Synthèse d'analogues acycliques de C-nucléosides

A direct method for the preparation of 2-hydroxyethoxymethyl derivatives of guanine and s-triazolo[1,5-a]pyridine in which the chain is joined into the heterocyclic base by a carbon-carbon bond is describe.     

On-line separation of short-lived tungsten isotopes from tantalum,hafnium and lutetium by adsorption on ion exchangers from aqueous ammonia solution

The title goal was achieved using a DOWEX 50Wx8 cation exchange column saturated with La(OH)₃ and ammonia solution as eluent. Hf, Ta and Lu were adsorbed on this column, whereas W remained in the solution. This chemical system may be used for fast on-line separations of element 106.     

Synthesis and characterization of some new 1,2,3-thiadiazole and 1,2,3-selenadiazole triterpene derivatives from allobetulone and 2-oxoallobetulin

Abstract

The synthesis of new 1,2,3-thiadiazole and 1,2,3-selenadiazole derivatives from triterpenoid ketones has been investigated via the corresponding semicarbazones. The intermediates 5, 8 have also been isolated, separated and their structures identified. The Hurd–Mori reaction and Lalezari method have been applied to synthesize a series of new substances 6 and 7. The regioselectivity of the functionalization mostly was centered at the C-3 position for the products 9 and 10. The structures of these compounds were confirmed by 2D-NMR spectroscopy.     

Palladium catalysis for the synthesis of hydrophobic C-6 and C-2 aryl 2'-deoxynucleosides. Comparison of C-C versus C-N bond formation as well as C-6 versus C-2 reactivity

Suzuki-Miyaura cross-coupling of haloaromatic compounds with arylboronic acids provides a simple entry to biaryl systems. Despite its ease, to date, there are no detailed investigations of this procedure for deoxynucleoside modification. As shown in this study, a wide variety of C-6 arylpurine 2'-deoxyriboside (C-6 aryl 2'-deoxynebularine analogues) and C-2 aryl 2'-deoxyinosine analogues can be conveniently prepared via the Pd-mediated cross-coupling of arylboronic acids with the C-6 halonucleosides, 6-bromo- or 6-chloro-9[2-deoxy-3,5-bis-O-(tert-butyldimethylsilyl)-beta-D-erythro-pentofuranosyl]purine (1 and 2), and the C-2 halonucleoside, 2-bromo-O(6)-benzyl-3',5'-bis-O-(tert-butyldimethylsilyl)-2'-deoxyinosine (3). Although bromonucleoside 1 proved to be a good substrate for the Pd-catalyzed Suzuki-Miyaura cross-couplings, we have noted that for several C-6 arylations, the chloronucleoside 2 provides superior coupling yields. Also described in this study is a detailed evaluation of catalytic systems that led to optimal product recoveries. Finally, a comparison of the C-C and C-N bond-forming reactions of deoxynucleosides is also reported. On the basis of this comparison, we provide evidence that C-N bond formation at the C-6 position, leading to N-aryl 2'-deoxyadenosine analogues, is more sensitive to the ligand used, whereas C-C bond-forming reactions at the same position are not. In contrast to the ligand dependency exhibited in C-N bond formation at the C-6 position, comparable reactions at the C-2 position of purine deoxynucleosides proceed with less sensitivity to the ligand used.     

Synthesis, structure, and reactivity of fluorous phosphorus/carbon/phosphorus pincer ligands and metal complexes

Reactions of the diphosphine 1,3-C6H4(CH2PH2)2 and fluorous alkenes H2C=CHR(fn)(R(fn)=(CF2)(n-1)CF3; n = 6, 8) at 75 degrees C in the presence of AIBN give the title ligands 1,3-C6H4(CH2P(CH2CH2R(fn))2)2(3-R(fn)) and byproducts 1,3-C6H4(CH3)(CH2P(CH2CH2R(fn))2)(4-R(fn)) in 1 : 3 to 1 : 5 ratios. Workups give -R(fn) in 4--17% yields. Similar results are obtained photochemically. Reaction of 1,3-C6H4(CH2Br)2 and HP(CH2CH2R(f8))2 (5) at 80 degrees C (neat, 1 : 2 mol ratio) gives instead of simple substitution the metacyclophane [1,3-C6H4(CH2P(CH2CH2R(f8))2 CH2-1,3-C(6)H(4)CH(2)P[lower bond 1 end](CH2CH2R(f8))2C[upper bond 1 end]H2](2+)2Br-, which upon treatment with LiAlH(4) yields 3-R(f8)(20%), 4-R(f8), and other products. Efforts to better access 3-R(f8), either by altering stoichiometry or using various combinations of the phosphine borane (H3B)PH(CH2CH2R(f8))2 and base, are unsuccessful. Reactions of 3-R(fn) with Pd(O2CCF3)2 and [IrCl(COE)2]2(COE=cyclooctene) give the palladium and iridium pincer complexes (2,6,1-C6H3(CH2 P(CH2CH2R(fn))(2)(2)Pd(O2CCF3)(10-R(fn); 80-90%) and (2,6,1-C6H3(CH2P(CH2CH2R(f8))2)2)Ir(Cl)(H)(11-R(f8); 29%), which exhibit CF3C6F(11)/toluene partition coefficients of >96 : <4. The crystal structure of 10-R(f8) shows CH2CH2R(f8) groups with all-anti conformations that extend in parallel above and below the palladium square plane to create fluorous lattice domains. NMR monitoring shows a precursor to 11-R(f8) that is believed to be a COE adduct.     

H(2)O Outgassing Properties of Fumed and Precipitated Silica Particles by Temperature-Programmed Desorption

Temperature-programmed desorption was performed at temperatures up to 850 K on as-received fumed and precipitated silica particles. Physisorbed water molecules on both types of silica had activation energies in the range of 38–61 kJ/mol. However, the activation energies of desorption for chemisorbed water varied from 80 to >247 kJ/mol for fumed silica, Cab-O-Sil-M-7D, and 96 to 155 kJ/mol for precipitated silica, Hi-Sil-233. Our results suggest that physisorbed water can be effectively pumped away at room temperature (or preferably at 320 K) in a matter of hours. Chemisorbed water with high activation energies of desorption (>126 kJ/mol) will not escape silica surfaces in 100 years even at 320 K, while a significant amount of the chemisorbed water with medium activation energies (80–109 kJ/mol) will leave the silica surfaces in that time span. Most of the chemisorbed water with activation energies <126 kJ/mol can be pumped away in a matter of days in a good vacuum environment at 500 K. We had previously measured about 0.1–0.4 wt% of water in silica-reinforced polysiloxane formulations containing 21% Cab-O-Sil-M-7D and 4% Hi-Sil-233. Comparing present results with these formulations, we conclude that the adsorbed H₂O and the Si–OH bonds on the silica surfaces are the major contributors to water outgassing from these types of silica-filled polymers.     

An optical pH sensor with extended detection range based on fluoresceinamine covalently bound to sol–gel support

An optical pH sensor was developed based on the fluorophor, fluoresceinamine isomer II (FA), covalently immobilized in a sol–gel matrix. This sol–gel matrix was created by the copolymerization of two precursors, methyltriethoxysilane (MTES) and 3-glycidoxypropyltrimethoxysilane (GPTMS), in an ethanolic solution. Fluoresceinamine was covalently bound to the glycidoxypropyl chain group of GPTMS, thereby preventing it from leaching. Moreover, the immobilization of the fluoresceinamine also extended its linear detection range. The sensor showed good repeatability, a short response time of less than 8 s, high long-term stability and no temperature effect in the biologically relevant range. In the pH range of 4–10, the sensor was very sensitive and its linear range was found to be between pH 6 and 9 (R² = 0.995).