Synthesis of Disaccharide Nucleosides by the O‐Glycosylation of Natural Nucleosides with Thioglycoside Donors

Disaccharide nucleosides constitute an important group of naturally‐occurring sugar derivatives. In this study, we report on the synthesis of disaccharide nucleosides by the direct O‐glycosylation of nucleoside acceptors, such as adenosine, guanosine, thymidine, and cytidine, with glycosyl donors. Among the glycosyl donors tested, thioglycosides were found to give the corresponding disaccharide nucleosides in moderate to high chemical yields with the above nucleoside acceptors using p‐toluenesulfenyl chloride (TolSCl) and silver triflate (AgOTf) as promoters. The interaction of these promoters with nucleoside acceptors was examined by ¹H NMR spectroscopic experiments.     

A new synthetic approach to aminolycoside antibiotics by use of oxidative decarboxylation and reductive deacetoxylation as key-reactions

By use of a decarboxylation reaction with lead tetraacetate and a deacetoxylation reaction with sodium borohydride as key-reactions, paromamine ($\text{12}$ and tri-N-benzyloxycarbonylparomamine ($\text{12a}$) were synthesized from D-glucosamine.     

Microwave‐Assisted Palladium‐Catalyzed Direct Arylation of 1,4‐Disubstituted 1,2,3‐Triazoles with Aryl Chlorides

Treatment of 1,4‐disubstituted 1,2,3‐triazoles with aryl chlorides in the presence of potassium carbonate under palladium catalysis and microwave irradiation at 250 °C for 15 min leads to arylation of the triazole at the 5‐position. A variety of functional groups, including ester and hydroxy groups, are compatible. The procedure is suitable for the regioselective preparation of trisubstituted triazoles. Microwave irradiation accelerates the reaction, thus allowing the rapid synthesis of trisubstituted triazoles, which are difficult to synthesize selectively.     

Three-dimensional scanning force/tunneling spectroscopy at room temperature

We simultaneously measured the force and tunneling current in three-dimensional (3D) space on the Si(111)-(7 × 7) surface using scanning force/tunneling microscopy at room temperature. The observables, the frequency shift and the time-averaged tunneling current were converted to the physical quantities of interest, i.e. the interaction force and the instantaneous tunneling current. Using the same tip, the local density of states (LDOS) was mapped on the same surface area at constant height by measuring the time-averaged tunneling current as a function of the bias voltage at every lateral position. LDOS images at negative sample voltages indicate that the tip apex is covered with Si atoms, which is consistent with the Si-Si covalent bonding mechanism for AFM imaging. A measurement technique for 3D force/current mapping and LDOS imaging on the equivalent surface area using the same tip was thus demonstrated.     

Enantioselective total synthesis of the potent anti-HIV nucleoside EFdA

A concise enantioselective total synthesis of 4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA), an extremely potent anti-HIV agent, has been accomplished from (R)-glyceraldehyde acetonide in 18% overall yield by a 12-step sequence involving a highly diastereoselective ethynylation of an α-alkoxy ketone intermediate.     

Aerobic oxidation of organic compounds catalyzed by vanadium compounds

Aerobic oxidation of α-hydroxy ketones catalyzed by dichloroethoxyoxovanadium in ethanol causes a carbon–carbon bond cleavage that produces diesters or diketones. This reaction is highly chemoselective, and disecondary glycols do not react at all. However, ditertiary glycols effectively react with dichloroethoxyoxovanadium or trichlorooxovanadium to provide the corresponding ketones. Aerobic oxidation of α-hydroxy ketones catalyzed by dichloroethoxyoxovanadium or trichlorooxovanadium in aprotic solvents almost quantitatively affords the corresponding α-diketones. The reaction of tertiary cyclopropanol compounds with vanadyl acetylacetonate under an oxygen atmosphere causes fragmentation of the cyclopropane moiety to produce β-hydroxy ketones and β-diketones. For the 6-substituted bicyclo[4.1.0]heptanol derivatives, the endoperoxides are also obtained together with β-hydroxy ketones. Conversely, 2-ethoxycarbonylcyclopropyl silyl ethers produce γ-oxocarboxylate derivatives given the same reaction conditions. Monothioacetals are easily deprotected into carbonyls using a catalytic amount of trichlorooxovanadium in 2,2,2-trifluoroethanol under an oxygen atmosphere. Thiols are converted into the corresponding disulfides by the aerobic oxidation catalyzed by trichlorooxovanadium in the presence of molecular sieves 3A. Polymer-supported vanadium compounds are synthesized by the reaction of vanadium oxytrichloride with polymers bearing hydroxyl moieties. The catalyst prepared from TentaGel S OH was highly active and reusable for the aerobic oxidations.     

Reaction of N-methyl-5-tributylstannyl-4-fluoro-1H-pyrazole and its application to N-methyl-chromeno[2,3-d]pyrazol-9-one synthesis

N-Methylation by sequential treatment of 5-tributylstannyl-4-fluoro-1H-pyrazole 1 with LDA and iodomethane regioselectively afforded the compound 2a in high yield. The addition reaction of 5-lithiated-4-fluoro-1H-pyrazole generated from 2a with a wide range of electrophiles allowed a facile introduction of different substituents at position 5 in good yields. The adduct 3d was efficiently converted to N-methyl-chromeno[2,3-d]pyrazol-9-one 9 in 3 steps.