title: Gold- or Silver-Catalyzed Syntheses of Pyrones and Pyridine Derivatives: Mechanistic and Synthetic Aspects.
abstract: 3-Oxo-5-alkynoic acid esters, on treatment with a carbophilic catalyst, undergo 6-endo-dig cyclization reactions to furnish either 2-pyrones or 4-pyrones in high yields. The regiochemical course can be dialed in by the proper choice of the alcohol part of the ester and the π-acid. This transformation is compatible with a variety of acid-sensitive groups as witnessed by a number of exigent applications to the total synthesis of natural products, including pseudopyronine A, hispidine, phellinin A, the radininol family, neurymenolide, violapyrone, wailupemycin and an unnamed brominated 4-pyrone of marine origin. Although the reaction proceeds well in neutral medium, the rate is largely increased when HOAc is used as solvent or co-solvent, which is thought to favor the protodeauration of the reactive alkenyl-gold intermediates as the likely rate-determining step of the catalytic cycle. Such intermediates are prone to undergo diauration as an off-cycle event that sequesters the catalyst; this notion is consistent with literature data and supported by the isolation of the gem-diaurated complexes 12 and 15. Furthermore, silver catalysis allowed access to be gained to 2-alkoxypyridine and 2-alkoxyisoquinoline derivatives starting from readily available imidate precursors.
authors.affiliation: Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
authors.forename: Johannes
authors.initials: J
authors.lastname: Preindl
authors.affiliation: Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
authors.forename: Kévin
authors.initials: K
authors.lastname: Jouvin
authors.affiliation: Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
authors.forename: Daniel
authors.initials: D
authors.lastname: Laurich
authors.affiliation: Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
authors.forename: Günter
authors.initials: G
authors.lastname: Seidel
authors.affiliation: Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany). fuerstner@kofo.mpg.de.
authors.forename: Alois
authors.initials: A
authors.lastname: Fürstner
keywords: diauration; gold; heterocycles; natural products; silver