Ru(II)-Catalyzed Site-Selective Hydroxylation of Flavone and Chromone Derivatives: The Importance of the 5-Hydroxyl Motif for the Inhibition of Aurora Kinases

2-Methoxyjuglone, a Promising Bioactive Compound for Pharmaceutical and Agricultural Purposes: A Review

2-Methoxyjuglone, a member of the 1,4-naphthoquinone family, was first obtained through semi-synthesis based on 2-hydroxyjuglone as the precursor in 1966. It has been isolated and identified from different plant species of the Juglandaceae, Sterculiaceae, and Proteaceae families. 2-Methoxyjuglone has been demonstrated to possess a wide range of biological activities, including antitumor, antifungal, and antibacterial activities; in addition, it has been shown to poison fish and inhibit seed germination. These properties make 2-methoxyjuglone a promising bioactive compound for pharmaceutical and agricultural purposes. This review article provides an overview of the current research progress on 2-methoxyjuglone for the first time, with a primary focus on the plant sources, extraction, identification, synthesis, and biological activities associated with this compound for further development.

Regioselective peri-C-H selenylation of aromatic compounds with weakly coordinating ketone groups

A novel and versatile method for peri-C-H selenylation of aromatic compounds bearing ketone groups, including chromones, xanthones, acridinones, quinolinones and naphthoquinones with diselenides under Ru(II) catalysis is presented. Various chromones and diselenides are applicable for this transformation, affording 5-selenyl chromones in a highly regioselective manner in good to excellent yields. This transformation is easy to scale up and the desired products can be further modified. Most importantly, this transformation allows the late-stage selenylation of bioactive compounds. Mechanistic studies show that radicals may be involved in this novel transformation.

Total Synthesis of the Antimycobacterial Natural Product Chlorflavonin and Analogs via a Late-Stage Ruthenium(II)-Catalyzed ortho-C(sp2)-H-Hydroxylation

The continuous, worldwide spread of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis (TB) endanger the World Health Organization’s (WHO) goal to end the global TB pandemic by the year 2035. During the past 50 years, very few new drugs have been approved by medical agencies to treat drug-resistant TB. Therefore, the development of novel antimycobacterial drug candidates to combat the threat of drug-resistant TB is urgent. In this work, we developed and optimized a total synthesis of the antimycobacterial natural flavonoid chlorflavonin by selective ruthenium(II)-catalyzed ortho-C(sp²)-H-hydroxylation of a substituted 3′-methoxyflavonoid skeleton. We extended our methodology to synthesize a small compound library of 14 structural analogs. The new analogs were tested for their antimycobacterial in vitro activity against Mycobacterium tuberculosis (Mtb) and their cytotoxicity against various human cell lines. The most promising new analog bromflavonin exhibited improved antimycobacterial in vitro activity against the virulent H37Rv strain of Mtb (Minimal Inhibitory Concentrations (MIC₉₀) = 0.78 μm). In addition, we determined the chemical and metabolic stability as well as the pK_a values of chlorflavonin and bromflavonin. Furthermore, we established a quantitative structure–activity relationship model using a thermodynamic integration approach. Our computations may be used for suggesting further structural changes to develop improved derivatives.

Synthetic and Computational Study of the Enantioselective [3+2]-Cycloaddition of Chromones with MBH Carbonates

Highly efficient and straightforward access to enantioenriched five-membered ring-fused chromanones is developed via [3+2]-cycloaddition of 3-cyanochromones with Morita-Baylis-Hillman carbonates. Densely functionalized chiral cyclopenta[b]chromanones with three continuous quaternary and tertiary stereogenic carbon centers were obtained in high yields with high ee and dr (≤97% yield, 97% ee, and >20:1 dr). Moreover, density functional theory calculations have been carried out to investigate the mechanism and regio- and diastereoselectivity of the reaction.

Novel CDK9 inhibitor oroxylin A promotes wild-type P53 stability and prevents hepatocellular carcinoma progression by disrupting both MDM2 and SIRT1 signaling

Hepatocellular carcinoma (HCC) is one of the most lethal tumours worldwide. However, the effects of first-line sorafenib treatment in advanced HCC fail to prolong patients' survival due to the highly heterogeneous characteristics of HCC etiology. Cyclin-dependent kinase 9 (CDK9) is an important target in the continuous development of cancer therapy. Here, we demonstrate that CDK9 is closely associated with the progression of HCC and can serve as an HCC therapeutic target by modulating the recovery of wild-type p53 (wt-p53) function. We prove that mouse double minute 2 homologue (MDM2) and Sirtuin 1 (SIRT1) are phosphorylated by CDK9 at Ser166 and Ser47, respectively. Inhibition of CDK9 not only reduces the MDM2-mediated ubiquitination and degradation of wt-p53 but also increases wt-p53 stability by suppressing deacetylase activity of SIRT1. Thus, inhibition of CDK9 promotes the wt-p53 stabilization and prevents HCC progression. However, excessive inhibition by high concentrations of specific CDK9 inhibitors counteracts the promotion of p53 stability and reduces their anti-HCC activity because of extreme general transcription repression. The effects of a novel CDK9 inhibitor named oroxylin A (OA) from Scutellaria baicalensis are explored, with the results indicating that OA shows moderate and controlled inhibition of CDK9 activity and expression, and stabilizes wt-p53 by inhibiting CDK9-regulated MDM2 and SIRT1 signaling. These outcomes indicate the high therapeutic potential of OA against HCC and its low toxicity in normal tissue. This study demonstrates a novel mechanism for the regulation of wt-p53 by CDK9 and indicates that OA is a potential candidate for HCC therapy.

Advances in the site-selective C-5, C-3 and C-2 functionalization of chromones via sp2 C-H activation

In this work, site-selective C-H activation at C-5, C-3 and C-2 positions of chromones for the introduction of structural diversity to the chromone scaffold was studied. The keto group of the chromone moiety acts as the directing group for the selective functionalization of chromones at the C-5 position. Furthermore, the C-H functionalization at the electron-rich C-3 position of the chromone can be achieved using electrophilic coupling partners. The C-H functionalization at the C-2 position can be possible using nucleophilic coupling partners. The direct functionalization methods provide a better pathway for the generation of C-5, C-3 and C-2-substituted chromones with good atom economy than that of classical pre-functionalized reaction protocols.

para-Selective hydroxylation of alkyl aryl ethers

para-Selective hydroxylation of alkyl aryl ethers is established, which proceeds with a ruthenium(II) catalyst, hypervalent iodine(III) and trifluoroacetic anhydride via a radical mechanism. This protocol tolerates a wide scope of substrates and provides a facile and efficient method for preparing clinical drugs monobenzone and pramocaine on a gram scale.

A Chiral-Pool-Based Strategy to Access trans-syn-Fused Drimane Meroterpenoids: Chemoenzymatic Total Syntheses of Polysin, N-Acetyl-polyveoline and the Chrodrimanins

trans-syn-Fused drimane meroterpenoids are unique natural products that arise from contra-thermodynamic polycyclizations of their polyene precursors. Herein we report the first total syntheses of four trans-syn-fused drimane meroterpenoids, namely polysin, N-acetyl-polyveoline, chrodrimanin C, and verruculide A, in 7-18 steps from sclareolide. The trans-syn-fused drimane unit is accessed through an efficient acid-mediated C9 epimerization of sclareolide. Subsequent applications of enzymatic C-H oxidation and contemporary annulation methodologies install the requisite C3 hydroxyl group and enable rapid generation of structural complexity to provide concise access to these natural products.

Palladium-Catalyzed Weakly Coordinating Lactone-Directed C-H Bond Functionalization of 3-Arylcoumarins: Synthesis of Bioactive Coumestan Derivatives

A palladium-catalyzed highly regioselective ortho-selective C-H functionalization of 3-arylcoumarins has been developed. The method utilizes the weakly coordinating lactone as a directing group. The versatility of the strategy is highlighted by developing methodologies for alkenylation, halogenation, fluoroalkoxylation, and hydroxylation. Different functional groups were well tolerated, and functionalized coumarins were obtained in moderate to high yields. The method also showed good selectivity for monofunctionalization versus difunctionalization. The generated ortho-hydroxy derivatives were cyclized in the presence of DDQ, thus developing a simple and fast method for the synthesis of bioactive coumestan from 3-arylcoumarins.

Rhoda-Electrocatalyzed Bimetallic C-H Oxygenation by Weak O-Coordination

Rhodium‐electrocatalyzed arene C−H oxygenation by weakly O‐coordinating amides and ketones have been established by bimetallic electrocatalysis. Likewise, diverse dihydrooxazinones were selectively accessed by the judicious choice of current, enabling twofold C−H functionalization. Detailed mechanistic studies by experiment, mass spectroscopy and cyclovoltammetric analysis provided support for an unprecedented electrooxidation‐induced C−H activation by a bimetallic rhodium catalysis manifold.

Ruthenium(II)-Catalyzed C-H Activation of Chromones with Maleimides to Synthesize Succinimide/Maleimide-Containing Chromones

An efficient route for the coupling of maleimides with chromones at the C5-position has been developed under Ru(II) catalysis. It could provide 1,4-addition products and oxidative Heck-type products by switching additives. Benzoic acid led to the formation of 1,4-addition products under solvent-free conditions, and silver acetate was promoted to the generation of oxidative Heck-type products. Various maleimides and chromones were suitable for this transformation, affording the desired products with good to excellent yields in a short reaction time. To understand the mechanism of this reaction, deuteration studies and control experiments have been performed.

Palladium-catalyzed ortho-selective C–H hydroxylation of carboxybenzyl-protected benzylamines

A convenient approach to synthesize o-hydroxybenzylamine via Pd-catalyzed hydroxylation of benzylamine with a removable Cbz-amide as the directing group was developed.

Directed Hydroxylation of sp2 and sp3 C-H Bonds Using Stoichiometric Amounts of Cu and H2O2

The use of copper for C-H bond functionalization, compared to other metals, is relatively unexplored. Herein, we report a synthetic protocol for the regioselective hydroxylation of sp² and sp³ C-H bonds using a directing group, stoichiometric amounts of Cu and H₂O₂. A wide array of aromatic ketones and aldehydes are oxidized in the carbonyl γ-position with remarkable yields. We also expanded this methodology to hydroxylate the β-position of alkylic ketones. Spectroscopic characterization, kinetics, and density functional theory calculations point toward the involvement of a mononuclear LCu^II(OOH) species, which oxidizes the aromatic sp² C-H bonds via a concerted heterolytic O-O bond cleavage with concomitant electrophilic attack on the arene system.

Divergent total syntheses of five illudalane sesquiterpenes and assignment of the absolute configuration

Concise, divergent total syntheses of five bioactive illudalane sesquiterpenes have been achieved. Our synthesis features an intermolecular [2+2+2] cycloaddition, and a lactone-directed aromatic C-H oxygenation to generate a temporary phenolic hydroxyl group which enables regioselective methylation. Furthermore, the absolute configuration of radulactone was assigned by chemical synthesis.

Catalyst-controlled positional-selectivity in C–H functionalizations

C–H bonds are ubiquitous in organic molecules and typically these bonds are chemically indistinct from each other and it would be highly advantageous for a synthetic chemist to have the ability to choose which C–H bond is functionalized in a given molecule.

Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K₂S₂O₈ as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.

Mixing O-Containing and N-Containing Directing Groups for C-H Activation: A Strategy for the Synthesis of Highly Functionalized 2,2'-Biaryls

A strategy combining O- and N-containing directing groups has been developed for the synthesis of 2,2'-biaryl via Pd-mediated C-H bond activation and oxidative coupling. This new transformation may proceed through a mechanism involving Pd(II) and Pd(IV) intermediates. We found the use of PTSA and HFIP to be critical for the reaction and suggest that these reagents could serve as efficient ligands for this C-C bond formation. This methodology provides broad functional group tolerance, excellent reactivity, and high yields.

Transition metal free regio-selective C–H hydroxylation of chromanones towards the synthesis of hydroxyl-chromanones using PhI(OAc)2 as the oxidant

Herein, we report a highly efficient PhI(OAc)₂ mediated regioselective, direct C–H hydroxylation of chromanones.

Weakly-coordinating N-oxide and carbonyl groups for metal-catalyzed C–H activation: the case of A-ring functionalization

This feature review is focused on recent challenges based on the functionalizations at C-8 and C-5 positions of heterocyclic and quinoidal compounds – a topic that is still rarely explored in the literature.

Transition-metal-catalyzed synthesis of phenols and aryl thiols

Phenols and aryl thiols are fundamental building blocks in organic synthesis and final products with interesting biological activities. Over the past decades, substantial progress has been made in transition-metal-catalyzed coupling reactions, which resulted in the emergence of new methods for the synthesis of phenols and aryl thiols. Aryl halides have been extensively studied as substrates for the synthesis of phenols and aryl thiols. In very recent years, C-H activation represents a powerful strategy for the construction of functionalized phenols directly from various arenes. However, the synthesis of aryl thiols through C-H activation has not been reported. In this review, a brief overview is given of the recent advances in synthetic strategies for both phenols and aryl thiols.

Synthesis of Succinimide-Containing Chromones, Naphthoquinones, and Xanthones under Rh(III) Catalysis: Evaluation of Anticancer Activity

The weakly coordinating ketone group directed C-H functionalizations of chromones, 1,4-naphthoquinones, and xanthones with various maleimides under rhodium(III) catalysis are described. These protocols efficiently provide a range of succinimide-containing chromones, naphthoquinones, and xanthones with excellent site selectivity and functional group compatibility. All synthetic compounds were screened for in vitro anticancer activity against human breast adenocarcinoma cell lines (MCF-7). In particular, compounds 7aa and 7ca with a naphthoquinone scaffold were found to be highly cytotoxic, with an activity competitive with anticancer agent doxorubicin.

Flavonoids as CDK1 Inhibitors: Insights in Their Binding Orientations and Structure-Activity Relationship

In the last years, the interactions of flavonoids with protein kinases (PKs) have been described by using crystallographic experiments. Interestingly, different orientations have been found for one flavonoid inside different PKs and different chemical substitutions lead to different orientations of the flavonoid scaffold inside one PK. Accordingly, orientation predictions of novel analogues could help to the design of flavonoids with high PK inhibitory activities. With this in mind, we studied the binding modes of 37 flavonoids (flavones and chalcones) inside the cyclin-dependent PK CDK1 using docking experiments. We found that the compounds under study adopted two different orientations into the active site of CDK1 (orientations I and II in the manuscript). In addition, quantitative structure-activity relationship (QSAR) models using CoMFA and CoMSIA methodologies were constructed to explain the trend of the CDK1 inhibitory activities for the studied flavonoids. Template-based and docking-based alignments were used. Models developed starting from docking-based alignment were applied for describing the whole dataset and compounds with orientation I. Adequate R2 and Q2 values were obtained by each method; interestingly, only hydrophobic and hydrogen bond donor fields describe the differential potency of the flavonoids as CDK1 inhibitors for both defined alignments and subsets. Our current application of docking and QSAR together reveals important elements to be drawn for the design of novel flavonoids with increased PK inhibitory activities.

Rh-catalyzed direct synthesis of 2,2′-dihydroxybenzophenones and xanthones

An efficient rhodium-catalyzed direct synthesis of 2,2′-dihydroxybenzophenones and xanthones was developed from functionalized salicylaldehydes.