2,2′-Bipyridine-Enabled Photocatalytic Radical [4+2] Cyclization of <i>N</i>-Aryl-α-amino Acids for Synthesizing Polysubstituted Tetrahydroquinolines
Ren-Xu Xiao, Ting Tian, Ting-You Yang, Ming‐Xing Lan, Shuo Lv, Xue‐Qing Mou, Yong‐Zheng Chen, Bao‐Dong Cui
Abstract
A facile photocatalytic radical [4+2] cyclization of N -aryl-α-amino acids with various alkenes to access structurally polysubstituted tetrahydroquinolines has been developed. Using a simple bipyridine as a catalyst, different N -aryl-α-amino acids could be utilized as the radical precursors to react with diverse electrophilic alkenes, including exocyclic terminal alkenes, acyclic terminal alkenes, and cycloalkenes, producing 10 types of nitrogen-containing heterocyclic compounds fused in multiple frameworks in generally moderate yields with good diastereoselectivities. Scale-up synthesis and transformations of the products further demonstrated the synthetic application of this protocol. Moreover, a decarboxylative radial pathway via a proton-coupled electron transfer process for illustration of this [4+2] cyclization was proposed on the basis of the control experiments. This process is highlighted by a simple bipyridine photocatalysis, mild reaction conditions, various N -aryl-α-amino acids and alkene materials, and application for the modification of natural products.