Synthesis of enantiopure prolines via exo-stereoselective 1,3-dipolar cycloadditions to acetone-derived chiral stabilised azomethine ylides

The chiral stabilised azomethine ylide formed from condensation of the dimethyl acetal of acetone with (5S)-5-phenylmorpholinone undergoes stereoselective exo-cycloaddition reactions with a range of doubly and singly activated dipolarophiles when generated in the presence of excess (MgBr2OEt2)-O-.. The cycloadducts can be degraded to yield enantiomerically pure proline derivatives.

Multiwavelength Observations of EXO 0748-676. II. Emission-Line Behavior

We present optical and ultraviolet spectra, light curves, and Doppler tomograms of the low-mass X-ray binary EXO 0748-676. Using an extensive set of 15 emission-line tomograms, we show that, along with the usual emission from the stream and ``hot spot,'' there is extended nonaxisymmetric emission from the disk rim. Some of the emission and Hα and Hβ absorption features lend weight to the hypothesis that part of the stream overflows the disk rim and forms a two phase medium. The data are consistent with a 1.35 Msolar neutron star with a main-sequence companion and hence a mass ratio q~0.34.

Multiwavelength Observations of EXO 0748-676. I. Reprocessing of X-Ray Bursts

We present high time-resolution multiwavelength observations of X-ray bursts in the low-mass X-ray binary UY Vol. Strong reprocessed signals are present in the ultraviolet and optical, lagged and smeared with respect to the X-rays. The addition of far-ultraviolet coverage for one burst allows much tighter constraints on the temperature and geometry of the reprocessing region than previously possible. A blackbody reprocessing model for this burst suggests a rise in temperatures during the burst from 18,000 to 35,000 K and an emitting area comparable to that expected for the disk and/or irradiated companion star. The lags are consistent with those expected. The single-zone blackbody model cannot reproduce the ratio of optical to ultraviolet flux during the burst, however. The discrepancy seems too large to explain with deviations from a local blackbody spectrum and more likely indicates that a range of reprocessing temperatures are required. Comparable results are derived from other bursts, and in particular the lag and smearing both appear shorter when the companion star is on the near side of the disk as predicted. The burst observed by HST also yielded a spectrum of the reprocessed light. It is dominated by continuum, with a spectral shape consistent with the temperatures derived from lightcurve modeling. Taken as a whole, our observations confirm the standard paradigm of prompt reprocessing distributed across the disk and companion star, with the response dominated by a thermalized continuum rather than by emission lines.

Exploring the Factors Affecting the Solubility of Chitosan in Water

We explore the role of crystallinity and inter- or intramolecular forces in chitosan for its solubility in water and demonstrate the expansion of its solubility to a wider pH range. Due to its semicrystalline nature, derived mainly from inter- and intramolecular hydrogen bonds, chitosan is water-soluble only at pH < 6. In acidic conditions, its amino groups can be partially protonated resulting in repulsion between positively charged macrochains, thereby allowing diffusion of water molecules and subsequent solvation of macromolecules. We show that chemical disruption of chitosan crystallinity by partial re-acetylation or physical disruption caused by the addition of urea and guanidine hydrochloride broadens the pH-solubility range for this biopolymer.

Why is Chitosan Mucoadhesive?

Chitosan is a biocompatible and biodegradable amino polysaccharide, which is soluble in aqueous solutions at pH < 6.5. It has been widely used for developing drug delivery systems because of its excellent mucoadhesive properties. Although many studies report on chitosan being mucoadhesive, the nature of interactions between chitosan and mucin remains poorly defined. Here, we have examined the role of primary amino groups and the role of electrostatic attraction, hydrogen bonding, and hydrophobic effects on aggregation of gastric mucin in the presence of chitosan. Reducing the number of amino groups through their half acetylation results in expansion of chitosan’s pH-solubility window up to pH 7.4 but also reduces its capacity to aggregate mucin. We demonstrated that electrostatic attraction forces between chitosan and gastric mucin can be suppressed in the presence of 0.2 mol/L sodium chloride; however, this does not prevent the aggregation of mucin particles in the presence of this biopolymer. The presence of 8 mol/L urea or 10% v/v ethanol in solutions also affects mucin aggregation in the presence of chitosan, demonstrating the role of hydrogen bonding and hydrophobic effects, respectively, in mucoadhesion.

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).

The first preparation of beta-lactones by radical cyclization

beta-Lactones have, for the first time, been prepared by 4-exo-trig radical cyclization. Thus, alpha-ethenoyloxy radicals react in the presence of tributylstannane in a photothermal process to give beta-lactones. Highest yields were obtained when groups capable of stabilizing a carboncentered radical were present at the 3-position of the alkenoate acceptor.

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.

Cycloadditions of chiral carbonyl ylides with imine dipolarophiles as a route to enantiomerically pure alpha-amino-beta-hydroxy acids

The preparation of enantiomerically pure threo-beta-amino-alpha-hydroxy acids via 1,3-dipolar cycloadditions of imine dipolarophiles with the chiral isomunchnone derived from (5R)-5-phenylmorpholin-3-one 1 is described. The cycloadducts were obtained with excellent diastereofacial- and exo-selectivity. Subsequent hydrolysis and chemoselective exocyclic amide cleavage afforded the threo-beta-amino-alpha-hydroxy acids with recovery of the initial chiral auxiliary. (C) 2009 Published by Elsevier Ltd.

Liquid crystal phase formation by biopolymers

This review discusses liquid crystal phase formation by biopolymers in solution. Lyotropic mesophases have been observed for several classes of biopolymer including DNA, peptides, polymer/peptide conjugates, glycopolymers and proteoglycans. Nematic or chiral nematic (cholesteric) phases are the most commonly observed mesophases, in which the rod-like fibrils have only orientational order. Hexagonal columnar phases are observed for several systems (DNA, PBLG, polymer/peptide hybrids) at higher concentration. Lamellar (smectic) phases are reported less often, although there are examples such as the layer arrangement of amylopectin side chains in starch. Possible explanations for the observed structures are discussed. The biological role of liquid crystal phases for several of these systems is outlined. Commonly, they may serve as a template to align fibrils for defined structural roles when the biopolymer is extruded and dried, for instance in the production of silk by spiders or silkworms, or of chitin in arthropod shells. In other cases, liquid crystal phase formation may occur in vivo simply as a consequence of high concentration, for instance the high packing density of DNA within cell nuclei.

Dynamics of shear-induced orientation transitions in block copolymers

This review discusses liquid crystal phase formation by biopolymers in solution. Lyotropic mesophases have been observed for several classes of biopolymer including DNA, peptides, polymer/peptide conjugates, glycopolymers and proteoglycans. Nematic or chiral nematic (cholesteric) phases are the most commonly observed mesophases, in which the rod-like fibrils have only orientational order. Hexagonal columnar phases are observed for several systems (DNA, PBLG, polymer/peptide hybrids) at higher concentration. Lamellar (smectic) phases are reported less often, although there are examples such as the layer arrangement of amylopectin side chains in starch. Possible explanations for the observed structures are discussed. The biological role of liquid crystal phases for several of these systems is outlined. Commonly, they may serve as a template to align fibrils for defined structural roles when the biopolymer is extruded and dried, for instance in the production of silk by spiders or silkworms, or of chitin in arthropod shells. In other cases, liquid crystal phase formation may occur in vivo simply as a consequence of high concentration, for instance the high packing density of DNA within cell nuclei.

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.

Reaction of 2-methyl-1,4-naphthoquinone and its 3-substituted derivatives with active methylene group anions and with diazo compounds

The reaction of 2-chloro-3-methyl-1,4-naphthoquinone (3) with the anion of ethyl cyanoacetate led to a mixture of two epimeric fused-ring cyclopropane compounds, characterised as exo- and endo-1-cyano-1 -ethoxycarbonyl-1a-methyl-1a,7a-dihydro-1H-cyclopropa[b]naphthalene-2,7-dione (8) and (9). Various hydrolysis products of these were prepared and an X-ray crystallographic analysis was carried out on one of them, 1-carbamoyl-1 -carboxy-1a-methyl-1a,7a-dihydro-1H-cyclopropa[b]-naphthalene-2,7-dione (17). The reaction of 2-methyl-1,4-naphthoquinone (1) with ethyl diazoacetate gave a fused pyrazoline derivative, 3-ethoxycarbonyl-4-hydroxy-9a-methyl-1,9a-dihydro-benz[f]indazol-9-one (22), while reaction of 2-methyl-3-nitro-1,4-naphthoquinone (5) with diazomethane led to a fused Δ2-isoxazoline N-oxide, 3a-methyl-3,3a-dihydroisoxazolo[3,4-b]naphthalene-4,9-dione 1-oxide (26).

Enhancing the crystallization and orientation of electrospinning poly (lactic acid) (PLLA) by combining with additives

PLLA is a thermoplastic biopolymer and can be used in industrial applications for medical and filtration applications. The brittleness of PLLA is attributed to slow crystallization rates and its glass transition temperature (Tg) is high (60 °C); for this reason, its applications are limited. The orientation, morphology, and crystal structure of the electrospun fibers was investigated by SEM, POM, DSC, FTIR, XRD, and SAXS. Combining with additives leads to a large decrease of fiber diameter, viscosity, and changes of fiber morphology and crystal structure compared to pure PLLA. DSC showed that the Tg of PLLA decreased about 15 °C and there was no change in relaxation enthalpy by the addition of plasticizer. FT-IR indicate a strong interaction between PLLA and additives; a new band appears in the PLLA blend at 1,756 cm−1 at room temperature as a crystalline band without any annealing. In addition, WAXD indicated that the intensities of the two peaks at (200/110) and (203) increased for the blend at room temperature without any annealing in comparison with PLLA; this means that PHB crystallizes in the amorphous region of PLLA. The POM experiments agree with the results from DSC, FTIR, and WAXS measurements, confirming that adding PHB results in an increase in the number of nuclei with much smaller spherulites and enhances the crystallization behavior of this material, thereby improving its potential for applications.