Pyridine-imide oligomers

Pyridine-imide oligomers created by incorporating imide and pyridine units alternatively in sequence were successfully synthesized and found to form highly compact and stable helical conformations contributed by intramolecular H-bonds between the imide and both adjacent pyridines, and by the structural characteristics of the imide units.     

Thiodiglycol-based oligomers

Sulfur-containing oligoethers were prepared by polycondensation of thiodiglycol in the presence of acid catalysts. These oligoethers are oligomers with terminal hydroxy groups, comprised of oxyalkyl sulfide units.     

Liquid properties of oligomers. Comparison of relaxational properties of propylene glycol oligomers with those of ethylene glycol oligomers

Liquid properties such as dielectric relaxation and viscous flow of the two structurally homologous propylene glycol oligomers HO(CH(CH₃)CH₂O)_nH (n=1, 2, 3, 4, 5 and 34) and ethylene glycol oligomers HO(C₂H₄O)_nH (n=1, 2, 3, 4, 5 and 6) are studied in pure liquid state to clarify the degree of polymerization dependences of chain molecules on their liquid properties. These oligomers are, at room temperature, viscous liquid which shows dielectric relaxations in the frequency range from 10 Hz to 3 MHz. Propylene glycol oligomers (n=from 1 to 5) show the Davidson-Cole-type relaxations, but the higher glycol (n=34) shows superposition of the two different relaxations, i. e., small Debye-type relaxation in the lower-frequency region and large principal Havriliak-Negami-type relaxation in the higher-frequency region. Relaxation times vs. degree of polymerization do not increase linearly, but vary in zigzag lines. Above all, the dimers (dipropylene glycol and diethylene glycol) show longer relaxation times than the other glycols. This dielectric result does not agree with the degree of polymerization dependence of viscous flow.     

Zinc-coordination oligomers of phenanthrolinylporphyrins

[structure: see text] We report the synthesis of mono- and bisphenanthrolinylporphyrins 5-(9-carboxy-1,10-phenanthroline-2),15-(p-tolyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin (MPPc) and 5,15-bis(9-carboxy-1,10-phenanthroline-2)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin (BPPc), respectively. The formation of dimers and oligomers, with MPPc and BPPc, respectively, upon exposure to zinc acetate is described. Oligomerization of the porphyrins was monitored spectrophotometrically, and a kinetic study was performed to estimate the average oligomer chain length.     

Electrorheology of aniline oligomers

Aniline oligomers were prepared by the oxidation of aniline with p-benzoquinone in aqueous solutions of methanesulfonic acid (MSA) of various concentrations. Their molecular structures were assessed by Fourier transform infrared spectroscopy. The electrorheological (ER) behavior of their silicone oil suspensions under applied electric field has been investigated. Shear stress at a low shear rate, τ _0.9, was used as a criterion of the rigidity of internal structures created by the application of an electric field. It was established from the fitting of the dielectric spectra of the suspensions with the Havriliak–Negami model that dielectric relaxation strength, as a degree of polarization induced by an external field contributing to the enhanced ER effect, increases and relaxation time, i.e., the response of the particle to the application of the field, decreases when a higher molar concentration of MSA is used. The best values were observed for suspensions of the sample prepared in the presence of 0.5 M of MSA. This suspension creates stiff internal structures under an applied electric field strength of 2 kV mm^?1 with τ _0.9 of nearly 50 Pa, which is even slightly of higher value than that obtained for standard polyaniline base ER suspension measured at the same conditions. The concentration of the MSA used in the preparation of oligomers seems to be a crucial factor influencing the conductivity, dielectric properties and, consequently, rheological behavior, and finally ER activity of their suspensions.     

Synthesis of ether oligomers

[structure: see text] Hydroxyaromatic aldehydes and ketones were used as building blocks to prepare ether oligomers. An iterative two-step protocol involving Mitsunobu coupling and carbonyl reduction provided a protecting-group-free route with high yields. Activity screening of an 84-member library against proteases led to the discovery of micromolar inhibitors for trypsin, chymotrypsin, and subtilisin.     

Protease-mediated fragmentation of p-amidobenzyl ethers: a new strategy for the activation of anticancer prodrugs

A new anticancer prodrug activation strategy based on the 1,6-elimination reaction of p-aminobenzyl ethers is described. Model studies were undertaken with the N-protected peptide benzyloxycarbonyl-valine-citrulline (Z-val-cit), which was attached to the amino groups of p-aminobenzyl ether derivatives of 1-naphthol and N-acetylnorephedrine. The amide bond that formed was designed for hydrolysis by cathepsin B, a protease associated with rapidly growing and metastatic carcinomas. Upon treatment with the enzyme, the Z-val-cit-p-amidobenzyl ether of 1-naphthol (2) underwent peptide bond hydrolysis with the rapid release of 1-naphthol. The aliphatic Z-val-cit-p-amidobenzyl ether of N-acetylnorephedrine (5) also underwent amide bond hydrolysis, but without the ensuing elimination of N-acetylnorephedrine. On the basis of these results, the phenolic anticancer drugs etoposide (6) and combretastatin A-4 (7) were attached to the Z-val-cit-p-amidobenzyl alcohol through ether linkages, forming the peptide-drug derivatives 8 and 9, respectively. Both compounds were stable in aqueous buffers and serum and underwent ether fragmentation upon treatment with cathepsin B, resulting in the release of the parent drugs in chemically unmodified forms. The released drugs were 13-50 times more potent than were the prodrug precursors on a panel of cancer cell lines. In contrast, the corresponding carbonate derivative of combretastatin A-4 (13) was unstable in aqueous environments and was as cytotoxic as combretastatin A-4. This result extends the use of the self-immolative p-aminobenzyl group for the fragmentation of aromatic ethers and provides a new strategy for anticancer prodrug development.     

Sheet-forming abiotic hetero foldamers

Abiotic hetero oligomers, adopting a well-defined extended self-assembled sheet-like structure, derived from conformationally constrained aliphatic and aromatic amino acid residues repeating at regular intervals are reported.     

From oligomers to polymer: convergence in the HOMO-LUMO gaps of conjugated oligomers

[Structure: see text] Extrapolation of HOMO-LUMO gaps for pi-conjugated oligomers at the B3LYP/6-31G(d) level of theory predict accurately (within 0.1-0.2 eV) the band gaps of conjugated polymers only when long (at least 20-mer) pi-conjugated oligomers are used for the extrapolation.     

Bromine-containing oligomers of dimethyl itaconate

The radical polymerization of dimethyl itaconate in the presence of carbon tetrabromide was investigated. Oligomers of low molecular weight and high bromine content were obtained; the yield depended mainly on initiator concentration and reached 100 per cent at high concentrations. It was found that the oligomers could be used as plasticizer extenders for polyvinyl chloride; their high bromine content reduced the flammability of polyester.     

Conformational behavior of beta-proline oligomers

Conformations of the monomer, dimer, and hexamer of beta-proline ((S) pyrrolidine-3-carboxylic acid) were determined using ab initio molecular orbital calculations at the RHF/6-31G level of theory. The calculated minima are in good agreement with experimental data for the system and imply that the conformations could be controlled through chemical modification at Calpha, Cgamma, or Cdelta. The monomer and dimer are small and flexible with many low-energy minima. In the hexamer, two forms of regular secondary structure are preferred: left-handed helices with cis-peptide bonds and right-handed helices with trans-peptide bonds. This is similar to the behavior of alpha-proline helices, except that the relationship between the peptide rotamer and the handedness of the helix is reversed. Therefore, helices of the enantiomer of beta-proline ((R)-pyrrolidine-3-carboxylic acid) should exhibit the same behavior as alpha-proline helices. Through understanding the conformational behavior of beta-proline in various environments, it may be possible to use these protein mimics to inhibit various protein-protein recognition events. To estimate these effects, SCRF energies for the conformers were determined in dielectrics corresponding to water, methanol, and chloroform. It appears that the cis helices are more favorably solvated than the trans helices, but the cause is not clear.     

Metal-directed assembly of coordination oligomers

Bridging ligands containing two spatially separated 2,2′:6′,2′-terpyridinyl metal-binding domains have been prepared and used for the controlled assembly of coordination oligomers incorporating ruthenium(II) and other metal ions. Two such bridging ligands have been structurally characterised. The systematic synthesis of a range of homo- and heterometallic polynuclear complexes incorporating these bridging ligands and a variety of terminator 2,2′:6′,2′- terpyridines is described.     

CE-MSn of complex pectin-derived oligomers

As pectin molecules are too large and heterogeneous to analyze as a whole, the polymer is usually degraded to smaller oligomers, which are often analyzed by high-performance anion exchange chromatography (HPAEC). However, the high salt concentration necessary to elute pectin oligomers by HPAEC is incompatible with online mass detection. To overcome such a disadvantage, a CE-IT-MS system was set up to further elucidate the fine structure of charged oligosaccharides. An effective separation of differently substituted galacturonic acid containing oligomers was obtained by low-pH CE-LIF analysis. By adapting the buffer and capillary online MS detection was enabled. Moreover, with MS/MS it was possible to localize sugar residues' substitutions. With this combined CE-MS approach LIF electropherograms of xylogalacturonan and rhamnogalacturonan I digests could be annotated. The method was further exemplified by a complex oligomer mixture of acid hydrolyzed apple pectin, which was separated and characterized by CE-MSn. Oligomers present in low amounts could be localized by their corresponding m/z, as was demonstrated by selected mass range representation.     

Self-assembled cyclic boron-dipyrrin oligomers

Self-assembled cyclic boron-dipyrrin oligomers 3-5 obtained by the reaction of catecholyldipyrrin with boron trichloride were characterized by 1H NMR, MALDI-TOF MS, and X-ray measurements and the interaction between 3 and alkali metal ions was examined.     

Reactive lyotropic polyimide oligomers

Amine-terminated and maleimide-terminated oligomers of molecular weight 1200–1800 based on 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl and 3,3′,4,4′-biphenylenetetracarboxylic dianhydride were synthesized and characterized for lyotropic liquid crystalline behavior. Several different synthetic procedures were evaluated and a one-step procedure in m-cresol was found to be the most effective for producing fully imidized materials. Lyotropic behavior was observed only for the as-prepared solutions in m-cresol and in dilutions of this solution. Oligomer thermal stability was excellent, onsets of decomposition were in excess of 550°C. © 1996 John Wiley & Sons, Inc.     

Crystallinity of ethylene oxide oligomers

Nonaethylene glycol and pentadecaethylene glycol and their dimethyl ethers have been prepared and characterized, with respect to crystallinity by wide- and small-angle X-ray scattering, Raman scattering, i.r. spectroscopy and differential scanning calorimetry. Wide-angle X-ray scattering is similar to that from high molecular weight poly(ethylene oxide). The crystal habit is lamella. The lamellae are highly crystalline and the surface layers are ordered. Comparison with crystalline poly(ethylene oxide) prepared conventionally, and having a distribution of chain lengths, shows that such samples crystallize into lamellae with disordered surface layers.     

Some reactions of tetrafluoroethylene oligomers

As part of a programme to prepare and evaluate a series of perfluoro- chemicals for use as inert fluids, the fluorinations of some tetrafluoroethylene oligomers over cobalt (III) fluoride have been studied.Fluorination of perfluoro-3,4-dimethylhex-3-ene (tetramer) and perfluoro-4-ethyl-3,4-dimethylhex-2-ene (pentamer) over CoF₃ at 230°C and l45°C respectively afforded the corresponding saturated fluorocarbons however, at 250°C, pentamer gave predominantly the saturated tetramer. The thermal behaviour of these saturated fluorocarbons alone and in the presence of bromine and toluene has been studied.Pyrolysis of pentamer over glass beads at 500°C gave perfluoro-1,2,3- trimethylcyclobutene and isomers of perfluoro-2,3-dimethylpenta-1,3- diene. Under similar conditions perfluoro-2-(1-ethyl-1-methylpropyl). 3-methylpent-1-ene (hexamer) gave perfluoro-1-methyl-2-(1-methyl- propyl)-cyclobut-1-ene and perfluoro-2-methyl-3-(1-methylpropyl)-buta- 1,3-diene.These reactions and the structural elucidation of the products will be discussed.