Synthesis of Isocoumarins and alpha-Pyrones via Palladium-Catalyzed Annulation of Internal Alkynes.

A number of 3,4-disubstituted isocoumarins and polysubstituted alpha-pyrones have been prepared in good yields by treating halogen- or triflate-containing aromatic and alpha,beta-unsaturated esters, respectively, with internal alkynes in the presence of a palladium catalyst. Synthetically, the methodology provides an especially simple and convenient, regioselective route to isocoumarins and alpha-pyrones containing aryl, silyl, ester, tert-alkyl, and other hindered groups. The reaction is believed to proceed through a seven-membered palladacyclic complex in which the regiochemistry of the reaction is controlled by steric factors. A number of 3,4-disubstituted isocoumarins and polysubstituted alpha-pyrones have been prepared in good yields by treating halogen- or triflate-containing aromatic and alpha,beta-unsaturated esters, respectively, with internal alkynes in the presence of a palladium catalyst. Synthetically, the methodology provides an especially simple and convenient regioselective route to isocoumarins and alpha-pyrones containing aryl, silyl, ester, tert-alkyl, and other hindered groups. The reaction is believed to proceed through a seven-membered palladacyclic complex in which the regiochemistry of the reaction is controlled by steric factors.