The application of intramolecular radical cyclizations of acylsilanes in the regiospecific formation of cyclic silyl enol ethers

Acylsilanes with terminal alpha -stannyl bromide or xanthate functionalities are prepared. alpha -Stannyl radicals generated from these acylsilanes undergo intramolecular cyclizations to give cyclic silyl enol ethers regiospecifically. The radical processes involve radical cyclization, Brook rearrangement, and beta -fragmentation in sequence. A tributylstannyl group serves as the radical leaving group. The newly formed sigma -bond and pi -bond are located between the same two carbon atoms. This approach is limited to the formation of five-membered rings. In another route, omega -bromo-alpha -phenylsulfonylacylsilanes are synthesized. The radical cyclizations of these alpha -sulfonylacylsilanes also give cyclic silyl enol ethers. The phenylsulfonyl moiety is the radical leaving group in this system. Furthermore, the newly formed sigma -bond and pi -bond are located at adjacent positions sharing a single carbon atom. The latter approach is effective for both five- and six-membered ring formation.