Linkage-Editing Pseudo-Glycans: A Reductive α-Fluorovinyl-C-Glycosylation Strategy to Create Glycan Analogs with Altered Biological Activities.

The acetal (O-glycoside) bonds of glycans and glycoconjugates are chemically and biologically vulnerable, and therefore C-glycosides are of interest as more stable analogs. We hypothesized that, if the O-glycoside linkage plays a vital role in glycan function, the biological activities of C-glycoside analogs would vary depending on their substituents. Based on this idea, we adopted a "linkage-editing strategy" for the creation of glycan analogs (pseudo-glycans). We designed three types of pseudo-glycans with CH2 and CHF linkages, which resemble the O-glycoside linkage in terms of bond lengths, angles, and bulkiness, and synthesized them efficiently by means of fluorovinyl C-glycosylation and selective hydrogenation reactions. Application of this strategy to isomaltose (IM), an inducer of amylase expression, and α-GalCer, which activates iNKT cells, resulted in the discovery of CH2-IM, which shows increased amylase production ability, and CHF-α-GalCer, which shows activity opposite that of native α-GalCer, serving as an antagonist of iNKT cells.

Radical C-Glycosylation Using Photoexcitable Unprotected Glycosyl Borate.

We have developed radical C-glycosylation using photoexcitable unprotected glycosyl borate. The direct excitation of glycosyl borate under visible light irradiation enabled the generation of anomeric radical without any photoredox catalysts. The in situ generated anomeric radical was applicable to the radical addition such as Giese-type addition and Minisci-type reaction to introduce alkyl and heteroaryl groups at the anomeric position. In addition, the radical-radical coupling between the glycosyl borate and acyl imidazolide provided unprotected acyl C-glycosides.

Linkage-Editing of Melibiosamine: Synthesis and Biological Evaluation of CH2- and CHF-Linked Analogs.

Melibiosamine (Gal-α(1,6)-GlcNH2), consisting of galactose and glucosamine linked by an α(1,6)-glycosidic bond, is an artificial disaccharide derivative that selectively inhibits the proliferation of K562 tumor cells relative to HUC-F2 normal cells. In this study, we employed a linkage-editing strategy to synthesize CH2- and CHF-linked melibiosamine analogs through chemo- and stereoselective hydrogenation of fluorovinyl-C-glycoside. (R)-CHF-Melibiosamine exhibited more potent antiproliferative activity than O-linked melibiosamine, while (S)-CHF-melibiosamine was less potent.

Structure-activity relationship study of nitrogen signaling factors.

We have synthesized 10 analogs of oxylipins, which are nitrogen signaling factors (NSFs) that mediate cell-to-cell communication in the fission yeast Schizosaccharomyces pombe, and evaluated their structure-activity relationships with the aim of developing molecular probes for NSFs. We found that the OH or OAc group at C10 could be replaced with a compact amide (17) or carbamate (19). Introducing an alkyne as a detection tag at C10 led to decreased, though still sufficient, activity. Introducing an alkyne at the C18 position showed a similar trend, suggesting tolerance is relatively low even for compact functional groups such as alkynes. Although introduction of a diazirine moiety as a photoreactive group at the C5 position decreased the activity, we found that introducing diazirine at the C13 position was acceptable, and compound 38 exhibited potent NSF activity. These findings will be helpful in the development of molecular probes for NSFs.

Copper-Catalyzed Stereoselective Borylation and Palladium-Catalyzed Stereospecific Cross-Coupling to Give Aryl C-Glycosides.

Metabolically stable C-glycosides are an essential family of compounds in bioactive natural products, therapeutic agents, and biological probes. For their application, development of synthetic methods by connecting glycosides and aglycons with strict stereocontrol at the anomeric carbon, as well as with high functional-group compatibility and environmental compatibility is a pivotal issue. Although Suzuki-Miyaura-type C(sp3 )-C(sp2 ) cross-coupling using glycosyl boronates is a potential candidate for the construction of C-glycosides, neither the cross-coupling itself nor the facile synthesis of the coupling precursor, glycosyl boronates, have been achieved to date. Herein, it was succeeded to develop a copper-catalyzed stereoselective one-step borylation of glycosyl bromides to glycosyl boronates and palladium-catalyzed stereospecific cross-coupling of ß-glycosyl borates with aryl bromides to give aryl ß-C-glycosides, in which the ß-configuration of the anomeric carbon of the glycosyl trifluoroborates is stereoretentively transferred to that of the resulting aryl C-glycosides.

Synthesis and biological activity of ganglioside GM3 analogues with a (S)-CHF-Sialoside linkage and an alkyne tag.

The alkyne tag, consisting of only two carbons, is widely used as a bioorthogonal functional group due to its compactness and nonpolar structure, and various probes consisting of lipids bearing an alkyne tag have been developed. Here, we designed and synthesized analogues of ganglioside GM3 bearing an alkyne tag in the fatty acid moiety and evaluated the effect of the alkyne tag on the biological activity. To eliminate the influence of other factors such as degradation of the glycan chain when evaluating biological activity in a cellular environment, we introduced the tag into sialidase-resistant (S)-CHF-linked GM3 analogues developed by our group. The designed analogues were efficiently synthesized by tuning the protecting group of the glucosylsphingosine acceptor. The growth-promoting effect of these analogues on Had-1 cells was dramatically altered depending upon the position of the alkyne tag.

Ligand-Controlled Stereoselective Synthesis and Biological Activity of 2-Exomethylene Pseudo-glycoconjugates: Discovery of Mincle-Selective Ligands.

Glycoconjugate analogues in which the sp3 -hybridized C2 position of the carbohydrate structure (normally bearing a hydroxy group) is converted into a compact sp2 -hybridized exomethylene group are expected to have unique biological activities. We established ligand-controlled Tsuji-Trost-type glycosylation methodology to directly prepare a variety of these 2-exomethylene pseudo-glycoconjugates, including glucosylceramide analogues, in an α- or ß-selective manner. Glucocerebrosidase GBA1 cleaves these synthetic pseudo-ß-glucosylceramides similarly to native glucosylceramides. The pseudo-glucosylceramides exhibit selective ligand activity towards macrophage-inducible C-type lectin (Mincle), but unlike native glucosylceramides, are inactive towards CD1d.

Effect of Alkynyl Group on Reactivity in Photoaffinity Labeling with 2-Thienyl-Substituted α-Ketoamide.

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.

Harringtonine Ester Derivatives with Enhanced Antiproliferative Activities against HL-60 and HeLa Cells.

Harringtonine (HT), produced from Cephalotaxus species, is known to exhibit potent antiproliferative activity against myeloid leukemia cells by inhibiting protein synthesis. A previous study using acute promyelocytic leukemia (HL-60) cells raised the possibility that the C-5' methyl group of HT plays an important role in regulating leukemia cell line antiproliferative activity. In order to investigate the effect of hydrocarbon chains at C-5' on the resultant activity, the C-5' methyl group was replaced with various straight- and branched-chain hydrocarbons using the corresponding alcohols, and their antiproliferative activity against HL-60 and HeLa cells was investigated. As a result, 4'-n-heptyl-4'-demethylharringtonine (1f, n-heptyl derivative) showed the most potent cytotoxicity among the HT ester derivatives produced, with IC50 values of 9.4 nM and 0.4 µM for HL-60 and HeLa cells, respectively. Interestingly, the cytotoxicity of derivative 1f against HL-60 and HeLa cells respectively was ∼5 (IC50 = 50.5 nM) and ∼10 times (IC50 = 4.0 µM) those of HT and ∼2 (IC50 = 21.8 nM) and ∼4 times (IC50 = 1.7 µM) more than homoharringtonine (HHT). These results demonstrate the potential of the derivative 1f as a lead compound against leukemia.

Design, synthesis, and functional evaluation of triazine-based bivalent agents that simultaneously target the active site and hot spot of phosphatase Cdc25B.

Cdc25B phosphatase catalyzes the dephosphorylation and activation of cyclin-dependent kinases 2 (CDK2/CycA) and their overexpression has been reported in cancers. Although Cdc25B has received much attention as a drug target, its flat and featureless surface makes it challenging to develop new agents targeting this protein. In this study, we investigated the rational design of a series of bivalent triazine-based derivatives with the aim of simultaneously targeting the active site and the remote hotspot critical for the interaction with CDK2/CycA. Compounds 1e and 10, containing aromatic residues, were shown to inhibit Cdc25B activity selectively over Cdc25A at low micromolar concentration.

Preparation of Oxysterols by C-H Oxidation of Dibromocholestane with Ru(Bpga) Catalyst.

Seven mono- and dihydroxycholesterols were prepared by direct C-H oxidation of the cholestane skeleton with a recently developed Ru(Bpga) catalyst (Ru(Bpga) = [RuCl (bpga) (PPh3)] Cl; bpga = 2-(bis(pyridin-2-ylmethyl)amino)-N-(2,6-dimethylphenyl)acetamide)). Due to the high selectivity of the Ru(Bpga) complex for tertiary C-H, the reaction afforded a mixture of 25-, 20-, 17-, and 14-oxygenated cholesterols that could be easily separated by high-performance liquid chromatography. These results suggest that late-stage C-H oxidation could be a viable strategy for preparing candidate metabolites of biologically important molecules.

Identification of novel fish sialidase genes responsible for KDN-cleaving activity.

2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) is a minor component of sialic acids detected in vertebrates, such as human cancer cells, rat liver, and fish tissues. Although the enzyme activity of KDN-cleaving sialidase (KDN-sialidase) has been detected in rainbow trout, the gene responsible for its expression has not been identified in vertebrates. We evaluated sialidases in human and various fish for their KDN-cleaving activity using an artificial substrate, methylumbelliferyl-KDN (MU-KDN). Four of the human sialidases tested (NEU1, NEU2, NEU3, and NEU4) did not hydrolyze MU-KDN. Although most fish Neu1s showed negligible KDN-sialidase activity, two Neu1b sialidases from Oreochromis niloticus and Astyanax mexicanus, a paralog of Neu1, exhibited a potent KDN-sialidase activity. Further, O. niloticus and Oryzias latipes Neu3a exhibited a drastically high KDN-sialidase activity, while Danio rerio Neu3.1 showed moderate activities and other Neu3 proteins exhibited little activity. All the Neu4 sialidases tested in fish cleaved KDN and Neu5Ac from MU-KDN and MU-Neu5Ac, respectively, with equivalent potential. To our knowledge, this is the first report to identify KDN-sialidase genes in vertebrates and we believe that KDN-sialidase activity could be conserved among fish Neu4s.

Synthesis of All Stereoisomers of RK460 and Evaluation of Their Activity and Selectivity as Abscisic Acid Receptor Antagonists.

The PYR/PYL/RCAR protein families have recently emerged as receptors of the phytohormone abscisic acid (ABA, 1), which regulates plant responses to environmental stress. These families have multiple members with different physiological actions, and so selective agonists or antagonists are needed both as tools to elucidate functional differences and as lead compounds for agrochemicals. We previously identified RK460 (rac-3 a) as a PYR1-selective antagonist, and showed that it possesses five stereocenters on a 6,5-cis-bicyclo skeleton. Here, we synthesized all the stereoisomers of RK460 and evaluated their activity towards a panel of receptors. Relative stereochemistry as well as absolute stereochemistry was important for selective action.

Synthesis of All Stereoisomers of Monomeric Spectomycin A1/A2 and Evaluation of Their Protein SUMOylation-Inhibitory Activity.

A synthetic methodology to access all possible stereoisomers of spectomycin A1 (SMA1) and A2 (SMA2) has been established through late-stage diversification. The key reaction for the construction of all four diastereomers is an intramolecular cyclization based on the umpolung of π-allyl palladium species with bis(pinacolato)diborane (B2 (pin)2 ). Silyl group assisted direct benzylic oxidation of each isomer enabled construction of the fragile ß-hydroxytetralone skeleton to provide the SMAs. The relative and absolute stereochemistry of SMA2 was also determined, and the absolute stereochemistry of SMA1 was extrapolated based on the optical rotation of SMA2. The axial chirality of SMAs is discussed based on circular dichroism spectra and DFT calculations, and it is concluded that the M isomer is predominant in solution. Biochemical assessment of all isomers in vitro revealed that the C9 hydroxyl group and dimeric structure were both important for protein SUMOylation-inhibitory activity.

Development of Turn-On Probes for Acids Triggered by Aromaticity Enhancement Using Tricyclic Amidine Derivatives.

Two fluorophores consisting of tricyclic amidine derivatives (DHIm and DHPy) were prepared as selective turn-on probes for acids, which were triggered by an aromaticity enhancement. Both amidine derivatives were expanded rings prepared by condensed reactions between the corresponding dibromoalkanes and an aminonaphthyridine analogue. In X-ray analyses, DHIm, in which the dihydroimidazole ring was condensed into aminonaphthyridine, showed high planarity, compared to DHPy, with condensed dihydropyrimidine. The fluorescence properties of DHIm exhibited a higher quantum yield than DHPy due to the difference in planarity. Under acidic conditions, such as in the presence of H+ and M(II), protonations and complexations occurred, exhibiting a higher quantum yield than the neutral DHX (X = Im or Py). The nucleus-independent chemical shift values from the density functional theory calculations suggested that the protonations and complexations caused an enhancement of the aromaticity within the frameworks. These aromaticity changes led to intense fluorescence, and DHX behaved as a selective turn-on probe for acids and metal ions. Interestingly, this fluorescence turn-on system triggered by the aromaticity-based enhancement is not a typical system, such as the photoinduced electron transfer, aggregation-induced enhanced emission, and twisted intramolecular charge transfer systems, but is classified as a novel turn-on system.

Internal-Edge-Substituted Coumarin-Fused [6]Helicenes: Asymmetric Synthesis, Structural Features, and Control of Self-Assembly.

π-Conjugated helicenes containing heteroatoms have attracted significant attention due to their diverse chemical and electronic structures, as well as tunable physical properties. It was rationally anticipated that the self-assembly of coumarin-fused helicenes would be controlled by the effects of a substituent on the internal edge of the helix. Here, this work reports the efficient syntheses of coumarin-fused helicenes 1 a,b (R=Ph, Me), and the enantioselective synthesis of 1 a (R=Ph) by chiral AuI -catalyzed hydroarylation. The helical structure of 1 was unambiguously determined by X-ray crystallography. Of particular note, the enantiomerically pure crystal of 1 a adopted a one-dimensional columnar structure based on π-π stacking interactions, as expected. Furthermore, a significant difference between the fluorescence quantum yields of the enantiomerically pure form and racemate of 1 a was observed.

Water-Proton Relaxivities of Radical Nanoparticles Self-Assembled via Hydration or Dehydration Processes.

Nanoparticles capable of accumulating in tumor tissues are promising materials for tumor imaging and therapy. In this study, two radical nanoparticles (RNPs), denoted as 1 and 2, composed of self-assembled ureabenzene derivatives possessing one or two amphiphilic side chains were demonstrated to be candidates for metal-free functional magnetic resonance imaging (MRI) contrast agents (CAs). Because of the self-assembly behavior of 1 and 2 in a saline solution, spherical RNPs of sizes ∼50-90 and ∼30-100 nm were detected. In a highly concentrated solution, RNP 1 showed considerably small water-proton relaxivity values (r1 and r2), whereas RNP 2 showed an r1 value that was around 5 times larger than that of RNP 1. These distinct r1 values might be caused by differences in the self-assembly behavior by a hydration or dehydration process. In vivo studies with RNP 2 demonstrated a slightly enhanced T1-weighted image in mice, suggesting that the RNPs can potentially be used as metal-free functional MRI CAs for T1-weighted imaging.

Synthesis of the Right-Side Structure of Type B Physalins.

We present a full account of our synthetic studies on the racemic DEFGH-ring moiety of physalins, featuring domino ring transformation of a tricyclic key intermediate. We also report the results of a detailed mechanistic examination of the domino ring transformation, as well as a reoptimization of the 2,3-Wittig rearrangement and methylation steps. Furthermore, we have newly established a method for the preparation of an optically active synthetic intermediate by enzymatic kinetic resolution. Our work provides access to both natural and nonnatural right-side physalin structures.

7-Hydroxy-3-methyleneisoindolin-1-one as a New ESIPT-Fluorescent Probe to Monitor Aqueous Environments.

A 7-hydroxy derivative of 3-methyleneisoindolin-1-one 1 was synthesized and its properties as a new fluorophore undergoing excited-state intramolecular proton transfer (ESIPT) were investigated. In alcohols and dimethylsulfoxide, 1 exhibited dual emission at ca. 380 and 525-540 nm when excited at ca. 336 nm, which agreed well with the density functional theory (DFT) and time-dependent (TD)-DFT-calculated emission predictions of 1 and its ESIPT tautomer. In aqueous solutions at near neutral pH, 1 exhibited a broad emission band at ca. 497 nm, presumably caused by the overlap of emissions from 1 and the excited state phenolate species of 1. In binary mixtures of H2O and EtOH, the wavelength and intensity of fluorescence maxima were dependent on the dielectric constant of the solvent, suggesting that 1 could be applied as a fluorescent probe to monitor aqueous environments.

A new carbamidemethyl-linked lanthanoid chelating tag for PCS NMR spectroscopy of proteins in living HeLa cells.

Structural analyses of proteins under macromolecular crowding inside human cultured cells by in-cell NMR spectroscopy are crucial not only for explicit understanding of their cellular functions but also for applications in medical and pharmaceutical sciences. In-cell NMR experiments using human cultured cells however suffer from low sensitivity, thus pseudocontact shifts from protein-tagged paramagnetic lanthanoid ions, analysed using sensitive heteronuclear two-dimensional correlation NMR spectra, offer huge potential advantage in obtaining structural information over conventional NOE-based approaches. We synthesised a new lanthanoid-chelating tag (M8-CAM-I), in which the eight-fold, stereospecifically methylated DOTA (M8) scaffold was retained, while a stable carbamidemethyl (CAM) group was introduced as the functional group connecting to proteins. M8-CAM-I successfully fulfilled the requirements for in-cell NMR: high-affinity to lanthanoid, low cytotoxicity and the stability under reducing condition inside cells. Large PCSs for backbone N-H resonances observed for M8-CAM-tagged human ubiquitin mutant proteins, which were introduced into HeLa cells by electroporation, demonstrated that this approach readily provides the useful information enabling the determination of protein structures, relative orientations of domains and protein complexes within human cultured cells.