Dual Directing-Groups-Assisted Redox-Neutral Annulation and Ring Opening of N-Aryloxyacetamides with 1-Alkynylcyclobutanols via Rhodium(III)-Catalyzed C–H/C–C Activations

The Construction of Novel Spirocyclic Frameworks with Cyclobutane through Rh(III)-Catalyzed [3 + 2]-Annulation between Quinoxalines and Alkynylcyclobutanols

An effective synthesis strategy for the preparation of 1'H-spiro[indene-1,2'-quinoxaline] has been developed. This involves a Rh(III)-catalyzed [3 + 2]-annulation of quinoxalines with alkynylcyclobutanols. The developed protocol offers a straightforward method for the preparation of versatile heterocyclic compounds with a four-membered ring and is compatible with a wide range of functional groups.

Hydroxy Group-Enabled Regio- and Stereoselective Hydroalkylation of Alkynyl Cyclobutanol via Palladium-Catalyzed C-C Bond Activation of Cyclopropanol: A One-Step Access to Vinyl Cyclobutanols

The regio-/stereoselective synthesis of vinyl cyclobutanols from alkynyl cyclobutanols is demonstrated. Here, selective C-C bond activation of the cyclopropyl alcohol ring has been achieved in the presence of the cyclobutanol ring. The KIE experiments indicated the noninvolvement of the O-H oxidative addition step in the rate-determining step. Further, the applicability of these vinyl cyclobutanols for the synthesis of 1,4-diketones and 1,6-diketone is demonstrated.

Kinetic Resolution of Azaarylethynyl Tertiary Alcohols by Chiral Brønsted Acid Catalysed Phosphine-Mediated Deoxygenation

A chiral Brønsted acid catalysed phosphine-mediated deoxygenation protocol is reported. This metal-free method provides a precise kinetic resolution platform for azaarylethynyl tertiary alcohols, which are a broad category of biologically and synthetically important azaarene derivatives. In addition to providing an efficient method for the first asymmetric preparation of these tertiary alcohols, the strategy facilitates the construction of azaaryl-functionalized allenes with good to excellent enantioselectivities. The high selectivity factors (s up to 235), broad substrate scope, and ability to convert azaaryl compounds into both chiral tertiary alcohols and allenes robustly underscore the efficiency and promising utility of this method. The practicability is further validated by the successful synthesis of deuterated allenes with high ee values and substantial incorporation of deuterium using inexpensive D₂ O as the deuterium source.

Merging Rh-Catalyzed C-H Functionalization and Cascade Cyclization to Enable Propargylic Alcohols as Three-Carbon Synthons

Reported herein is a reactivity of propargyl alcohols as "Three-Carbon Synthons" in a Rh(III)-catalyzed C-H functionalization of acetanilides, leading to the synthesis of core structures of isocryptolepine, γ-carbolines, dihydrochromeno[2,3-b]indoles, and diindolylmethanes (DIM) derivatives. The transformation involves a rhodium(III)-catalyzed C-H functionalization and heteroannulation to yield indoles followed by a cascade cyclization with both external and internal nucleophiles to afford diverse products. The role of the hydroxy group, the key function of the silver additive, the origin of the reverse regioselectivity and the rate-determining step, are rationalized in conformity with the combination of experimental, noncovalent interaction analysis and DFT studies. This protocol is endowed with several salient features, including one-pot multistep cascade approach, exclusive regioselectivity, good functional group tolerance and synthesis of variety of molecular frameworks.

Rh(III)-Catalyzed chemo-, regio- and stereoselective carboamination of sulfonyl allenes with N-phenoxy amides or N-enoxy imides

The Rh(III)-catalyzed chemo-, regio- and stereoselective carboamination of sulfonyl allenes has been realized by virtue of either N-phenoxy amides or N-enoxy imides simultaneously acting as the C- and N-sources, via redox-neutral tandem C-H activation/allene insertion/oxidative addition/C-N bond formation for the direct construction of allylamine derivatives equipped with an α-quaternary carbon center. This protocol features high atom-economy with good substrate compatibility and exhibits profound synthetic potential for late-stage C-H modification of complex molecules.

Coupling partner-dependent unsymmetrical C–H functionalization of N-phenoxyacetamides leading to sophisticated spirocyclic scaffolds

In this paper, a coupling partner-dependent unsymmetrical C–H functionalization of N-phenoxyacetamides leading to the formation of sophisticated spirocyclic scaffolds is presented.

Specific assembly of dihydrobenzofuran frameworks via Rh(iii)-catalysed C–H coupling of N-phenoxyacetamides with 2-alkenylphenols

The synergistic dual directing group-enabled and Rh(iii)-catalysed redox-neutral C–H functionalization/[3+2] annulation has been realized for the synthesis of dihydrobenzofurans.

Synthesis of Indolyl-Tethered Spiro[cyclobutane-1,1'-indenes] through Cascade Reactions of 1-(Pyridin-2-yl)-1H-indoles with Alkynyl Cyclobutanols

Presented herein is an efficient and unprecedented synthesis of indolyl-tethered spiro[cyclobutane-1,1'-indenes] through the cascade reaction of 1-(pyridin-2-yl)-1H-indoles with alkynyl cyclobutanols. Mechanistic experiments implicate a sequential process in which 1-(pyridin-2-yl)-1H-indole first undergoes an alkenylation with alkynyl cyclobutanol followed by an intramolecular Friedel-Crafts reaction to give the title products. The utility of this novel protocol was reflected by the ample substrate scope, high chemo- and regioselectivity, removable directing group, and scalable preparation. In addition, the product thus obtained can be further derivatized quite efficiently.

Rh(III)-Catalyzed and synergistic dual directing group-enabled redox-neutral [3+3] annulation of N-phenoxyacetamides with α-allenols

By virtue of α-allenols as innovative three-carbon annulation components, the Rh(III)-catalyzed redox-neutral C-H coupling of N-phenoxyacetamides with α-allenols has been realized for the assembly of 4-alkylidene chroman-2-ol frameworks via an unusual [3+3] annulation. This transformation features good functional group tolerance, specific regio-/chemoselectivity and potential synthetic utility. Mechanistic studies reveal that synergistic coordination modes between the dual directing groups (-ONHAc and -OH) and the rhodium metal center account for the observed exclusive selectivity.

Rh(III)-Catalyzed Regioselective Annulations of 3-Arylisoxazolones and 3-Aryl-1,4,2-dioxazol-5-ones with Propargyl Alcohols: Access to 4-Arylisoquinolines and 4-Arylisoquinolones

The Rh(III)-catalyzed dual directing group assisted C-H activation/annulation of 3-arylisoxazolones with propargyl alcohols has been developed, which expands the application scope of isoxazolones in organic synthesis. This protocol also worked well with 3-aryl-1,4,2-dioxazol-5-ones to produce synthetically and biologically important 4-arylisoquinolones.

Fully Substituted Conjugate Benzofuran Core: Multiyne Cascade Coupling and Oxidation of Cyclopropenone

An unprecedented C═C double bond cleavage of cyclopropenone and dioxygen activation by multiyne cascade coupling has been developed. This chemistry provides a novel, simple, and efficient approach to synthesize fully substituted conjugate benzofuran derivatives from simple substrates under mild conditions. The density functional theory (DFT) calculations reveal that the unique homolytic cleavages of cyclopropenone and molecular oxygen are crucial to the success of this reaction.

Chemodivergent assembly of ortho-functionalized phenols with tunable selectivity via rhodium(III)-catalyzed and solvent-controlled C-H activation

Ortho-functionalized phenols and their derivatives represent prominent structural motifs and building blocks in medicinal and synthetic chemistry. While numerous synthetic approaches exist, the development of atom-/step-economic and practical methods for the chemodivergent assembly of diverse ortho-functionalized phenols based on fixed catalyst/substrates remains challenging. Here, by selectively controlling the reactivities of different sites in methylenecyclopropane core, Rh(III)-catalyzed redox-neutral and tunable C-H functionalizations of N-phenoxyacetamides are realized, providing access to both ortho-functionalized phenols bearing linear dienyl, cyclopropyl or allyl ether groups, and cyclic 3-ethylidene 2,3-dihydrobenzofuran frameworks under mild cross-coupling conditions. These divergent transformations feature broad substrate compatibility, synthetic applications and excellent site-/regio-/chemoselectivity. Experimental and computational mechanistic studies reveal that distinct catalytic modes involving selective β-C/β-H elimination, π-allylation, inter-/intramolecular nucleophilic substitution cascade and β-H' elimination processes enabled by different solvent-mediated and coupling partner-controlled reaction conditions are crucial for achieving chemodivergence, among which a structurally distinct Rh(V) species derived from a five-membered rhodacycle is proposed as the corresponding active intermediates.

Synthesis of 2-aminobenzofurans via base-mediated [3 + 2] annulation of N-phenoxy amides with gem-difluoroalkenes

Efficient metal-free [3 + 2] annulation of N-phenoxy amides with gem-difluoroalkenes has been realized for the assembly of 2-aminobenzofuran derivatives with potent cytotoxicity against cancer cell lines and application potential for DELs.

Rhodium(III)-Catalyzed Alkenyl C-H Functionalization to Dienes and Allenes

An oxyacetamide-directed Rh(III)-catalyzed Z-type alkenyl C-H functionalization through a rare exo-rhodacyle intermediate is described, forming multisubstituted dienes and allenes. A variety of alkenes and propargylic carbonate coupling partners are suitable for this transformation with high regio- and stereoselectivity. The synthetic utility is demonstrated by the selective late-stage modification of the Z-type natural products as well as the synthesis of the unnatural β-amino acid.

Ru(II)-Catalyzed and acidity-controlled tunable [5+1]/[5+2] annulation for building ring-fused quinazolines and 1,3-benzodiazepines

The Ru(ii)-catalyzed tunable [5+1]/[5+2] annulation of N-benzo[d]imidazole indolines with propargyl carbonates has been realized for the divergent synthesis of ring-fused quinazolines and 1,3-benzodiazepines bearing various functional groups. These transformations represent an efficient and practical strategy in constructing complex heterocycles via diversified C-H functionalization. A distinctive acidity-controlled reaction manner has been clarified to account for the chemoselectivity.

Selective Synthesis of Pyrazolo[1,2-a]pyrazolones and 2-Acylindoles via Rh(III)-Catalyzed Tunable Redox-Neutral Coupling of 1-Phenylpyrazolidinones with Alkynyl Cyclobutanols

An unprecedented divergent synthesis of pyrazolo[1,2-a]pyrazolones and 2-acylindoles via Rh(III)-catalyzed [4 + 1] or [3 + 2] annulation of 1-phenylpyrazolidinones with alkynyl cyclobutanols through redox-neutral multiple bond activation by using -NH and -OH units as directing groups is presented. Notably, different annulation reactions were selectively achieved by simply adjusting the reaction conditions. With features such as simple procedures, easily accessible substrates, and high regio/chemoselectivity, these methods may find wide applications in related areas.

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C-C and/or C-O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a-O bond formation: (route a). (2.2) O-C2 bond formation: (route b). (2.3) C2-C3 bond formation: (route c). (2.4) C3-C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3-C3a bond formation (route a + d). (3.2) O-C2 and C2-C3 bond formation: (route b + c). (3.3) O-C2 and C3-C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion.

Deconstructive Reorganization: De Novo Synthesis of Hydroxylated Benzofuran

An unprecedented deconstructive reorganization strategy for the de novo synthesis of hydroxylated benzofurans from kojic acid- or maltol-derived alkynes is reported. In this reaction, both the benzene and furan rings were simultaneously constructed, whereas the pyrone moiety of the kojic acid or maltol was deconstructed and then reorganized into the benzene ring as a six-carbon component. Through this strategy, at least one free hydroxyl group was introduced into the benzene ring in a substitution-pattern tunable fashion without protection-deprotection and redox adjustment. With this method, a large number of hydroxylated benzofuran derivatives with different substitution-patterns have been prepared efficiently. This methodology has also been shown as the key step in a collective total synthesis of hydroxylated benzofuran-containing natural products (11 examples).

Rh(iii)-catalyzed, hydrazine-directed C–H functionalization with 1-alkynylcyclobutanols: a new strategy for 1H-indazoles

Rh(iii)-catalyzed coupling of phenylhydrazines with 1-alkynylcyclobutanols was realized through a hydrazine-directed C–H functionalization and [4+1] annulation pathway.

Solvent-free and room temperature microwave-assisted direct C7 allylation of indolines via sequential C–H and C–C activation

A Ru or Rh-catalyzed efficient and atom-economic C7 allylation of indolines with vinylcyclopropanes was developed via sequential C–H and C–C activation. A wide range of substrates were well tolerated to afford the corresponding allylated indolines in high yields and E/Z selectivities under microwave irradiation. The obtained allylated indolines could further undergo transformations to afford various value-added chemicals. Importantly, this reaction proceeded at room temperature under solvent-free conditions.

A Ru or Rh-catalyzed direct C7 allylation of indolines with vinylcyclopropanes via sequential C–H/C–C activation under microwave irradiation has been disclosed.

Rh(III)-Catalyzed Redox-Neutral [4+2] Annulation for Direct Assembly of 3-Acyl Isoquinolin-1(2H)-ones as Potent Antitumor Agents

By virtue of an efficient rhodium(III)-catalyzed redox-neutral C-H activation/ring-opening of a strained ring/[4+2] annulation cascade of N-methoxybenzamides with propargyl cycloalkanols, diverse 3-acyl isoquinolin-1(2H)-ones were directly obtained in good yields and with excellent functional group compatibility. Additionally, their antitumor activities against various human cancer cells including HepG2, A549, MCF-7 and SH-SY5Y were evaluated and the action mechanism of the selected compound was also investigated in vitro. The results revealed that these products possessed a potent efficacy, by inhibiting proliferation and inducing apoptosis in a time-dependent and dose-dependent manner, suggesting that such compounds can serve as promising candidates for anti lung cancer drug discovery.

Rhodium-Catalyzed Successive C-H Bond Functionalizations To Synthesize Complex Indenols Bearing a Benzofuran Unit

An efficient rhodium-catalyzed redox-neutral annulations of N-phenoxyacetamides and ynones via successive double C-H bond activations has been developed. A series of novel and complex indenols bearing a benzofuran unit were generated with moderate to excellent regioselecetivities under mild conditions. Adding N-ethylcyclohexanamine (CyNHEt) could restrict the formation of the mono C-H bond activation byproduct, which is not the intermediate of the reaction demonstrated via the mechanistic investigations.

Rh(III)-Catalyzed C-H Activation/Cycloisomerization of N-Phenoxyacetamides with Enynones for One-Pot Assembly of Furylated 2-Alkenylphenols

An efficient and practical procedure for one-pot assembly of furylated 2-alkenylphenols has been achieved via the Cp*^CyRh-catalyzed regioselective redox-neutral C-H activation/5-exo-dig cyclization cascade using N-phenoxyacetamides and enynones as the viable substrates. The synthetic application of such a protocol has also been demonstrated to highlight the versatility of this transformation.

Rhodium(iii)-catalysed cascade [3 + 2] annulation of N-aryloxyacetamides with 3-(hetero)arylpropiolic acids: synthesis of benzofuran-2(3H)-ones

Herein, a cascade [3 + 2] annulation of N-aryloxyacetamides with 3-(hetero)arylpropiolic acids affording benzofuran-2(3H)-ones via rhodium(iii)-catalyzed redox-neutral C-H functionalization/isomerization/lactonization using an internal oxidative directing group O-NHAc was achieved. This catalytic system provides a regio- and stereoselective approach to synthesize (Z)-3-(amino(aryl)methylene)benzofuran-2(3H)-ones with exclusive Z configuration selectivity, acceptable yields and good functional group tolerance. Preliminary investigations on ultraviolet-visible and fluorescence behaviors reveal that the annulation products may be applied as a promising fluorescent probe for sensing metal cations, especially for cerium (Ce3+).

Stereoselective β-F Elimination Enabled Redox-Neutral [4 + 1] Annulation via Rh(III)-Catalyzed C-H Activation: Access to Z-Monofluoroalkenyl Dihydrobenzo[ d]isoxazole Framework

An efficient and practical Rh(III)-catalyzed redox-neutral [4 + 1] annulation of N-phenoxy amides with α, α-difluoromethylene alkynes has been realized to give direct access to the Z-configured monofluoroalkenyl dihydrobenzo[ d]isoxazole framework with broad substrate compatibility and good functional group tolerance, which was further enhanced by the late-stage C-H modification of complex bioactive compounds. Subsequent density functional theory calculations revealed that the stereoselective β-F elimination involving an allene species played a decisive role in determining the reaction outcome and such Z-selectivity.