7-<i>endo</i> Selective Aryl Radical Cyclization onto Enamides Leading to 3-Benzazepines: Concise Construction of a Cephalotaxine Skeleton

Abstract

Bu3SnH-mediated radical cyclizations of 2-(2-bromophenyl)-N-ethenylacetamide gave 6-exo cyclization product 15 as the major product, whereas N-[2-(2-bromophenyl)ethyl]-N-ethenylamides gave almost exclusively 7-endo cyclization products. These results indicated that the position of the carbonyl group on enamide played an important role in deciding the course of the cyclization. The 7-endo selective cyclization was applied to concise construction of a cephalotaxine skeleton.