By choosing suitable ligand-directed gold catalysts, two types of gold-containing all-carbon 1,4-dipoles could be generated selectively from the gold(I)-catalyzed cycloisomerizations of allenyl ketones bearing a cyclopropyl moiety, which undergo [4 + 3] cycloadditions with nitrones to produce two regiomers of furan-condensed N,O-seven-membered rings in moderate to excellent yields highly selectively.I ntermolecular cycloaddition represents a powerful tool to create a myriad of functionalized carbocyclic and heterocyclic frameworks in a highly regio-and stereocontrolled fashion. 1 While traditional protocols by applying stable conjugated 1,3-dipoles to react with unsaturated bonds to produce cyclic motifs are well documented, 2 studies on utilizing nonclassical all-carbon 1,4-dipoles, a class of highly reactive variants without a fully conjugated system, 3 for cycloaddition reactions remain a critical challenge and are of high value in organic chemistry.Benefiting from the development of the transition-metal catalysis in organic synthesis, 4 a few approaches have been established to enter the polarized four-carbon units, binding a relatively strong carbon−metal bond via elaborate design of substrates. 5 For example, in 2007, an elegant work involving the generation of 1,4-zwitterionic dipoles with a π-allylpalladium moiety from a palladium(0)-catalyzed oxidative addition into the γ-methylidene-δ-valerolactones and decarboxylation process was first disclosed by Hayashi 6 and co-workers (Scheme 1, eq 1). Later, vinyl benzoxazinanone derivatives have also been successfully used as valuable precursors of an aromatic ring containing 1,4-dipoles undergoing a similar decarboxylation process. 7 The homogeneous gold catalysis has emerged as a continuously growing field of investigation. 8 Owing to the exceptional π-acidity of cationic gold(I) complexes to activate unsaturated carbon−carbon bonds, Zhang's group 9 developed the first gold(I)-catalyzed cycloisomerization of 1-(1-alkynyl)cyclopropyl ketones toward
Herein, we describe a gold(I)-catalyzed generation of nonclassical gold-containing 1,4-all-carbon dipoles from cycloisomerization/1,2carbene transfer/ring opening cascade reactions of readily accessible allenyl ketones bearing a cyclopropyl moiety and its cyclization with tropone. This method features an unprecedented formal [8+4] high-order cycloaddition under mild conditions for delivering structurally complex 7,7,5-tricycles in generally moderate to high yields.
The inherently strained furan-fused cyclobutenes, in situ generated via cycloisomerizations of allenyl ketones bearing cyclopropyl moiety under gold catalysis, have been utilized as reactive building blocks toward cross cycloadditions. The [4 + 2] and [3 + 2] annulations of these species with benzo[c]isoxazoles and N-iminoquinazolinium ylides furnish various three-dimensional cyclobutane-bridged polyheterocycles in good yields. A wide range of typically electron-deficient 1,3-dienes, heterodienes, and 1,3-dipoles can trap furan-fused cyclobutenes to afford several polycyclic architectures.
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