Stereoselection in 5-exo radical cyclizations of polysubstituted 2- oxahex-5-enyl radicals: A systematic study of the combination substituent effect

Abstract

The nitrate radical induced oxidative cyclization of the alkynyl ethers cis/trans-1-3 proceeds with high diastereroselectivity and leads to the annellated tetrahydrofurans cis/trans-13-15. The stereoselectivity in this reaction sequence is likely determined in the 5-exo radical cyclization step of the intermediate polysubstituted 2-oxahex-5-enyl radicals cis/trans-7-9. The respective transition state geometries of these cyclizations are discussed within the framework of the Beckwith-Houk model. A dependence of the stereoselection on the configuration and size of the fused cycloalkyl ring as well as on the stability of the 2-oxahex-5-enyl radicals cis/trans-7- 9 is observed.