Abstract

With recent advances in photoredox chemistry, a variety of methods for generating carbon radicals have emerged. This review highlights recent approaches for radical generation utilizing zirconocene(III), a species infrequently employed in organic syntheses. Of particular interest are methods employing visible light irradiation, which induce C–Zr bond homolysis of alkyl zirconium species or activate a photoredox catalyst to reduce zirconocene(IV).

The development of chemoselective defluorination reactions is highly desirable due to the exceptional stability of the C–F bond compared to other functional groups. Recent advances in photocatalysis have enabled cataytic single-electron transfer (SET) processes, offering an alternative to stoichiometric methods that rely on strong reducing agents. However, these strategies have primarily focused on trifluoromethyl substrates, with limited success for compounds containing fewer fluorine atoms, which are inherently more resistant to SET. Herein, we report a novel defluorination strategy for α-fluorocarbonyl compounds, employing zirconocene and photoredox catalysis. Our method leverages the strong fluorine affinity of zirconocene and bypassed reliance on reduction potential, focusing instead on the bond dissociation energy of the fluorinated molecules. This methodology offers a complementary ap-proach for catalytic C–F bond cleavage under visible-light conditions.

Abstract

Oxetanes are frequently utilized in organic synthesis, both as target products and as fairly reactive intermediates. Whereas ring cleavage of oxetanes through polar mechanisms has been extensively investigated, their radical-based counterparts remain underexplored. We used zirconocene and photoredox catalysis to open an oxetane ring in a radical manner. In our protocol, the reaction selectively delivers the more-substituted alcohols via putative less-stable radicals. This method not only affords the corresponding hydrogenated products, but also provides unique benzylidene acetal products.

Abstract

Circadian clocks are biological timekeeping systems that coordinate genetic, metabolic and physiological behaviors with the external day–night cycle. The clock in plants relies on the transcriptional-translational feedback loops transcription-translation feedback loop (TTFL), consisting of transcription factors including PSUEDO-RESPONSE REGULATOR (PRR) proteins, plant lineage–specific transcriptional repressors. Here, we report that a novel synthetic small-molecule modulator, 5-(3,4-dichlorophenyl)-1-phenyl-1,7-dihydro-4H-pyrazolo[3,4-d] pyrimidine-4,6(5H)-dione (TU-892), affects the PRR7 protein amount. A clock reporter line of Arabidopsis was screened against the 10,000 small molecules in the Maybridge Hitfinder 10K chemical library. This screening identified TU-892 as a period-lengthening molecule. Gene expression analyses showed that TU-892 treatment upregulates CIRCADIAN CLOCK–ASSOCIATED 1 (CCA1) mRNA expression. TU-892 treatment reduced the amount of PRR7 protein, a transcriptional repressor of CCA1. Other PRR proteins including TIMING OF CAB EXPRESSION 1 were altered less by TU-892 treatment. TU-892-dependent CCA1 upregulation was attenuated in mutants impaired in PRR7. Collectively, TU-892 is a novel type of clock modulator that reduces the levels of PRR7 protein.

Abstract

The circadian clock, an internal time-keeping system with period of about 24 h, coordinates many physiological processes with the day-night cycle. We previously demonstrated that BML-259 (N-(5-Isopropyl-2-thiazolyl) phenylacetamide), a small molecule with mammal CYCLIN DEPENDENT KINASE 5 (CDK5)/ CDK2 inhibition activity, lengthens Arabidopsis thaliana (Arabidopsis) circadian clock periods. BML-259 inhibits Arabidopsis CDKC kinase, which phosphorylates RNA polymerase II in the general transcriptional machinery. To accelerate our understanding of the inhibitory mechanism of BML-259 on CDKC, we performed structure-function studies of BML-259 using circadian period lengthening activity as an estimation of CDKC inhibitor activity in vivo. The presence of a thiazole ring is essential for period lengthening activity, whereas acetamide, isopropyl, and phenyl groups can be modified without effect. BML-259 analogue TT-539, as known a mammal CDK5 inhibitor, did not lengthen the period nor did it inhibit Pol II phosphorylation. TT-361, an analogue having a thiophenyl ring instead of a phenyl ring, possesses stronger period lengthening activity and CDKC;2 inhibitory activity than BML-259. In silico ensemble docking calculations using Arabidopsis CDKC;2 obtained by a homology modeling indicated that the different binding conformations between these molecules and CDKC;2 explains the divergent activities of TT539 and TT361.

Circadian clocks regulate the diel rhythmic physiological activities of plants, enabling them to anticipate and adapt to day–night and seasonal changes. Genetic and biochemical approaches have suggested that transcription–translation feedback loops (TTFL) are crucial for Arabidopsis clock function. Recently, the study of chemical chronobiology has emerged as a discipline within the circadian clock field, with important and complementary discoveries from both plant and animal research. In this review, we introduce recent advances in chemical biology using small molecules to perturb plant circadian clock function through TTFL components. Studies using small molecule clock modulators have been instrumental for revealing the role of post-translational modification in the clock, or the metabolite-dependent clock input pathway, as well as for controlling clock-dependent flowering time.

"Dance reaction" on the aromatic ring is a powerful method in organic chemistry to translocate functional groups on arene scaffolds. Notably, dance reactions of halides and pseudohalides offer a unique platform for the divergent synthesis of substituted (hetero)aromatic compounds when combined with transition-metal-catalyzed coupling reactions. Herein, we report a tandem reaction of ester dance and decarbonylative coupling enabled by palladium catalysis. In this reaction, 1,2-translocation of the ester moiety on the aromatic ring is followed by decarbonylative coupling with nucleophiles to enable the installation of a variety of nucleophiles at the position adjacent to the ester in the starting material.

The reductive ring opening of epoxides is a powerful transformation to convert readily accessible epoxides into a diverse array of valuable alcohols, including pharmaceuticals, agrochemicals, and functional polymers. Although significant progress has been made, the established methods were limited to titanocene-catalyzed reactions. Herein, we report an unprecedented zirconocene-catalyzed ring opening of epoxide enabled by photoredox catalysis. Compared with the conventional ring-opening methods, the present protocol exhibited reverse regioselectivity to afford more substituted alcohols via putative less-stable radicals. This reaction is remarkably mild and smoothly cleaves C–O bonds in molecules with a variety of functional groups, including natural products. We believe that the finding that changing the metal center in metallocene influences the energy profile of ring opening is a significant advance and provides a new perspective in radical chemistry with group IV metals.

Minimalist photo-reactive probes, which consist of a photo-reactive group and a tag for detection of target proteins, are useful tools in chemical biology. Although several diazirine-based and aryl azide-based minimalist probes are available, no keto-based minimalist probe has yet been reported. Here we describe minimalist probes based on a 2-thienyl-substituted α-ketoamide bearing an alkyne group on the thiophene ring. The 3-alkyne probe showed the highest photo-affinity labeling efficiency.

Glycoconjugates are an important class of biomolecules that regulate numerous biological events in cells. However, these complex, medium-size molecules are metabolically unstable, which hampers detailed investigations of their functions as well as their potential application as pharmaceuticals. Here we report sialidase-resistant analogues of ganglioside GM3 containing a monofluoromethylene linkage instead of the native O-sialoside linkage. Stereoselective synthesis of CHF-linked disaccharides and kinetically controlled Au(I)-catalyzed glycosylation efficiently furnished both stereoisomers of CHF-linked as well as CF2- and CH2-linked GM3 analogues. Like native GM3, the C-linked GM3 analogues inhibited the autophosphorylation of epidermal growth factor (EGF) receptor induced by EGF in vitro. Assay of the proliferation-enhancing activity toward Had-1 cells together with NMR-based conformational analysis showed that the (S)-CHF-linked GM3 analogue with exo-gauche conformation is the most potent of the synthesized compounds. Our findings suggest that exo-anomeric conformation is important for the biological functions of GM3.

We report here a novel strategy for the synthesis of 1,2,3,4,9-pentaarylcarbazole. In this method, an N-alkynylation of sulfonamide facilitated by an iodonium salt, and spontaneous cycloaddition of the resulting ynamide were crucial for the rapid construction of the central carbazole core.

A synthesis of decaarylanthracene with nine different substituents has been accomplished by a coupling/ring-transformation strategy. The oxidation of tetraarylthiophenes with four different substituents to the corresponding thiophene S-oxides and a [4 + 2] cycloaddition with a double benzyne precursor afforded a multiply arylated naphthalene derivative. Subsequently, the naphthalene derivative was converted into a naphthalyne, and then a [4 + 2] cycloaddition of another thiophene S-oxide provided decaarylanthracenes with nine different aryl groups.

Copyright © 2019 American Chemical Society. A Rh-catalyzed asymmetric intramolecular Buchner ring expansion of α-alkyl-α-diazoesters has been developed. The present protocol generates a 5,7-fused ring system in an enantioselective manner while minimizing β-hydrogen migration, which has been a competing reaction when using α-alkyl-α-diazoesters. The ester functionality at the bridgehead position would be a useful synthetic handle for further derivatization to complex molecules including natural products.

© 2019 American Chemical Society. We report a general protocol for the light-driven isomerization of cyclic aliphatic alcohols to linear carbonyl compounds. These reactions proceed via proton-coupled electron-transfer activation of alcohol O-H bonds followed by subsequent C-C β-scission of the resulting alkoxy radical intermediates. In many cases, these redox-neutral isomerizations proceed in opposition to a significant energetic gradient, yielding products that are less thermodynamically stable than the starting materials. A mechanism is presented to rationalize this out-of-equilibrium behavior that may serve as a model for the design of other contrathermodynamic transformations driven by excited-state redox events.

© 2018 American Chemical Society. Photoaffinity labeling (PAL) is an important tool in chemical biology research, but application of α-ketoamides for PAL has been hampered by their photoinstability. Here, we show that 2-thienyl-substituted α-ketoamide is a superior photoreactive group for PAL. Studies with a series of synthetic mannose-conjugated α-ketoamides revealed that 2-thienyl substitution of α-ketoamide decreased the electrophilicity of the keto group and reduced the rate of photodegradation. Mannose-conjugated thienyl α-ketoamide showed greater concanavalin A labeling efficiency than other alkyl or phenyl-substituted α-ketoamides. In comparison with representative conventional photoreactive groups, 2-thienyl ketoamide showed reduced labeling of nontarget proteins, probably owing to its lower hydrophobicity.

© 2017 Elsevier Ltd We describe a total synthesis of a polyunsaturated fatty acid (PUFA)-containing glucuronosyldiacylglycerol (GlcADG), which is a surrogate glycolipid whose synthesis is remarkably upregulated in plant membranes under phosphorus-depleted conditions. Glycosylation between the glucuronide donor bearing 3,4-dimethoxybenzyl (DMPM) protecting groups and di-acylglycerol acceptor proceeded smoothly in the presence of gold(I) catalyst to provide the protected α-isomer of GlcADG as the major product.

© Georg Thieme Verlag. We developed a photo-chemical coupling reaction of α-keto esters with several simple alcohols, alkanes, ethers, and amides. Use of tertiary alkyl ester, α-cumyl ester, is the key for avoiding the known photo-degradation process. Intermolecular C-H bond activation and subsequent C-C bond formation were promoted by irradiation with an LED lamp (365 nm) without any additives. Among the coupling partners, reactions with sterically less demanding amides proceeded efficiently to provide unique N-acyl-β-amino-α-hydroxy acid derivatives. In benzene or acetone as a solvent, the reaction with a solid amino acid derivative provided a precursor of tetrahydro-1,4-diazepine-2,5-dione derivatives.

© 2015 Elsevier Ltd. A novel type of photocyclization of α-ketoamides was developed, affording unique cyclopropanols bearing amide functionality. N-tert-Butyl, N-trityl, or N-non-substituted α-ketoamides with a bulky substituent at the β-position of the amide functionality were efficiently converted to corresponding cyclopropanols through the activation of the γ-C-H bond followed by C-C bond formation between the α- and γ-positions of the amide. Hydrogen abstraction from the γ-position of the amide was considered to be the rate-determining step of cyclopropanol formation, based on the kinetic isotope effect. Cyclopropanols could be converted to two different types of functionalized α-ketoamides depending on the method of ring-opening.

© 2015 FCCA (Forum: Carbohydrates Coming of Age). This mini-review focuses on the synthesis and biological activities of sialic acid (SA) derivatives with a C-glycosidic linkage in place of the standard O-glycosidic bond at the C-2 position. We summarize reported synthetic methodologies for monoand di-saccharides in separate sections. Then, we introduce our strategy for the construction of C-sialoside linkages utilizing Ireland-Claisen rearrangement reaction and its application to the synthesis of a ganglioside GM4 analogue. Although C-sialosides are expected to be useful as sialidase-resistant analogues of sialoglycoconjugates (sialoGCs), their biological activities have not yet been investigated in detail. Herein we briefly discuss the potential applications of C-sialosides.

Molecular probes based on 3-[(dodecylthiocarbonyl)methyl]glutarimide (DTCM-glutarimide) were synthesized and assessed for inhibitory activity against LPS-induced NO production. Among the probes examined, several derivatives exhibited potential for use in determining the target proteins of DTCM-glutarimide. © 2011 Elsevier Ltd. All rights reserved.

We previously reported that 9-methylstreptimidone, a piperidine compound isolated from a culture filtrate of Streptomyces, induces apoptosis selectively in adult T-cell leukemia cells. It was screened for a compound that inhibits LPS-induced NF-κB and NO production in mouse macrophages. However, 9-methystreptimidone is poorly obtained from the producing microorganism and difficult to synthesize. Therefore, in the present research, we studied the structure-activity relationship to look for new selective inhibitors. We found that the structure of the unsaturated hydrophobic portion of 9-methylstreptimidone was essential for the inhibition of LPS-induced NO production. Among the 9-methylstreptimidone-related compounds tested, (±)-4,α-diepi-streptovitacin A inhibited NO production in macrophage-like cells as potently as 9-methylstreptimidone and without cellular toxicity. Moreover, this compound selectively induced apoptosis in adult T-cell leukemia MT-1 cells. Copyright © 2012 Cognizant Comm. Corp. All rights reserved.