Desilylative Transannular Cyclization of Nine-Membered α-Silyl (E)-γ-Epoxy Sulfones

Article abstract of DOI:10.1021/jo00031a024

Anionic desilylation of α-silylated (E)-4,5-epoxythiacyclononane 1,1-dioxides results in formation of transannular cyclization products whose distribution is independent of the stereochemistry of the starting material.Unlike the species formed by direct BuLi metalation, the intermediate arising from the 9-silyl regioisomer rapidly and indiscriminately attacks either epoxidic carbon forming both cis- and trans-joined bicyclic products.This lack of selectivity suggests the reactive intermediate is not a hypervalent silicon species, but is more likely a carbanionwhich, being associated to a large cation (K⁺ or Bu4N⁺) is less discriminating than that arising from BuLi metalation.The intermediate formed by desilylation of the 2-silyl regioisomer appears to cyclize relatively slowly, such that transfer to the carbanionic functionality from C-2 to C-9 becomes competitive with cyclization.With this substrate an unexpected phenomenon has emerged, namely that t-BuOK may act as a base or a desilylating agent depending on the temperature.