ARYL RADICAL ADDITIONS TO ALDEHYDES AND OXIME ETHERS - THE TANDEM ENEDIYNE-RADICAL CYCLIZATION

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 6 and 9 and the oxime ethers 7 and 10 were heated to 190-degrees-C in chlorobenzene in the presence of 1,4-cyclohexadiene as a hydrogen atom source to yield the tandem enediyne-radical cyclization products 11a, 11b, 14, 21, and 22, and the simple enediyne cyclization products 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, or intramolecular 1,5- and 1,6-hydrogen abstractions. In contrast, there actions with oxime ether precursors provide the tandem enediyne-radical cyclization products in good yield and provide a useful alternative to the tandem enediyne-6-exo-radical cyclization onto olefins.