Highly Regioselective Radical Cyclizations of Allenamides

The first radical cyclizations of allenamides are described. These reactions are highly regioselective for the central carbon of the allenic moiety, leading to an efficient preparation of nitrogen heterocycles such as isoquinolines, and carbocycles such as indane and naphthalene derivatives. The exo-cyclization mode could also be achieved in some cases, leading to the synthesis of isoindoles. The feasibility of a tandem radical cyclization using allenamide is also established. [reaction: see text]

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

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.