The Annuloselectivity And Stereochemistry In Thio-staudinger Cycloaddition

In this dissertation,the Thio-Staudinger cycloaddition is systematically studied,which mainly include the[2t+2i]cycloadditions of one molecule of ketenes and one molecule of imines and the[2t+2i+2i]cycloadditions of one molecule of ketenes and two molecules of imines.Based on the experimentation and theoretical calculations,a model for explaining and predicting the selectivity and stereochemistry of Thio-Staudinger cycloaddition is proposed.In the cycloadditions of[2t+2i],the steric effect of thioketene substituents plays a key role in deciding the diastereoselectivity through controlling exo-and endo-attacks.The conrotatory process of zwitterionic intermediates are the rate-determining step of cycloadditions.In the[2t + 2i + 2i]annulations,the zwitterionic intermediates generated from mono-substituted thioketenes and cyclic imines undergo stepwise endo-addition/Si-attack pathway with a second imine molecule,giving initially(2,4)-cis-(4,5)-cis-[2t + 2i + 2i]annuladducts,which completely epimerize into the corresponding(2,4)-cis-(4,5)-trans-annuladducts under basic reaction conditions.