RADICAL CYCLIZATION REACTIONS OF ALPHA-SILYL AMINE ALPHA,BETA-UNSATURATED KETONE AND ESTER SYSTEMS PROMOTED BY SINGLE ELECTRON-TRANSFER PHOTOSENSITIZATION

The results of a broad investigation of the preparative and mechanistic aspects of single electron transfer (SET) promoted photocyclization reactions of alpha-silyl amino and amido alpha,beta-unsaturated esters and ketones are presented. A number of unique and synthetically useful features of these processes, driven by alpha-silyl amine and amide cation radical desilylation and by intramolecular conjugate addition of intermediate alpha-amino and alpha-amido carbon-centered radicals to unsaturated esters and ketones, are described. Comparisons of the SET-sensitized and direct irradiation promoted reactions of these systems have shown how the former method is superior in inducing photocyclization reactions in cases where the alpha,beta-unsaturated ketone or ester excited states are too reactive to be quenched by SET from the tethered amine donors and where diradicals produced as intermediates in the direct-irradiation reactions undergo fragmentation rather than cyclization. The current efforts have also demonstrated that problems associated with the ready oxidation of intermediate alpha-amino radicals can be avoided by the proper selection of photosensitizer or amine N substituents. Lastly, the synthetic versatility of this chemistry, exemplified by its application to the preparation of a number of N-heterocyclic substances by pathways involving either exo or endo radical cyclization, is presented.