Lewis Acid-Mediated Acetal Substitution Reactions: Mechanism and Application to Asymmetric Catalysis
作者:Shū Kobayashi、Kenzo Arai、Takeshi Yamakawa、Yi-Jing Chen、Matthew M. Salter、Yasuhiro Yamashita
DOI:10.1002/adsc.201100346
日期:2011.8
Substitution reactions of acetals with carbon nucleophiles are fundamental and conventional organic reactions. We succeeded in the preparation of an optically active acetal, which reacted with a silyl enol ether smoothly to afford the desired adducts in racemic forms. By comparison of the ees of the products with the ees of the recovered acetals, we concluded that the aldol-type reactions proceeded not
缩醛与碳亲核试剂的取代反应是基本的和常规的有机反应。我们成功地制备了旋光性缩醛,该缩醛与甲硅烷基烯醇醚平稳反应,得到所需外消旋形式的加合物。通过将产物的ee与回收的乙缩醛的ee进行比较,我们得出的结论是,醛醇型反应不是通过直接置换(S N 2)或接触离子对(亲密离子对)(S N 1)进行的。但由游离的氧碳鎓离子(S N1)机制。接下来,进行了研究以实现缩醛取代反应的不对称催化。经过多次试验,发现由五甲氧基铌[Nb(OMe)5 ]和四齿BINOL衍生物制备的手性铌络合物可以实现高对映选择性。缩醛与甲硅烷基烯醇醚的不对称醛醇型反应顺利进行,以高收率和高对映选择性提供了相应的醛醇型加合物。