Ring Expansion of Benzothiaselenazole-1-oxides via Five-Membered Selenonium Ion-Mediated Alkene Insertion into Se–N Bonds

Abstract Ring expansion of heterocyclic frameworks provides an efficient approach to structurally diverse bioactive molecules, yet such transformations of organoselenium heterocycles remain largely unexplored. Herein, we report a calcium-catalyzed insertion of alkenes into the Se–N bonds of benzothiaselenazole-1-oxides, enabling modular ring expansion of these selenium-containing heterocycles. Mechanistic studies and DFT calculations support the involvement of an unprecedented five-membered selenonium ion intermediate that governs the regio- and site-selectivity. The reaction accommodates diverse 1,3-dienes and benzothiaselenazole-1-oxides, affording ring-expanded products in high efficiency with exclusive E-selectivity. Late-stage functionalization of natural products and drug molecules further demonstrates the synthetic utility of this method for the diversification of organoselenium scaffolds.