Suppression of β-hydride elimination in the intramolecular hydrocarboxylation of alkynes leading to the formation of lactones

Palladium-catalyzed intramolecular cyclization of the alkynoic acids was studied which gave the five- and six-membered lactones in moderate yields. The reaction can be conducted simply by heating a toluene solution of alkynoic acids at 100 °C in the presence of catalytic amounts of Pd(PPh₃) ₄ and (o-tol)₃P. The key for this transformation is the use of phosphines, instead of carboxylic acids, as an additive. Similar to our previously developed catalytic system the use of carboxylic acid, instead of (o-tol)₃P, resulted into the exclusive formation of the diene.