An efficient synthesis of the constrained peptidomimetic 2-oxo-3-(N-9-fluorenyloxycarbonylamino)-1-azabicyclo[4.3.0]nonane-9-carboxylic acid from pyroglutamic acid

[reaction: see text] Azabicyclo[X.Y.0]alkane amino acids are rigid dipeptide mimetics that are useful tools for structure-activity studies in peptide-based drug discovery. Herein, we report an efficient synthesis of three diastereomers of 9-tert-butoxycarbonyl-2-oxo-3-(N-tert-butoxycarbonylamino)-1-azabicyclo[4.3.0]nonane (3S,6S,9S, 3S,6R,9R, and 3S,6R,9S). Methyl N-Boc-pyroglutamate is cleaved with vinylmagnesium bromide to produce an acyclic gamma-vinyl ketone. Michael addition of N-diphenylmethyleneglycine tert-butyl ester produces the N-Boc-delta-oxo-alpha,omega-diaminoazelate intermediate, which, on hydrogenloysis, gives the fused ring system. Acidolytic deprotection followed by Fmoc-protection provided building blocks suitable for solid-phase synthesis.     

Oxygen quenching of tris(2,2′-bipyridine) ruthenium(II) complexes in thin organic films

A study is presented of the quenching, by oxygen, of the luminescence of tris(2,2′-bipyridine) ruthenium(II) complexes immobilized in thin, transparent, polymer-based films. The film media consist of a water-insoluble linear polymer plasticized with a trialkylphosphate ester, in which the complex ruthenium cations are solubilized by ion pairing with organophilic anions such as tetraphenylborate.

Luminescence lifetimes were studied in relation oxygen concentration in a gas stream contiguous with the film medium, film thickness and concentration of the metal complex within the film medium. It is shown that the microheterogeneous environment of the luminescent complex, which has recently been implicated in the non-linear quenching responses of polymer-immobilized, transition metal complex oxygen sensors, may arise simply as a consequence of the limited solubility of the complex in the film medium. When solubility is limited, the partial precipitation of the complex results in a colloidal of luminescent particles which exhibit non- uniform susceptibilities to quenching by oxygen. Good solubility, and therefore linear quenching characteristics, are promoted by methyl substitution of the bipyridyl ligand and by use of a plasticizer (tributylphosphate) with marked cation solvating powers.     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

Photochemistry in ordered physisorbed monolayers of dinitrogen tetroxide

The effects of physisorption and two-dimensional ordering on the photochemistry of N₂O₄ were investigated. Ordered monolayers were prepared by adsorption of NO₂ at 100 K on a water-ice surface. Irradiation with a continuous light source in the wavelength region 300–400 nm or with pulsed laser radiation at 355 nm resulted in exclusive desorption of NO₂. This desorption was induced by electronic absorption directly in the adsorbate via a transition corresponding to the ( )¹B_2u←( )¹A_g transition in N₂O₄, as in the gas phase. However, the subsequent dynamics in the excited state were markedly different from the gas-phase counterpart. Time-of-flight mass spectrometry of NO₂ photodesorbed at 355 nm revealed a most probable fragment translational energy of ca. 17 meV; and the angular distribution of the nascent NO₂ was peaked sharply in a direction around 10° from the normal. It is apparent that, despite the weak interaction with the substrate, significant energy transfer occurs in the ordered physisorbed monolayer to yield nascent NO₂ with very low translational energy and a constrained angle of escape which is consistent with a high degree of adsorbate order and alignment.     

Fast atom bombardment positive-ion mass spectra of 1,2,2-tris(sulfonyl)hydrazines

Several 1,2,2-tris(sulfonyl)hydrazines, conceived as prodrugs of 1,2-bis(sulfonyl)hydrazines and synthesized as potential antineoplastic and trypanocidal drugs, were analyzed by mass Spectrometry as part of the confirmation of the structure of these compounds. Since these compounds showed activity against several transplanted animal tumors, an understanding of the mass spectral behavior of these agents is important to gain information should clinical trials and metabolic studies be undertaken. In all spectra, protonated molecular ions were observed. Collision activation resulted in considerable fragmentation in the fast atom bombardment spectra. Rearrangement ions formed by elimination of substituted diimides from the middle of the molecule were observed. Other rearrangements involving the alkyl substituent and sulfonyl oxygen also occurred.     

Neutron total,scattering and inelastic Gamma-ray cross sections of yttrium at few MeV energies

Neutron total, scattering and (n; n′,γ) cross sections of elemental yttrium (⁸⁹Y) were measured in the few-MeV region. The neutron total-cross-section measurements were made with broad resolutions from ≈0.5 to 4.2MeV in steps of ?0.1 MeV. Neutron elastic- and inelastic-scattering cross sections were measured from ≈1.5 to 4.0 MeV, at incident-neutron energy intervals of ≈50keV and at ten or more scattering angles distributed between 20 and 160 degrees using neutron detection. Inelastic-scattering cross sections were also determined using the (n; n′,γ) reaction at incident energies from 1.6 to 3.8 MeV at intervals of 0.1 MeV. Gamma-rays and/or inelastically-scattered neutrons were observed corresponding to the excitation of levels at: 909.0±0.5, 1,507.4 ±0.3, 1,744.5±0.3, 2,222.6±0.5, 2,530±0.8, 2,566.4±1.0, 2,622.5±1.0, 2,871.9 ±1.5, 2,880.6±2.0, 3,067.0±2.0, 3,107.0±2.0, 3,140.0±2.0, 3,410.0±2.0, 3,450.0±2.0, 3,504.0 ±1.5, 3,514.0±2.0, 3,556.0±2.0, 3,619.0±3.0, 3,629.0±3.0 and 3,715.0±3.0 keV. The experimental results are discussed in terms of the spherical-optical-statistical, coupledchannels, and core-coupling models, and in the context of previously reported excitedlevel structure.     

The level structures of the nuclei232Th and238U

Time-of-flight gating techniques have been used to study the decay γ-rays from states excited by inelastic scattering of neutrons from²³²Th and²³⁸U. Neutron energies up to 1900 MeV have been used. From accurate determinations of the γ-ray energies, intensities and thresholds, detailed level and decay schemes have been obtained for²³²Th and²³⁸U. New levels in both nuclei are observed at larger excitations than before and the present work is incompatible with some previously accepted spin and collective band assignments derived from Coulomb excitation studies.