Oxidative De-aromatization ofpara-Alkyl Phenols intopara-Peroxyquinols andpara-Quinols Mediated by Oxone as a Source of Singlet Oxygen

The Kornblum DeLaMare rearrangement in natural product synthesis: 25 years of innovation

Covering: 1998 up to the end of 2023Since its initial disclosure in 1951, the Kornblum DeLaMare rearrangement has proved an important synthetic transformation and has been widely adopted as a biomimetic step in natural product synthesis. Utilising the base catalysed decomposition of alkyl peroxides to yield a ketone and alcohol has found use in many syntheses as well as a key strategic step, including the unmasking of furans, as a biomimetic synthetic tool, and the use of the rearrangement to install oxygen enantioselectively. Since ca. 1998, its impact as a synthetic transformation has grown significantly, especially given the frequency of use in natural product syntheses, therefore this 25 year time period will be the focus of the review.

A Green Blue LED-Driven Two-Liquid-Phase One-Pot Procedure for the Synthesis of Estrogen-Related Quinol Prodrugs

Quinol derivatives of estrogens are effective pro-drugs in steroid replacement therapy. Here, we report that these compounds can be synthesized in one-pot conditions and high yield by blue LED-driven photo-oxygenation of parent estrogens. The oxidation was performed in buffer and eco-certified 2-methyltetrahydrofuran as the two-liquid-phase reaction solvent, and in the presence of meso-tetraphenyl porphyrin as the photosensitizer. Two steroidal prodrugs 10β, 17β-dihydroxyestra-1,4-dien-3-one (DHED) and 10β-Hydroxyestra-1,4-diene-3,17-dione (HEDD) were obtained with high yield and selectivity.

Photocatalytic Synthesis of para-Peroxyquinols: Total Synthesis of (±)-Stemenone B and (±)-Parvistilbine B

A photocatalytic method to selectively synthesize 4-hydroperoxy-2,5-cyclohexadienones from para-alkyl phenols is disclosed. This photosensitized singlet oxygen approach functionalized a variety of electronically diverse para-alkyl phenols in 27-99% isolated yields. Utilizing this dearomative oxidation, (±)-stemenone B and (±)-parvistilbine B were synthesized in 9 and 11 steps, respectively, from commercially available starting materials. Additional experiments revealed the dramatic influence of base and solvent on the selectivity while providing insight into the mechanism of this transformation.

A One-Pot Approach for Bio-Based Arylamines via a Combined Photooxidative Dearomatization-Rearomatization Strategy

The synthesis of arylamines from renewable resources under mild reaction conditions is highly desired for the sustainability of the chemical industry, where the production of hazardous waste is a prime concern. However, to date, there are very few tools in chemists' toolboxes that are able to produce arylamines in a sustainable manner. Herein, a robust one-pot approach for constructing bio-based arylamines via a combined photooxidative dearomatization-rearomatization strategy is presented. The developed methodology enables the synthesis of structurally complex amines in moderate-to-good isolated yields using biomass-derived phenols, natural α-amino acids, and naphthols under remarkably mild reaction conditions. For the photooxygenation of phenols, a novel chrysazine-based catalyst system was introduced, demonstrating its efficiency for the synthesis of natural products - hallerone, rengyolone, and the pharmaceutically relevant prodrug DHED.

Telescoped Continuous Flow Synthesis of 2-Substituted 1,4-Benzoquinones via Oxidative Dearomatisation of para-Substituted Phenols Using Singlet Oxygen in Supercritical CO2

This paper describes a continuous multi-step synthesis in supercritical CO2. A continuous flow synthesis of 2-substituted 1,4-benzoquinones is reported, and details of the high-pressure reactors are given. This proceeds via the telescoped dearomatisation of p-substituted phenols using singlet oxygen in supercritical CO2 and an acid-mediated C–C migration. The process has a short residence time of 30 minutes, with overall yields and projected productivities of up to 83% and 9 g/day, respectively. This methodology enables a safe and efficient synthesis of 2-substituted 1,4-benzoquinones from photo-generated singlet oxygen, and cheap and readily available p-substituted phenols. The procedure has high atom efficiency, low photocatalyst loading, and substitutes potentially hazardous and corrosive reagents and solvents for molecular oxygen, CO2, and the less hazardous solid-supported acid Amberlyst-15.

Dioxiranes: A Half-Century Journey

Dioxiranes are multi-tasking reagents inheriting mild and selective oxygen transfer attributes. These oxidants are accessed from the reaction of ketones with an oxidant and are employed stoichiometrically or catalytically (in...

Iron salt-promoted oxidation of steroidal phenols by m-chloroperbenzoic acid: a route to possible antitumor agents

A new oxidant, containing m-chloroperoxybenzoic acid (MCPBA) and an iron salt, was developed and used for oxidation of steroidal phenols to a quinol/epoxyquinol mixture. Reaction was optimized for estrone, by varying initiators (Fe-salts), reaction temperature, time and mode of MCPBA application. A series of five more substrates (17β-estradiol and its hydrophobized derivatives) was subjected to the optimized oxidation, providing corresponding p-quinols and 4β,5β-epoxyquinols in good to moderate yields. The obtained epoxyquinols were additionally transformed by oxidation, as well as the acid-catalyzed oxirane opening. In a preliminary study of the antiproliferative activity against human cancer cell lines, all newly synthesized compounds expressed moderate to high activity.

Iron salt-promoted reaction of estrone and its derivatives with meta-chloroperoxybenzoic acid was developed and epoxyquinols were further transformed. Most compounds showed in vitro antiproliferative activity.

Atom Economical Photocatalytic Oxidation of Phenols and Site-Selective Epoxidation Toward Epoxyquinols

The discovery of a multiple-bond-forming process merging the singlet oxygen-mediated dearomatization of 3,4-disubstitued phenols and diastereo- and regioselective epoxidation is described. This one-pot strategy using a transition metal-free multicatalytic system comprised of rose bengal and cesium carbonate allowed the efficient formation of functionalized epoxyquinol products under mild conditions. Mechanistic investigations have been performed to shed the light on the key species involved in this transformation.

Organophotocatalytic Aerobic Oxygenation of Phenols in a Visible-Light Continuous-Flow Photoreactor

A mild photocatalytic phenol oxygenation enabled by a continuous-flow photoreactor using visible light and pressurized air is described herein. Products for wide-ranging applications, including the synthesis of vitamins, were obtained in high yields by precisely controlling principal process parameters. The reactor design permits low organophotocatalyst loadings to generate singlet oxygen. It is anticipated that the efficient aerobic phenol oxygenation to benzoquinones and p-quinols contributes to sustainable synthesis.

Singlet Oxygen Generation from a Water-Soluble Hypervalent Iodine(V) Reagent AIBX and H2O2: An Access to Artemisinin

Herein, we report an efficient method for the chemical generation of ¹O₂ by treatment of H₂O₂ with AIBX, a highly water-soluble, bench-stable, recyclable hypervalent iodine(V) reagent developed by our group. The generation of ¹O₂ was confirmed by the following results: (1) capture of ¹O₂ with the sodium salt of anthracene-9,10-bis(ethanesulfonate) produced the corresponding endoperoxide and (2) TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) produced by the oxidation of 2,2,6,6-tetramethylpiperidine with ¹O₂ generated using the AIBX/H₂O₂ system was detected by electron spin resonance spectroscopy. To illustrate the potential utility of this method for organic synthesis, we used the AIBX/H₂O₂ system to perform typical reactions of ¹O₂: [2 + 2]/[4 + 2] cycloadditions, Schenck ene reactions, and heteroatom oxidation reactions, which afforded the corresponding products in high yields. Moreover, we used the method to synthesize the antimalarial drug artemisinin. Finally, we demonstrated that AIBX could be regenerated after the reaction by means of a workup involving extraction and removal of water to obtain a precursor of AIBX, which could then be re-oxidized.

Selective Extraction of Bioactive Phenylethanoids from Digitalis obscura

Cardenolide-free extracts from Digitalis obscura showed significant antifeedant effects against the aphid Myzus persicae and this activity correlated with their phenylethanoid content. The content in phenylethanoids of Digitalis obscura has been studied. Maceration of the aerial parts of D. obscura was used for the selective extraction of the natural compound rengyolone (1) and the aglycone of cornoside (compound 3). Pure rengyolone (1) can be obtained from D. obscura in approximately 90% purity from fresh plant from the CHCl3 soluble fraction of the ethanolic extract (0.8% yield). The ethanol extraction of freshly collected D. obscura showed the presence of compound 3 as the only phenylethanoid. Compound 3 was proven to easily evolve to rengyolone. Due to this instability, and although its presence in plants has been previously reported, the spectroscopical data of 3 are reported herein for the first time. Selective mono-acetylation of compound of 3 led to the active natural compound hallerone (5). The aphid antifeedant (against Myzus persicae) and nematicidal (against root-knot nematode Meloidogyne javanica) activities of these compounds have been evaluated. Here we report for the first time on the aphid antifeedant effects of 1, 3, and 5. Additionally, the nematicidal activity of hallerone (5) is described here for the first time.

Microwave-assisted green synthesis of bile acid derivatives and evaluation of glucocorticoid receptor binding

Herein, we present microwave-assisted AlCl3 catalyzed oxidation of bile acid hydroxyl groups in the presence of Oxone® in water media. Significant rate enhancements were observed for Wolff-Kishner reduction of synthesized bile acids oxo derivatives to the 5β-cholanic acid. Reaction of amidation of the simplest bile acid and aminolysis of the deoxycholic acid was accomplished in the absence of solvent and catalysts under sealed vessel microwave conditions. Because 5β-cholanic acid reportedly modulates glucocorticoid receptor signaling in cell models of Parkinson's disease, we tested the affinity of 5β-cholanic acid and deoxycholic acid derivatives for the glucocorticoid receptor in vitro using a yeast-based fluorescent screen. Treatment of GR-expressing yeast with prednisolone resulted in a dose-dependent increase in fluorescence; whereas 5β-cholanic acid binds to the glucocorticoid receptor with more moderate affinity. Similarly, molecular docking also suggests that 5β-cholanic acid can bind to the glucocorticoid receptor, with similar geometry to known GR ligands.

Synthetic Study on Acremoxanthone A, Part 2: Model Study on the EFG Xanthone Moiety through a Nitrile Oxide Cycloaddition–SNAr Sequence

Toward a total synthesis of acremoxanthone A, we report a model study on the construction of the EFG ring system. The key steps include (1) an intermolecular 1,3-dipolar cycloaddition of an aryl nitrile oxide with a dienone, and (2) an SNAr reaction for construction of the F ring.

Metal-free visible-light-induced aerobic oxidation of α-diazoesters leading to α-ketoesters in air

A metal-free and visible-light-induced strategy has been established for the construction of α-ketoesters via aerobic oxidation of α-diazoesters with dioxygen in air at room temperature.

Hydroxyl group-directed, tartaric acid-catalyzed synthesis of meta-functionalized aryl ethers and phenols through domino conjugate addition/aromatization of para-quinols

A tartaric acid-catalyzed three-component reaction of para-quinols, organoboronic acids, and alcohols affords meta-alkenylated aryl alkyl ether in good yields. In the absence of alcohols, meta-functionalized phenols could be obtained selectively.

Oxidative Photocatalytic Homo- and Cross-Coupling of Phenols: Nonenzymatic, Catalytic Method for Coupling Tyrosine

The oxidative photocatalytic method for phenol-phenol homo-coupling and cross-coupling is described and isolated yields of 16-97% are obtained. Measured oxidation potentials and computed nucleophilicity parameters support a mechanism of nucleophilic attack of one partner onto the oxidized neutral radical form of the other partner. Understanding of this model permitted development of cross-coupling reactions between nucleophilic phenols/arenes and easily oxidized phenols with high selectivity and efficiency. A highlight of this method is that one equivalent of each coupling partner is utilized. Building on these findings, a non-enzymatic, catalytic method for coupling tyrosine was also developed.

One-Pot Synthesis of 1-Hydroxyacridones from para-Quinols and ortho-Methoxycarbonylaryl Isocyanates

A variety of substituted acridones were synthesized via a one-pot, metal-free cascade reaction. In this event, the DBU-mediated addition between quinols and ortho-methoxycarbonylaryl isocyanates formed a bicyclic oxazolidinone, followed by a sequence of intramolecular condensation, tautomerization, and decarboxylation, which led to the formation of acridones. The acridones showed mild activity against the human cytomegalovirus.

Enantioselective Kinetic Resolution/Desymmetrization of Para-Quinols: A Case Study in Boronic-Acid-Directed Phosphoric Acid Catalysis

A chiral phosphoric acid-catalyzed kinetic resolution and desymmetrization of para-quinols operating via oxa-Michael addition was developed and subsequently subjected to mechanistic study. Good to excellent s-factors/enantioselectivities were obtained over a broad range of substrates. Kinetic studies were performed, and DFT studies favor a hydrogen bonding activation mode. The mechanistic studies provide insights to previously reported chiral anion phase transfer reactions involving chiral phosphate catalysts in combination with boronic acids.

XIAP as a Target of New Small Organic Natural Molecules Inducing Human Cancer Cell Death

X-linked inhibitor of apoptosis protein (XIAP) is an emerging crucial therapeutic target in cancer. We report on the discovery and characterisation of small organic molecules from Piper genus plants exhibiting XIAP antagonism, namely erioquinol, a quinol substituted in the 4-position with an alkenyl group and the alkenylphenols eriopodols A–C. Another isolated compound was originally identified as gibbilimbol B. Erioquinol was the most potent inhibitor of human cancer cell viability when compared with gibbilimbol B and eriopodol A was listed as intermediate. Gibbilimbol B and eriopodol A induced apoptosis through mitochondrial permeabilisation and caspase activation while erioquinol acted on cell fate via caspase-independent/non-apoptotic mechanisms, likely involving mitochondrial dysfunctions and aberrant generation of reactive oxygen species. In silico modelling and molecular approaches suggested that all molecules inhibit XIAP by binding to XIAP-baculoviral IAP repeat domain. This demonstrates a novel aspect of XIAP as a key determinant of tumour control, at the molecular crossroad of caspase-dependent/independent cell death pathway and indicates molecular aspects to develop tumour-effective XIAP antagonists.

Multicatalytic dearomatization of phenols into epoxyquinols via a photooxygenation process

A multicatalytic photooxygenation of substituted phenols in the presence of rose bengal and cesium carbonate under green LED light is reported. This transformation enabled the introduction of both atoms of singlet oxygen and led to the one-pot synthesis of epoxyquinols in a stereoselective way.

An Enzymatic Route to α-Tocopherol Synthons: Aromatic Hydroxylation of Pseudocumene and Mesitylene with P450 BM3

Aromatic hydroxylation of pseudocumene (1 a) and mesitylene (1 b) with P450 BM3 yields key phenolic building blocks for α-tocopherol synthesis. The P450 BM3 wild-type (WT) catalyzed selective aromatic hydroxylation of 1 b (94 %), whereas 1 a was hydroxylated to a large extent on benzylic positions (46-64 %). Site-saturation mutagenesis generated a new P450 BM3 mutant, herein named "variant M3" (R47S, Y51W, A330F, I401M), with significantly increased coupling efficiency (3- to 8-fold) and activity (75- to 230-fold) for the conversion of 1 a and 1 b. Additional π-π interactions introduced by mutation A330F improved not only productivity and coupling efficiency, but also selectivity toward aromatic hydroxylation of 1 a (61 to 75 %). Under continuous nicotinamide adenine dinucleotide phosphate recycling, the novel P450 BM3 variant M3 was able to produce the key tocopherol precursor trimethylhydroquinone (3 a; 35 % selectivity; 0.18 mg mL^-1 ) directly from 1 a. In the case of 1 b, overoxidation leads to dearomatization and the formation of a valuable p-quinol synthon that can directly serve as an educt for the synthesis of 3 a. Detailed product pattern analysis, substrate docking, and mechanistic considerations support the hypothesis that 1 a binds in an inverted orientation in the active site of P450 BM3 WT, relative to P450 BM3 variant M3, to allow this change in chemoselectivity. This study provides an enzymatic route to key phenolic synthons for α-tocopherols and the first catalytic and mechanistic insights into direct aromatic hydroxylation and dearomatization of trimethylbenzenes with O₂ .

Novel series of 1,2,4-trioxane derivatives as antimalarial agents

Among three series of 1,2,4-trioxane derivatives, five compounds showed good in vitro antimalarial activity, three compounds of which exhibited better activity against P. falciparum resistant (RKL9) strain than the sensitive (3D7) one. Two best compounds were one from aryl series and the other from heteroaryl series with IC₅₀ values of 1.24 µM and 1.24 µM and 1.06 µM and 1.17 µM, against sensitive and resistant strains, respectively. Further, trioxane derivatives exhibited good binding affinity for the P. falciparum cysteine protease falcipain 2 receptor (PDB id: 3BPF) with well defined drug-like and pharmacokinetic properties based on Lipinski's rule of five with additional physicochemical and ADMET parameters. In view of having antimalarial potential, 1,2,4-trioxane derivative(s) reported herein may be useful as novel antimalarial lead(s) in the discovery and development of future antimalarial drug candidates as P. falciparum falcipain 2 inhibitors against resistant malaria.

"Dark" Singlet Oxygen and Electron Paramagnetic Resonance Spin Trapping as Convenient Tools to Assess Photolytic Drug Degradation

Forced degradation studies are an important tool for a systematic assessment of decomposition pathways and identification of reactive sites in active pharmaceutical ingredients (APIs). Two methodologies have been combined in order to provide a deeper understanding of singlet oxygen-related degradation pathways of APIs under light irradiation. First, we report that a "dark" singlet oxygen test enables the investigation of drug reactivity toward singlet oxygen independently of photolytic irradiation processes. Second, the photosensitizing properties of the API producing the singlet oxygen was proven and quantified by spin trapping and electron paramagnetic resonance analysis. A combination of these techniques is an interesting addition to the forced degradation portfolio as it can be used for (1) revealing unexpected degradation pathways of APIs due to singlet oxygen, (2) clarifying photolytic drug-drug interactions in fixed-dose combinations, and (3) synthesizing larger quantities of hardly accessible oxidative drug degradants.

Synthesis of o-Carboxyarylacrylic Acids by Room Temperature Oxidative Cleavage of Hydroxynaphthalenes and Higher Aromatics with Oxone

A simple procedure for the synthesis of a variety of o-carboxyarylacrylic acids has been developed with Oxone (2KHSO5·KHSO4·K2SO4); the oxidation reaction involves the stirring of methoxy/hydroxy-substituted naphthalenes, phenanthrenes, anthracenes, etc. with Oxone in an acetonitrile-water mixture (1:1, v/v) at rt. Mechanistically, the reaction proceeds via initial oxidation of naphthalene to o-quinone, which undergoes cleavage to the corresponding o-carboxyarylacrylic acid. The higher aromatics are found to yield carboxymethyl lactones derived from the initially formed o-carboxyarylacrylic acids.