Photoreductive Removal of O-Benzyl Groups from Oxyarene N-Heterocycles Assisted by O-Pyridine-pyridone Tautomerism

Five new glycosides from the culms of Phyllostachys nigra var. henonis

Five new glycosides, namely methyl 3-methoxybenzoate-4,5-di-O-β-D-glucopyranoside (1), (1aS,3aS,3R)-3-(4'-O-β-D-glucopyranosyl-3'-methoxyphenyl)-5,6-dioxa-bicyclo[3.3.0]octane-1-one (2), quinolin-4(1H)-one-3-O-β-D-glucopyranoside (3), 3-methoxy-propiophenone 4-O-(6'-β-D-xylopyranosyl)-β-D-glucopyranoside (4), methyl 3-methoxybenzoate 4-O-(6'-β-D-xylopyranosyl)-β-D-glucopyranoside (5), and one known compound, bambulignan B (6) were isolated from the culms of Phyllostachys nigra var. henonis. Their structures were determined using spectroscopic analysis. All compounds were evaluated for their DPPH radical scavenging activity. Compound 6 exhibited antioxidant activity with IC50 value of 59.5 μM (positive control, L-ascorbic acid, IC50 = 12.4 μM; 2,6-ditertbutyl-4-methyl phenol, IC50 = 11.8 μM).

Efficient destruction of basic organo-nitrogenous compounds in liquid hydrocarbon fuel using ascorbic acid/H2O2 system under ambient condition

Due to the limitations of the conventional refinery methods, development of a new method such as oxidative denitrogenation (ODN) is highly desirable. This study described a novel ODN to remove organo-nitrogenous compounds (ONCs) in liquid fuel by ascorbic acid (AscH2) and H2O2 redox system under ambient conditions. Seven ONCs including pyridine, quinoline, acridine, 7,8-benzoquinoline, indole, N-methylpyrrolidone (NMP), and N,N-dimethylformamide (DMF) were chosen to assess the fuel-denitrified ability of the AscH2/H2O2 system. The results showed that the basic group of ONCs (pyridine, quinoline, and acridine) can be effectively removed (removal ratio > 95 %) while the removal efficiency of water-soluble compounds (7,8-benzoquinoline, NMP, and DMF) was moderate (61-68 %) under a mild temperature (30 °C) and atmospheric pressure. Free radical quenching and electron paramagnetic resonance experiments confirmed that hydroxyl and AscH2 radicals played a major role in the degradation of ONCs. The degraded products of quinoline were analyzed by gas chromatography-mass spectroscopy and ion chromatography. Based on the identified intermediate products, a putative reaction pathway majorly involving three steps of N-onium formation, transfer hydrogenation, and free radical oxidative ring-opening was suggested for the quinoline degradation. The presented approach can be performed at a normal temperature and pressure and will live up to expectations in the pre-denitrogenation and selective removal of basic ONCs in fuel oils.

Photoinduced Borylation of the Inert C(sp3)-O Bond of Alkyl Heteroaryl Ethers

A photoinduced reductive Calkyl-O borylation of alkyl heteroaryl ethers with very negative reduction potential in the presence of 4-dimethylaminopyridine (DMAP) and bis(catecholato)diborane(B2cat2) was developed. Despite the high reducing power, various substrates with liable functional groups were well-tolerated as well as ethers derived from natural products and medicinal-relevant compounds. Mechanistic investigation implied that an intra-single electron transfer process in an electron donor-acceptor complex formed from ethers with the adduct of B2cat2 and DMAP should be involved.

Nucleophilic Dearomatization Strategy to Synthesize Disubstituted 3-Isoquinolinones under Transition Metal-Free Conditions

Herein, a one-pot protocol for constructing the disubstituted isoquinolinone derivatives via the three-component reactions of 3-haloisoquinolines, alkyl halides, and indoles under transition-metal-free conditions is described. The reaction realized the trifunctionalization of isoquinoline via a dearomatization strategy, which displayed high chemical selectivity, excellent functional group tolerance, and a wide range of substrates, and is environmentally friendly. The three-component coupling involves the construction of new C-N, C═O, and C-C bonds in one step.

Visible Light-Induced Deoxygenation and Allylation/Vinylation of Pyridyl Ethers

The generation of alkyl radicals by deoxygenation of unactivated ethers under visible light catalysis is a hitherto unmet challenge. Herein, we report a visible light-induced deoxygenation of pyridyl ethers via formation of their pyridinium salts. The generated benzylic radicals further react with allyl/alkenyl sulfones to provide a series of coupling products in good to moderate yields. This process is proposed to undergo a reductive quenching cycle, which was elucidated by chemical, optical, and electrical experiments.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Intramolecular tetrazine-acryloyl cycloaddition: chemistry and applications

An unprecedented intramolecular [4 + 2] tetrazine-olefin cycloaddition with α,β-unsaturated substrates was discovered. The reaction produces unique coumarin-dihydropyridazine heterocycles that exhibited strong fluorescence with large Stokes shifts and excellent photo- and pH-stability. This property can be used for reaction analysis. The rate of cycloaddition was found to be solvent dependent and was determined using experimental data with a kinetic modeling software (COPASI) as well as DFT calculations (k₁ = 0.64 ± 0.019 s⁻¹ and 4.1 s⁻¹, respectively). The effects of steric and electronic properties of both the tetrazine and α,β-unsaturated carbonyl on the reaction were studied and followed the known trends characteristic of the intermolecular reaction. Based on these results, we developed a “release-then-click” strategy for the ROS triggered release of methylselenenic acid (MeSeOH) and a fluorescent tracer. This strategy was demonstrated in HeLa cells via fluorescence imaging.

Tetrazines rapidly react with tethered acrylates/acrylamides to produce fused coumarin derivatives. This template can be used in prodrug designs by depleting toxic α,β-unsaturated byproducts while also producing an imaging agent.

Synthetic Strategies in the Preparation of Phenanthridinones

Phenanthridinones are important heterocyclic frameworks present in a variety of complex natural products, pharmaceuticals and displaying wide range of pharmacological actions. Its structural importance has evoked a great deal of interest in the domains of organic synthesis and medicinal chemistry to develop new synthetic methodologies, as well as novel compounds of pharmaceutical interest. This review focuses on the synthesis of phenanthridinone scaffolds by employing aryl-aryl, N-aryl, and biaryl coupling reactions, decarboxylative amidations, and photocatalyzed reactions.

Radical Annulation of 2-Cyanoaryl Acrylamides via C═C Double Bond Cleavage: Access to Amino-Substituted 2-Quinolones

A novel annulation of 2-cyanoaryl acrylamides via C═C double bond cleavage has been developed for facile and efficient access to a broad spectrum of functionalized 4-amino-2-quinolones, which are important N-heterocycles. In this transformation, the solvent THF is demonstrated to play a crucial role, and the addition of alkyl radicals to nitrile, 1,5-hydride shift, and cleavage of the C-C bond are involved in the mechanism.

Design, synthesis and biological evaluations of a long-acting, hypoxia-activated prodrug of fasudil, a ROCK inhibitor, to reduce its systemic side-effects

ROCK, one of the downstream regulators of Rho, controls actomyosin cytoskeleton organization, stress fiber formation, smooth muscle contraction, and cell migration. ROCK plays an important role in the pathologies of cerebral and coronary vasospasm, hypertension, cancer, and arteriosclerosis. Pharmacological-induced systemic inhibition of ROCK affects both the pathological and physiological functions of Rho-kinase, resulting in hypotension, increased heart rate, decreased lymphocyte count, and eventually cardiovascular collapse. To overcome the adverse effects of systemic ROCK inhibition, we developed a bioreductive prodrug of a ROCK inhibitor, fasudil, that functions selectively under hypoxic conditions. By masking fasudil's active site with a bioreductive 4-nitrobenzyl group, we synthesized a prodrug of fasudil that is inactive in normoxia. Reduction of the protecting group initiated by hypoxia reveals an electron-donating substituent that leads to fragmentation of the parent molecule. Under normoxia the fasudil prodrug displayed significantly reduced activity against ROCK compared to its parent compound, but under severe hypoxia the prodrug was highly effective in suppressing ROCK activity. Under hypoxia the prodrug elicited an antiproliferative effect on disease-afflicted pulmonary arterial smooth muscle cells and pulmonary arterial endothelial cells. The prodrug displayed a long plasma half-life, remained inactive in the blood, and produced no drop in systemic blood pressure when compared with fasudil-treated controls. Due to its selective nature, our hypoxia-activated fasudil prodrug could be used to treat diseases where tissue-hypoxia or hypoxic cells are the pathological basis of the disease.

Photocatalytic Reductive C-O Bond Cleavage of Alkyl Aryl Ethers by Using Carbazole Catalysts with Cesium Carbonate

Methods to activate the relatively stable ether C-O bonds and convert them to other functional groups are desirable. One-electron reduction of ethers is a potentially promising route to cleave the C-O bond. However, owing to the highly negative redox potential of alkyl aryl ethers (E^red < -2.6 V vs SCE), this mode of ether C-O bond activation is challenging. Herein, we report the visible-light-induced photocatalytic cleavage of the alkyl aryl ether C-O bond using a carbazole-based organic photocatalyst (PC). Both benzylic and non-benzylic aryl ethers underwent C-O bond cleavage to form the corresponding phenol products. Addition of Cs₂CO₃ was beneficial, especially in reactions using a N-H carbazole PC. The reaction was proposed to occur via single-electron transfer (SET) from the excited-state carbazole to the substrate ether. Interaction of the N-H carbazole PC with Cs₂CO₃ via hydrogen bonding exists, which enables a deprotonation-assisted electron-transfer mechanism to operate. In addition, the Lewis acidic Cs cation interacts with the substrate alkyl aryl ether to activate it as an electron acceptor. The high reducing ability of the carbazole combined with the beneficial effects of Cs₂CO₃ made this otherwise formidable SET event possible.

3-(Benzo[d]thiazol-2-yl)-4-aminoquinoline derivatives as novel scaffold topoisomerase I inhibitor via DNA intercalation: design, synthesis, and antitumor activities

Twenty-seven 3-(benzo[d]thiazol-2-yl)-4-aminoquinoline derivatives have been designed and synthesized as topoisomerase I inhibitors.

Visible-Light-Photocatalyzed Reductions of N-Heterocyclic Nitroaryls to Anilines Utilizing Ascorbic Acid Reductant

A photoreductive protocol utilizing [Ru(bpy)₃]²⁺ photocatalyst, blue light LEDs, and ascorbic acid (AscH₂) has been developed to reduce nitro N-heteroaryls to the corresponding anilines. Based on experimental and computational results and previous studies, we propose that the reaction proceeds via proton-coupled electron transfer between AscH₂, photocatalyst, and the nitro N-heteroaryl. The method offers a green catalytic procedure to reduce, e.g., 4-/8-nitroquinolines to the corresponding aminoquinolines, substructures present in important antimalarial drugs.

Synthesis of N-Bridged Pyrido[4,3-d]pyrimidines and Self-Assembly into Twin Rosette Cages and Nanotubes in Organic Media

Two N-bridged pyrido[4,3-d]pyrimidine derivatives were synthesized toward realization of a self-assembled bis-rosette cage, in organic media. Starting from commercially available malononitrile dimer and dimethyl 5-aminoisophthalate, the target molecules were synthesized in 11 steps using a convergent approach. The final bridged compounds were characterized by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry. The hierarchical self-assembly of the nanocages into rosette nanotubes and nanobundles was established by electron microscopy and molecular modelling studies.

S,S-Chiral Linker Induced U Shape with a Syn-facial Sensitizer and Photocleavable Ethene Group

There is a major need for light-activated materials for the release of sensitizers and drugs. Considering the success of chiral columns for the separation of enantiomer drugs, we synthesized an S,S-chiral linker system covalently attached to silica with a sensitizer ethene near the silica surface. First, the silica surface was modified to be aromatic rich, by replacing 70% of the surface groups with (3-phenoxypropyl)silane. We then synthesized a 3-component conjugate [chlorin sensitizer, S,S-chiral cyclohexane and ethene building blocks] in 5 steps with a 13% yield, and covalently bound the conjugate to the (3-phenoxypropyl)silane-coated silica surface. We hypothesized that the chiral linker would increase exposure of the ethene site for enhanced ¹ O₂ -based sensitizer release. However, the chiral linker caused the sensitizer conjugate to adopt a U shape due to favored 1,2-diaxial substituent orientation; resulting in a reduced efficiency of surface loading. Further accentuating the U shape was π-π stacking between the (3-phenoxypropyl)silane and sensitizer. Semiempirical calculations and singlet oxygen luminescence data provided deeper insight into the sensitizer's orientation and release. This study has lead to insight on modifications of surfaces for drug photorelease and can help lead to the development of miniaturized photodynamic devices.

Novel 8-nitroquinolin-2(1H)-ones as NTR-bioactivated antikinetoplastid molecules: Synthesis, electrochemical and SAR study

To study the antiparasitic 8-nitroquinolin-2(1H)-one pharmacophore, a series of 31 derivatives was synthesized in 1-5 steps and evaluated in vitro against both Leishmania infantum and Trypanosoma brucei brucei. In parallel, the reduction potential of all molecules was measured by cyclic voltammetry. Structure-activity relationships first indicated that antileishmanial activity depends on an intramolecular hydrogen bond (described by X-ray diffraction) between the lactam function and the nitro group, which is responsible for an important shift of the redox potential (+0.3 V in comparison with 8-nitroquinoline). With the assistance of computational chemistry, a set of derivatives presenting a large range of redox potentials (from -1.1 to -0.45 V) was designed and provided a list of suitable molecules to be synthesized and tested. This approach highlighted that, in this series, only substrates with a redox potential above -0.6 V display activity toward L. infantum. Nevertheless, such relation between redox potentials and in vitro antiparasitic activities was not observed in T. b. brucei. Compound 22 is a new hit compound in the series, displaying both antileishmanial and antitrypanosomal activity along with a low cytotoxicity on the human HepG2 cell line. Compound 22 is selectively bioactivated by the type 1 nitroreductases (NTR1) of L. donovani and T. brucei brucei. Moreover, despite being mutagenic in the Ames test, as most of nitroaromatic derivatives, compound 22 was not genotoxic in the comet assay. Preliminary in vitro pharmacokinetic parameters were finally determined and pointed out a good in vitro microsomal stability (half-life > 40 min) and a 92% binding to human albumin.