Stereoselective synthesis of (E)-2-en-4-ynoic acids with ynolates: catalytic conversion to tetronic acids and 2-pyrones.

A highly torquoselective olefination of alkynoates to provide functionalized tetrasubstituted olefins, (E)-2-en-4-ynoic acids, is described. Addition of Brønsted acids dramatically switched the mode of the Ag(I)-catalyzed cyclization of the resulting enyne carboxylic acids to give either tetronic acids or 2-pyrones.