Computational and Experimental Studies of Phthaloyl Peroxide-Mediated Hydroxylation of Arenes Yield a More Reactive Derivative, 4,5-Dichlorophthaloyl Peroxide

Tailoring photocatalysts to modulate oxidative potential of anilides enhances para-selective electrochemical hydroxylation

Phenolic compounds have long captivated the interest of organic synthesis, particularly in their quest for selective hydroxylation of arenes using H2O as a hydroxyl source. However, the inherent high reactivity and low redox potential of phenols often lead to undesirable overoxidation byproducts. To address this challenge, herein, we develop an electrophotochemical approach, finetuning substrate oxidative potential and enabling para-selective hydroxylation of anilides. This method showcases versatility, accommodating a wide array of substrates, while revealing high regional selectivity and compatibility with diverse functional groups. Moreover, the protocol allows facile late-stage functionalization of biologically active molecules. Mechanistic investigations demonstrate the activation of anilides by the excited state photocatalyst, effectively decreasing their oxidative potential and enhancing regional selectivity during hydroxylation. By using this protocol, important drug molecules such as Paracetamol, Fenretinide, Practolol, and AM404 could be synthesized, demonstrating the applicability of this approach in drug synthesis and late-stage functionalization.

Deoxyfluorination of phenols for chemoselective 18F-labeling of peptides

The challenge of forming C-18F bonds is often a bottleneck in the development of new 18F-labeled tracer molecules for noninvasive functional imaging studies using positron emission tomography (PET). Nucleophilic aromatic substitution is the most widely employed reaction to functionalize aromatic substrates with the radioactive fluorine-18 but its scope is restricted to arenes containing electron-withdrawing substituents. Furthermore, many protic functional groups are incompatible with basic fluoride anions. Peptide substrates, which are highly desirable targets for PET molecular imaging, are particularly challenging to label with fluorine-18 because they are densely functionalized and sensitive to high temperatures and basic conditions. To expand the utility of nucleophilic aromatic substitution with fluorine-18, we describe two complementary procedures for the radiodeoxyfluorination of bench-stable and easy-to-access phenols that ensure rapid access to densely functionalized electron-rich and electron-poor 18F-aryl fluorides. The first procedure details the synthesis of an 18F-synthon and its subsequent ligation to the cysteine residue of Arg-Gly-Asp-Cys in 10.5 h from commercially available starting materials (189-min radiosynthesis). The second procedure describes the incorporation of commercially available CpRu(Fmoc-tyrosine)OTf into a fully protected peptide Lys-Met-Glu-(CpRu-Tyr)-Leu via solid-phase peptide synthesis and subsequent ruthenium-mediated uronium deoxyfluorination with fluorine-18 followed by deprotection, accomplished within 7 d (116-min radiosynthesis). Both radiolabeling methods are highly chemoselective and have conveniently been automated using commercially available radiosynthesis equipment so that the procedures described can be employed for the synthesis of peptide-based PET probes for in vivo imaging studies according to as low as reasonably achievable (ALARA) principles.

Concise Total Syntheses of the 6-7-5 Hamigeran Natural Products

Herein, we report the total syntheses of four hamigeran natural products featuring a 6-7-5 tricyclic carbon skeleton. We utilized a palladium-catalyzed intramolecular cyclopropanol ring opening cross-coupling to build the central seven-membered ring and a series of oxidations including a challenging aromatic C-H oxidation to introduce the peripheral functionalities. This approach enabled us to achieve the first total syntheses of hamigeran C (14 steps), debromohamigeran I (12 steps), and hamigeran I (13 steps). Our synthesis also resulted in hamigeran G in 13 steps, which is significantly shorter than the previously reported one (24 steps, longest linear sequence).

4,4′-(Butane-1,4-diyl)bis(4-methyl-1,2-dioxolane-3,5-dione)

Over the past decades, studies of cyclic diacyl peroxides have shown superior or even fundamentally new reactivity compared to their acyclic counterparts in various reactions. Previously, the scope of cyclic diacyl peroxides was limited to the mono peroxy compounds. The first doubled cyclic diacyl peroxide is presented herein. The diperoxide was characterized by NMR spectroscopy, mass spectrometry, and IR spectroscopy. The structure of 4,4′-(butane-1,4-diyl)bis(4-methyl-1,2-dioxolane-3,5-dione) was confirmed by X-ray diffraction analysis. The novel diperoxide was prepared in a 55% overall yield in three steps from dibromobutane and diethyl methylmalonate.

Activation of O-Electrophiles via Structural and Solvent Effects: SN2@O Reaction of Cyclic Diacyl Peroxides with Enol Acetates

The reactions of O-electrophiles, such as organic peroxides, with carbon nucleophiles are an umpolung alternative to the common approaches to C-O bond formation. Nucleophilic substitution at the oxygen atom of cyclic diacyl peroxides by enol acetates with the following deacylation leads to α-acyloxyketones with an appended carboxylic acid in 28-87% yields. The effect of fluorinated alcohols on the oxidative functionalization of enol acetates by cyclic diacyl peroxides was studied experimentally and computationally. Computational analysis reveals that the key step proceeds as a direct substitution nucleophilic bimolecular (S_N2) reaction at oxygen (S_N2@O). CF₃CH₂OH has a dual role in assisting in both steps of the reaction cascade: it lowers the energy of the S_N2@O activation step by hydrogen bonding to a remote carbonyl and promotes the deacylation of the cationic intermediate.

Electrochemical aromatic C-H hydroxylation in continuous flow

The direct hydroxylation of arene C-H bonds is a highly sought-after transformation but remains an unsolved challenge due to the difficulty in efficient and regioselective C-H oxygenation and high reactivity of the phenolic products leading to overoxidation. Herein we report electrochemical C-H hydroxylation of arenes in continuous flow for the synthesis of phenols. The method is characterized by broad scope (compatible with arenes of diverse electronic properties), mild conditions without any catalysts or chemical oxidants, and excellent scalability as demonstrated by the continuous production of 1 mol (204 grams) of one of the phenol products.

C-H Acyloxylation of Polycyclic Aromatic Hydrocarbons

The C-H acyloxylation of polycyclic aromatic hydrocarbons (PAHs) is described. This reaction constructs aryl acyloxylate scaffolds from PAHs with equimolar hypervalent iodine compounds under mild reaction conditions. Interestingly, the blue light irradiation accelerated this transformation. Additionally, the synthesis of structurally new symmetric and unsymmetric diaroyloxylated fluoranthenes was accomplished with a ruthenium photoredox catalyst.

Direct Acetoxylation of Arenes

Acetoxylation of arenes is an important reaction and an unmet need in chemistry. We report a metal-free, direct acetoxylation reaction using sodium nitrate under an anhydrous environment of trifluoroacetic acid, acetic acid, and acetic anhydride. Arenes (31 examples), with oxidation potentials (E_ox, in V vs SCE) lower than benzene (2.48 V), were acetoxylated with good yields and regioselectivity. A stepwise, single electron-transfer mechanism is proposed.

Iron-catalyzed arene C-H hydroxylation

[Figure: see text].

Development of a Convergent Enantioselective Synthetic Route to (-)-Myrocin G

Myrocins are a family of antiproliferative antibiotic fungal metabolites possessing a masked electrophilic cyclopropane. Preliminary chemical reactivity studies imputed the bioactivity of these natural products to a DNA cross-linking mechanism, but this hypothesis was not confirmed by studies with native DNA. We recently reported a total synthesis of (-)-myrocin G (4), the putative active form of the metabolite myrocin C (1), that featured a carefully orchestrated tandem fragment coupling-annulation cascade. Herein, we describe the evolution of our synthetic strategy toward 4 and report the series of discoveries that prompted the design of this cascade coupling. Efforts to convert the diosphenol (-)-myrocin G (4) to the corresponding 5-hydroxy-γ-lactone isomer myrocin C (1) are also detailed. We present a preliminary evaluation of the antiproliferative activities of (-)-myrocin G (4) and related structures, as well as DNA cross-linking studies. These studies indicate that myrocins do not cross-link DNA, suggesting an alternative mode of action potentially involving a protein target.

Site-Selective C-H Oxygenation via Aryl Sulfonium Salts

Herein, we report a two-step process forming arene C-O bonds in excellent site-selectivity at a late-stage. The C-O bond formation is achieved by selective introduction of a thianthrenium group, which is then converted into C-O bonds using photoredox chemistry. Electron-rich, -poor and -neutral arenes as well as complex drug-like small molecules are successfully transformed into both phenols and various ethers. The sequence differs conceptually from all previous arene oxygenation reactions in that oxygen functionality can be incorporated into complex small molecules at a late stage site-selectively, which has not been shown via aryl halides.

Late-Stage Aromatic C-H Oxygenation

Synthetic methods for oxidative aromatic C-O bond formation are sparse, despite their demand in metabolite synthesis for drug discovery and development. We report a novel methodology for late-stage C-O bond formation of arenes. The reaction proceeds with excellent functional group tolerance even for highly functionalized substrates. The resulting aryl mesylates provide access to potential human metabolites of pharmaceuticals, and may be used directly to install a C-F bond to block metabolic hotspots. A charge-transfer interaction between the reagent bis(methanesulfonyl) peroxide and the substrate arenes may be relevant for the chemoselective functionalization of arenes over other functional groups.

Bioactive Phenolic Components from the Twigs of Atalantia buxifolia

Five new compounds named buxifoximes A-C (1-3), buxifobenzoate (4), and 7- O-(7'-peroxygeranyl) coumarin (5), together with 25 known compounds, were identified from the twigs of Atalantia buxifolia. Compounds 1-3 are unique secondary metabolites with the aldoxime functionality. The structures of the isolates were determined on the basis of spectroscopic data analyses, and the structure of 1 was confirmed by an X-ray single-crystallographic analysis. With respect to bioactivity, antidengue virus, anti-inflammatory, and cytotoxic activities of all purified compounds were tested and evaluated. Compound 1 showed a significant anti-inflammatory effect by inhibiting superoxide anion generation with an IC₅₀ value of 4.8 ± 0.7 μM. Among the acridone alkaloids, 5-hydroxy- N-methylseverifoline (23) exhibited antidengue activity (IC₅₀ = 5.3 ± 0.4 μM), and atalaphyllinine (20) demonstrated cytotoxicity (IC₅₀ = 6.5 ± 0.0 μM) against the human liver cancer cell line, HepG2.

Photoredox radical C–H oxygenation of aromatics with aroyloxylutidinium salts

The peroxide-free aroyloxylation of aromatics with N-aroyloxylutidinium salts by photoredox catalysis has been developed.

Silica gel mediated oxidative C–O coupling of β-dicarbonyl compounds with malonyl peroxides in solvent-free conditions

For the first time silica gel was observed to activate peroxides in oxidative coupling reactions. Here we report silica gel mediated oxidative C–O coupling of β-dicarbonyl compounds with cyclic diacyl peroxides affording α-acyloxy derivatives with 100% atom efficiency. The highest yields of coupling products were achieved in solvent free conditions. C–O coupling products were prepared in yields up to 86%.

Stereoselective Synthesis of Alkylidene Phthalides

The N,O-diacylhydroxylamine derivative 4 has been prepared and its reactivity with nucleophiles investigated. On reaction with lithium enolates of cyclic or acyclic ketones, 4 is converted stereoselectively to the corresponding alkylidene phthalide. The stereochemical outcome of the transformation can be modified by changing the polarity of the reaction medium and the products isomerized under acidic conditions.