Biomimetic synthesis of (+/-)-pinnatal and (+/-)-sterekunthal A

Concise biomimetic syntheses of the antimalarial naphthoquinones (+/-)-pinnatal and (+/-)-sterekunthal A are described.     

Liquid chromatographic-mass spectrometric studies on the in vitro degradation of a poly(ether ester) block copolymer

A detailed study on the in vitro degradation of a poly(ethylene glycol) and poly(butylene terephthalate) (PEOT/PBT) segmented block copolymer was carried out using liquid chromatography/electrospray-mass spectrometry. Accelerated hydrolysis of PEOT/PBT was achieved by placing the material for 14 days in a refluxing phosphate buffered saline (pH 7.4) solution. All major degradation products and several side-products were identified using both the positive and the negative ion mode. The data indicate that degradation does not only occur in the "soft", but also in the "hard" segments of the polymer. Liquid chromatographic separation is required to distinguish between degradation products with different sequences but identical molecular mass. The addition of ammonium and sodium ions provided important complementary information on the number of monomer units present in the degradation products.     

Trichloropalladate(II) Complexes with Pyridine Ligands – Molecular Structure and Conformational Analysis of [K(18C6)][PdCl3(py)]

[K(18C6)]₂[Pd₂Cl₆] ( 1 ) (18C6 = 18‐crown‐6) was found to react with pyridines in a strictly stoichiometric ratio 1 : 2 in methylene chloride or nitromethane to yield trichloropalladate(II) complexes [K(18C6)][PdCl₃(py*)] (py* = py, 2a ; 4‐Bnpy, 2b ; 4‐tBupy, 2c ; Bn = benzyl; tBu = tert‐butyl). The reaction of 1 with pyrimidine (pyrm) in a 1 : 1 ratio led to the formation of the pyrimidine‐bridged bis(trichloropalladate) complex [K(18C6)]₂[(PdCl₃)₂(μ‐pyrm)] ( 3 ). The identities of the complexes were confirmed by means of NMR spectroscopy (¹H, ¹³C) and microanalysis. The X‐ray structure analysis of 2a reveals square‐planar coordination of the Pd atom in the [PdCl₃(py)]^? anion. The pyridine plane forms with the complex plane an angle of 55.8(2)°. In the [K(18C6)]⁺ cation the K⁺ lies outside the mean plane of the crown ether (defined by the 6 O atoms) by 0.816(1) Å. There are tight K···Cl contacts between the cation and the anion (K···Cl1 3.340(2) Å, K···Cl2 3.166(2) Å). To gain an insight into the conformation of the [PdCl₃(py)]^? anion, DFT calculations were performed showing that the equilibrium structure ( 6eq ) has an angle between the pyridine ligand and the complex plane of 35.3°. Rotation of the pyridine ligand around the Pd–N vector exhibited two transition states where the pyridine ligand lies either in the complex plane ( 6TS _pla, 0.87 kcal/mol above 6eq ) or is perpendicular to it ( 6TS _per, 3.76 kcal/mol above 6eq ). Based on an energy decomposition analysis the conformation of the anion is discussed in terms of repulsive steric interactions and of stabilizing σ and π orbital interactions between the PdCl₃^? moiety and the pyridine ligand.     

Atropo-enantioselective total synthesis of knipholone and related antiplasmodial phenylanthraquinones

The "lactone concept" has been efficiently employed for the first atropo-enantioselective synthesis of knipholone and related natural phenylanthraquinones. Besides the regio- and stereoselective construction of the biaryl axis, another important step was the "synthetically late" introduction of the C-acetyl group, either by a Friedel-Crafts type acetylation or by an ortho-selective Fries rearrangement first tested on simplified model systems and subsequently applied to the highly atroposelective preparation of the natural products and of simplified analogs thereof for biotesting. The synthetic availability of these natural biaryls, their precursors, and their unnatural analogs permitted a broader investigation of the antiplasmodial activities of these interesting biaryls.     

13C and (15)N chemical shift tensors in adenosine,guanosine dihydrate, 2'-deoxythymidine,and cytidine

The (13)C and (15)N chemical shift tensor principal values for adenosine, guanosine dihydrate, 2'-deoxythymidine, and cytidine are measured on natural abundance samples. Additionally, the (13)C and (15)N chemical shielding tensor principal values in these four nucleosides are calculated utilizing various theoretical approaches. Embedded ion method (EIM) calculations improve significantly the precision with which the experimental principal values are reproduced over calculations on the corresponding isolated molecules with proton-optimized geometries. The (13)C and (15)N chemical shift tensor orientations are reliably assigned in the molecular frames of the nucleosides based upon chemical shielding tensor calculations employing the EIM. The differences between principal values obtained in EIM calculations and in calculations on isolated molecules with proton positions optimized inside a point charge array are used to estimate the contributions to chemical shielding arising from intermolecular interactions. Moreover, the (13)C and (15)N chemical shift tensor orientations and principal values correlate with the molecular structure and the crystallographic environment for the nucleosides and agree with data obtained previously for related compounds. The effects of variations in certain EIM parameters on the accuracy of the shielding tensor calculations are investigated.     

Layer structures. VI. Chiral sanidic polyesters derived from 2,5-bis (dodecyloxy) terephthalic acids and 4,4′-dihydroxybiphenyl

Copolycondensations of (S,S)-2,5-bis(2-methylbutyloxy) terephthaloylchloride with 2,5-bis(dodecyloxy)terephthaloylchloride and with 4,4′-bistrimethylsiloxybiphenyl yielded a series of novel chiral thermotropic copolyesters. These polyesters were characterized by elemental analyses, inherent viscosities, ¹H-NMR spectroscopy, optical rotations, optical microscopy, DSC measurements, and WAXS powder patterns recorded with synchrotron radiation under variation of the temperature. All homo- and copolyesters formed a solid sanidic layer structure with melting temperatures (T_m) ≥ 200°C. A broad enantiotropic nematic or cholesteric phase is formed above T_m with isotropization temperatures (T_is) in the range of 275–325°C. Yet, the T_m of the chiral homopolyester is so high (378°C) that the melting process is immediately followed by rapid degradation. The cholesteric phases of the copolyesters displayed unusual mobile schlieren textures, but a stable Grandjean texture was never obtained. Cholesteric domains consisting of loose bundles of more or less helical main chains are discussed as supramolecular order responsible for the observed textures and their pronounced temperature dependence. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 947–957, 1997     

Diastereoselective formation of homochiral flexible perylene bisimide cyclophanes and their hybrids with fullerenes

Cyclophanes of different ring sizes featuring perylene-3,4:9,10-tetracarboxylic acid bisimide (PBI) linked by flexible malonates were designed, synthesized, and investigated with respect to their structural, chemical and photo-physical properties. It is predominantly the number of PBIs and their geometric arrangement, which influence dramatically their properties. For example, two-PBI containing cyclophanes reveal physico-chemical characteristics that are governed by strong co-facial π–π interactions. This is in stark contrast to cyclophanes with either three or four PBIs. Key to co-facial π–π stackings are the flexible malonate linkers, which, in turn, set up the ways and means for diastereoselectivity of the homochiral PBIs at low temperatures, on one hand. In terms of selectivity, diastereomeric (M,M)/(P,P) : (M,P)/(P,M) pairs with a ratio of approximately 10 : 1 are discernible in the ¹H NMR spectra in C₂D₂Cl₄ and a complete diastereomeric excess is found in CD₂Cl₂. On the other hand, symmetry-breaking charge transfer as well as charge separation at room temperature are corroborated in steady-state and time-resolved photo-physical investigations. Less favourable are co-facial π–π stackings in the three-PBI containing cyclophanes. For statistical reasons, the diastereoisomers (M,M,M)/(P,P,P) and (M,M,P)/(P,P,M) occur here in a ratio of 1 : 3. In this case, symmetry-breaking charge transfer as well as charge separation are both slowed down. The work was rounded-off by integrating next to the PBIs, for the first time, hydrophobic or hydrophilic fullerenes into the resulting cyclophanes. Our novel fullerene–PBI cyclophanes reveal unprecedented diastereoselective formation of homochiral (M,M)/(P,P) pairs exceeding the traditional host–guest approach. Hybridization with fullerenes allows us to modulate the resulting solubility, stacking, cavity and chirality, which is of tremendous interest in the field.

Perylene bisimide (PBI) cyclophanes linked by flexible malonates were functionalized with fullerenes. Modulation of the chemical environment enhances the chiral self-sorting, leading exclusively to the homochiral diastereomeric pair (M,M)/(P,P).     

Investigation of swelling and diffusion in polymers by NMR imaging

The kinetics of diffusion in polymers ranges from simple Fickian diffusion to higher order diffusion, such as Case II diffusion^1-2). The conventional method for determining the characteristics of solvents into polymer matrices is by measuring the mass uptake of the polymer as the solvent penetrates the matrix. However, since such measurements perform observations at a macroscopic level, little information has been obtained relating to the nature of the solvent in the polymer matrix and the mechanisms of the processes that control the diffusion. Nuclear magnetic resonance (NMR) imaging (‘MRI’) has been used to observe the penetration of solvents into solid systems in realtime. The method provides a one- or more-dimensional image of the density and the mobility of the solvent in a material or of the network changes of the material itself due to the softening influence of the solvent. The first (imaging of the solvent) can be used for a quantitative measurement of the diffusion whereas the observation of the network gives information about the changing of the network (mobility, de-crystallization…) during the swelling process. For example the diffusion of organic solvents in some polymeric materials (natural rubber, water gels (PNIPAAm), and nematic diblock-copolymers) are investigated.