Remarkable Template Effect of a Lewis Acidic Receptor in Intramolecular Radical Cyclizations

摘要

Over the last decade free-radical cyclization has developed as a powerful method for constructing ting systems by C-C bond-forming processes and it is now routinely utilized as one of the most reliable tools in organic synthesis. Although the chemo-,regio-, and stereoselectivities of many classes of radical cycliza-tions are well understood, the full potential of this reaction including stereochemical control at the newly created centers is yet to be realized, and hence only limited structural typesof cyclization products have been accessible by this reaction. In this context, we were interested in achieving stereoselective intramolecular radical additions to multiple bonds of halo ethers as model substrates by complexation with a rationally designed Lewis acid. Our recently developed, structurally defined aluminum tris(2,6-diphenylphenoxide) (ATPH, Figure 1) seemed quite suitable for this purpose, since it should allow an appropriate proximity of carbon radical to multiple bond in the transitionstate, thereby srnoothly facilitating the otherwise difficult cyclization and yet reversing the stereoselectivity.