Palladium iodide catalyzed carbonylative double cyclization to a new class of S,O-bicyclic heterocycles

A Pd-catalyzed carbonylative double cyclization approach to a novel class of S,O-bicyclic heterocycles is presented. It is based on the reaction between readily available 5-(methylthio)pent-1-yn-3-ols with CO in the presence of the PdI2/KI catalytic system carried out under oxidative conditions with oxygen as external oxidant. The process takes place through an ordered sequence of steps involving an initial 5-exo-dig S-cyclization (by intramolecular nucleoplilic attack by the thiomethyl group to the triple bond activated by coordination to the palladium center), followed by S-demethylation (by attack by the iodide anion), carbon monoxide insertion, O-cyclization (by intramolecular nucleophilic displacement of the hydroxyl group to the ensuing acylpalladium intermediate), and Pd(0) reoxidation. The method allows the construction of previously unreported 6,6a-dihydrothieno[3,2-b]furan-2(5H)-ones starting from simple substrates (5-(methylthio)pent-1-yn-3-ols and CO) with the formation of 3 new bonds and two cycles in one step, and has been successfully applied to the synthesis of variously substituted products (57–81 %, 17 examples).