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.