Catalytic Regioselectivity Control in Ring-Opening Cycloisomerization of Methylene- or Alkylidenecyclopropyl Ketones

2-Methylene- or alkylidenecyclopropanyl ketones were easily prepared by the regioselective cyclopropanation of allenes or the reaction of methylene-/alkylidenecyclopropanyllithium with N,N-dimethyl carboxylic acid amides. Due to the presence of the exo-cyclic C=C bond and the strained cyclopropane, their highly selective ring-opening cycloisomerization using PdCl(2)(CH(3)CN)(2), NaI (or PdCl(2)(CH(3)CN)(2) + NaI), and Pd(PPh(3))(4) as catalysts provided five different products, i.e., 4H-pyrans, 2,3,4-trisubstituted furans (or 4,5-disubstituted-3-alkylidene-2,3-dihydrofurans), and 2,3,4,5-tetrasubtituted furans (or 2,4,5-trisubstituted-3-alkylidene-2,3-dihydrofurans) in good yields, respectively, depending on the nature of the catalyst and reaction conditions. The less-substituted C=C bonds in these products can be highly selectively hydrogenated or hydroborated to afford new heterocyclic products stereoselectively. These three types of different reactions may proceed through a highly regioselective cleavage of a carbon-carbon single bond in the cyclopropane ring triggered by regioselective halometalation of the C=C bond and beta-decarbopalladation, halogen anion attack on the nonsubstituted carbon atom of the cyclopropane ring, or the direct oxidative addition of the distal carbon-carbon single bond of the cyclopropane ring with Pd(0). In some cases substituent effects were successfully applied to synthesize 2H-pyrans 8 and 3-alkylidene-2,3-dihydrofurans 5, which also provided some mechanistic information.

An Expedient Route for the Stereoselective Construction of Bridged Polyheterocyclic Ring Systems Using the Tandem "Pincer" Diels-Alder Reaction

The tandem "pincer" Diels-Alder reaction, consisting of two consecutive [4 + 2] cycloadditions between two dienes and an acetylenic bis-dienophile, has been applied toward the rapid construction of bridged polyoxacyclic ring systems when furan derivatives are used as the diene components. The study has demonstrated the stereoselectivity (exo-exo adduct), the chemoselectivity ("pincer" vs "domino"), as well as the regioselectivity of the reaction. The reaction has been successfully applied to a variety of 2-substituted furans and tethered bis-furans in combination with monoactivated and diactivated dienophiles. The synthesis of unsymmetrical cycloadducts starting from the aza- and oxanorbornadiene-type intermediate has also been realized.