endo-Selective (3+2) cycloaddition polymerizations of nitrone monomers with olefins utilising high pressure conditions

A high pressure mediated (3+2) cycloaddition polymerization strategy has been employed to afford linear poly(isoxazolidine) architectures. Under these high pressure conditions this cycloaddition process was found to afford primarily endoheterocycles which when translated to the polymerization should ultimately affect the tacticity and resultant properties of the polymer. The stereoselectivity occurred as a result of a lower volume of activation for the endo-transition state and the application of a 'type-I' regime (HOMODipole-LUMODipolarophile) cycloaddition process that features secondary orbital interactions within the extended molecular orbitals. A variety of linker segments were employed in an attempt to affect the physical properties of the polymeric cycloadducts such as T-g and solubility in order to tailor these materials for use in coating applications. (C) 2007 Elsevier Ltd. All rights reserved.

A new class of ammonium ylid for [2,3]-sigmatropic rearrangement reactions: ene-endo-spiro ylids

The first examples of sigmatropic rearrangements of ene-endo-spirocyclic, tetrahydropyridine-derived ammonium ylids are reported. Thus, spiro[6.7]-ylids rearrange primarily by a [2,3]-pathway, whereas the analogous [6.6]-ylids rearrange by [1,2]- and [2,3]-mechanisms in roughly equal proportions. This method serves as a rapid entry to the core of a range of alkaloids bearing a pyrrolo[1,2-a]azepine or octahydroindolizidine nucleus.

Preparation of transition metal complexes of bicyclo[2.2.1]hept-5-ene-2-carboxylic acid (C7H9CO2H) and X-ray crystal structure of [Cu2?(±)-endo-μ-O2CC7H9?4 (CH3OH)2]·2CH3OH

Metathesis reactions were used to prepare a range of dicopper(II), monocopper(I), diruthenium(II, III), dimolybdenum(II,II) and dirhodium(II,II) complexes of either racemic or resolved forms of endo- and exo-bicyclo[2.2.1]hept-5-ene-2-carboxylic acid (C7H9CO2H). The X-ray crystal structure of [Cu2{(±)-endo-μ-O2CC7H9}4(CH3OH)2]·2CH3OH shows the two copper(II) ions bridged by two (+)-endo-bicyclo[2.2.1]hept-5-ene-2-carboxylate anions and two (−)-endo-bicyclo[2.2.1]hept-5-ene-2-carboxylate anions. Methanol molecules occupy the two trans axial sites, and there are also two methanol molecules hydrogen bonded to opposite carboxyl oxygens.

The use of culture-independent tools to characterize bacteria in endo-tracheal aspirates from pre-term infants at risk of bronchopulmonary dysplasia

Although premature infants are increasingly surviving the neonatal period, up to one-third develop bronchopulmonary dysplasia (BPD). Despite evidence that bacterial colonization of the neonatal respiratory tract by certain bacteria may be a risk factor in BPD development, little is known about the role these bacteria play. The aim of this study was to investigate the use of culture-independent molecular profiling methodologies to identify potential etiological agents in neonatal airway secretions. This study used terminal restriction fragment length polymorphism (T-RFLP) and clone sequence analyses to characterize bacterial species in endo-tracheal (ET) aspirates from eight intubated pre-term infants. A wide range of different bacteria was identified in the samples. Forty-seven T-RF band lengths were resolved in the sample set, with a range of 0-15 separate species in each patient. Clone sequence analyses confirmed the identity of individual species detected by T-RFLP. We speculate that the identification of known opportunistic pathogens including S. aureus, Enterobacter sp., Moraxella catarrhalis, Pseudomonas aeruginosa and Streptococcus sp., within the airways of pre-term infants, might be causally related to the subsequent development of BPD. Further, we suggest that culture-independent techniques, such as T-RFLP, hold important potential for the characterization of neonatal conditions, such as BPD.

Differentiation status of limbal epithelial cells cultured on intact and denuded amniotic membrane before and after air-lifting

We have investigated differences in bovine limbal epithelial cell differentiation when expanded upon intact (amniotic epithelial cells and basement membrane remaining) and denuded human amniotic membrane, a commonly used substrate in ophthalmic surgery for corneal stem cell transplantation. Ex vivo expansion of the epithelial cells, in supplemented media, continued for 2 weeks followed by 1 week under ‘air-lifting’ conditions. Before and after air-lifting the differentiated (K3/K12 positive) and undifferentiated (K14 positive) cells were quantified by immunohistochemistry, Western blotting and quantitative PCR. Limbal epithelial cells expanded upon amniotic membrane formed 4-6 stratified layers, both on intact and denuded amniotic membrane. On denuded amniotic membrane the proportion of differentiated cells remained unaltered following airlifting. Within cells grown on intact amniotic membrane, however, the number of differentiated cells increased significantly following air-lifting. These results have important implications for both basic and clinical research. Firstly, they show that bovine limbal epithelia can be used as an alternative source of cells for basic research investigating ex vivo limbal stem cells expansion. Secondly, these findings serve as a warning to clinicians that the affect of amniotic membrane on transplantable cells is not fully understood; the use of intact or denuded amniotic membrane can produce different results in terms of the amount of differentiation, once cells are exposed to the air.

Quartic anharmonic resonances in acetylene molecules

Formulas are derived for the quartic anharmonic resonance coefficients observed to be important between C–H stretching and the combination of one quantum of C≡C stretching and two quanta of H–C≡C bending in a number of acetylene molecules. Examples of this resonance are ν3 with ν2+ν4+ν5 in 12C2H2, ν1 with ν2+2ν5 in 13C2H2, and ν1 with ν2+2ν4 in monofluoroacetylene and monochloroacetylene. The coefficients characterizing the resonances in these examples, which we denote K3,245, K1,255, and K1,244, arise from cubic and quartic terms in the anharmonic force field, in the normal coordinate representation, through second order and first order perturbation treatments respectively, where the second order resonances are calculated by a Van Vleck resonance formalism. The experimentally determined values of these coefficients are compared with values calculated from model anharmonic force fields.

Phases of dormancy in yam tubers (Dioscorea rotundata)

center dot Background and Aims The control of dormancy in yam (Disocorea spp.) tubers is poorly understood and attempts to shorten the long dormant period (i.e. cause tubers to sprout or germinate much earlier) have been unsuccessful. The aim of this study was to identify and define the phases of dormancy in Dioscorea rotundata tubers, and to produce a framework within which dormancy can be more effectively studied. center dot Methods Plants of 'TDr 131' derived from tissue culture were grown in a glasshouse simulating temperature and photoperiod at Ibadan (7 degrees N), Nigeria to produce tubers. Tubers were sampled on four occasions: 30 d before shoot senescence (149 days after planting, DAP), at shoot senescence (179 DAP), and twice during storage at a constant 25 degrees C (269 and 326 DAP). The development of the apical shoot bud was described from tissue sections. In addition, the responsiveness of shoot apical bud development to plant growth regulators (gibberellic acid, 2-chloroethanol and thiourea) applied to excised tuber sections was also examined 6 and 12 d after treatment. center dot Key Results and Conclusions Three phases of tuber dormancy are proposed: Phase I, from tuber initiation to the appearance of the tuber germinating meristem; Phase II, from the tuber germinating meristem to initiation of foliar primordium; and Phase III, from foliar primordium to appearance of the shoot bud on the surface of the tuber. Phase I is the longest phase (approx. 220 d in 'TDr 131'), is not affected by PGRs and is proposed to be an endo-dormant phase. Phases II and III are shorter (< 70 d in total), are influenced by PGRs and environmental conditions, and are therefore endo-/eco-dormant phases. To manipulate dormancy to allow off-season planting and more than one generation per year requires that the duration of Phase I is shortened.

Phases of dormancy in yam tubers (Dioscorea rotundata)

Background and Aims The control of dormancy in yam (Disocorea spp.) tubers is poorly understood and attempts to shorten the long dormant period (i.e. cause tubers to sprout or germinate much earlier) have been unsuccessful. The aim of this study was to identify and define the phases of dormancy in Dioscorea rotundata tubers, and to produce a framework within which dormancy can be more effectively studied. center dot Methods Plants of 'TDr 131' derived from tissue culture were grown in a glasshouse simulating temperature and photoperiod at Ibadan (7 degrees N), Nigeria to produce tubers. Tubers were sampled on four occasions: 30 d before shoot senescence (149 days after planting, DAP), at shoot senescence (179 DAP), and twice during storage at a constant 25 degrees C (269 and 326 DAP). The development of the apical shoot bud was described from tissue sections. In addition, the responsiveness of shoot apical bud development to plant growth regulators (gibberellic acid, 2-chloroethanol and thiourea) applied to excised tuber sections was also examined 6 and 12 d after treatment. center dot Key Results and Conclusions Three phases of tuber dormancy are proposed: Phase I, from tuber initiation to the appearance of the tuber germinating meristem; Phase II, from the tuber germinating meristem to initiation of foliar primordium; and Phase III, from foliar primordium to appearance of the shoot bud on the surface of the tuber. Phase I is the longest phase (approx. 220 d in 'TDr 131'), is not affected by PGRs and is proposed to be an endo-dormant phase. Phases II and III are shorter (< 70 d in total), are influenced by PGRs and environmental conditions, and are therefore endo-/eco-dormant phases. To manipulate dormancy to allow off-season planting and more than one generation per year requires that the duration of Phase I is shortened.

Alkene-arene meta photocycloadditions with a four-carbon-atom tether: efficient approach toward the polycyclic ring systems of aphidicolin and stemodinone

At the end of its tether! The fusion of a six-membered ring onto the four-carbon-atom tether of substrate 1 provides an efficient approach toward the polycyclic ring systems of the natural products aphidicolin and stemodinone. The reaction represents a unique example of a preference for product formation from an endo exciplex in an intramolecular system (exo:endo 2:3=1.0:1.2).

Mono- and binuclear bipyridyl derivatives of the Mo(eta(3)-C3H5)(CO)(2) fragment: structural studies and fluxionality in solution

New mono- and binuclear complexes of the Mo(eta(3)-C3H5)(CO)(2) fragment, containing bipyridyl ligands (2,2'-bpy, 4,4'-Me-2-2,2'-bpy) as chelates, and mono- (4-CNpy, 4-Mepy, NCMe, Br) or bidentate nitrogen ligands (4,4'-bpy, bipyridylethylene, pyrazine) as terminal or bridging ligands, respectively, were prepared. The binuclear complex [{Mo(eta(3)-C3H5)(CO)(2)(2,2'-bpy)}(2)(mu-4,4'-bpy)][PF6](2) (2) was shown by X-ray diffraction to assemble in the crystal forming large channels with a rectangular section. A longer bridge, such as bipyridylethylene, led to a different structure (3). 4-CNpy behaved as monodentate ligand (4), coordinating through the pyridine nitrogen as a terminal ligand. NMR spectroscopy studies showed that the complexes exhibited a fluxional behavior in solution, the endo and exo forms of the more symmetrical equatorial isomers being usually present and interconverting in solution. The solid state structures of the complexes revealed a preference for the more symmetrical equatorial isomer, with the two chelate ligands in trans positions in the binuclear species. The rings tended to become parallel in the organized crystal. (C) 2003 Elsevier B.V. All rights reserved.

New polynuclear Mo-Fe complexes with ferrocenylamidobenzimidazole ligands

The reaction between [Mo(eta(3)-C3H5)(CO)(2)(NCMe)(2)Br] (1) and the ferrocenylamidobenzimidazole ligands FcCO(NH(2)benzim) (L1) and (FcCO)(2)(NHbenzim) (L2) led to a binuclear (2) and a trinuclear (3) Mo-Fe complex, respectively. The single-crystal X-ray structure of [Mo(eta(3)-C3H5)(CO)(2)(L2)Br] [L2 = {[(eta(5)-C5H5)Fe(eta(5)-C5H4CO)](2)(2-NH-benzimidazol-yl)}] shows that L2 is coordinated to the endo Mo(eta(3)-C3H5)(CO)(2) group in a kappa(2)-N,O-bidentate chelating fashion whereas the Mo-II centre displays a pseudooctahedral environment with Br occupying an equatorial position. Complex 2 was formulated as [MO(eta(3)-C3H5)(CO)(2)(L1)Br] on the basis of a combination of spectroscopic data, elemental analysis, conductivity and DFT calculations. L1 acts as a kappa(2)-N,N-bidentate ligand. In both L1 and L2, the HOMOs are mainly localised on iron while the C=O bond(s) contribute to the LUMO(s) and the next highest energy orbitals are Fe-allyl antibonding orbitals. When the ligands bind to Mo(eta(3)-C3H5)(CO)(2)Br, the greatest difference is that Mo becomes the strongest contributor to the HOMO. Electrochemical studies show that, in complex 2, no electronic interaction exists between the two ferrocenyl ligands and that the first electron has been removed from the Mo-II-centred HOMO. (c) Wiley-VCH Verlag GmbH & Co. KGaA.

Mechanical effects of plant cell wall enzymes on xyloglucan/cellulose composites

Xyloglucan-acting enzymes are believed to have effects on type I primary plant cell wall mechanical properties. In order to get a better understanding of these effects, a range of enzymes with different in vitro modes of action were tested against cell wall analogues (bio-composite materials based on Acetobacter xylinus cellulose and xyloglucan). Tomato pericarp xyloglucan endo transglycosylase (tXET) and nasturtium seed xyloglucanase (nXGase) were produced heterologously in Pichia pastoris. Their action against the cell wall analogues was compared with that of a commercial preparation of Trichoderma endo-glucanase (EndoGase). Both 'hydrolytic' enzymes (nXGase and EndoGase) were able to depolymerise not only the cross-link xyloglucan fraction but also the surface-bound fraction. Consequent major changes in cellulose fibril architecture were observed. In mechanical terms, removal of xyloglucan cross-links from composites resulted in increased stiffness (at high strain) and decreased visco-elasticity with similar extensibility. On the other hand, true transglycosylase activity (tXET) did not affect the cellulose/xyloglucan ratio. No change in composite stiffness or extensibility resulted, but a significant increase in creep behaviour was observed in the presence of active tXET. These results provide direct in vitro evidence for the involvement of cell wall xyloglucan-specific enzymes in mechanical changes underlying plant cell wall re-modelling and growth processes. Mechanical consequences of tXET action are shown to be complimentary to those of cucumber expansin.

Localisation studies of cell-wall degrading enzymes in soybean

Postembedding immunoelectron microscopy has been used to investigate the diffusibility of an endo-beta-1,4-glucanase and a xylanase from A. niger in soybean. The results showed more specific localisation of the enzymes into the protein and lipid bodies of soybean cells. This was against our hypothesis that suggested that the enzymes should be localised in the cell wall.

Homoleptic Diphosphacyclobutadiene Complexes [M(η4-P2C2R2)2]x− (M=Fe, Co; x=0, 1)

The preparation and comprehensive characterization of a series of homoleptic sandwich complexes containing diphosphacyclobutadiene ligands are reported. Compounds [K([18]crown-6)(thf)2][Fe(hapto4-P2C2tBu2)2] (K1), [K([18]crown-6)(thf)2][C(h4-P2C2tBu2)2] (K2), and [K([18]crown-6)(thf)2][Co(hapto4-P2C2Ad2)2] (K3, Ad=adamantyl) were obtained from reactions of [K([18crown-6)(thf)2][M(hapto4-C14H10)2] (M=Fe, Co) with tBuCP (1, 2), or with AdCP (3). Neutral sandwiches [M(hapto4-P2C2tBu2)2] (4: M=Fe 5: M=Co) were obtained by oxidizing 1 and 2 with [Cp2Fe]PF6. Cyclic voltammetry and spectro-electrochemistry indicate that the two [M(hapto4-P2C2tBu2)2]-/[M(hapto4-P2C2tBu2)2] moieties can be reversibly interconverted by one electron oxidation and reduction, respectively. Complexes 1–5 were characterized by multinuclear NMR, EPR (1 and 5), UV/Vis,and Moessbauer spectroscopies (1 and 4), mass spectrometry (4 and 5), and microanalysis (1–3). The molecular structures of 1–5 were determined by using X-ray crystallography. Essentially D2d-symmetric structures were found for all five complexes, which show the two 1,3-diphosphacyclobutadiene rings in a staggered orientation. Density functional theory calculations revealed the importance of covalent metal–ligand pi bonding in 1–5. Possible oxidation state assignments for the metal ions are discussed.

Formation of novel photoadducts from irradiation of 5(S)-5-O-tert-butyldimethylsiloxymethylfuran-2(5H)-one

Irradiation of 5S-5-O-tert-butyldimethylsiloxymethylfuran-2(5H)-one 1 in acetonitrile yields the C2-symmetric bis(lactone), 1S,2S,6S,7S-[3S,10S]-bis-tert-butyldimethylsiloxymethyl-4,9-dioxatricyclo[5.3.0.02,6]deca-5,8-dione 6, and a 3-substituted intramolecular product resulting from an apparent 8-endo-trig cyclisation.