Synthesis of 4-aminomethyl-tetrahydrofuran-2-carboxylates with 2,4-cis and 2,4-trans relationships

Templated tetrahydrofuran-based γ-azido esters were prepared with the C-2 and C-4 functionalities in cis and trans relative configurations. This was achieved by ring contraction of the suitably protected 2-O-triflates of pentono-1,5-lactones (d-ribose and l-arabinose) with subsequent introduction of the azide via the 4-O-triflate. Access to a corresponding β-azido ester was achieved in good yield. Little elimination product was observed by introduction of the azide via the 3-O-triflate. These azido esters are scaffolds, which may be predisposed to adopt secondary structural motifs, for example, for use as peptidomimetics; they may also be utilised for the preparation of stereodiverse compound libraries.     

Preparation of sugar derived β,β′-dihydroxy α,α-disubstituted α-amino acids

A novel strategy for the preparation of β,β′-dihydroxy α,α-disubstituted α-amino acids bearing a sugar moiety has been developed. The procedure is based on two Henry reactions: the first Henry reaction involves a sugar aldehyde and nitroethanol to furnish a sugar derived α-hydroxymethyl α-nitroalkanol while the second Henry reaction is between this nitro sugar and formaldehyde. This sequence provided the expected epimers of sugar derived α,α-dihydroxymethyl α -nitroalkanols, from which the corresponding β,β′-dibenzyloxy α-N-benzyloxycarbonylamino esters were easily obtained. All rights reserved.     

Temperature hysteretic effect and its influence on colossal magnetoresistance of La0.33Nd0.33Ca0.33MnO3

Electrical resistance (R) measurements of a bulk La_0.33Nd_0.33Ca_0.33MnO₃ perovskite in magnetic fields up to 40 kOe have revealed anomalous temperature hysteretic effects both in 0 Oe and 20 kOe magnetic fields. The sharp peak observed in the R vs. T plot indicates the occurrence of metal-to-insulator (M-I) transition at a temperature of T _MI=110 K and 140 K, for cooling and warming paths, respectively. An applied magnetic field of 20 kOe reduces the resistance and shifts T _MI to 160 K and 185 K for cooling and warming, respectively. We have observed a much higher resistance in the cooling path than in the warming path leading to the hysteretic resistance ratio (R _cool/R _warm) of 200 at 110 K and 1.8 at 160 K for 0 Oe and 20 kOe, respectively. Record values of colossal magnetoresistance (CMR) have been achieved. The CMR value reaches nearly 99% in the temperature ranges of 90 K to 140 K and 90 K to 170 K for 20 kOe and 40 kOe magnetic fields in the cooling mode, respectively. The observed unusual behavior is attributed to the co-existence of La-rich and Nd-rich domains assumed to be distributed randomly in the compound.     

ac Conductivity of mixed spinel NiAl0.7Cr0.7Fe0.6O4

ac Conductivity measurements are carried out across the metal to insulator transition in NiAl_0.7Cr_0.7Fe_0.6O₄. The low frequency data is analyzed using Summerfield scaling theory for hopping conductivity. The exponent of the scaling behavior has significantly different values in the conducting and insulating regimes. The hopping frequency and the zero frequency conductivity are found to increase with temperature, slowly in the metallic regime and rapidly in the insulating regime.     

Synthesis and biological activity of analogues of vanchrobactin, a siderophore from Vibrio anguillarum serotype O2

Several analogues of vanchrobactin, a catechol siderophore isolated from the bacterial fish pathogen Vibrio anguillarum serotype O2 strain RV22, have been synthesized. The biological evaluation of these novel compounds showed that most of them are active as siderophores, as determined by growth promotion assays using the producer strain, as well as V. anguillarum serotype O1, Salmonella enterica, and Erwinia chrysanthemi. These compounds also gave a positive chrome azurol-S (CAS) test. On the basis of these results, we were able to deduce some structure-activity relationships. Furthermore, we found an analogue with siderophore activity that has appropriate functionality (an amino group) for use as an antibiotic vector to be employed in a "Trojan horse strategy".     

Total synthesis of (-)-dysithiazolamide

The tetrachlorinated natural product (-)-dysithiazolamide was synthesized from l-glutamic acid in a convergent way, confirming the previously proposed (2S,4S,6S,8S) absolute stereochemistry.     

On the theory of slowing down gracefully

We discuss the transport of a tracer particle through the Bose?CEinstein condensate of a Bose gas. The particle interacts with the atoms in the Bose gas through two-body interactions. In the limiting regime where the particle is very heavy and the Bose gas is very dense, but very weakly interacting (??mean-field limit??), the dynamics of this system corresponds to classical Hamiltonian dynamics. We show that, in this limit, the particle is decelerated by emission of gapless modes into the condensate (Cerenkov radiation). For an ideal gas, the particle eventually comes to rest. In an interacting Bose gas, the particle is decelerated until its speed equals the propagation speed of the Goldstone modes of the condensate. This is a model of ??Hamiltonian friction??. It is also of interest in connection with the phenomenon of ??decoherence?? in quantum mechanics. This note is based on work we have carried out in collaboration with D Egli, I M Sigal and A Soffer.     

Local canted spin behaviour in Co1.4−x Zn x Ge0.4Fe1.2O4 spinels: A macroscopic, mesoscopic and microscopic study

DC magnetization, neutron depolarization and neutron diffraction (with both polarized and unpolarized neutrons) measurements have been reported for the Co_1.1−x Zn _x Ge_0.1Fe_1.2O₁ spinels with x=0.5, 0.6 and 0.7. Neutron depolarization and neutron diffraction measurements confirm the presence of a long range ferrimagnetic ordering of the local canted spins in these ferrite samples. The observed features of low field magnetization have been explained under the framework of thermally activated domain wall movement of ferrimagnetic arrangement of local canted spins. An important role of magnetic anisotropy (due to the presence of Co²⁺ ions) in establishing the magnetic ordering and domain kinetics in these ferrites has been observed.     

Transformation of D-glucose into 1D-3-deoxy-3-hydroxymethyl-myo-inositol by stereocontrolled intramolecular Henry reaction

[reaction: see text] An intramolecular Henry cyclization provides a promising new strategy for the transformation of nitroheptofuranoses into deoxyhydroxymethylinositols. This method has allowed a stereospecific transformation of d-glucose into 1D-3-deoxy-3-hydroxymethyl-myo-inositol.     

On left conjugacy closed loops in which the left multiplication group is normal

A loopQ is said to be left conjugacy closed (LCC) if the left translations form a set of permutations that is closed under conjugation. This papers investigates those LCC loops where the group generated by left translations is normal in the group generated by both left and right translations.