Osmium(III) and Osmium(V) Complexes Bearing a Macrocyclic Ligand: A Simple and Efficient Catalytic System for cis‐Dihydroxylation of Alkenes with Hydrogen Peroxide

A simple protocol that uses [Os^III(OH)(H₂O)(L ‐N₄Me₂)](PF₆)₂ ( 1 ; L ‐N₄Me₂=N,N′‐dimethyl‐2,11‐diaza[3.3](2,6)pyridinophane) as a catalyst and H₂O₂ as a terminal oxidant for efficient cis‐1,2‐dihydroxylation of alkenes is presented. Unfunctionalized (or aliphatic) alkenes and alkenes/styrenes containing electron‐withdrawing groups are selectively oxidized to the corresponding vicinal diols in good to excellent yields (46–99 %). In the catalytic reactions, the stoichiometry of alkene:H₂O₂ is 1:1, and thus the oxidant efficiency is very high. For the dihydroxylation of cyclohexene, the catalytic amount of 1 can be reduced to 0.01 mol % to achieve a very high turnover number of 5500. The active oxidant is identified as the Os^V(O)(OH) species ( 2 ), which is formed via the hydroperoxide adduct, an Os^III(OOH) species. The active oxidant 2 is successfully isolated and crystallographically characterized.     

Ni-catalyzed α-arylation of secondary α-bromo-α-fluoro-β-lactam: cross-coupling of a secondary fluorine-containing electrophile

A highly diastereoselective cross-coupling reaction of an α-bromo-α-fluoro-β-lactam with a wide range of aryl Grignard reagents was catalyzed by Ni/bis(oxazoline) in yields of up to 98%. The product was obtained diastereoselectively as an anti-isomer. This is the first successful α-arylation of an α-fluoro-β-lactam to produce diverse α-aryl-α-fluoro-β-lactams.     

Synthetic Studies on Sialoglycoconjugates 60: α-Stereocontrolled,Glycoside Synthesis of Trimeric Sialic Acid with Galactose and Lactose Derivatives

Abstract

α-Stereocontrolled, glycoside synthesis of trimeric sialic acid is described toward a systematic approach to the synthesis of sialoglycoconjugates containing an α-sialyl-(2→8)-α-sialyl-(2→8)-sialic acid unit α-glycosidically linked to O-3 of a galactose residue in their oligosaccharide chains. Glycosylation of 2-(trimethylsilyl)ethyl 6-O-benzoyl-β-d-galactopyranoside (4) or 2-(trimethylsilyl)ethyl 2,3,6,2′,6′-penta-O-benzyl-β-lactoside (5), with methyl [phenyl 5-acetamido-8-O-[5-acetamido-8-O-(5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-d-glycero-α-d-galacto-2-nonulopyranosylono-1”, 9′-lactone)-4,7-di-O-acetyl-3,5-dideoxy-d-glycero-α-d-galacto-2-nonulopyranosylono-1′, 9-lactone]-4,7-di-O-acetyl-3,5-dideoxy-2-thio-d-glycero-d-galacto-2-nonulopyranosid]onate (3), using N-iodosuccinimide-trifluoromethanesulfonic acid as a promoter, gave the corresponding α-glycosides 6 and 8, respectively. The glycosyl donor 3 was prepared from trimeric sialic acid by treatment with Amberlite IR-120 (H⁺) resin in methanol, O-acetylation, and subsequent replacement of the anomeric acetoxy group with phenylthio. Compounds 6 and 8 were converted into the per-O-acyl derivatives 7 and 9, respectively.     

Studies on 1-Deoxynojirimycin-Containing Glycans: Synthesis of Novel Disaccharides Related to Lactose,Lactosamine, and Chitobiose

Abstract

Suitably protected 1-deoxynojirimycin (l, 5-dideoxy-l, 5-imino-D-glucitol; DNJ) and its 2-acetamido derivative, i.e., 2, 3, 6-tri-O-benzyl-.N-benzyloxycarbonyl-l, 5-dideoxy-1, 5-imino-D-glucitol (6) and 2-acetamido-3, 6-di-O-benzyl-N-benzyloxycarbonyl-1, 2, 5-trideoxy-l, 5-imino-D-glucitol (14) were each coupled with methyl 2, 3, 4, 6-tetra-O-acetyl-1-thio-β-D-galactopyranoside (15) in the presence of dimethyl(methylthio)-sulfonium triflate (DMTST) as a promoter, to give 16 and 18, which were converted to the novel disaccharides (20, 21) related to lactose and lactosamine. Coupling of 14with methyl 3, 4, 6-tri-O-acetyl-2-deoxy-2-phthalimido-l-thio-β-D-glucopyranoside (22) gave achitobiose analog (25). O-(β-D-Galactopyranosyl)-(l→3)-DNJ derivatives (38, 39) and O-(β-D-glucopyranosyl)-(l→3)-DNJ (45) were also synthesized. Conformational analysis of a variety of DNJ derivatives, based on the ¹H NMR data, is also discussed.     

Laser-diode pumped heavy-doped Yb:YAG ceramic lasers

Laser performance of heavy-doped Yb:YAG ceramics was investigated using a two-pass pumping miniature laser configuration. Slope efficiency of 52% and optical-to-optical efficiency of 48% have been achieved for 1-mm-thick YAG ceramic doped with 20 at.% ytterbium ions. Laser spectra of Yb:YAG ceramic and single-crystal lasers were addressed under different intracavity laser intensities. Heavy-doped Yb:YAG ceramic is more suitable for a thin disk laser than a single-crystal with the same Yb(3+)-ion lasants.     

Catalytic Activity of Molecular Rhenium Sulfide Clusters [Re6S8(OH)6−n (H2O) n ](4−n)− (n = 0, 2, 4, 6) with Retention of the Octahedral Metal Frameworks: Dehydrogenation and Dehydration of 1,4-Butanediol

A series of molecular rhenium sulfide clusters [Re₆S₈(OH)_6?n (H₂O) _n ]^(4?n)? (n = 0, 2, 4, 6) catalyze dehydrogenation of alcohols, and hydrogenation of ketones and olefins in a hydrogen stream at 350 °C. The catalytic activities of the dianionic and neutral clusters (n = 2, 4) are lower than those of tetraanionic and dicationic clusters (n = 0, 6) for all the reactions. When 1,4-butanediol is allowed to react over K₄[Re₆S₈(OH)₆], dehydrogenation proceeds to yield 2-hydroxytetrahydrofuran and successively γ-butyrolactone above 300 °C. Over [Re₆S₈(H₂O)₆]SO₄ dehydration proceeds to yield tetrahydrofuran above 250 °C. The thermal activation mechanisms of these clusters were studied by powder X-ray diffraction analyses, Raman spectrometry, extended X-ray absorption fine structure spectrometry, thermogravimetry, and differential thermal analyses. The catalytically active site of K₄[Re₆S₈(OH)₆] is an uncoordinated metal site (Lewis acid site) developed by the loss of a water molecule from two hydroxo ligands. The active site of [Re₆S₈(H₂O)₆]SO₄ is a Brønsted acid site; the anhydrous aqua cluster dication disproportionates to a hydroxo cluster monocation and a proton. Both of the octahedral cluster frameworks are retained up to 500 °C.     

Effect of Particle Size on Electro-Optic Properties of Liquid Crystal Devices Doped with γ-Cyclodextrin Stabilized Barium Titanate Nanoparticles

Barium titanate stabilized by γ-cyclodextrin nanoparticles were prepared by using a microwave reactor equipped with ultrasonic nozzle mixing at 240°C in a tetraethylene glycol solution of barium ethoxide and titanium ethoxide in the presence of γ-cyclodextrin. Particles in γ-cyclodextrin-stabilized BaTiO₃ nanoparticles had an average diameter of 2.1 nm and mainly distributed within the range of about 1 to 4 nm. The γ-cyclodextrin-stabilized BaTiO₃ nanoparticles were mixed with 4-cyano-4′-pentylbiphenyl at room temperature resulting in a liquid crystal sol of 4-cyano-4′-pentylbiphenyl. The response time of liquid crystal devices in the presence of γ-cyclodextrin-stabilized BaTiO₃ nanoparticles was faster than that in the absence.     

Direct Asymmetric Mannich‐Type Reaction of α‐Isocyanoacetates with Ketimines using Cinchona Alkaloid/Copper(II) Catalysts

The enantioselective direct Mannich‐type reaction of ketimines with α‐isocyanoacetates has been developed. Excellent yields and enantioselectivity were observed for the reaction of various ketimines and α‐isocyanoacetates using cinchona alkaloid/Cu(OTf)₂ and a base. Both enantiomers of the products could be obtained by using pseudoenantiomeric chiral catalysts. This process offers an efficient route for the synthesis of α,β‐diamino acids.     

pH-responsive non-surface-active/surface-active transition of weakly ionic amphiphilic diblock copolymers

The weakly ionic amphiphilic diblock copolymer polystyrene-b-poly(acrylic acid) was synthesized by nitroxy radical-mediated living radical polymerization with precise control of block length, block ratio, and polydispersity. Systematical surface tension experiments and foam formation observations revealed that this polymer was non-surface active under neutral and alkaline (pH 10) conditions, while it was surface active under an acidic condition (pH 3). This result supports our proposed origin of non-surface activity; the image charge repulsion at the air/water interface is essential in addition to very stable micelle formation in the bulk solution. At a higher pH (pH 12), the polymer showed slight surface activity since the added NaOH played a role as an added salt. The critical micelle concentration (cmc) was estimated by static light scattering. Cmc increased with increasing added salt (NaCl) concentration as was observed for other strongly ionic non-surface-active polymers. Hence, this trend is characteristic for non-surface-active polymers. The pH dependence of cmc was minimum at pH 8–10. Since the acrylic acid block is fully ionized under this condition, the strong image charge repulsion at this condition accelerated micelle formation at a low polymer concentration, which consequently decreased cmc. Micelles in bulk solution were confirmed by dynamic light scattering, and the salt concentration and pH dependencies of the hydrodynamic radius of the micelles were also estimated. The pH-responsive non-surface-active/surface-active transition observed in this study strongly supports the fact that the image charge repulsion is an essential factor for non-surface activity in addition to stable micelle formation in solution.