An improved synthesis of pyrido[2,3-d]pyrimidines

An improved and efficient synthesis of 1,3-dialkylpyrido[2,3-d]pyrimidine-2,4-(1H,3H)-diones from 6-methylaminouracils and methyl propiolate or diethyl ethoxymethylenemalonate is described.     

Synthesis of novel 3-(α-arylhydrazono-1,3,4-oxadiazol-2-ylmethyl)-2-oxo-1,2-dihydroquinoxalines and their characteristic tautomerism between the hydrazone imine and diazenyl enamine forms

The reactions of 3-(α-arylhydrazono)hydrazinocarbonylmethyl-2-oxo-1,2-dihydroquinoxalines 1a,b with triethyl orthoesters resulted in the intramolecular cyclization to give the 3-(α-arylhydrazono-1,3,4-oxadiazol-2-ylmethyl)-2-oxo-1,2-dihydroquinoxalines 4a–d , but not the 1,2,4,5-tetrazepinylquinoxalines 5a–d . The cyclization mode into the 1,3,4-oxadiazole ring was confirmed by the alternate syntheses of 4a,c from the reactions of 3-(1,3,4-oxadiazol-2-ylmethylene)-2-oxo-1,2,3,4-tetrahydroquinoxalines 6a,b with o-chlorophenyl diazonium salts, respectively. Moreover, 4a–d exhibited an interesting tautomerism between the hydrazone imine form A and diazenyl enamine form B.     

Characterization of the IF Noise of a Submillimeter SIS Receiver

A fitting method is presented here for the accurate characterization of the IF noise contribution of a sub-millimeter SIS receiver. By fitting the mixer's IF output power response and junction's IV curve of an SIS mixer without LO pumping, we can obtain the IF noise contribution, the physical temperature of the isolator connected just behind the SIS mixer, the output mismatching of the mixer, and the total gain of the IF chain. Differing from a conventional method, which only uses the normal-state (linear) branch of the junction's IV curve, the method proposed here also includes the nonlinear portion around the gap voltage. The dynamic resistance in this portion is varied dramatically, providing us a good probe to characterize the output mismatching of the mixer, as well as other parameters.     

Simulation study on microstructure formations in magnetic fluids

We propose the Langevin-type microscopic equations of motion for magnetic fluids. Magnetic fluids are modeled as an ensemble of interacting ferromagnetic nanoparticles suspended in a viscous fluid. The present model is described in terms of position vectors of nanoparticles and orientation vectors of their magnetic dipole moments. In this model, forces and torques arising from the magnetic origin and the surrounding fluid flow are included. Effects of non-spherical particle shape are also taken into account. From the Brownian dynamics simulations of the model, it is found that the present model exhibits various microstructure formation processes in magnetic fluids.     

Effects of encounters on phase separation dynamics from diffusion-controlled growth to coarsening

We investigate the dynamics of phase separations in quenched binary mixtures, such as binary alloys and polymer blends with low mobilities, at higher volume fractions. Computer simulations are performed using simple model equations in the presence of encounters of droplets. Four characteristics stages with three crossovers, including dynamical scaling behavior and temporal power laws, are found beyond a volume fraction of approximately 0.14, where each crossover is attributed to a transition from one growth mechanism to the other. The encounter effect is thus shown to strongly influence a transition from the diffusion-controlled (t^1/2) stage to the coarsening (t^1/3) stage. A simple analytical argument for the simulation results is also presented.     

Preparation of polyethylene block copolymers by a combination of postmetallocene catalysis of ethylene polymerization and atom transfer radical polymerization

The synthesis of block copolymers consisting of a polyethylene segment and either a poly(meth)acrylate or polystyrene segment was accomplished through the combination of postmetallocene-mediated ethylene polymerization and subsequent atom transfer radical polymerization. A vinyl-terminated polyethylene (number-average molecular weight = 1800, weight-average molecular weight/number-average molecular weight =1.70) was synthesized by the polymerization of ethylene with a phenoxyimine zirconium complex as a catalyst activated with methylalumoxane (MAO). This polyethylene was efficiently converted into an atom transfer radical polymerization macroinitiator by the addition of α-bromoisobutyric acid to the vinyl chain end, and the polyethylene macroinitiator was used for the atom transfer radical polymerization of n-butyl acrylate, methyl methacrylate, or styrene; this resulted in defined polyethylene-b-poly(n-butyl acrylate), polyethylene-b-poly(methyl methacrylate), and polyethylene-b-polystyrene block copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 496–504, 2004     

1H NMR spectral analysis of physalin A,a 13,14-seco-16,24-cyclosteroid,based on crystal structure of (25R)-2,3,25,27-tetrahydrophysalin a dimethanol solvate

(25R)-2,3,25,27-Tetrahydrophysalin A dimethanol solvate, C₃₀H₄₂O₁₂, a derivative of 13,14-seco-16,24-cyclosteroid physalin A, crystallizes in the orthorhombic space groupP2₁2₁2₁, witha=15.486(5),b=14.794(4), 12.248(6) Å andZ=4. The crystal structure has been solved by direct methods, and refined to an R-value of 0.053 for 2345 observed reflections.¹H NMR spectral data of physalin A and its hydrogenated derivatives measured in dimethylsulfoxide-d₆ solutions have been elucidated, based on the crystal structure of the tetrahydrophysalin A, enabling the assignment of all the proton signals of physalin A. The-proton of C(11)-methylene has been shown to resonate at remarkably higher field due to anisotropic shielding effect of carbonyl group at C(15).     

Subshell-pair correlation coefficients of atoms in momentum space

Angular correlation coefficients τ ^{nl,n^′ l^′} [p] between linear momenta of an electron in a subshell nl and another electron in a subshell n′ l′ are studied for the 102 neutral atoms He through Lr in their ground states, where n and l are the principal and azimuthal quantum numbers, respectively. We theoretically find that electron momenta are negatively correlated or uncorrelated; τ ^{nl,n^′ l^′} [p] < 0 when |l − l′|=1, while τ ^{nl,n^′ l^′} [p]=0 when |l − l′| ≠ 1. Numerical examinations of the atoms show that except for the He–B atoms, negative correlations are largest between 1s and 2p subshells, which have the most diffuse electron distributions in momentum space.     

A convenient synthesis of novel spiro[benzoxazole-2′,4(1H,3′H)-pyrazolo[5,1-c][1,2,4]triazines] by ring transformation of novel pyrazolo[1′,5′:3,4][1,2,4]triazino[5,6-b][1,5]benzoxazepines

Novel pyrazolo[1′,5′:3,4][1,2,4]triazino[5,6-b][1,5]benzoxazepines 5, 6 and 8 were synthesized, and these compounds were converted into novel spiro[benzoxazole-2′,4(1H,3′H)-pyrazolo[5,1-c][1,2,4]triazines] 7 and 9 by ring transformation.     

Synthesis and biological activity of 1,5-dihydropyridazino-[3,4-b]quinoxalines and a new class of quinolones

A new class of quinolones, 1,4-dihydro-4-oxopyridazino[3,4-b]quinoxaline-3-carboxylic acids and related compounds, were synthesized via oxidation of 1,5-dihydropyridazino[3,4-b]quinoxalines obtained from 2-hydrazinoquinoxaline 4-oxides. Some of the 1,5-dihydropyridazino[3,4-b]quinoxalines, 1,4-dihydro-4-oxopyridazino[3,4-b]quinoxaline-3-carboxylic acids, and related compounds showed biological activity.