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On the 6-Exo Atom Transfer Radical Cyclization Reactions of 3-Butenyl 2-Iodoalkanoates

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posted on 31.10.2002, 00:00 by Xingang Fang, Hairong Xia, Hui Yu, Xicheng Dong, Minbo Chen, Quanrui Wang, Fenggang Tao, Chaozhong Li
Bis(tributyltin)-initiated atom transfer cyclization reactions of 3-butenyl iodoalkanoates in the presence of BF3·OEt2 as the catalyst afforded the 6-exo cyclization products as a mixture of 3,4-cis- and trans-substituted tetrahydro-2H-pyran-2-ones in 53−71% yield with the major isomers being the cis ones. Ab initio calculations at the B3LYP/6-31G* level on the transition states of the radical cyclization and on the cyclized products revealed that the reactions are kinetically controlled and the transition states for the 6-exo radical cyclization are in boat conformations. Moreover, the cis-oriented transition states are of lower energy than the corresponding trans-oriented ones, which are in excellent agreement with experimental results.