Synthesis and photolysis of doxyl derivatives of 5β-androstan-3-ones

In an unsuccessful attempt to effect remote functionalization at C9, 4,4-dimethyloxazolidine-N-oxyl (doxyl) derivatives at C3 of 5β-androstan-3,17-dione and 5β-androstan-17β-ol-3-one were prepared and photolyzed. The former doxyl yielded its C13 stereoisomer; the latter was converted very slowly to intractable material.     

A copper-catalyzed C-N bond formation involving sp-hybridized carbons. A direct entry to chiral ynamides via N-alkynylation of amides

A copper-catalyzed new C-N bond formation involving a sp-hybridized carbon is described here leading to a facile entry for syntheses of chiral ynamides. This direct N-alkynylation of amides should have a significant impact on the future development of synthetic methodologies employing ynamides.     

Structures of lanthanide shift reagent complexes by molecular mechanics computations

Molecular mechanics calculations have been applied to the structure determination of 7-coordinate lanthanide complexes. To circumvent problems in defining oxygen—lanthanide—oxygen bond angles, the energy of angle deformations at the metal center are not evaluated explicitly. Instead the standard approach to molecular mechanics calculations is modified by including 1,3-nonbonded interactions between atoms that are both bonded to the metal center. Geometry optimization for two known lanthanide complexes afforded structures that are in reasonable agreement with X-ray crystal structures, and small discrepancies are attributed to cyrstal packing forces.     

Two families of mixed finite elements for second order elliptic problems

Summary Two families of mixed finite elements, one based on triangles and the other on rectangles, are introduced as alternatives to the usual Raviart-Thomas-Nedelec spaces. Error estimates inL ² () andH ^–5 () are derived for these elements. A hybrid version of the mixed method is also considered, and some superconvergence phenomena are discussed.     

The convergence of spline collocation for strongly elliptic equations on curves

Summary Most boundary element methods for two-dimensional boundary value problems are based on point collocation on the boundary and the use of splines as trial functions. Here we present a unified asymptotic error analysis for even as well as for odd degree splines subordinate to uniform or smoothly graded meshes and prove asymptotic convergence of optimal order. The equations are collocated at the breakpoints for odd degree and the internodal midpoints for even degree splines. The crucial assumption for the generalized boundary integral and integro-differential operators is strong ellipticity. Our analysis is based on simple Fourier expansions. In particular, we extend results by J. Saranen and W.L. Wendland from constant to variable coefficient equations. Our results include the first convergence proof of midpoint collocation with piecewise constant functions, i.e., the panel method for solving systems of Cauchy singular integral equations.Dedicated to Prof. Dr. Dr. h.c. mult. Lothar Collatz on the occasion of his 75th birthdayThis work was begun at the Technische Hochschule Darmstadt where Professor Arnold was supported by a North Atlantic Treaty Organization Postdoctoral Fellowship. The work of Professor Arnold is supported by NSF grant BMS-8313247. The work of Professor Wendland was supported by the Stiftung Volkswagenwerk     

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.     

Synthesis of fluorinated 1,8-naphthyridinone derivatives

Processes for the synthesis of fluorinated 1,8-naphthyridinone derivatives including 6,7-difluoro-1,8-naphthyridin-2-one are described.     

Novel mass spectral fragmentation of heptafluorobutyryl derivatives of acyl analogues of platelet-activating factor

Direct derivatization of the acyl analogue of platelet-activating factor (acyl-PAF) with heptafluorobutyric anhydride results in replacement of the phosphocholine moiety with a heptafluorobutyryl (HFB) group. Electron capture (EC) mass spectrometric analysis of this compound that makes use of negative ion detection along with subsequent accurate mass measurement and tandem mass spectrometry studies revealed that in addition to expected fragmentation due to losses of elements of HF, ketene, and/or acetic acid, there is a rearrangement reaction between the HFB group and the substituent on carbon-2 of the glycerol backbone. For 2-acetyl isomers, this fragmentation yields a characteristic ion at m/z 237; for 1-acetyl isomers, the analogous ion is observed at [M-135]⁻, along with a corresponding carboxylate anion. The use of the HFB derivative is invaluable for analysis of PAF homologues and analogues because it provides detailed structural information in combination with the high sensitivity of a gas chromatography combined with EC-mass spectrometry assay.     

Chemical modification of a viral cage for multivalent presentation

Here we present generalized methods for chemically modifying the surface of a viral protein cage; this exploits the chemistry of native and engineered surface exposed functional groups for multivalent presentation of ligands.