Gold-Catalyzed Cyclization Leads to a Bridged Tetracyclic Indolenine that Represses β-Lactam Resistance

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp3-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

Synthesis of Spiroindolenine-3,3'-pyrrolo[2,1-b]quinazolinones through Gold(I)-Catalyzed Dearomative Cyclization of N-Alkynyl Quinazolinone-Tethered Indoles

A variety of functionalized spiroindolenine-3,3'-pyrrolo[2,1-b]quinazolinones were prepared in good to excellent yields through a gold(I)-catalyzed dearomative cyclization of N-alkynyl quinazolinone-tethered C2-substituted indoles. The reaction features broad substrate scope, good functional...

Diastereoselective synthesis of indolenine-based spiro[pyrazolone-4,2'-pyrrolidine] scaffolds via 1,3-dipolar cycloaddition of 4-aminopyrazolones, aldehydes, and indolenines

In this work, we report a one-pot [3 + 2] cycloaddition of 4-aminopyrazolones, indolenines, and aldehydes. The reaction utilized in situ generated azomethine ylides as 1,3-dipoles and 2-alkenylindolenines as dipolarophiles affording indolenine-derived spiro[pyrazolone-4,2'-pyrrolidine] scaffolds with four contiguous stereocenters with excellent yields (up to 95%) and diastereoselectivities (up to >20 : 1 dr) under simple conditions. The in situ generation of azomethine ylides and dipolarophiles in one pot is a unique feature of this process.

Au(I)-Catalyzed Domino Cyclization of 1,6-Diynes Incorporated with Indole

We disclose herein a Au(I)-catalyzed domino cyclization of 1,6-diynes incorporated with indole. This protocol enabled the diastereoselective buildup of indole-fused azabicyclo[3.3.1]nonanes from linear precursors. Density functional theory calculations showed that the reaction proceeded via an unprecedented cascade dearomatization/rearomatization/dearomatization process. Independent gradient model analysis revealed that a noncovalent attractive interaction between the distal alkyne and the Au/proximal complex was responsible for the chemoselectivity of the first spirocyclization step.

Iron(III) Chloride as a Mild Catalyst for the Dearomatizing Cyclization of N-Acylindoles

A catalytic approach for the preparation of indolines by dearomatizing cyclization is presented. FeCl₃ acts as a catalyst to afford tetracyclic 5a,6-dihydro-12H-indolo[2,1-b][1,3]benzoxazin-12-ones in good yields. The cyclization also proceeds with tosylamides forming C-N bonds in 53% yield.

Selectivity Control in Gold-Catalyzed Hydroarylation of Alkynes with Indoles: Application to Unsymmetrical Bis(indolyl)methanes

Gold-catalyzed hydroarylation of unactivated alkynes with indoles have previously been reported to proceed with double indole addition to produce symmetrical bis(indolyl)methanes (BIMs). We demonstrate for the first time that the selectivity of the gold-catalyzed reaction can be fully switched to allow for isolation of the vinylindole products instead. Furthermore, this selective reaction can be utilized to synthesize the more difficult to access unsymmetrical BIMs from readily available starting materials.

Gold-Catalyzed Spirocyclization Reactions of N-Propargyl Tryptamines and Tryptophans in Aqueous Media

N-Propargyl tryptamine and tryptophan derivatives that are readily available from both synthetic and biocatalytic approaches undergo gold-catalyzed dearomative cyclizations in aqueous media to the corresponding spirocyclic derivatives. In addition to the efficiency of the method, operating in aqueous media affords a selective entry to C2-unsubstituted spiroindolenines that have long remained unattainable by Au(I) catalysis. Moderate to excellent yields of the desired spirocyclic products bearing various substituents were obtained.

Gold-Catalyzed Intramolecular Dearomatization Reactions of Indoles for the Synthesis of Spiroindolenines and Spiroindolines

A gold-catalyzed dearomatization reaction of indole derivatives was realized in the presence of JohnPhosAuCl/AgOMs to afford a series of spiroindolenines in excellent yields (≤99%). In addition, when the Hantzsch ester was used as the hydrogen transfer reagent, various spiroindolines were obtained in a cascade fashion starting from readily available indole derivatives in modest to good yields (≤79%). Both reactions feature readily available substrates, mild conditions, and good functional group tolerance.

Gold(I)-Catalyzed Cascade Cyclization of Anilines with Diynes: Controllable Formation of Eight-Membered Ring-Fused Indoles and Propellane-Type Indolines

Heterocycle-fused indoles or indolines are distributed widely in a variety of natural products, bioactive agents, and pharmaceuticals. Herein, we describe the development of gold-catalyzed cascade reactions of anilines with diynes to form eight-membered ring-fused indoles and propellane-type indolines, both of which proceed through an intramolecular 5-endo-dig hydroamination followed by an 8-endo-dig cycloisomerization. Controllable formation of eight-membered ring-fused indoles and propellane-type indolines was achieved through selection of the ligands and/or solvents. Protic solvents such as alcohols or IPr ligand favored the formation of eight-membered ring-fused indoles, whereas the use of Buchwald's type ligands and/or nonpolar solvents gave propellane-type indoline predominantly. This reaction provides rapid access to two types of fused nitrogen heterocycles from simple aniline derivatives.

Gold-catalyzed enantioselective functionalization of indoles

This review documents the potential of enantioselective gold catalysis for the functionalization of indoles.

Tryptoline-based benzothiazoles re-sensitize MRSA to β-lactam antibiotics

Resistance-modifying agents (RMAs) offer a promising solution to combat bacterial antibiotic resistance. Here we report the discovery and structure-activity relationships of a new class of RMAs with a novel tryptoline-based benzothiazole scaffold. Our most potent compound in this series (4ad) re-sensitizes multiple MRSA strains to cephalosporins at low concentrations (2 μg/mL) and has low mammalian cytotoxicity with a half growth inhibitory concentration (GI50) > 100 μg/mL in human cervical carcinoma (HeLa) cells. In addition, the same core scaffold with different substitutions also gives good antibacterial activity against MRSA.

Efficient synthesis and antioxidant activity of novel N-propargyl tetrahydroquinoline derivatives through the cationic Povarov reaction

New N-propargyl tetrahydroquinolines 6a-g have been synthesized efficiently through the cationic Povarov reaction (a domino Mannich/Friedel-Crafts reaction), catalyzed by Indium (III) chloride (InCl3), from the corresponding N-propargylanilines preformed, formaldehyde and N-vinylformamide, with good to moderate yields. All tetrahydroquinoline derivatives obtained were evaluated in vitro as free radical scavengers. Results showed that compound 6c presents a potent antioxidant effect compared with ascorbic acid, used as a reference compound. ADME predictions also revealed favorable pharmacokinetic parameters for the synthesized compounds, which warrant their suitability as potentials antioxidant. Additionally, a theoretical study using Molecular Quantum Similarity and reactivity indices were developed to discriminate different reactive sites in the new molecules in which the oxidative process occurs.

Zn(OTf)2-mediated annulations of N-propargylated tetrahydrocarbolines: divergent synthesis of four distinct alkaloidal scaffolds

Experimental and computational studies gained insights into Zn(OTf)₂-mediated annulations and allowed programmable synthesis of four distinct alkaloidal scaffolds.

Intramolecular hydroarylations of N-propargylated tetrahydrocarbolines were efficiently mediated using a unique combination of Zn(OTf)₂ with t-BuOH under neutral conditions. Use of the artificial force induced reaction method in the global reaction route mapping strategy provided insights into the Zn(OTf)₂-mediated hydroarylations and the associated intriguing solvent effects of t-BuOH facilitating a protodezincation process without a Brønsted acid activator. We systematically implemented three distinct hydroarylations as well as an unanticipated α-alkenylation of a carbonyl group to obtain the four alkaloidal scaffolds 2–4, and 18. Zn(OTf)₂-mediated annulation of 1c proceeded through kinetic formation of the spiroindole 3c followed by an alkenyl shift and concomitant retro-Mannich-type fragmentation to furnish azepino[4,5-b]indole 2 framework. Substituents on substrate 1 in the vicinity of the reaction sites substantially affected the mode of the divergent annulations. Judicious choices of the substituents, solvent and reaction conditions enabled programmable divergent synthesis of the four distinct skeletons.

Where does Au coordinate to N-(2-pyridiyl)benzotriazole: gold-catalyzed chemoselective dehydrogenation and borrowing hydrogen reactions

Pyridyltriazole gold(i) complexes proved to be an efficient precatalyst for the most challenging gold-catalyzed borrowing hydrogen reaction and dehydrogenation of alcohols and amines.

Engaging 2-methyl indolenines in a tandem condensation/1,5-hydride transfer/cyclization process: construction of a novel indolenine–tetrahydroquinoline assembly

An efficient BF₃·Et₂O-catalyzed diastereoselective tandem condensation/1,5-hydride transfer/cyclization strategy for the synthesis of a novel indolenine–tetrahydroquinoline assembly was reported.

Small-Molecule Potentiators for Conventional Antibiotics against Staphylococcus aureus

Antimicrobial resistance constitutes a global health problem, while the discovery and development of novel antibiotics is stagnating. Methicillin-resistant Staphylococcus aureus, responsible for the establishment of recalcitrant, biofilm-related infections, is a well-known and notorious example of a highly resistant micro-organism. Since resistance development is unavoidable with conventional antibiotics that target bacterial viability, it is vital to develop alternative treatment options on top. Strategies aimed at more subtle manipulation of bacterial behavior have recently attracted attention. Here, we provide a literature overview of several small-molecule potentiators for antibiotics, identified for the treatment of Staphylococcus aureus infection. Typically, these potentiators are not bactericidal by themselves and function by reversing resistance mechanisms, by attenuating Staphylococcus aureus virulence, and/or by interfering with quorum sensing.

Enantioselective Tandem Cyclization of Alkyne-Tethered Indoles Using Cooperative Silver(I)/Chiral Phosphoric Acid Catalysis

Reported is the enantioselective synthesis of tetracyclic indolines using silver(I)/chiral phosphoric acid catalysis. A variety of alkyne-tethered indoles are suitable for this process. Mechanistic studies suggest that the in situ generated silver(I) chiral phosphate activates both the alkyne and the indole nucleophile in the initial cyclization step through an intermolecular hydrogen bond and the phosphate anion promotes proton transfer. In addition, further modifications of the cyclization products enabled stereochemistry-function studies of a series of bioactive indolines.

A 7-Step Formal Asymmetric Total Synthesis of Strictamine via an Asymmetric Propargylation and Metal-Mediated Cyclization

Herein is shown how a novel catalytic asymmetric propargylation of 3,4-dihydro-β-carboline, followed by a designed Au(I)/Ag(I)-mediated 6-endo-dig cyclization, can directly deliver the indolenine-fused methanoquinolizidine core of the akuammiline alkaloid strictamine in its native oxidation state, ultimately achieving a 7-step formal asymmetric total synthesis. Also demonstrated are how the cyclization products can rearrange into vincorine-type skeletons and a further use for the developed propargylation with the first catalytic asymmetric total synthesis of decarbomethoxydihydrogambirtannine.

A Tryptoline Ring-Distortion Strategy Leads to Complex and Diverse Biologically Active Molecules from the Indole Alkaloid Yohimbine

High-throughput screening (HTS) is the primary driver to current drug-discovery efforts. New therapeutic agents that enter the market are a direct reflection of the structurally simple compounds that make up screening libraries. Unlike medically relevant natural products (e.g., morphine), small molecules currently being screened have a low fraction of sp³ character and few, if any, stereogenic centers. Although simple compounds have been useful in drugging certain biological targets (e.g., protein kinases), more sophisticated targets (e.g., transcription factors) have largely evaded the discovery of new clinical agents from screening collections. Herein, a tryptoline ring-distortion strategy is described that enables the rapid synthesis of 70 complex and diverse compounds from yohimbine (1); an indole alkaloid. The compounds that were synthesized had architecturally complex and unique scaffolds, unlike 1 and other scaffolds. These compounds were subjected to phenotypic screens and reporter gene assays, leading to the identification of new compounds that possessed various biological activities, including antiproliferative activities against cancer cells with functional hypoxia-inducible factors, nitric oxide inhibition, and inhibition and activation of the antioxidant response element. This tryptoline ring-distortion strategy can begin to address diversity problems in screening libraries, while occupying biologically relevant chemical space in areas critical to human health.

Gold-Catalyzed Regiodivergent [2 + 2 + 2]-Cycloadditions of Allenes with Triazines

Gold-catalyzed regiodivergent cycloadditions of functionalized allenes with 1,3,5-triazines, providing diverse N-heterocycles in moderate to excellent yields under mild reaction conditions, are reported. Importantly, different types of allenes exhibit distinct selectivity and reactivity for the reactions. Mechanistic investigations reveal that all of the cycloadditions proceed through a stepwise [2 + 2 + 2]-cycloaddition process.

Divergent Synthesis of Hydro-γ-Carbolines and Multisubstituted Indoles through Grob Fragmentation/Mannich Cyclization

A distinct strategy for the divergent synthesis of hydro-γ-carbolines and multisubstituted indoles is reported. The stereochemical outcomes and a control experiment indicate that the reactions likely proceed through Grob fragmentation/Mannich cyclization rather than a concerted aza-pinacol rearrangement.

Tetracyclic indolines as a novel class of β-lactam-selective resistance-modifying agent for MRSA

Antibiotic-resistant bacterial infections have seen a marked increase in recent years, while antibiotic discovery has waned. Resistance-modifying agents (RMA) offer an intriguing alternative strategy to fight against resistant bacteria. Here we report the discovery, antibiotic profiling, and structure-activity relationships of a novel class of RMAs, tetracyclic indolines. These selectively potentiate β-lactam antibiotics in methicillin-resistant Staphylococcus aureus (MRSA) without antibacterial or β-lactamase inhibitory activity on their own. The most potent analogue, 6a, showed strong potentiation of amoxicillin/clavulanic acid in a variety of hospital-acquired and community-acquired MRSA strains with low mammalian toxicity. These compounds may be further developed to extend the clinic life span of β-lactam antibiotics.

Property-Guided Synthesis of Aza-Tricyclic Indolines: Development of Gold Catalysis En Route

Antibiotic resistance is a worldwide public health threat that needs to be addressed by improved antibiotic stewardship and continuing development of new chemical entities to treat resistant bacterial infections. Compounds that work alongside known antibiotics as combination therapies offer an efficient and sustainable approach to counteract antibiotic resistance in bacteria. Guided by property-based analysis, a series of aza-tricyclic indolines (ATIs) were synthesized to optimize their physiochemical properties as novel combination therapies with β-lactams to treat methicillin-resistant S. aureus (MRSA) infections. A novel and highly efficient gold-catalyzed tandem cyclization was developed to facilitate the synthesis of these ATIs. One guanidine-containing ATI was discovered to possess both improved anti-MRSA activity and lower mammalian toxicity both in vitro and in vivo. In addition, it also showed significantly enhanced aqueous solubility and metabolic stability. These results indicated that the ATIs are a novel class of anti-MRSA agents suitable for further evaluations as adjuvant therapy in animal model studies.

Formal Total Synthesis of (±)-Strictamine Based on a Gold-Catalyzed Cyclization

A gold-catalyzed cyclization of 1-propargyl-1,2,3,4-tetrahydro-β-carboline led to formation the D-ring of strictamine. Functional group modifications of the resulting tetracyclic indolenine led to the formal total synthesis of (±)-strictamine.

A Study on the Photoreaction of 2(5H)-Furanones with Substituted Acetylenes: Evidence for a Mechanistic Reformulation

The photoreaction of 2(5H)-furanones with alkynes has been investigated. The complexity of this process is evidenced by the variety of isolated products, which have allowed disclosing interesting mechanistic aspects. When the reaction is performed in acetonitrile under direct excitation, in addition to the primary [2+2] cycloadducts, products derived from an 1,3-acyl shift rearrangement are also formed. For unsymmetrical alkynes, the rearrangement of the head-to-tail primary adducts produces new regioisomers and, when the starting furanone is chiral, this rearrangement inverts the relative anti/syn geometry of the primary cycloadducts. In the reactions performed in acetone under photosensitized conditions, rearranged products were never detected, supporting that the 1,3-acyl shift takes place from the singlet excited state S1 of the β,γ-unsaturated lactone. When bis(trimethylsilyl)acetylene is used as the alkyne partner, the major photoproducts are monocyclic bis(trimethylsilyl)lactones.

Cu-catalyzed β-functionalization of saturated ketones with indoles: a one-step synthesis of C3-substituted indoles

One-pot synthesis of β-indolylketones from saturated ketones and indoles was reported, which was useful in the synthesis of numerous heterocycles.