Synthesis of 2-Pyrones and Tetronic Acids via Silver(I) Catalysis

Significance

Reported is the highly E-selective olefination of alkynoates 2 with ynolates 1 to tetrasubstituted alkenes 3 (M. Shindo, S. Mori Synlett 2008, 2231) and their silver(I)-catalyzed cyclization to give either tetronic acids 4 or pyrones 5. Optimization studies of silver catalysts have shown that Ag₂CO₃ activates the carboxyl group stereoelectronically to favor a 5-exo cyclization, whereas AgSbF₆, via π-complexation, favors the 6-endo cyclization mode. The combination of Ag₂CO₃ and Brønsted acid (AcOH or TFA) was shown to promote the exclusive formation of pyrones 5.