SAMARIUM-MEDIATED ACYCLIC STEREOSELECTION IN A RADICAL REACTION

Ketyls (anion radicals) generated from both cyclic and acyclic ketones by one electron transfer from SMI2 can be directly trapped with tributyltin hydride or with activated olefins, thus making C-H or C-C bonds respectively under extremely mild conditions. By analysis of the products, one can deduce the stereoconfiguration of the ketyl intermediates. Configuration of the ketyls generated from beta-hydroxy ketone derivatives, whose hydroxyl groups are protected as diethyl phosphate or N,N,N',N'-tetramethylphosphorodiamidate, can be fixed through samarium-involved eight-membered chelate formation. The subsequent C-C bond-forming reaction with methyl acrylate took place in a highly stereoselective manner. Aldehydes were coupled with beta-monosubstituted acrylates to give the corresponding 3,4-cis-disubstituted-gamma-lactones almost exclusively by using SmI2 in tetrahydrofuran. Thus a high degree of stereoselection in some intermolecular radical reactions was attained by using the distinct property of samarium.