Metal-free methodology for the preparation of ynones using potassium alkynyltrifluoroborates and its application to the synthesis of natural products
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A straightforward procedure for the preparation of ynones from potassium alkynyltrifluoroborate salts and acyl chlorides or anhydrides has been developed. The one-pot reaction is advantageous in that it does not require exclusion of air or water, is operationally simple, has broad substrate scope, and proceeds rapidly under mild conditions in the presence of a Lewis acid. 2,6-dimethoxy substituted anhydrides afford the corresponding mono-demethylated o-alkynoylphenol precursors of aurone and flavone natural product scaffolds in good yields. A variety of flavones were obtained via 6-endo cyclization of o-alkynoylphenol intermediates in the presence of trifluoromethanesulfonic acid (TfOH). Cesium carbonate was discovered to promote the rapid 5-exo cyclization of o-alkynoylphenols to form aurone products in excellent yields.