Stereochemistry and mechanistic insights in the [2t+2i+2i] annulations of thioketenes and imines

Stereochemical models and mechanistic insights are proposed for [2(t) + 2(i) + 2(i)] annulations of thioketenes and imines on the basis of experimental and computational investigations. In the [2(t) + 2(i) + 2(i)] annulations involving cyclic imines, the zwitterionic intermediates generated from monosubstituted thioketenes and the cyclic imines undergo a stepwise nucleophilic endo-addition/Si-face attack pathway with a second imines molecule, giving initially (2,4)-cis-(4,5)-cis-[2(t) + 2(i) + 2(i)] annuladducts, which completely epimerize into the corresponding (2,4)-cis-(4,5)-trans-annuladducts under basic reaction conditions. The annulo-selectivity of thio-Staudinger cycloadditions is dependent on the substituents of both thioketenes and imines. The reactivities and annuloselectivities of Staudinger, thio-Staudinger, and sulfa-Staudinger intermediates are compared.