The application of C-N.M.R. spectroscopy to the determination of the stereochemistry of Diels-Alder adducts

Long-range (³J) ¹³C-H coupling is a reliable probe to evaluate the stereochemistry of cycloaddition products. The method is best applied to carbonyl containing adducts but not restricted to them. Several structures have been revised and new ones evaluated.     

The polymerization of 1,4-pentadiene to [3.3.1]bicyclo-linked chains

1, 4-Pentadiene has been polymerized by Ziegler-Natta catalysts to give amorphous polymers which are up to 64 wt.-% soluble. The polymers all have residual unsaturation which is substantially less than one double bond per mer, some as low as 0.1 double bond per mer. The polymerizations proceed by an unusual double inter-, double intramolecular cyclopolymerization mechanism leading to the formation of [3.3.1] bicyclic repeat units. Some main-chain (internal) unsaturation as well as pendant-group (external) unsaturation is observed. The latter results from incompletely cyclized 1,4-pentadiene units. The former is due to some concurrent isomerization of 1,4- to 1,3 pentadiene, which then copolymerizes with the 1,4 diene. The extent of isomerization varied with the catalyst system used. The soluble polymer fractions were brittle for internal unsaturations of less than about 0.05 double bond per mer and were viscous for higher values. The insoluble fractions were brittle and are believed to be lightly crosslinked.     

QUANTUM YIELD OF PHOTOHYDRATION FOR DIMETHYLURACIL. CONCENTRATION AND WAVELENGTH DEPENDENCE*

Abstract— The quantum yields for the photohydration of dimethyluracil were determined for concentrations in the range 5 × 10^--¹-^-1 × 10^--³M by use of 240–280 nm irradiation. The average quantum yield (0.0139 f 0.0005) was independent of both concentration and irradiation wavelength.     

The reactions of iminosulfur difluorides and chlorine monofluoride

The reactions of perfluoroalkyl- and perfluoroacyl-iminosulfur difluorides with chlorine monofluoride result in the preparation of perfluoroalkyldichloroamines and a new class of compounds N,N-dichloroperfluoroamides, R_fC(O)NCl₂, via the elimination of SF₄. The amides, FC(O)NCl₂ and CF₃C(O)NCl₂, in addition to 1,2-bis-(dichloroamino)tetrafluoroethane, Cl₂NCF₂CF₂NCl₂, are reported and characterized. The reactions of CIF with other sulfur(IV) imines proceed in an analogous manner to form perfluoroalkyl-dichloroamines via the elimination of the corresponding sulfur(IV) fluoride.     

X=Y-ZH systems as potential 1,3-dipoles. The stereochemistry and regiochemistry of cycloaddition reactions of imines of α-amino-acid esters.

The stereochemistry and regiochemistry of cycloadditions of α-amino acid ester imines is dependent both on imine structure and on the reactivity of the dipolarophile. Phenylglycine imines undergo competing dipole stereomutation and cycloaddition with some dipolarophiles.     

Nioboxidfluoride mit Blockstrukturen — Elektronenmikroskopische Durchstrahlungsaufnahmen und ihre Simulation

Niobiumoxidefluorides Nb₅₉O₁₄₇F, Nb₃₁O₇₇F, Nb₆₅O₁₆₁F₃ and Nb₃₄O₈₄F₂ were prepared by reaction of Nb₂O₅ and Nb₃O₇F at 1 270°C. These niobiumoxidefluorides have blockstructures which were examined by high resolution electron microscopy. The observed images of the crystal structures were compared with computer simulated images.

     

Mathematical properties of the vacuum polarization function

This paper is an investigation of certain mathematical properties of the vacuum polarization function (s). We show that (s) is a Herglotz function, has no complex zeroes, and belongs to the class of functions called typically real. In addition, we obtain upper bounds on the higher derivatives of (s), at s=0, given that we know the value of the first derivative at that point.Research supported in part by NASA Grant NSG-8035     

Filtered backprojection for the reconstruction of a high-resolution (4,2)D CH3-NH NOESY spectrum on a 29 kDa protein

Projection-reconstruction (PR) NMR enables rapid collection of multidimensional NMR data. NOESY represents a particularly difficult challenge for currently existing reconstruction algorithms, as it requires the quantitative reconstruction of an unknown number of peaks, at full sensitivity. We have demonstrated the successful application of PR-NMR to NOESY by determining the 4D methyl/amide NOESY spectrum of a 29 kDa protein, human carbonic anhydrase II, from 2D projections, using filtered backprojection for reconstruction. Compared with a 3D control spectrum, all expected peaks were faithfully reconstructed, with correct volumes and with no artifacts. Filtered backprojection thus provides a way to obtain high-resolution 4D NOESY data in the time required for conventional 3D data collection.     

Molecular orbital theoretical prediction of the isomeric products formed from reactions of arene oxides and related metabolites of polycyclic aromatic hydrocarbons

MO theoretical calculations based on the perturbational method of Dewar provide good correlation between predicted and observed structures of products formed during: (1) isomerization of arene oxides to phenols; (2) hydration and nucleophilic addition to arene oxides; and (3) dehydration of arene dihydrodiols. The method is equally applicable to the arene oxides, dihydrodiols, etc. derived from carcinogenic and noncarcinogenic polycyclic hydrocarbons. Extension to the related enzymatic reactions occurring during metabolism of carcinogenic hydrocarbons and to the reactions of the biologically active arene diolepoxides and aryloxirenes suggests the potential utility of this approach in predicting (a) metabolite structure and (b) the structural requirements for carcinogenic and mutagenic activity.     

X=Y=ZH Systems as potential 1,3-dipoles. Part 58: Cycloaddition route to chiral conformationally constrained (R)-pro-(S)-pro peptidomimetics

Imines of (1S,9S)-t-butyl-9-amino-octahydro-6,10-dioxo-6H-pyridazino[1,2-a][1,2]diazepine-1-carboxylate undergo thermal (toluene, 110°C) or LiBr-DBU catalysed (MeCN, room temperature) regio- and stereo-specific cycloaddition to a range of chiral dipolarophiles giving enantiopure spiro-cycloadducts in excellent yield. The reactions proceed via intermediate NH azomethine ylides and litho azomethine ylides, respectively and results in the multiplication of chiral centres from 2 (one of which is lost in the process) to 5.