leading to medium- and large-ring heterocycles--reaction modes and regio- and stereoselectivity of the 1,n-biradicals.
abstract: A new photocyclization strategy by using intermolecular tandem reactions between N-(omega-hydroxyalkyl)-4,5,6,7-tetrachlorophthalimides (1, 2, and 3) and a series of acyclic and cyclic alkenes is reported. Electron transfer of the triplet-excited phthalimide with the alkene and regioselective trapping of the alkene cation radical by the hydroxyl group at the phthalimide side chain gives a triplet 1,n-biradical, which after intersystem crossing (ISC) leads to regio- and diastereoselective synthesis of polycyclic heterocycles with an N,O-containing medium to large ring. Regio- and diastereoselectivity in the cyclizations are clarified by unambiguous steric structure assignments of the products by X-ray diffraction or extensive 2D NMR measurements. The diastereoselectivity is decided by the stereochemical course of the ISC process of the triplet 1,n-biradicals. These intermolecular photoreactions also furnish a new strategy to generate triplet 1,n-biradicals. Therefore, in photoreactions of 1 and 2 with phenylcyclohexene, the unprecedented stereoselective formation of products by intramolecular hydrogen-atom transfer in the 1,n-biradical intermediate was found (9 and 23). These facts provide direct verification to the reaction pathways of the 1,n-biradicals and give a new insight into the factors deciding reaction-pathway partitioning and stereoselectivity.
authors.affiliation: Institute of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.
authors.forename: Yong-Miao
authors.initials: YM
authors.lastname: Shen
authors.forename: Xiao-Liang
authors.initials: XL
authors.lastname: Yang
authors.forename: Da-Qing
authors.initials: DQ
authors.lastname: Chen
authors.forename: Jie
authors.initials: J
authors.lastname: Xue
authors.forename: Liang
authors.initials: L
authors.lastname: Zhu
authors.forename: Hoong-Kun
authors.initials: HK
authors.lastname: Fun
authors.forename: Hong-Wen
authors.initials: HW
authors.lastname: Hu
authors.forename: Jian-Hua
authors.initials: JH
authors.lastname: Xu