2-Ethoxy-4-(phenylchalcogeno)but-3-ynyl ketones 1-10 were reduced with LiBH(4) in Et(2)O diastereoselectively to give 5-(phenylchalcogeno)pent-4-yn-1-ols 11-20. Treatment of the phenylchalcogen-substituted alkynyl alcohols 11-20 with t-BuOK in t-BuOH provided useful (Z)-2-((phenylchalcogeno)methylene)tetrahydrofurans 21-31 stereoselectively.
γ-Chalcogen-substituted prop-2-ynyl cations are generated by the reactions of diethyl acetals 1 and 2 with BF3âEt2O and react with various mild nucleophiles without isomerisation to allenyl cations to afford the prop-2-ynylated products 3aâe and 5aâc in good yields.