Palladium(II)-mediated cascade carbonylative annulation of o-alkynyl-phenols on silyl linker-based macrobeads: a combinatorial synthesis of a 2,3-disubstituted benzo[b]furan library

Direct access to furan and cyclopropane derivatives via palladium-catalyzed C-H activation/alkene insertion/annulation

A practical and effective palladium-catalyzed C-H activation/alkene insertion/annulation has been reported for the synthesis of furans and cyclopropanes from cyclic 1,3-diketones or 1,3-indandione and diverse alkenes, resulting in moderate to good yields. This protocol demonstrates excellent selectivity and is well-compatible with a wide range of alkene substrates, exhibiting exceptional regioselectivities, high efficiency, and good functional group tolerance.

Transition-Metal-Catalyzed Carbonylative Multifunctionalization of Alkynes

Currently, the construction of new carbon-carbon bonds and value-added structures in an atom- and step economical manner has become a continuous pursuit in the synthetic chemistry community. Since the first transition-metal-catalyzed hydroformylation of ethylene was reported by Otto Roelen in the 1930s, impressive progress has been achieved in the carbonylative functionalization of unsaturated C-C bonds. In contrast to alkenes, the carbonylative functionalization of alkynes offers tremendous potential for the construction of multisubstituted carbonyl-containing derivatives because of their two independently addressable π-systems. This review provides a timely and necessary investigation of transition-metal-catalyzed carbonylative mutifunctionalization of alkynes with the exclusion of carbonylative hydrofunctionalizations. Different transition metals including palladium, rhodium, iridium, ruthenium, iron, copper, etc. were applied to the development of novel carbonylative transformation. Various C-C, C-N, C-O, C-S, C-B, C-Si, and carbon-halogen bonds were formed efficiently and give the corresponding tri- or tetrasubstituted α,β-unsaturated ketones, diesters, and heterocycles.

Synthesis of medium-sized (6-7-6) ring compounds by iron-catalyzed C-H activation/annulation [corrected]

In this report, we have described a FeCl3-catalyzed process involving intramolecular annulation of o-phenoxy diarylacetylenes via hydroarylation to afford a series of biologically potent fused seven-membered (6-7-6) ring compounds under mild reaction conditions. This reaction was believed to proceed through Friedel-Crafts type sequential carbometallation followed by protonation to produce phenyldibenz[b,f]oxepines. This method was also extended to synthesize seven-membered rings that are fused with coumarins.

Rapid synthesis of bicyclic lactones via palladium-catalyzed aminocarbonylative lactonizations

A novel and efficient palladium-catalyzed aminocarbonylative lactonization of amino propargylic alcohols has been developed to provide rapid access to various bicyclic lactones especially dihydropyrrole-fused furanones, which are novel structures and have not been explored in biological and medicinal settings. This method can also be used to access β-lactone products such as 16. Preliminary biological evaluations revealed that compounds 13h and 13s demonstrated promising activity against Clostridium difficile and compounds 13h, 13k, 13s, and 16b showed activity against several important fungal pathogens.

Hydrazone–Pd-catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates

The hydrazone–palladium catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates gave the corresponding 2-substituted-3-allylbenzofuran derivatives at room temperature.

Visible-light-induced oxidant and metal-free dehydrogenative cascade trifluoromethylation and oxidation of 1,6-enynes with water

Unprecedented light-induced oxidant and metal-free tandem radical cyclization–trifluoromethylation and dehydrogenative oxygenation of 1,6-enynes have been achieved using a photoredox catalyst, CF₃SO₂Na, and water as the oxygen source.

Generally, oxy-trifluoromethylation in olefins is achieved using oxidants and transition metal catalysts. However, labile olefins remain unexplored due to their incompatibility with harsh reaction conditions. Here, unprecedented light-induced oxidant and metal-free tandem radical cyclization–trifluoromethylation and dehydrogenative oxygenation of 1,6-enynes have been achieved using a photoredox catalyst, CF₃SO₂Na, and phenanthrene-9,10-dione (PQ), Langlois’ reagent (CF₃SO₂Na) and water as the oxygen source. This benign protocol allows for access to various CF₃-containing C₃-aryloyl/acylated benzofurans, benzothiophenes, and indoles. Moreover, the oxidized undesired products, which are inherently formed by the cleavage of the vinylic carbon and heteroatom bond, have been circumvented under oxidant free conditions. The mechanistic investigations by UV-visible and ESR spectroscopy, electrochemical studies, isotope labelling and density functional theory (DFT) suggest that light induced PQ produced a CF₃ radical from CF₃SO₂Na. The generated CF₃ radical adds to the alkene, followed by cyclization, to provide a vinylic radical that transfers an electron to PQ and generates a vinylic cation. Alternatively, electron transfer may occur from the CF₃-added alkene moiety, forming a carbocation, which would undergo cationic cyclization to generate a vinylic carbocation. The subsequent addition of water to the vinylic cation, followed by the elimination of hydrogen gas, led to the formation of trifluoromethylated C₃-aryloyl/acylated heterocycles.

One-Pot Synthesis of Substituted Benzo[b]furans and Indoles from Dichlorophenols/Dichloroanilines Using a Palladium-Dihydroxyterphenylphosphine Catalyst

Disubstituted benzo[b]furans were synthesized by ortho-selective Sonogashira coupling of dichlorophenols and terminal alkynes, followed by cyclization and Suzuki-Miyaura coupling in one pot, using a palladium-dihydroxyterphenylphosphine (Cy-DHTP) catalyst. The use of substoichiometric amounts of tetrabutylammonium chloride was effective in accelerating the Suzuki-Miyaura coupling. This protocol was also successfully applied to the one-pot synthesis of disubstituted indoles from dichloroaniline derivatives.

An Expedient Protocol to the Synthesis of Benzo(b)furans by Palladium Induced Heterocyclization of Corresponding 2-Allylphenols Containing Electron Rich and Electron Capturing Substituents in the Arene Ring

A facile and rapid accessibility to the libraries containing several mono, and disubstituted benzo(b)furan derivatives substituted with a variety of electron rich and electron capturing groups on 2, 4, 5, 6, and 7 positions of its nucleus has been explored using the PdCl2(CH3CN)2catalyzed heterocyclization of the corresponding 2-allylphenols to afford2(a–m)in good yield and high purity.

Synthesis of diverse indole libraries on polystyrene resin - Scope and limitations of an organometallic reaction on solid supports

The synthesis of diverse substituted indole structures on solid supports is described. The immobilization of nitrobenzoic acid onto Merrifield resin and the subsequent treatment with alkenyl Grignard reagents delivered indole carboxylates bound to solid supports. In contrast to results in the liquid phase, ortho,ortho-unsubstituted nitroarenes also delivered indole moieties in good yields. Subsequent palladium-catalyzed reactions (Suzuki, Heck, Sonogashira, Stille) delivered, after cleavage, the desired molecules in moderate to good yields over four steps. The scope and limitations are presented.

Parallel synthesis of a multi-substituted benzo[b]furan library

The solution-phase parallel synthesis of a 121-member library of multi-substituted benzo[ b]furans is described. 2,3,5-Trisubstituted benzo[ b]furans have been prepared by the palladium-catalyzed substitution of 3-iodobenzofurans by Suzuki-Miyaura, carbonylative Suzuki, Sonogashira, Heck, and carboalkoxylation chemistry. The 3-iodobenzofurans are readily prepared in good to excellent yields by the palladium/copper-catalyzed cross-coupling of various o-iodoanisoles and terminal alkynes, followed by electrophilic cyclization with ICl.

RhI-Catalyzed Bimolecular Cyclization between Two Different 1,5-Bisallenes: A Combinatorial One-Step Approach to Heterosteroids and Mechanistic Implications

In this paper, a bimolecular-cyclization reaction between two different bis(allene)s with at least one heteroatom as the tether under the catalysis of trans-[RhCl(CO)(PPh3)2] is described. This protocol provides an efficient entry to different heterocyclic 18,19-norsteroid-like scaffolds. The tricyclic product was formed highly selectively from the cyclization reaction of bis(2,3-butadienyl)sulfide with dimethyl 2-bis(2',3'-butadienyl)malonate, which sheds light on the mechanism involving the metalla-[4.3.0]-bicyclic intermediate formed by the cyclometallation of the terminal and the internal C=C bonds of each of the two allene moieties in 2-bis(2',3'-butadienyl)malonate.

Synthesis of naphthalenes and 2-naphthols by the electrophilic cyclization of alkynes

[reactions: see text] A wide variety of substituted naphthalenes are readily prepared regioselectively under mild reaction conditions by the 6-endo-dig electrophilic cyclization of appropriate arene-containing propargylic alcohols by ICl, I2, Br2, NBS, and PhSeBr. 3-Iodo-2-naphthols have also been prepared in excellent yields by the cyclization of analogous 1-aryl-3-alkyn-2-ones. This methodology readily accommodates various functional groups and has been successfully extended to the synthesis of substituted carbazoles and dibenzothiophenes.

Novel PdII-Mediated Cascade Carboxylative Annulation to Construct Benzo[b]furan-3-carboxylic Acids

[reaction: see text] Benzo[b]furan-3-carboxylic acid (2) was generated from 1 by forming three new bonds in one step via a Pd(II)-mediated cascade carboxylative annulation. The proposed mechanism was supported by the observation of an unusual acetylation of 1 as a side reaction together with an (18)O-labeling study.

Libraries from natural product-like scaffolds

Natural products are an attractive source of varied structures that exhibit potent biological activities, and desirable pharmacological profiles. Since the relatively recent advent of high-throughput organic synthesis in the drug discovery process, several design approaches have been applied to the construction of screening libraries. Libraries of natural-product derivatives, natural-product-like compounds prepared by total synthesis, and libraries derived from natural-products are several types that have been reported.

Synthesis of Conformationally Restricted 2,3-Diarylbenzo[b]furan by the Pd-Catalyzed Annulation of o-Alkynylphenols: Exploring a Combinatorial Approach

The palladium/bpy-catalyzed annulation of o-alkynylphenol with various aryl halides to generate diversified 2,3-diarylbenzo[b]furan is herein described. This method provides an efficient synthetic pathway for the combinatorial synthesis of conformationally restricted 2,3-diarylbenzo[b]furan for drug discovery.

Total Synthesis of Wedelolactone

The total synthesis of wedelolactone, a naturally occurring direct inhibitor of IKK complex that can suppress LPS-induced caspase-11 expression, using a convergent synthetic approach, is described. The key steps involved in this synthesis include the palladium-catalyzed Sonogashira reaction and the palladium-catalyzed carbonylative annulation reaction. This approach allows access to diversified analogues of wedelolactone.

Palladium-catalyzed cross-coupling reactions of 4-tosyl-2(5H)-furanone with boronic acids: a facile and efficient route to generate 4-substituted 2(5H)-furanones

An efficient and facile synthesis of 4-substituted 2(5H)-furanones using palladium catalyzed cross-coupling reactions between 4-tosyl-2(5H)-furanone and boronic acids is reported herein.