Silver-Mediated exo-Selective Tandem Desilylative Bromination/Oxycyclization of Silyl-Protected Alkynes: Synthesis of 2-Bromomethylene-Tetrahydrofuran

摘要

Tandem reactions are one-pot multi-step processes and are thus among the most useful synthetic methods. Recently, the use of transition metals in tandem reactions has received considerable attention and has revolutionized the synthetic strategies in organic chemistry. Among the transition metal-based tandem reactions, the processes that involve heterocyclization reactions of unsaturated substrates are of particular interest, as they provide a direct method for the construction of synthetically and biologically valuable heterocycles. The intramolecular heterocyclization of gamma or delta-hydroxyalkynes promises to be a convenient way to access the functionalized tetrahydrofurans and pyrans, which are common motifs in a variety of natural products. This cyclization can be driven by transition metal catalysts, and the regioselectivity of these reactions varies depending on the metal and the conditions employed. Among the metals thus far reported for these reactions, silver and gold have been demonstrated to be the most effective and selective for the formation of exo-dig products. These metals can promote heterocyclization of both internal and terminal alkynes. Even halogenated and silylated alkynes can be converted into the corresponding exo-dig products using these metals.