N Mischenko

Stereochemically defined C-substituted glutamic acids and their derivatives. 1. An efficient asymmetric synthesis of (2S,3S)-3-methyl- and -3-trifluoromethylpyroglutamic acids

Abstract An efficient asymmetric synthesis of biologically important (2 S ,3 S )-3-methyl- and (2 S ,3 S )-3-trifluoromethylpyroglutamic acid has been developed. The method consists of diastereoselective Michael addition reaction between ethyl crotonate or ethyl 4,4,4-trifluorocrotonate and a Ni(II) complex of the chiral non-racemic Schiff base of glycine with ( S )- o -[ N -( N -benzylprolyl)amino]benzophenone (BPB) followed by decomposition of the addition products by aq. HCl and treatment of the resultant glutamic acid derivatives with NH 4 OH to afford the target pyroglutamic acids along with recovery of the chiral auxiliary BPB. The stereochemical outcome of the addition reactions was found to be subjected to kinetic control. A mechanistic rationale for the observed stereochemical preferences is discussed.

Highly diastereoselective aza-aldol reactions of a chiral Ni(II) complex of glycine with imines. An efficient asymmetric approach to 3-perfluoroalkyl-2,3-diamino acids

Abstract The LiCl/base-assisted asymmetric aldol-type addition reaction between the N -( p -methoxyphenyl)imine of trifluoroacetaldehyde and the chiral non-racemic Ni(II) complex of the Schiff base of glycine with ( S )- o -[ N -( N -benzylprolyl)amino]benzophenone was found to proceed with excellent chemical and stereochemical outcomes allowing for an efficient access to hitherto unknown stereochemically defined β-perfluoroalkyl-α,β-diamino carboxylic acids. A mechanistic rationale for the observed stereochemical preferences is proposed.

An efficient asymmetric synthesis of (2S,3S)-3-trifluoromethylpyroglutamic acid

Abstract The stereochemical outcome of the Michael reaction between ethyl 4,4,4-trifluorocrotonate and a Ni(II) complex of the Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone was found to be subjected to kinetic and thermodynamic control. Thus, under kinetically controlled conditions high values of diastereo-selectivity, up to 94% de, allowing for an efficient asymmetric synthesis of (2S,3S)-3-trifluoromethylpyroglutamic acid, could be obtained, while the thermodynamically controlled stereoselectivity is rather modest (54–60% de).

Complex melting of semi-crystalline chicory (Cichorium intybus L.) root inulin

When concentrated solutions (30-45% by weight) of inulin (degree of polymerization 2-66, number average degree of polymerization 12) are cooled at 1 degree C/min or 0.25 degree C/min from 96 degrees C to 20 degrees C, suspensions of semi-crystalline material in water are formed. A thermal nucleation process was detected by optical microscopy: the 8-like shaped crystallites resulting from primary nucleation at higher temperature are larger than those resulting from secondary nucleation at lower temperature. Differential scanning calorimetry (DSC) thermograms display melting profiles with three to four partly overlapping endotherms that vary as a function of concentration, cooling rate during crystallization and storage time at 25 degrees C of the crystallite suspension. Recrystallization during melting was observed. The wide-angle X-ray scattering patterns of the samples at 25 degrees C correspond to those of the hydrated crystal polymorph. The structural changes during melting indicated the existence of a single crystal polymorph throughout melting. A periodicity of 95 A, arising from alternating regions of different electron density, is detected in the small angle X-ray scattering patterns at 25 degrees C. The stepwise increase of the long period upon heating is related to the existence of two types of lamellar stacks: one with a higher long period, resulting from the primary nucleation and thus crystallized at high temperature, and a second one with a smaller long period, formed by crystallization at lower temperature. The lamellae formed at low temperature melt at a lower temperature than those formed at high temperature, explaining the existence of the two DSC-endotherms.

Ordering phenomena in ABA triblock copolymer gels

Temperature and concentration dependencies of the degree of order in ABA triblock copolymer gels are discussed. Two factors can influence the ordering phenomena: the conformation of the midblocks (links of the network) and the polydispersity of the endblock domains (nodes of the network). The latter is defined by the scattering density profile (polymer chain packing) at the domain boundary: a sharp boundary corresponds to less polydisperse domains and to increased order in the arrangement of the network nodes. The structure of the network can be described in terms of a highly distorted crystalline lattice with close-packed spheres or with cubic (presumably BCC) equilibrium morphology. The appearance of the latter is never detected in the gels with a stretched conformation of the midblock.

SANS observations on weakly flocculated dispersions

Abstract Structural changes occurring in colloidal dispersions of poly-(methyl metacrylate) (PMMA) particles, sterically stabilized with poly-(12-hydroxystearic acid) (PHSA), while varying the solvent quality, temperature and shear rate, are investigated by small-angle neutron scattering (SANS). For a moderately concentrated dispersion in a marginal solvent the transition on cooling from the effective stability to a weak attraction is monitored. The degree of attraction is determined in the framework of the sticky spheres model (SSM). SANS and rheological results are correlated.

Morphology of physical polymer gels as observed by SAXS and SANS

The morphology of the triblock copolymer gels ABA type, where A constitutes a thermoplastic material (e.g. PS) and B an elastomer (e.g. PB), are presented. The triblock copolymer gels can be described as follows: PS endblocks form spherical domains with small polydispersity of radius in a homogeneous of dissolved midblock and solvent. The conformation of the midblock can be close to that of statistical coil in good solvents or a bit compressed and stretched. These elastic links between the PS domains define the short-range order and through the densepaching principle finally influence the morphology of the gel. The order is liquid-like with a mean interdomain distance that is 2 - 3 times larger than the mean diameter of the domains. The results of stretching experiments are consistent with a model of a polymer network exhibiting local coordination (short-range) order.

SANS contrast in iota-carrageenan gels and solutions in D2O

Abstract SANS of Na + -iota-carrageenan in D 2 O/saline solutions was measured as a function of concentration, temperature and type of counterions (K + or Na + ). High and low scattering-contrasted gels and solutions were detected. High contrast is caused by aggregation of low-hydrated chains at high concentration of carrageenan in the presence of gel-promoting counterions. The aggregates do not disappear at 60°C, although at this temperature the molecules should partialy lose their ordered conformation. The network formation, observed by SANS, does not happen in high-contrast system immediately after cooling and takes a few hours to be completed at 12°C. The minimum concentration sufficient for the high-contrast regime in 0.1 M KCl lies between 15 and 25 mg/ml.