Metal-free methodology for the preparation of sterically hindered alkynoylphenols and its application to the synthesis of flavones and aurones

A metal-free synthesis for the preparation of sterically demanding ortho-demethylated ynones from mixed anhydrides and potassium alkynyltrifluoroborate salts has been developed. The one-pot reaction proceeds rapidly in the presence of a Lewis acid without the exclusion of air and moisture. This method is advantageous in that it is operationally simple, proceeds under mild conditions, and has a broad substrate scope. 2,6-Dimethoxy substituted anhydrides afford the corresponding mono-demethylated ynone products in good yields. In particular, 2-hydroxy substituted ynone products are valuable synthetic intermediates because their conversion to biologically active natural product scaffolds is straightforward: Flavones were obtained via 6-endo cyclization of the o-alkynyoylphenol intermediates under acidic conditions. Cesium carbonate was found to promote rapid 5-exo cyclization to furnish aurone products. (c) 2015 Elsevier Ltd. All rights reserved.