A palladium iodide catalyzed regioselective carbonylative route to isocoumarin and thienopyranone carboxylic esters

The reactivity of 2-alkynylbenzoic acids under PdI2/KI-catalyzed oxidative alkoxycarbonylation conditions, with oxygen (from air) as external oxidant and an alcohol as external nucleophile, has been studied. It was found that substrates with triple bond substituted with a bulky alkyl group, such as tert-butyl, selectively underwent a 6-endo-dig cyclization - alkoxycarbonylation pathway with ethanol or isopropanol as nucleophile, to give high value added isocoumarin-4-carboxylic esters in good to high yields (67–87%). When applied to alkynylthiophencarboxylic acids bearing an internal triple bond, the reaction turned out to be completely regioselective toward the formation of the corresponding alkyl thienopyranonecarboxylates ensuing from 6-endo-dig cyclization-alkoxycarbonylation, regardless the nature of the substituent on the triple bond and the external alcohol (54–91% yields). To confirm the structure of the products deriving from different kinds of substrates, and therefore to corroborate the proposed mechanistic pathways, the crystalline structures of 11 new compounds have been resolved by XRD analysis.