Zur Analyse eines Polymerisates aus Methylisopropenylketon, Butadien und Acrylnitril

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Kinetic control of proline amide rotamers: total synthesis of trans,trans- and cis,cis-ceratospongamide

Ceratospongamide is a cyclic peptide natural product that is biosynthesized as a mixture of two proline rotamers. Remarkably, these rotamers do not detectably interconvert at temperatures up to 100 degress C. Here we report high-yielding syntheses of each rotamer and demonstrate that the threonine-derived oxazoline plays a critical role in determining the kinetic distribution of conformational isomers.     

Thermal behavior of trans,trans,trans-1,2,3,4-tetravinylcyclobutane

Above 42°C the title compound decomposes to trans-hexatriene and cyclohexadiene-1,3. At temperatures above 770°C only cyclohexadiene-1,3 and benzene are found.     

An extensive study of bromination of cis,trans,trans-1,5,9-cyclododecatriene: product structures and conformations

Bromine has been added to cis,trans,trans-1,5,9-cyclododecatriene under various reaction conditions. All expected direct addition products have been isolated, and their structures have been determined by microanalysis, NMR and X-ray crystallography. Advanced NMR techniques were used to determine solution conformations of several of the compounds, enabling comparison with the solid-state conformations obtained by crystallography.     

Stereochemistry of the [4 + 2] cycloadditions of trans,trans- and cis,trans-2,4-hexadiene to C(60)

The [4 + 2] cycloaddition of trans,trans-2,4-hexadiene with C(60) proceeds via a concerted mechanism with retention of stereochemistry in the cycloadduct 1a. However, when cis,trans-2,4-hexadiene reacts with C(60), isomerization of the cis,trans to the thermodynamically more stable trans,trans isomer occurs. Subsequently, the cis,trans diene isomerized to the trans,trans isomer and cycloadds to C(60), to form adduct 1a. When the reaction is carried out at higher temperatures, the formation of cycloadduct 1b is also obtained. This result is consistent with a concerted cycloaddition of cis,trans-2,4-hexadiene with C(60), which is more reactive at elevated temperatures and leads to the formation of the Diels-Alder adduct 1b.     

Kinetics of the cis,cis to trans,trans isomerization of 1,1,2,2,5,5,6,6-octamethyl-1,2,5,6-tetrasilacycloocta-3,7-diene

The kinetics of the ruthenium-promoted cis,cis to trans,trans isomerization of 1,1,2,2,5,5,6,6-octamethyl-1,2,5,6-tetrasilacycloocta-3,7-diene were investigated. Incubation of a ruthenium alkylidene complex, (Cy(3)P)RuCl(2)(==CHPh)Ru(p-cymene)Cl(2), in CD(2)Cl(2) for 5 days at 40 degrees C afforded a catalytically active ruthenium species that was shown to be responsible for promoting the isomerization. The isomerization was observed to proceed in two steps: (1) conversion of the starting cis,cis isomer to a proposed cis,trans intermediate and (2) subsequent conversion of the intermediate to the product trans,trans isomer. Kinetic studies demonstrated that the two steps are first-order with respect to the concentrations of the cis,cis isomer, the intermediate, and the ruthenium alkylidene complex. The data were further consistent with a mechanism involving bimolecular hydride addition-elimination during the two isomerization steps.     

An ab initio structural and vibrational analysis of gauche,trans,trans- and gauche,cis,trans-hexa-1,3,5-trienes

The results of complete geometry optimizations of the high energy stable gauche,Trans,trans- and gauche,Cis,trans- rotamers of hexa-1,3,5-trienes are reported at the RHF/6-31G//RHF/B-31G level. The angles of rotation around one of the single C-C bonds are found to be 33.7° and 45.5°, respectively. The corresponding harmonic force fields of these molecules are also reported at this level and corrected using scale factors transferred from buta 1,3-diene. Aspecial scale factor was used for the central C=C double bond stretching coordinate to take into account vibronic coupling. The theoretical vibrational frequencies, calculated with the scaled quantum mechanical (SQM) force fields, allow a complete interpretation of the experimental vibrational spectra of these molecules.Preliminary results were reported at the Austin XII Symposium on Molecular Structure, Austin, TX, February 28 through March 3, 1988, S 18, p. 111 (USA) and at the XIXth European Congress on Molecular Spectroscopy, Dresden, September 4 through September 8, 1989, p. 226 (GDR).     

Polyamide aus trans-2,5-Dimethylpiperazin und Bestimmung deren Feuchtigkeitsaufnahme

Ohne Zusammenfassung