SYNTHESIS OF ALPHA-PYRONES VIA PALLADIUM-CATALYZED ANNULATION OF INTERNAL ALKYNES

A number of 3,5,6-tri- and 3,4,5,6-tetrasubstituted alpha-pyrones have been prepared in good yields by the reaction of vinylic iodides, brom ides or triflates bearing ester functionality with internal alkynes in the presence of a palladium catalyst. The methodology provides an esp ecially simple, convenient, and regioselective route to alpha-pyrones containing aryl, silyl, tert.-alkyl and other hindered groups. The rea ction is believed to proceed through a seven-membered palladacyclic sa lt in which the regiochemistry of the reaction is controlled by steric factors. (C) 1998 Elsevier Science Ltd. All rights reserved.