A new insight into the understanding of the collapsed form of poly(N-isopropylacrylamide) molecules

Possible collapsed forms of poly(N-isopropylacrylamide) molecules are reviewed on the basis of first principle calculations. Various configurations and associated conformations are detailed. The calculated optimized structures exhibit different possibilities of creating networks of intra-molecular bonds of the hydrogen type. We show that the most remarkable one is able to form a local, self-saturated and well ordered helix. We also indicate in which direction the synthesis of the molecule should be oriented to improve its global behavior in term of hydrophobic/hydrophilic behavior.     

Spectroscopic studies on 2,3,4,5-tetraphenylpyrylium salts with and without silver (I)-bridged structures

Two novel phenylated pyrylium compounds, silver (I)-bridged 2,3,4,5-tetraphenylpyrylium perchlorate (P1) and its silver (I)-free pyrylium ligand (P2) were prepared from 1,2,3,4-tetraphenylcyclopentadiene to examine their spectroscopic behaviors. The UV/vis absorption and fluorescent emission spectra of P1 and P2, measured in three solvents (acetonitrile, dichloromethane and toluene), reveal that the photophysical behaviors are closely related to silver (I) fragment, and strongly dependent on solvent polarity. In polar acetonitrile, P1 displays longer absorption wavelength and much lower fluorescent emission intensity than P2, although they exhibit much similarity in shape. In contrast, in nonpolar toluene, while P2 shows an apparent absorption band at 338 nm, P1 displays a tail-like line without absorption band observed. All the spectra obtained indicate a better coplanarity and a stronger intra-molecular charge transfer in P1 due to the effect of silver (I) fragment. Additionally, the 1H NMR spectra of P1 and P2, which were recorded under the same conditions, indicate that the silver (I) fragment reinforces pyrylium ring's capacity to localize the formal positive charge within the heterocyclic ring.     

Intramolecular aglycon delivery for (1 → 2)-β-mannosylation: towards the synthesis of phospholipomannan of Candida albicans

A high yielding method for 1,2-cis-β-D-mannosylation by intra-molecular aglycon delivery (IAD) through p-methoxy benzyl ether/acetal exchange and phenylsulfoxide donor is reported, along with its application in iterative assembly of antigenic (1 → 2)-β-pentamannoside domain of phospholipomannan (PLM) of fungal pathogen Candida albicans.     

邻氨基二苯醚类重氮盐的水解及分子内缩合反应

用邻氨基二苯醚类化合物进行重氮化水解反应制备邻羟基二苯醚类化合物, 对影响氯代邻氨基二苯醚重氮盐水解反应和分子内关环反应的因素进行了系统研究, 讨论了取代基、金属及其离子催化等对两类反应的影响规律, 揭示了在金属离子催化下, 邻氨基二苯醚类化合物重氮盐发生分子内关环反应的规律, 并推测了反应机理.     

Four new dimeric triterpene glucosides from Sanguisorba officinalis

In search for bioactive compounds from the roots of Sanguisorba officinalis L. (Rosaceae), four new dimeric triterpene glucosides, namely sanguidioside A, B, C, and D (1-4) were isolated. Alkaline hydrolysis of 1-2 afforded the corresponding dimeric aglycones (1a and 2a). Meanwhile, a ready intra-molecular transesterification was observed, providing dimeric triterpenes 1b and 2b. Alkaline hydrolysis of the crude dimmeric saponin also provided a new dimeric triterpene, sanguidiogenin (9). The structures of all these compounds are elucidated via spectroscopic and chemical methods, and are further confirmed by the X-ray diffraction analysis of the dimeric aglycone 2a. Compound 3 represents the first dimeric saponin of an oleanolic acid and an ursonic acid derivative, while compound 4 is the first dimeric saponin of oleanolic acid derivatives.     

Enhanced gelation property due to intra-molecular hydrogen bonding in a new series of bis(amino acid)-functionalized pyridine-2,6-dicarboxamide organogelators

A series of pyridine-2,6-dicarboxamide derivatives containing two α-amino acid pendant groups was prepared and characterized. Three of the synthesized compounds obtained from this series, all having aromatic amino acid side chains, were found to be excellent organogelators toward aromatic solvents (mgc∼10-20 mg/mL), alcoholic solvents (mgc∼4-15 mg/mL), and CCl₄ (mgc∼4-10 mg/mL). It was found that the intra-molecular hydrogen bonds between the pyridine dicarboxamide N-Hs and the pyridine N atom were the key structural elements for gel formation. This series of compounds represented one of the rare examples where both inter- and intra-molecular hydrogen bonds were needed for effective gel formation. FTIR, ¹H NMR, and CD spectroscopy revealed that both hydrogen bonding and π-π aromatic stacking were the driving forces for gelation.     

A simulation algorithm based on Bloch equations and product operator matrix: application to dipolar and scalar couplings

A product operator matrix is proposed to describe scalar couplings in liquid NMR. Combination of the product operator matrix and non-linear Bloch equations is employed to describe effects of chemical shift, translational diffusion, dipolar field, radiation damping, and relaxation in multiple spin systems with both scalar and dipolar couplings. A new simulation algorithm based on this approach is used to simulate NMR signals from dipolar field effects in the presence of scalar couplings. Several typical coupled spin systems with both intra-molecular scalar couplings and inter-molecular dipolar couplings are simulated. Monte Carlo methods are incorporated into simulations as well to analyze diffusion process in these complicated spin systems. The simulated results of diffusion and relaxation parameters and 2D NMR spectra are coincident with the experimental measurements, and agree with theoretical predictions as well. The simulation algorithm presented herein therefore provides a convenient means for designing pulse sequences and quantifying experimental results in complex coupled spin systems.     

Absorption and emission behaviour of trans-p-coumaric acid in aqueous solutions and some organic solvents

The absorption and fluorescence behaviour of trans-p-coumaric acid (trans-4-hydroxycinnamic acid) is investigated in buffered aqueous solution over a wide range from pH 1 to pH 12, in un-buffered water, and in some organic solvents. Absorption cross-section spectra, fluorescence quantum distributions, fluorescence quantum yields, and degrees of fluorescence polarisation are measured. p-Coumaric acid exists in different ionic forms in aqueous solution depending on the pH. There is an equilibrium between the neutral form (p-CAH₂) and the single anionic form (p-CAH⁻) at low pH (pK_na ≈ 4.9), and between the single anionic and the double anionic form (p-CA²⁻) at high pH (pK_aa ≈ 9.35). In the organic solvents studied trans-p-coumaric acid is dissolved in its neutral form. The fluorescence quantum yield of trans-p-coumaric acid in aqueous solution is ?_F ≈ 1.4 × 10⁻⁴ for the neutral and the single anionic form, while it is ?_F ≈ 1.3 × 10⁻³ for the double anionic form. For trans-p-coumaric acid in organic solvents fluorescence quantum yields in the range from 4.8 × 10⁻⁵ (acetonitrile) to 1.5 × 10⁻⁴ (glycerol) were measured. The fluorescence spectra are 7700–10,000 cm⁻¹ Stokes shifted in aqueous solution, and 5400–8200 cm⁻¹ Stokes shifted in the studied organic solvents. Decay paths responsible for the low fluorescence quantum yields are discussed (photo-isomerisation and internal conversion for p-CA²⁻, solvent-assisted intra-molecular charge-transfer or ππ^∗ to nπ^∗ transfer and internal conversion for p-CAH₂ and p-CAH⁻). The solvent dependence of the first ππ^∗ electronic transition frequency and of the fluorescence Stokes shift of p-CAH₂ is discussed in terms of polar solute–solvent interaction effects. Thereby the ground-state and excite-state molecular dipole moments are extracted.     

ESIPT-regulated Mechanoresponsive Luminescence Process byIntroducing Intramolecular Hydrogen Bond in NaphthalimideDerivatives#br#

Herein, two compounds, 4-2′-hydroxybenzylidenehydrazinyl-N-butyl-1,8-naphthalimide(BN-1) and 4-benzylidenehydra-zinyl-N-butyl-1,8-naphthalimide(BN-2), were synthesized to explore the hydrogen bonding effect on mechanoresponsive luminescent(MRL). The results showed that compound BN-1 exhibited strong emission in solution and solid-state compared with compound BN-2. After grinding, the emission intensity of compound BN-1 sharply decreased by as much as 15 times with an obvious red-shift from 552 nm to 577 nm. The control compound BN-2, by contrast, did not change so much before and after grinding. Single crystal analysis suggests that BN-1 molecule formed strong intramolecular interaction via ―N=N···H―O hydrogen bond with a distance of 0.2632 nm. An excited-state intramolecular proton transfer(ESIPT) based fluorophore featured this intramolecular hydrogen bond. The intramolecular hydrogen bond as well as other intermolecular interactions can rigidify the molecular conformation of compound BN-1 in solid-state, and thus suppress the nonradiative pathways, resulting in strong emission. These intra- and intermolecular interactions were destroyed by mechanical stimuli, accompanied by molecular conformation change that decreases the luminescence and blocks the ESIPT process. The MRL process was also demonstrated by scanning electron microscopy and powder X-ray diffraction. The molecular stacking mode changed from crystalline to a disordered amorphous state after grinding.     

A Stereospecific Approach to cis-anti-cis Triquinanes

Abstract

A methodology for the synthesis of compounds containing the cis-anti-cis fused triquinane system has been developed starting from tricyclo[5.2.1.0^2,6]deca-4,8-dien-3-(exo)ol 6 involving Ireland ester Claisen rearrangement and intramolecular diazo ketone cyclopropanation reactions as key steps.