Aryl radical additions to aldehydes and oxime ethers: the tandem enediyne-radical cyclization

Abstract

The previously unreported cyclization of aryl radicals onto aldehyde and oxime ether acceptors is described. The aryl radicals were generated from a cyclization of enediyne substrates. The aldehydes 5-(2-ethynylphenyl)pent-4-ynal (6) and 6-(2-ethynylphenyl)hex-5 -ynal (9), and the oxime ethers derivatives (7) and (10) were heated to 190 o C in chlorobenzene in the presence of 1,4-cyclohexadiene as a hydrogen atom source to yield the tandem enediyne-radical cyclization products benzo[e]indene (11a, 11b, 21 and 22 ), dihydrophenanthrene (14), and the simple enediyne cyclization products naphtalene derivatives (12, 13, 15, 16, and 23). For the enediyne aldehyde substrates tandem enediyne-radical cyclization does not appear to be a synthetically useful process and a mixture of products was obtained. The aryl radicals generated in these enediyne cyclizations subsequently undergo either a radical cyclization or other reactions such as hydrogen abstraction from 1,4-cyclohexadiene, decarbonylation. In contrast, the reactions with oxime ether precursors provide the tandem enediyne-radical cyclisation products in good yield and provide a useful alternative to the tandem enediyne-6-exo-radical cyclization into olefins