Molecular Recognition of Aromatic Nitro Compounds at Cyclodextrin Dithiocarbamate-modified Electrodes

Silver electrodes were modified by the adsorption of α-, β-, and γ-cyclodextrin dithiocarbamates (2α–γ) and characterized by reductive desorption experiments. Their molecular recognition properties were studied by cyclic voltammetry using three families of positional isomers of aromatic nitro compounds. Electrodes modified with 2α and 2β were selective for meta and para isomers while 2γ showed little selectivity. These observations are explained in terms of cavity sizes and guest structure. Computational studies suggest that the main reason for the observed selectivity is the different position of the NO² group in ortho and para isomers with respect to the CD cavity.     

Sorption of phenols by single and mixed cyclodextrin polymers

Cyclodextrin (CD) hydrogels were synthesized by a crosslinking reaction with the same cyclodextrin/epichlorohydrin mole ratio (1/11) using αCD, βCD, γCD, and 50:50 mixtures of α/βCD and β/γCD. In order to investigate the sorption capacity of these hydrogels to different solutes, five model molecules have been selected: phenol, 3-nitrophenol, 4-nitrophenol, 1-naphthol, and the antiinflamatory drug diflunisal. The amounts sorbed have been related to the different affinities of the solutes. 1-naphthol shows the highest affinity for these polymers, especially in the case of sorbents containing βCD. The sorption is considerably poorer for phenol than for its nitro derivatives. The two structural isomers 3- and 4-nitrophenol show significant differences in their affinities towards αCD, βCD and α/βCD. Finally, in the case of diflunisal, a bulkier model molecule, remarkable differences were found on the sorption behaviour by polymers whose cyclodextrins have a similar affinity for this solute (βCD, γCD, and β/γCD).     

Isomerization and fragmentation of aliphatic ether radical cations: Interconversion of distonic ions and cyclopropane intermediates

The reactions of propyl ether radical cations close to threshold are initiated by (reversible) formation of γ-disitonic isomers, R$ \mathop {\rm O}\limits^ + $ (H)CH₂CH₂CH₂^·. The three methylene groups in these ions lose their positional identity by ring closure/ring opening via [cyclopropane + alcohol]^+· intermediates. Extensive hydrogen exchange occurs within the C₃-chain. When R is not methyl the γ-distonic isomer undergoes further intramolecular hydrogen atom transfer reactions that lead to formation of α- and β-distonic ions. The α-distonic isomers expel ethyl and propyl radicals by C? O bond cleavage.     

Construction of α-phosphonolactams via rhodium (II)-catalyzed intramolecular C?H insertion reactions

The Rh(II)-catalyzed intramolecular C H insertion reactions of N,N-dialkyl-α-diazo-α-(diethylphosphono)acetamides 2a , f–j in CHCl₃ or ClCH₂CH₂Cl were found to give monocyclic and bicyclic α-phosphono-β-lactams, 3a and 3f–j , in 43–67% yields via regiospecific α-C H insertion of the N-alkyl groups. Similar treatment of N-benzyl-N-isopropyl-α-diazo-α-(diethylphosphono)acetamide ( 2b ) and the corresponding N-isobutyl-N-methylacetamide 2d in ClCH₂CH₂Cl afforded mixtures of β-lactams 3b (35%) and and 3b ′ (16%), β-lactam 3d (47%), and γ-lactam 4d (10%), respectively, each of which is formed by the competitive C H insertion reaction between benzylic and isopropyl α-C H bonds and between methyl α-C H and methine β-C H bonds, respectively. For the formation of β-lactams, the selectivity in the rhodium-mediated C H insertion in ClCH₂CH₂Cl follows the order methyl > methine > benzylic α-C H bond on N-substituents. The N,N-dibutyl-α-diazo homologue 2c and Nα[α-diazo-α-(diethylphosphono)acetyl]-2-methylindoline ( 2k ) exclusively produced γ-lactams 4c (67%) and 4k (81%) via insertion into the methylene β-C H and methyl β-C H bonds. tert-Butyl N-[α-diazo-α-(dibenzylphosphono)acetyl]-piperidine-2-carboxylate ( 2m ) on similar treatment, followed by deprotection of the benzyl ester afforded the 7-phosphono carbacepham 6 in 32% overall yield. Similar Rh(II)-catalyzed cyclization of N-methyl-N[4-benzyloxy-α-(diethylphosphono)-phenyl(diethyl-phosphono)methyl]-α-diazo-acetamide ( 2n ) led to 1-[4′-benzylphenyl(diethylphosphono)methyl] -3-(diethyl-phosphono)azetidin-2-one ( 3n ) in 78% yicld. The phosphono group at C-7 of 3f was converted into the acetylamino group via a four-step reaction. Application of chiral rhodium(II) carboxylates 12a–c to the insertion reactions of 2b , c produced α-phosphono-β-and γ-lactams, 3b and 4c , in 6–24% ee and 25–29% ee, respectively.     

Capillary gas chromatographic separation of C10–C12 secondary phenylalkanes on modified cyclodextrin stationary phases

The separation of isomers and enantiomers of branched C₁₀-C₁₂ phenylalkanes by gas chromatography on fused silica capillary columns coated with some modified β- and γ-cyclodextrins was studied. It was shown that the separation of positional isomers of C₁₀-C₁₂ phynylalkanes on modified cyclodextrin capillary columns is not better than that on a column coated with modified polyethylene glycol. Differences were found in the enantioselectivity of modified β- and γ-cyclodextrins for the separation of C₁₀-C₁₂ secondary phenylalkane enantiomers. While alkylderivatives of β-CDs resolve enantiomers of 3-phenylalkanes, alkyl derivatives of γ-CD resolve enantiomers of 2-phenylalkanes. Since shape selectivity factors of modified cyclodextrins have indicated no inclusion of the considered solutes in cyclodextrin cavities, enantioselective interactions most probably occur on the outer sphere of cyclodextrins.     

Lewis acid-promoted reaction of β,γ-unsaturated α,α-dimethoxy esters with silyl nucleophiles

The Lewis acid-promoted reaction of β,γ-unsaturated α,α-dimethoxy esters, which are easily prepared by the acetalization of β,γ-unsaturated α-keto esters, with silyl nucleophiles is presented. By employing trimethylsilyl enolate and allyltrimethylsilane as nucleophiles, the BF₃-promoted reactions of a series of β,γ-unsaturated α,α-dimethoxy esters bearing aromatic and aliphatic substituents proceeded at the γ-position in an SN2′ manner to furnish γ-substituted α,β-unsaturated α-methoxy esters in good yields with high regioselectivity. In contrast, the reaction using trimethylsilyl cyanide predominantly occurred at the α-position, and the reaction of silyl hydride resulted in a mixture of α- and γ-regioisomers in favor of the γ-substitution products.     

3-位烯丙基及甲基衍生的β-环糊精固定相的合成及气相色谱性能研究

制备了一种新的β-环糊精衍生物固定相2,6-二-O-戊基-3-O-[(甲基)5(烯丙基)2]-β-CD,并对其气相色谱分离性能进行研究。实验显示,该固定相具有良好的柱表面性能和较强的色谱分离能力,对一些芳香族位置异构体的分离能力优于2,6-二-O-戊基-3-O-烯丙基-β-CD衍生物,对10余种手性物质显示出较好的选择性能,且对α-位取代的丙酸酯类手性分离效果明显优于杂环类β-CD衍生物。与2,6-二-O-戊基-3-O-烯丙基-β-CD固定相的分离性能比较表明,β-CD 3位羟基部分甲基基团的引入能增强对芳香族位置异构体的选择能力,但不能明显改变烯丙基衍生物的手性分离能力。     

Effects of some phosphorus substituents on the carbon-13 chemical shifts of alkyl chains

The carbon-13 NMR spectra of 16 aliphatic phosphorus compounds with -PCl₂, -P(OCH₃)₂, -P(CH₃)₂ or -PS(CH₃)₂ groups were determined and interpreted on the basis of the usual α, β-deshielding and γ-shielding effects. The α-effects of all the groups were quite large (15 to 30 ppm), though the β-effects were small (0 to 3 ppm). The γ-effects were also small (0·2 to 1·6 ppm), but of significance in suggesting reduced steric compression relative to that found for methyl and most of the common organic functional groups. This may be attributed to the greater length of the C? P bond. The phosphorus-31 NMR spectra of these compounds also show the existence of interaction with γ-carbons by exhibiting small additive upfield shifts.     

Separations of a-hexachlorocyclohexane (a-HCH) and pentachlorocyclohexene (PCCH) enantiomers on a cyclodextrin-phase (Cyclodex-B) by HRGC/ECD

The separation of a-hexachlorocyclohexane and of all eight possible pentachlorocyclohexene isomers into their enantiomers by high-resolution gas chromatography and electron capture detection was achieved by using permethylated β-cyclodextrin in DB 1701 (Cyclodex-B, J & W) as chiral stationary phase. The first step in the metabolism of 1,2,3,4,5,6-hexachlorocyclohexane (HCH) is the formation of 1,3,4,5,6-pentachlorocyclohexene-1 (PCCH). Various degradation experiments were carried out with α-, β-, γ-, and δ-HCH. Whereas β-HCH gave no degradation products, the isomers α-, γ-, and δ-HCH degraded into various PCCHs and trichlorobenzenes, when using pyridine as dehydrochlorination reagent in xylene at 60°C. Isomerization into other HCH isomers was observed only for δ-HCH. Enantioselective dehydrochlorination was observed for a- and γ-HCH using the alkaloid (?)-brucine, R-(+)- or S-(?)-a-phenylethylamine as chiral base.     

Syntheses and characterisation of novel cyclodextrin vinyl derivatives from cyclodextrin-nitrophenol-derivatives

In this study novel reactive α-, β- and γ-cyclodextrin-esters (acrylate, pent-4-enoate and undec-10-enoate) have been synthesised and characterised. The syntheses were carried out by using nitrophenol-esters with the ability to form inclusion complexes with cyclodextrins, thereby aiming at a better control of the substitution degree and number of positional isomers of the cyclodextrin derivatives. Derivatives of α-, β- and γ-cyclodextrins modified with three different lengths of carbon-chains were successfully synthesised and characterised by MALDI-TOF MS, HPLC and LC-MS/MS, revealing some differences in LC elution patterns, substitution degrees and number of produced positional isomers. Differences were seen as an effect of changing the size of the cyclodextrin as well as the size of the side-chain being attached. The inclusion complexes between the nitrophenol esters and the different cyclodextrins were studied by ITC and selected ones by 2D ROESY NMR, showing some interesting differences in strength and structure of the complexes. These differences are speculated to be the origin of the different substitution patterns of the derivatives as observed by LC-MS/MS.     

New enantiopure 7-azanorbornane β-substituted prolines by SN2 displacements at the Cγ of the side chain

Continuing our work on building new β-substituted prolines in enantiomerically pure form as new surrogates to be incorporated into model peptides, we describe functional group conversions and carbon–carbon bond forming reactions by S_N2 displacements at the γ-position of the α-amino acid side chain of an azanorbornane derivative. This work extends efforts on the elaboration of the side arm at carbon C2 of the 7-azabicyclo[2.2.1]heptane core.     

Effect of alkaline earth metal dopants on the thermal decomposition of the CaCO3-SiO2 system: Part II. Formation of dicalcium silicate

Mixtures of calcium oxide (taken as carbonate) and silica in 2:1 molar ratio containing varying amounts of MgO, SrCO₃ and BaCO₃ as dopants were subjected to thermal treatment up to 1450°C. The exothermic peaks at 1200°C and above (beyond the decomposition temperature of calcium carbonate) have been examined to elucidate the phases formed. The exothermic peak at 1210°C without dopant was found to conform to the β-dicalcium silicate phase with a significant amount of free lime and cristobalite along with small amounts of the γ-C₂S phase. MgO at 0.1–1% leads to the formation of β- and γ-dicalcium silicate phases at 1420–1430°C, while 5% MgO results in the formation of the β-C₂ S phase at 1360°C. SrCO₃, in the concentration range studied, leads to the stabilization of β-C₂S, but does not lower its temperature of formation. BaCO₃ at 0.1–1% assists in the formation of the β-dicalcium silicate phase, but 5% BaO forms a mixture of β- and α'_H-C₂S phases at a lower temperature.     

The vibrational spectra and stability of dipyrrolylmethene hydrobromides and their oxa and thia derivatives

This paper presents the IR spectra of solid alkyl substituted α,α-, α,β-, and β,β-dipyrrolylmethene hydrobromides and their oxa and thia derivatives recorded in KBr pellets over the frequency range 400–4000 cm^?1. The conclusion was drawn that the NH bonds in the pyrrole fragments of α,α-dipyrrolylmethene were identical, as distinct from the α,β and β,β isomers. The influence of functional substitution in the dipyrrolylmethene molecule on the frequencies of NH stretching vibrations was discussed. A satisfactory correlation between NH bond vibrational frequencies and the enthalpies of vaporization of gaseous hydrogen bromide from crystalline samples under heating was observed. This allows the IR data to be used as a criterion of the stability of dipyrrolylmethene salts and their oxa and thia derivatives.     

Naphthyl- vs. anthrylpyridine-2,6-dicarboxamides in cation binding studies. Synthesis and spectroscopic properties

Pyridine-2,6-carboxamides bearing α- or β-naphthyl- and α- or β-anthryl residues were prepared using simple method from pyridine-2,6-carboxylic acid dichloride and the respective aromatic amines. For the obtained receptors, selective binding of lead(II) and copper(II) was found. Ion–receptor interactions were studied using UV–vis spectroscopy, spectrofluorimetry, ¹H NMR and FTIR spectroscopy. The reversible lead(II) and copper(II) binding was discussed in regard of type of aromatic residue and amide bond localisation in aromatic ring, and binding model was proposed.     

Design,synthesis and structural analysis of mixed α/β-peptides that adopt stable cyclic hairpin-like conformations

The strategic replacement of four α-amino acid residues of a cyclo-(ααααα)₂ peptide by β-, β²- or β³-amino acids residues provided a series of novel 2:1 α/β-mixed peptides that were designed to adopt cyclic hairpin-like structures. It was shown that conformationally stable cyclo-(αβαβα)₂ isomers can be obtained using both enantiomers of the central two basic α-amino acid residues, a known α-amino acid turn sequence and several combinations of facing β-amino acid residues with no side chain or a hydrophobic side chain having specific regio- and stereochemistry. The X-ray analysis of two derivatives provides molecular details of the intra-molecular hydrogen bonding interaction, dihedral angles of the backbone and side chain positioning of the novel cyclic hairpin-like structures. One of these isomers forms an unprecedented hexagon-shaped nano-channel assembly in the crystal structure. Well-defined cyclic hairpin-like structures as described here and derivatives that can be readily designed based on this research can be used as scaffolds onto which functional groups can be grafted in a spatially controlled manner and as β-hairpin mimics with specific biological properties.     

S‐(4‐Nitrophenyl) 4‐nitrobenzenethiosulfonate

The title compound, C₁₂H₈N₂O₆S₂, (I), is a positional isomer of S‐(2‐nitrophenyl) 2‐nitrobenzenethiosulfonate [Glidewell, Low & Wardell (2000). Acta Cryst. B 56 , 893–905], (II). The most obvious difference between the two isomers is the rotation of the nitro groups with respect to the planes of the adjacent aryl rings. In (I), the nitro groups are only slightly rotated out of the plane of the adjacent aryl ring [2.4 (6) and 6.7 (7)°], while in (II) the nitro groups are rotated by between 37 and 52°, in every case associated with S—S—C—C torsion angles close to 90°. Other important differences between the isomers are the C—S—S(O₂)—C torsion angle [78.39 (2)° for (I) and 69.8 (3)° for (II) (mean)] and the dihedral angles between the aromatic rings [12.3 (3)° for (I) and 28.6 (3)° for (II) (mean)]. There are two types of C—H...O hydrogen bond in the structure [C...O = 3.262 (7) Å and C—H...O = 144°; C...O = 3.447 (7) Å and C—H...O = 166°] and these link the molecules into a two‐dimensional framework. The hydrogen‐bond‐acceptor properties differ between the two isomers.     

Preparation and properties of fluorinated carboxylic acid/silica nanocomposite-encapsulated low molecular weight compounds

Perfluoro-2-methyl-3-oxahexanoic acid/silica nanocomposites [R_F-CO₂H/SiO₂] were prepared by the sol–gel reaction of tetraethoxysilane in the presence of silica nanoparticles and the corresponding fluorinated carboxylic acid under alkaline conditions. R_F-CO₂H/SiO₂ nanocomposites were found to exhibit no weight loss in proportion to the contents of fluorinated carboxylic acid in the composites even after calcination at 800 °C. The modified glass surface treated with the R_F-CO₂H/SiO₂ nanocomposites was shown to give a good oleophobicity with superhydrophilicity imparted by fluorinated carboxylic acid in the composites. R_F-CO₂H/SiO₂ nanocomposites were also applied to the encapsulation of a variety of low molecular weight aromatic and aliphatic compounds such as bisphenol AF [BPAF], bisphenol A [BPA], 4,4′-biphenol [BPOH], octafluoro-4,4′-biphenol [FBPOH], 4,4′-bis(triethoxysilyl)-1,1′-biphenyl [BTSBP], 3-(trihydroxysilyl)propane-1-sulfonic acid [THSP], α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), and γ-cyclodextrin (γ-CD). Encapsulated aromatic compounds possessing acidic hydroxyl groups such as BPAF, BPA, and FBPOH in the R_F-COOH/SiO₂ nanocomposites were found to exhibit no weight loss corresponding to the contents of aromatic compounds in the composites even after calcination at 800 °C. On the other hand, encapsulated aromatic compounds possessing no acidic hydroxyl groups such as BTSBP and aliphatic compounds (THSP, α-, β-, and γ-CD) gave a clear weight loss corresponding to the contents of these compounds in the composites after calcination. In addition, the fluorinated silica nanocomposite-encapsulated these compounds were applied to the surface modification of glass to exhibit a good oleophobicity with superhydrophilicity imparted by fluorinated carboxylic acid on the surface.

1H-NMR Studies on (6R)- and (6S)-deuterated (1–6)-linked disaccharides: assignment of the preferred rotamers about C5–C6 bond of (1–6)-disaccharides in solution

Conformational properties of the C5–C6 bond in the α- and β(1–6)-linked disaccharides in solution were clarified based on the ¹H-NMR spectra of the (6R)- and (6S)-deuterated derivatives.     

Vibrational analysis of sodium α-, β- and γ-hydroxybutyrates. Inter- and intramolecular hydrogen bonds

The infrared and Raman spectra of sodium α-, β- and γ-hydroxybutyrates and their deuterated analogues are examined in the 4000-100 cm⁻¹ range and an assignment of the fundamental vibrations is given. Based on the localization of the asymmetric stretching vibrations ν_asOH and the out-of-plane vibration γOH, inter- and/or intramolecularly hydrogen-bonded forms are proposed: the low frequencies of ν_asOH (<3200 cm⁻¹) and high frequencies of γOH (≈800 cm⁻¹) argue in favour of the existence of intramolecular hydrogen bonding. Sodium α-hydroxybutyrate exhibits as a chelate ring with an intramolecular hydrogen bond between hydroxyl and carboxyl groups, whereas sodium, β-hydroxybutyrate has the two association forms with inter- and intramolecular hydrogen bonds. Sodium γ-hydroxybutyrate exists as a hydrogen-bonded polymer, with an intermolecular hydrogen bond between the hydroxyl groups and between the hydroxyl and carbonyl groups. At a crystallization temperature above 50°C, only the α- salt showed a structural change indicating the existence of intra- and intermolecular hydrogen bonds. This result is confirmed by differential scanning analysis.     

A Facile One‐Step Synthesis of 2‐Arylindazoles via Reductive Cyclization of N‐(2‐nitroarylidene)amines

A mild and efficient synthesis of 2‐arylindazole derivatives via the reductive cyclization of nitro‐aryl substrates mediated by a low‐valent titanium reagent (TiCl₄/Sm/Et₃N) has been developed. The attractive features of the current method include an N–N bond formation and the selective reduction of the C = N bond and nitro group, both of which were easily achieved in one‐pot by controlling the pH of the reaction mixture.