Experimental antiulcer drugs. 3. Some unusual transformations of the cyclopenta[c] pyrroles

The cyclopenta[c]pyrrole-4-carbonitrile ( 3 ) is transformed by hyrlroxylamine in hot alcohol to 4,5,6,6a-tetrahydro-l-imino-3,5,5,6a-tetramethyl-4-cyclopenta[c]pyrrole methyl ketone oxime ( 5 ) in contrast to the cyclopentapyrrole 10 which afforded 1,2-diacetyl-3,3,5,5-tetramethyl-cyclopentene dioxime ( 11 ). The cyclopenta[c]pyrrole-4-carboxamide 1 (R = C₆H₅) yielded the isomeric 2a,3,4,4a,5,6,6a,6b-octahydro-2a,4,4,6a-tetramethyl-5-(phenylimino)pentaleno[1,6-bc]-pyrrol-2-(1H)one ( 12 ) in hot dilute hydrochloric acid or hot 99% phosphoric acid. The amide 1 (R = C₆H₅) was transformed in the solid state by oxygen over a period of several months to a mixture of the isomeric anils, 13 and 14 of 1,7-diacetyl-7-hydroxy-4,6,6-trimethyl-2-azabicyclo-[2.2.l.]heptan-3-one( 16 ).     

Mass spectrometry of nucleoside derivatives. Seleno analogs of purine and pyrimidine bases and nucleosides

Mass spectra of certain selenobases and selenonucleosides, and some of their trimethylsilyl and O,N-permethyl derivatives have been studied from the standpoint of structural characterization, and in order to ascertain the influence of selenium on normal fragmentation patterns. Molecular ion abundances of the selenouracils are intermediate between those of the corresponding oxygen and sulfur analogs. Fragmentation processes are similar to those of the corresponding normal bases and nucleosides but with additional ions resulting from expulsion of Se or SeH in most cases. Trimethylsilylation occurs at approximately the same rate as for normal bases and nucleosides but the products show decreasing stability with prolonged heating. A least squares procedure is demonstrated which generates monoisotopic mass patterns and assists in interpretation of the mass spectra.     

Segregation in aqueous methanol enhanced by cooling and compression

Molecular segregation in methanol-water mixtures is studied across a wide concentration range as a function of temperature and pressure. Cluster distributions obtained from both neutron diffraction and molecular dynamics simulations point to significantly enhanced segregation as the mixtures are cooled or compressed. This evolution toward greater molecular heterogenity in the mixture accounts for the observed changes in the water-water radial distribution function and there are indications also of a change in the topology of the water clusters. The observed behavior is consistent with an approach to an upper critical solution point. Such a point would appear to be "hidden" below the freezing line, thereby precluding observation of the two-fluid region.     

Methanol-water solutions: a bi-percolating liquid mixture

An extensive series of neutron diffraction experiments and molecular dynamics simulations has shown that mixtures of methanol and water exhibit extended structures in solution despite the components being fully miscible in all proportions. Of particular interest is a concentration region (methanol mole fraction between 0.27 and 0.54) where both methanol and water appear to form separate, percolating networks. This is the concentration range where many transport properties and thermodynamic excess functions reach extremal values. The observed concentration dependence of several of these material properties of the solution may therefore have a structural origin.     

Cellular solid behaviour of liquid crystal colloids 1. Phase separation and morphology

We study the phase ordering colloids suspended in a thermotropic nematic liquid crystal below the clearing point and the resulting aggregated structure. Small () PMMA particles are dispersed in a classical liquid crystal matrix, 5CB or MBBA. With the help of confocal microscopy we show that small colloid particles densely aggregate on thin interfaces surrounding large volumes of clean nematic liquid, thus forming an open cellular structure, with the characteristic size of inversely proportional to the colloid concentration. A simple theoretical model, based on the Landau mean-field treatment, is developed to describe the continuous phase separation and the mechanism of cellular structure formation. Received 13 March 2000 and Received in final form 6 June 2000