Preparation of a deoxynucleoside thiophosphoramidite intermediate in the synthesis of nucleoside phosphorodithioates

A general synthetic scheme for preparation of thiophosphoramidite intermediates, chloro-N,N-diisopropylaminothiomethoxyphosphine, 2, and 3′-O-deoxythymidine derivatives of N,N-diisopropylaminothiomethoxyphosphine, 4, is described. These intermediates can be used to synthesize dithiophosphate deoxyoligonucleotide analogues.     

Conformation of acetate derivatives of sugars and other cyclic alcohols. Crystal structures, NMR studies, and molecular mechanics calculations of acetates. When is the exocyclic C-O bond eclipsed?

[reaction: see text] A study of published crystal structures (of O-acetylated sugars for the most part) suggests that the exocyclic C-O bond in acetate esters of cyclic alcohols intrinsically prefers a staggered conformation, although the eclipsed conformation is only slightly less stable. When the acetate is flanked by two equatorial substituents the preferred conformation is close to eclipsed. Over 1500 C-OAc bonds have been analyzed. Diagnostic NMR criteria for torsion angles and MM3 calculations are reported and confirm these conclusions.     

Photochemical reactivity of trifluoromethyl aromatic amines: the example of 3,5-diamino-trifluoromethyl-benzene (3,5-DABTF)

This work presents the application of an on-line photoreactor to the detection of 3,5-diamino-trifluoromethyl-benzene (3,5-DABTF) in aqueous solutions. When irradiated at 310 nm, this compound is defluorinated to 3,5-diaminobenzoic acid by a nucleophilic substitution of the fluoride by water. Concomitantly, defluorination intermediates polymerize through amide bonds to give dark-colored compounds. We take advantage of the photocatalyzed defluorination and the subsequent decrease in pH to develop an original and specific photoreactor. Continuous recording of pH and temperature in the outlet of the reactor by a dual electrode gives us an opportunity to optimize the system. In the photoreactor, 3,5-DABTF is immediately and totally transformed as attested by the rapid drop of the flowing solution pH from 6.2 to 3.2 and the chromatographic analysis of the outgoing solutions. The detection remains effective from 1 to 1000 parts per million.     

Search for improved host architectures: application of de novo structure-based design and high-throughput screening methods to identify optimal building blocks for multidentate ethers

This paper presents a computational approach to the deliberate design of improved host architectures. The approach, which involves the use of computer-aided design software, is illustrated by application to cation hosts containing multiple aliphatic ether oxygen binding sites. De novo molecule building software, HostDesigner, is interfaced with molecular mechanics software, GMMX, providing a tool for generating and screening millions of potential bidentate building block structures. Enhanced cation binding affinity can be achieved when highly organized building blocks are used to construct macrocyclic hosts.     

Dicyclopentadienylnickel-alkyne complexes: photochemical formation and mass spectra

Irradiation of toluence or n-hexanne solutions of dicyclopentadienyldicarbonyldinickel complexes at λ ? 350 nm gives good yields of bridged dinuclear complexes (RCCR′)(CpNi)₂. Their bonding and mass spectra are discussed.     

A redetermination of the lifetime of the 4s 5s (3S1) level in Ca by high-rate electron—photon stepwise pulse excitation

The radiative lifetime of the 4s 5s (³S₁) level in Ca was measured using two-step excitation of atoms in a beam which traverses a cold-cathode Penning discharge, where, by electron impact the atoms are excited to the metastable 4s 5p (³P_{2, 1, 0}) states. Immediately thereafter, the beam is crossed with a pulse-modulated cw dye laser beam to populate the final state selectively. Using a pulse-modulation frequency of 0.5 MHz, a preliminary value of 12.4 ± 0.5 ns was obtained with a statistical error of 0.4%. The wide limits of systematic error reflect an assumed uncertainty in the excitation function and possible distortion in the fluorescence decay requiring further study.     

Die ionisierenden Potentialwellen beim Funkendurchschlag

Extending earlier studies on spark breakdown, the development of the impulse prebreakdown streamers leading to complete breakdown were investigated. By taking advantage of the proportionality between point potential and length of the positive Lichtenberg figure produced, the potential of the streamer tips progressing from a point anode towards a plane cathode were determined. It was found that the streamer tip potential decreases as streamer advances. At voltages much lower than sparking potential this decrease is rapid. By approaching the sparking potential the loss of tip potential diminishes considerably near the anode. The decrease of potential remains very rapid near the cathode until sparking potential is reached. At the cathode the tip potential increases with the applied potential. At a certain value electrons are emitted from the cathode under the influence of high tip fields. These electrons are accelerated towards the tips forming avalanches by collision. These avalanches appear on the photographic film as minute dots whose number and density depend on the intensity of tip fields. If the applied potential exceeds the sparking voltage, tip potentials are recorded on the cathode between 15 and 25 kV. The avalanches formed in the high intensity field region reach the critical size and form negative, anode — directed streamers. These streamers appear on the film as negative Lichtenberg figures. This ionising potential wave, known as return stroke, travels along the channel of the positive primary streamer. It increases its ionisation to such a degree that the main stroke can take place.     

Induced supercontinuum and steepening of an ultrafast laser pulse

The problem of the propagation of an intense ultrashort pulse in a cubic (χ³) nonlinear medium is generalized to include coupling between the primary and second harmonics signals. It is shown that the presence of a strong primary signal induces the superbroadening of the spectrum of a weak second harmonic signal and the deformation of its pulse shape.     

Investigation of the structural directing effect of (2-hydroxyethyl)trimethylammonium on distribution of the silicate anions using 29Si NMR spectroscopy

The effect of (2-hydroxyethyl)trimethylammonium (2-HETMA) cation on the equilibrium of silicate oligomers in aqueous alkaline silicate solutions was investigated using (29)Si NMR spectra. The results indicate role of structural directing of 2-HETMA in which it particularly directs the silicate species to the Q(4)(1)Q(4)(3)Q(4)(4) silicate anion. Results reveal that composition of the alcohols in solution affect the distribution of anionic species. The effect of methanol concentration is also discussed.