Diastereoselective, Zinc-Catalyzed Alkynylation of α‑Bromo Oxocarbenium Ions

We
have developed a bromination/alkynylation sequence that enables
efficient transformation of simple cyclic enol ethers to difunctionalized
products. The success of this strategy relies on a highly diastereselective,
zinc-catalyzed addition of terminal alkynes to α-bromo oxocarbenium
ions, formed in situ via ionization of acetal precursors. Using this
method, trans-α-alkynyl-β-halo pyran
and furan derivatives can be prepared with high diastereoselectivity
and excellent functional group tolerance.