Et2AlCl-promoted asymmetric phenylseleno group transfer radical cyclization reactions of unsaturated β-hydroxy esters

Abstract

We have developed a new method for asymmetric phenylseleno group transfer radical cyclization of unsaturated β-hydroxy esters. Various unsaturated α-phenylseleno β-hydroxy esters underwent radical cyclization in the presence of Et2AlCl in benzene with sunlamp irradiation at 25-30 °C to give mono- and bicyclic group-transferred products in an efficient and highly regioselective and diastereoselective manner. To rationalize the high diastereoselectivities observed in this reaction, we propose a model based on chelation control of the aluminum alkoxides that are formed in situ. We devised a general method to prepare chiral radical precursors from which we obtained highly optically pure mono- and bicyclic group transfer products. The synthetic advantages of this method are demonstrated by our formal total synthesis of (-)-wilforonide. This paper presents the first examples of stereoselective group transfer radical cyclizations that occur via 1,2-asymmetric induction.