E- and Z-stereoselectivity in the preparation of enamides from glycidyl sulfonamides and carbamates

Treatment of glycidyl sulfonamides with LDA delivers the corresponding enesulfonamide with good selectivity for the E-isomer, whereas the corresponding carbamates exhibit selectivity for the Z-enecarbamate. An E1cB elimination mechanism proceeding from a substrate-base chelate complex is advanced as rationalisation of the latter set of Z-selective outcomes.     

Electrical detection of spin pumping due to the precessing magnetization of a single ferromagnet

We report direct electrical detection of spin pumping, using a lateral normal-metal/ferromagnet/normal-metal device, where a single ferromagnet in ferromagnetic resonance pumps spin-polarized electrons into the normal metal, resulting in spin accumulation. The resulting backflow of spin current into the ferromagnet generates a dc voltage due to the spin-dependent conductivities of the ferromagnet. By comparing different contact materials (Al and/or Pt), we find, in agreement with theory, that the spin-related properties of the normal metal dictate the magnitude of the dc voltage.     

Synthesis and conformational analysis of 2'-fluoro-5-methyl-4'-thioarabinouridine (4'S-FMAU)

An improved synthesis of 2'-deoxy-2'-fluoro-5-methyl-4'-thioarabinouridine (4'S-FMAU) is described. Participation of the 3'-O-benzoyl protecting group in the thiosugar precursor influenced the stereochemistry of the N-glycosylation reaction in nonpolar solvents, permitting a higher beta/alpha ratio than previously observed for similar Lewis acid catalyzed glycosylations. Conformational analysis of the nucleoside using 3JHH and 3JHF NMR coupling constants together with the PSEUROT program showed that it adopted a predominantly northern conformation in contrast to 2'-deoxy-2'-fluoro-5-methylarabinouridine (FMAU), whose PSEUROT conformational analysis is presented here for the first time, which showed a dominantly southeast conformation. The sharp conformational switch attained by replacing the ring heteroatom is attributed to a decrease in relevant steric and stereoelectronic effects.     

Mode coherence measurements across a 1.5 THz spectral bandwidth of a passively mode-locked quantum dash laser

The mode coherence of adjacent and non-adjacent spectral modes of a passively mode locked quantum dash (QDash) semiconductor laser are deduced through radio frequency beat-tone linewidth measurements. A wavelength conversion scheme that uses degenerate four wave mixing in a semiconductor optical amplifier is proposed which considerably extends the mode spacing beyond the limit imposed by conventional fast-photodetection and electrical spectrum analysis of around 100 GHz. Using this scheme, the mode coherence of the QDash laser was measured out to the thirty-first harmonic, or a mode separation of 1.5 THz.     

ChemInform Abstract: Stabilization of the Pseudo-Benzene N6 Ring with Oxygen.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Application of phosphonate and thiophosphate analogues of nucleotides to studies of some enzyme reactions

The enzymatic cleavage of a scissile P O bond can be blocked by recourse to phosphonate analogues of biological phosphate esters. α-Fluorophosphonates have an enhanced electronegativity at the bridging carbon, which, in many cases, makes them superior to simple methylene phosphonates for the study of enzyme reactions. Thus, the β,γ-difluoro-methylene analogue of ATP is a good substrate for the interferon-induced (2→5)A_n synthetase, which converts it into a (2→5)A₄ species having a 5′-β,γ-difluoromethylenetriphosphate. This binds strongly to RNase L but does not activate it. The unsymmetrical Ap₄Aases from Artemia and Lupin are strongly inhibited by P^1,P⁴-dithiophosphate analogues of diadenosyl-5′,5‴-P¹,P⁴-tetraphosphate although nonregiospecific cleavage of certain P²,P³-methylene analogues can be observed. Some of these analogues are remarkably effective inhibitors of platelet aggregation and are effective inhibitors in vivo of arterial blood-clotting in rabbits. Separation of all diastereoisomers of P¹,P⁴-dithiophosphate analogues of Ap₄A is achieved cleanly using reverse-phase hplc chromatography and this provides direct access to β,γ-CHF-bridged analogues of ATP with resolved stereochemistry at the CHF center. Lastly, growing cells of Dictyostelium discoideum not only tolerate a range of substituted methylene bisphosphonates in their growth medium but actually incorporate them into nucleotide analogues of ATP and Ap₄A.