Yang Dan, Gao Qiang, Zheng Bao-Fu, Zhu Nian-Yong
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China.
J Org Chem. 2004 Dec 10;69(25):8821-8. doi: 10.1021/jo048322z.
We have developed a new method for asymmetric phenylseleno group transfer radical cyclization of unsaturated beta-hydroxy esters. Various unsaturated alpha-phenylseleno beta-hydroxy esters underwent radical cyclization in the presence of Et(2)AlCl in benzene with sunlamp irradiation at 25-30 degrees 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.
我们已经开发出一种用于不饱和β-羟基酯的不对称苯基硒基转移自由基环化反应的新方法。各种不饱和α-苯基硒基β-羟基酯在Et(2)AlCl存在下于苯中，在25 - 30℃用日光灯照射进行自由基环化反应，以高效、高区域选择性和非对映选择性的方式得到单环和双环基团转移产物。为了解释该反应中观察到的高非对映选择性，我们提出了一个基于原位形成的烷氧基铝的螯合控制的模型。我们设计了一种制备手性自由基前体的通用方法，从中获得了高光学纯度的单环和双环基团转移产物。(-)-wilforonide的形式全合成证明了该方法的合成优势。本文展示了通过1,2-不对称诱导发生的立体选择性基团转移自由基环化反应的首例。
