Synthetic enantiopure aziridinomitosenes: preparation, reactivity, and DNA alkylation studies

An enantiocontrolled route to aziridinomitosenes had been developed from l-serine methyl ester hydrochloride. The tetracyclic target ring system was assembled by an internal azomethine ylide cycloaddition reaction based on silver ion-assisted intramolecular oxazole alkylation and cyanide-induced ylide generation via a labile oxazoline intermediate (62 to 66). Other key steps include reductive detritylation of 26, methylation of the N-H aziridine 56, oxidation of the sensitive cyclohexenedione 68 to quinone 70, and carbamoylation using Fmoc-NCO. Although the aziridinomitosene tetracycle is sensitive, a range of protecting group manipulations and redox chemistry can be performed if suitable precautions are taken. A study of DNA alkylation by the first C-6,C-7-unsubstituted aziridinomitosene 11a has been carried out, and evidence for DNA cross-link formation involving nucleophilic addition to the quinone subunit is described.     

The influence of fine structure on the pyrolysis of cellulose. I. Vacuum pyrolysis

The effects of crystallinity, orientation and degree of polymerization on the vacuum pyrolysis of cellulose have been investigated. Natural, modified and manmade celluloses were characterized and the kinetics of their pyrolysis at 251°C studied. The high-temperature pyrolysis of these samples was also investigated by means of DSC and TGA. At 251°C all of the samples showed a rapid, initial weight loss followed by a linear (weight loss)^1/2 versus time dependency. Both the initial weight loss and the subsequent reaction rate were dependent upon the crystallinity of the sample. The reaction rate was shown to be inversely proportional to (DP)^1/2 and related to the orientation. Effects of crystallinity and orientation could also be seen in the DSC thermograms and in the apparent energies of activation. Results are interpreted in terms of probable mechanisms of the uncatalyzed pyrolysis of cellulose.     

An efficient synthesis of the novel triazoloquinazoline adenosine antagonist,CGS 15943

An efficient synthesis of the novel triazoloquinazoline adenosine antagonist, CGS 15943, is reported in five steps in approximately 50% overall yield. A key reaction in the synthetic sequence is the double cyclization of an N-(substituted-2-cyanophenyl)carbamate with a carboxylic acid hydrazide to afford a [1,2,4]triazolo-[1,5-c]quinazolin-5(6H)-one in high yield without either a Dimroth or “translocative” rearrangement occurring. Another key reaction is the condensation of a 2-(1H-1,2,4-triazol-5-yl)benzenamine with cyanamide under acidic conditions to prepare a guanidine.     

Inorganic-Organic Hybrid Polymers from the Polymerisation of Methacrylate-Substituted Oxotantalum Clusters with Methylmethacrylate: A Thermomechanical and Spectroscopic Study

New inorganic-organic hybrid materials were prepared by free-radical polymerization of methyl methacrylate (MMA) with methacrylate-substituted oxotantalum cluster [Ta₄O₄(OEt)₈(OMc)₄] and their properties evaluated. The cluster was prepared by the reaction of the parent alkoxide with methacrylic acid. Samples of the hybrid materials were produced with Ta-cluster to methyl methacrylate in the ratios of 1:50 and 1:100 and were characterized by thermal and spectroscopic techniques. The glass transition temperatures of the hybrid materials are shifted to higher temperatures than pure PMMA as a result of cross-linking of the polymer by the oxotantalum clusters. The increase in T_g is also observed from the dynamic mechanical analysis (DMA). Evidence of crosslinking between the Ta-cluster and PMMA is obtained from infrared spectroscopic study. Surface studies performed by X-ray photoelectron spectroscopy (XPS) provide information about the atomic concentrations of the surface and indicate tantalum bonded to oxygen.     

An HPLC and EPR investigation on the stability of DMPO and DMPO spin adducts in vivo

Application of the spin trapping technique in intact animals requires an understanding of the stability and distribution of the spin traps and their spin adducts in vivo. We studied the stability of DMPO in vivo in mice using HPLC and the stability of spin adducts of DMPO by EPR in plasma, whole blood, peritoneal fluid, and homogenized heart tissue of the rat. At 15 minutes after intraperitoneal injection DMPO had similar concentrations in the liver, heart, and blood of the mice and 40% remained in the organs 2 hours after the injection. In contrast, the spin adduct DMPO-OH was short lived, with a half-life of 3.0 minutes in plasma, and was not detectable 1 minute after formation in whole blood and homogenized heart tissue. The carbon centered spin adduct DMPO-CH(OH)CH₃ was more stable, having half-lives of 16, 11, 3.6, and 0.79 minutes in plasma, peritoneal fluid, whole blood, and homogenized heart tissue, respectively. The spin adduct DMPO-SO₃ was sufficiently stable for the adduct to be observed directly from living mice.     

A spectrophotometric assay for solid phase primary amino groups

A rapid, sensitive, and convenient spectrophotometric assay was developed for the measurement of amino groups on solid supports. This method is based on the reaction of amino groups of solids with an excess ofo-phthaldialdehyde (OPA) and subsequent quantitative determination of unreacted OPA by reaction with glycine. Four solids possessing variable quantities of amino groups were examined. Results indicate that about 70% of the total surface amino concentration (determined by the microKjeldahl method) are available for ligand attachment. Unlike the spectrophotometric 2,4,6-trinitrobenzenesulfonic acid method, the OPA spectrophotometric assay is more rapid, sensitive, and convenient, and unlike the spectrofluorimetric OPA, it does not require sophisticated instrumentation.     

Effects of structure on properties of some new aromatic-aliphatic polybenzimidazoles

Polybenzimidazoles were prepared in poly(phosphoric acid) from isophthalic, m- and p-phenylene diacetic, succinic, adipic, suberic, and sebacic acids and 3,3′-diaminobenzidine, 3,3′,4,4′-tetraaminodiphenyl ether and 3,3′,4,4′-tetraaminodiphenylmethane. The thermal, mechanical, and bonding properties were studied. A 3:1 copolymer of isophthalic and m-phenylenediacetic acid with 3,3′-diaminobenzidine showed the best results as far as isothermal oxidation resistance and thermal and processing characteristics.     

Synthesis and characterization of highly luminescent CdSe-core CdS/Zn0.5Cd0.5S/ZnS multishell nanocrystals

We report on the preparation and structural characterization of CdSe nanocrystals, which are covered by a multishell structure from CdS and ZnS. By using the newly developed successive ion layer adhesion and reaction (SILAR) technique, we could gradually change the shell composition from CdS to ZnS in the radial direction. Because of the stepwise adjustment of the lattice parameters in the radial direction, the resulting nanocrystals show a high crystallinity and are almost perfectly spherical, as was investigated by X-ray diffraction and electron microscopy. Also, due to the radial increase of the respective valence- and conduction-band offsets, the nanocrystals are well electronically passivated. This leads to a high fluorescence quantum yield of 70-85% for the amine terminated multishell particles in organic solvents and a quantum yield of up to 50% for mercapto propionic acid-covered particles in water. Finally, we present experimental results that substantiate the superior photochemical and colloidal stability of the multishell particles.     

Orbital nature in the electronic states of cyclopropene

Self-consistent field results for the excited electronic states of cyclopropene, with regard to the spatial extent of the π* orbital in the lowest energy ¹(π, π*) state, are found to be identical with that for ethylene. Due to the forced planarity of the ring structure, dihedrally twisted conformers of the upper electronic state cannot contribute significantly to the oscillator strength which must therefore receive a substantial contribution from the vertical transition; this conclusion is contrary to the self-consistent field results. For a proper theoretical treatment of the problem the importance of a multi-configuration approach in an extended basis with ¹(σ, σ*)-type states is emphasized.