PdII-Catalyzed Reaction of Phenols with Propiolic Esters. A Single-Step Synthesis of Coumarins

The intermolecular reaction of phenols with propiolic esters in TFA in the presence of a Pd(OAc)2 catalyst, affording coumarin derivatives, is described. An exclusive formation of 5,6,7-trimethoxy-4-phenylcoumarin was observed in the reaction of 3,4,5-trimethoxyphenol and ethyl phenylpropiolate with a catalytic amount of Pd (OAc)2. The reaction in the absence of Pd(OAc)2 did not give any coumarin at all. Coumarin derivatives were obtained in high yields in the cases of electron-rich phenols, such as 3,4-methylenedioxyphenol, 3-methoxyphenol, 2-naphthol, and 3,5-dimethoxyphenol. Furthermore, the reaction of some phenols with various 2alkynoates or with diethyl acetylenedicarboxylate gave the corresponding coumarins in good-to-high yields.