An investigation of structure-reactivity relationships of d-alkenyl oximes; competitive thermal reactions leading to cyclic nitrones and/or N-unsubstituted bicyclic isoxazolidines

Thermal reactions of C-aryl d-alkenyl oximes give N-unsubstituted bicylic lactone, lactam and pyrrolidine fused isoxazolidines by an intramolecular oxime olefin cycloaddition pathway (IOOC) and/or cyclic nitrones by an azaprotio cyclotransfer (APT) route; a number of factors, including the nature of the aryl group, the oxime geometry and the structure of the linker between the oxime and the terminal alkene, contribute to the competition

Conjugation at the oligonucleotide level based on isoxazole phosphoramidites generated by click chemistry

The versatility of the isoxazole generating nitrile oxide–alkyne Huisgen cycloaddition for provision of chemically modified oligonucleotides has been extended; in a novel approach isoxazole conjugated oligodeoxyribonucleotides have been constructed by phosphoramidite chemistry of isoxazole derivatives previously generated by nitrile oxide–alkyne click chemistry. The conjugation involves manual solid phase synthesis at room temperature in aqueous ethanol and proceeds in high yield

Supporting Information: Fast, Copper-Free Click Chemistry, A Convenient Solid-Phase Approach to Oligonucleotide Conjugation

General experimental Analytical TLC was performed on precoated (250 μm) silica gel 60 F-254 plates from Merck. All plates were visualized by UV irradiation, and/or staining with 5% H2SO4 in ethanol followed by heating. Flash chromatography grade silica gel 60 (230-400 mesh) was obtained from Merck. Mass analysis was performed on an Ettan MALDI-TOF Pro from Amersham Biosciences or LASER-TOF LT3 from Scientific Analytical Instruments with 3- hydroxypicolinic acid or 2,’ 4’, 6’-trihydroxyacetophenone as matrix. The NMR spectra were obtained at 1H (300 MHz), 13C (75 MHz) and 31P (121 MHz) on a Bruker instrument at 25 ºC. Chemical shifts are reported in ppm downfield from TMS as standard. HPLC was carried out using a Gilson instrument equipped with a UV detector and a Nucleosil C18 column (4.0 × 250 mm) or Phenomenex Clarity. Fluorescence spectra were recorded on a Varian Cary Eclipse instrument. All other chemical agents were purchased from Aldrich Chemical Company unless otherwise noted

Within brood trade-offs in reproductive effort: An experimental study on the common tern: Sterna hirundo

The optimal allocation of effort during reproduction is a key component of life history theory, with trade-offs predicted to operate both within and between reproductive attempts. Experimental work in this field has largely concentrated on the latter. The need to partition investment between the different phases of the current reproductive event, and how this varies between individuals, has received little empirical investigation. The primary aims of this project were to investigate the capacity of birds to adjust their reproductive effort in response to increasing demand, and to investigate within brood trade-offs between the different phases of the reproductive attempt. To do this the project involved experimental manipulation of effort (within the natural range) at the stages of egg production, incubation and chick rearing. Only those pairs that were given a free chick (incurring chick rearing costs only) were able to rear a significantly larger brood than unmanipulated controls. When parents incurred the full costs (egg production, incubation and chick rearing) of producing additional young to their intended clutch size, their capacity to rear an enlarged brood was negated. A within clutch trade-off was identified between producing and rearing extra young, with experimental parents subsequently showing reduced chick provisioning, growth and survival. In an experiment to increase the costs of incubation alone, which have often been considered relatively trivial, study birds showed a significantly depressed performance in the later stage of chick rearing. Again a within clutch trade-off in reproductive effort was identified with second hatched chicks in experimental nests growing at a significantly lower rate and Hedging at a significantly lighter mass than those in control broods. The effects of experimental manipulation on aspects of the dynamics of parental foraging suggested that an increase in reproductive demand may affect strategy, but the relationship was not clear. An increase in the costs of egg production alone was not found to decrease parental performance or the quality of the additional egg or chick. These results cast doubt on the interpretation of previous brood enlargement experiments as providing empirical evidence that observed clutch sizes are often less than the Lack value, as such experiments have failed to include the costs of egg production and much of the incubation costs also. They lend support to the Individual Optimisation Hypothesis, in that the parents appear to be raising the clutch size that maximises their number of recruits. The effect of increased cost in relation to individual quality was also examined. The capacity of individuals to compensate for deviations from their allocation of effort to different reproductive phases was found to differ. The negative fitness effects of an increase in incubation demand were most marked in lower quality pairs. Also, the capacity to lay additional eggs in response to experimental egg removal differed between individuals and between years. Only birds in a good enough condition prior to, and during, egg laying appeared able to increase their allocation of effort to the egg production phase and replace the removed egg. A significant proportion of (presumably poorer quality) birds simply deserted the nest site. The adaptive significance of a conditional response to egg loss is discussed, particularly in relation to the finding that egg production alone did not result in any significant within clutch trade-offs

Isoxazole linked oligonucleotide conjugates by on resin and previously clicked nitrile oxide alkyne cycloadditions

Bioconjugation protocols in environments free from residual copper or other catalytic components are important for therapeutic and biomedical applications as well as in living systems. In this communication we discuss the versatility of the catalyst free, isoxazole generating nitrile oxide alkyne Huisgen cycloaddition for provision of chemically modified oligonucleotide conjugates. Two distinct approaches will be demonstrated. In the first we discuss on resin cylcoaddition between in situ generated nitrile oxide

Metal free click chemistry on nucleosides and oligonucleotides

Chemoselective ligation of biologically significant moieties through azide alkyne Click Chemistry has recently received much attention1. The reaction is attractive in that it regioselectively affords stable triazole linked bioconjugated products under mild conditions. However, from the view point of the synthetic oligonucleotide chemist, a significant disadvantage is that the non-thermal reaction requires an in situ generated Cu (I) catalyst. Unwanted Cu (I) mediated chemistry, specifically oxidative degradation etc

Orthogonal modification of polymer chain-ends via sequential nitrile oxide–alkyne and azide–alkyne Huisgen cycloadditions

The α- and ω-chain-ends of well-defined polystyrene chains were functionalized using consecutive Huisgen cycloadditions. Firstly, an α-alkyne, ω-azido heterotelechelic polystyrene precursor was synthesized in three steps: (i) atom transfer radical polymerization in the presence of (1,1,1-trimethylsilyl)-2-propynyl 2-bromo-2-isobutyrate, (ii) deprotection of the alkyne function of the initiator and (iii) nucleophilic substitution of the bromine chain-end of the polymer with sodium azide. Afterwards, the chain-ends of the polymer were modified by successive nitrile oxide–alkyne cycloaddition (NOAC) and copper-catalyzed azide–alkyne cycloaddition (CuAAC). 2 Model building blocks were tested for NOAC, while 4 building blocks were studied for CuAAC. In all cases, the orthogonal combination of NOAC and CuAAC allowed the preparation of tailored heterotelechelic polymers

Highly Selective Fluorimetric Turn-Off Detection of Copper (II) by Two Different Mechanisms in Calix[4]arene-Based Chemosensors and Chemodosimeters

Isoxazolo‐pyrene tethered calix[4]arenes selectively detect copper(II) ions without interference from related perchlorate ions. The fluorescence emission of the probes, synthesised by nitrile oxide alkyne cycloaddition, and characterised by spectroscopic and crystallographic data, is rapidly reduced by Cu(II) ions. Detection limits are in the micromolar or sub‐micromolar range (0.3–3.6 μM) based on a 1 : 1 sensor:analyte interaction. Voltammetric behaviour and 1H NMR data provide new insights into the sensing mechanism which is dependent on the calixarene substitution pattern. When the calixarene lower rim is fully substituted, Cu(II) detection occurs through a traditional chelation mechanism. In contrast, for calixarenes 1,3‐disubstituted on the lower rim, detection takes place through a chemodosimetric redox reaction. The isolation of a calix[4]diquinone from the reaction with excess Cu(ClO4)2 provides confirmation that the sensor–analyte interaction culminates in irreversible sensor oxidation

Modulation of interleukin 2 high affinity binding to human T cells by a pyrimidodiazepine insect metabolite

AbstractAn insect metabolite containing the little known pyrimido[4,5-b][l,4]diazepine ring system has been found to act as an effective mimic of tetrahydrobiopterin in its ability to modulate the affinity of interleukin 2 (IL-2) for its receptors on human T cells. Semi-empirical molecular orbital calculations reveal that while tetrahydrobiopterin has considerable flexibility, the pyrimidodiazepine has rather few conformational options and offers a useful model for exploring the nature of the pterin binding site

Highly Selective Fluorimetric Turn-Off Detection of Copper(II) by Two Different Mechanisms in Calix[4]arene-Based Chemosensors and Chemodosimeters

Isoxazolo-pyrene tethered calix[4]arenes selectively detect copper(II) ions without interference from related perchlorate ions. The fluorescence emission of the probes, synthesised by nitrile oxide alkyne cycloaddition, and characterised by spectroscopic and crystallographic data, is rapidly reduced by Cu(II) ions. Detection limits are in the micromolar or sub-micromolar range (0.3–3.6 μM) based on a 1:1 sensor:analyte interaction. Voltammetric behaviour and 1 H NMR data provide new insights into the sensing mechanism which is dependent on the calixarene substitution pattern. When the calixarene lower rim is fully substituted, Cu(II) detection occurs through a traditional chelation mechanism. In contrast, for calixarenes 1,3-disubstituted on the lower rim, detection takes place through a chemodosimetric redox reaction. The isolation of a calix[4] diquinone from the reaction with excess Cu(ClO4)2 provides confirmation that the sensor–analyte interaction culminates in\ud irreversible sensor oxidation