Complexes of diphosphaferrocenes with Lewis acids and Cu(I), Ag(I) salts

Complexes of diphosphaferrocenes have been prepared using (a) Lewis acids BF₃·Et₂O, A1C1₃ and (b) Cu(I) and Ag(I) salts. The ³¹P spectra showed that BF₃·Et₂O complexes at phosphorus (characterised by a large downfield shift of 103 ppm) whereas all the other adducts involved binding at iron (characterised by large upfield shifts of 36–94 ppm). Deep purple adducts were isolated and characterised for the reaction of 2, 2′, 5, 5′-tetraphenyldipho sphaferrocene with silver trifluoroacetate and triflate and also with copper(I) iodide. Similar complexes were isolated from 3, 3′, 4, 4′-tetramethyldiphosphaferrocene. ⁵⁷Fe Mössbauer spectroscopy of the adducts showed that unlike the ferrocene system, quadrupole splittings generally decreased on iron complexation. The results are discussed in terms of current MO theory of ferrocene and diphosphaferrocene.     

Analysis of linkage positions in saccharomyces cerevisiaed-mannans by the reductive-cleavage method

The positions of linkage in the d-mannans derived from Saccharomyces cerevisiae X2180 and its mutants, mnn1, mnn2, and mnn4, were established by perethylation and subsequent reductive cleavage with triethylsilane in the presence of boron trifluoride etherate (BF₃ · Et₂O) or trimethylsilyl trifluoromethanesulfonate. With the latter as the catalyst, all glycosidic carbon-oxygen bonds were cleaved, to produce a mixture of ethylated 1,5-anhydro-d-mannitol derivatives. With BF₃ · Et₂O as the catalyst, 2-, 3-, and 6-linked residues were incompletely cleaved, and residues linked at both O-2 and O-6 were not cleaved at all. It was concluded that reductive cleavage is an attractive method for determination of the structure of polysaccharides.     

Biogenetic-type synthesis of hydroxylated tricyclic diterpenes in the pimarane class

The biogenetic-type synthesis of various diterpenoids (in the racemic form) possessing the pimarane backbone was achieved through nonenzymic cyclization of the oxide of methyl geranylgeranyl carbonate (46). Treatment of oxide 46 with BF₃ · Et₂O in CH₃NO₂ effected formation of pimaradienol 36, isopimaradienol 39, and the naturally occurring tricycle 40. Acid treatment of either 39 or 40 led to an equilibrium mixture which includes isomer 41, also of natural origin. Side-chain oxidation of dienol 40 afforded araucarol (16), a third plant product. Other substances formed in the cyclization of oxide 46 are described, and a mechanistic interpretation of the overall reaction course is presented.     

A high yield procedure for the preparation of arsonolipids (2,3-diacyloxypropylarsonic acids)

The crucial step in the preparation of the title arsonolipids starting from the dichloromethane-soluble dithioarsonite CH₂(OH)CH(OH)CH₂–As(SPh)₂ is to avoid an internal cyclization during the acylation which protects the primary –OH group from being acylated. This was to a large extent accomplished by using fatty acyl chloride in the presence of the weak base pyridine and controlling the temperature and rate of the acyl chloride addition, giving ～70% yields of arsonolipids. The presence of catalytic amounts of 4-dimethylaminopyridine boosted the yields to 82–85%. This yield is a great improvement over the yields (20–55%) previously achieved. The acylating systems (RCO)₂O or RCOCl and BF₃·Et₂O gave only moderate yields (25–60%) of arsonolipids.     

Synthesis and properties of some O-[2,2-bis(alkylthio)ethyl]-glycolaldehydes

O-[2,2-Bis(alkylthio)ethyl]glycoaldehydes (1a–e; alkyl = Et, Pr, Prⁱ, Bu^t, and -CH₂-, respectively) have been prepared from the corresponding O-[2,2-bis(alkylthio)ethyl]glycolaldehyde dimethyl acetals (2a–e) by acid hydrolysis. In anhydrous 1,4-dioxane in the presence of BF₃ · (Et₂O)₂,1a–c were partially transformed into glycolaldehyde bis(dialkyl dithioacetals),1d afforded trans-2,6-bis(tert-butylthio)-1,4-dioxane and 3,5-bis(tert-butylthio)-1,4-oxathiane, and1e did not react. The acetals2a–e) were prepared from the appropriate glycolaldehyde dialkyl dithioacetal by O-alkylation with bromoacetaldehyde dimethyl acetal.     

Synthesis of a ‘direct-linked’ C-disaccharide from a pyranulose glycoside

Syntheses of five ‘direct linked’ C-disaccharides 8a–e were reported. The (Et₃SiH/BF₃·Et₂O) reduction of pyranulose glycoside 1 yielded (6S)- and (6R)-6-(2,3,5-tri-O-benzoyl-β-d-ribofuranosyl)pyran-3(2H,6H)-one (2a and 2b) in a ratio of ca. 2:1 and in 88% combined yield. The absolute stereochemistry of each was determined from its CD spectrum. The reduction of 2a with NaBH₄ in methanol afforded two allylic alcohols 6a and 6b in 14 and 73% yield, respectively. The reduction of 2b with NaBH₄ afforded 6c and 6d in 30 and 56% yield, respectively. Cis hydroxylation of the double bond in compounds 6a–d with osmium tetroxide gave 7a–e. The stereoisomers 7a–e were separated and their configuration was established by ¹H NMR spectroscopy. Debenzoylation of compounds 7a–e with aqueous sodium carbonate produced deprotected C-disaccharides 8a–e.     

Fragmentation of realgar,As4S4, controlled by transition metalligand systems

The chemical behavior of the realgar molecule, As₄S₄, toward various (triphos)M moieties has been investigated. The reaction of As₄S₄ with [{MCl(cod)}₂] (M=Rh or Ir; cod=1,5-cyclooctadiene) in the presence of the ligand triphos [triphos=1,1,1- tris(diphenylphosphinomethyl)ethane] yields compounds of formula [(triphos)M(η³-As₃S₃)]·C₆H₆ containing the new As₃S₃ unit, which is trihapto bonded to the metal atom through one sulfur and two arsenic atoms. Such a As₃S₃ fragment is the largest one so far extruded from the realgar molecule. The As₄S₄ molecule undergoes more drastic disruptions in the reactions with Co(BF₄)₂·6H₂O and Ni(BF₄)₂·6H₂O in the presence of triphos. These results suggest that the fragmentation of the As₄S₄ molecule is controlled by the nature of the metal atom involved in the reaction.     

Insight into dihalogenation of E-ring of podophyllotoxins,and their acyloxyation derivatives at the C4 position as insecticidal agents

Unexpected sequential E-ring dihalogenation of podophyllotoxin analogues is reported. It demonstrated that a chlorine/bromine atom was prior introduced at the C2′ position of podophyllotoxin, and the corresponding free rotation of E-ring around the C1–C1′ bond of 2′-chloro or 2′-bromopodophyllotoxin was restricted. When 2′-chloro or 2′-bromopodophyllotoxin reacted with N-chlorosuccinimide (NCS), the chlorine atom was regioselectively introduced at their C6′ position on the E-ring. Whereas 2′-chloro or 2′-bromopodophyllotoxin reacted with NBS, the bromine atom was regioselectively introduced at their C5 position on the B-ring. When 2′-chloropodophyllotoxin reacted with different carboxylic acids in the presence of BF₃·Et₂O, the steric effect of its E-ring for stereoselective synthesis of 4β-acyloxy-2′-chloropodophyllotoxin derivatives was observed. The insecticidal activity of 2′(2′,6′)-(di)halogen-substituted podophyllotoxin derivatives were evaluated with Mythimna separata Walker.     

The influences of water solvent on the structures and stabilities of Na+–AD conformers

The influences of water solvent on the structures and stabilities of the complex ion conformers formed by the coordination of alanine dipeptide (AD) and Na⁺ have been investigated using supramolecular and polarizable continuum solvation models at the level of B3LYP/6-311++G**, respectively; 12 monohydrated and 12 dihydrated structures of Na⁺–AD complex ion were obtained after full geometrical optimization. The results showed that H₂O molecules easily bind with Na⁺ of Na⁺–AD complex ion, forming an ion-lone pair interaction with the Na–O bond length of 2.1–2.3 Å. Besides, H₂O molecules also can form hydrogen bonds O_W–H_W···O(1), O_W–H_W···O(2), N(1)–H(1)···O_W or N(2)–H(2)···O_W with O or N groups of the Na⁺–AD backbone. The most stable gaseous bidentate conformer C7AB of Na⁺–AD is still the most stable one in the solvent of water. However, the structure of the most unstable gaseous conformer α′B of Na⁺–AD collapses under the attack of H₂O molecules and changes into C7AB conformation. Computations with IEFPCM solvation model of self-consistent reaction field theory give that aqueous C5A is more stable than C7_eqB and that the stabilization energies of water solvent on monodentate conformers of Na⁺–AD complex ion (about 272–294 kJ/mol) are more than those on bidentate ones (about 243 kJ/mol).     

Reduction of Escherichia coli O157:H7 on radish seeds by sequential application of aqueous chlorine dioxide and dry‐heat treatment

Aims: To assess the effectiveness of sequential treatments of radish seeds with aqueous chlorine dioxide (ClO₂) and dry heat in reducing the number of Escherichia coli O157:H7. Methods and Results: Radish seeds containing E. coli O157:H7 at 5·5 log CFU g^?1 were treated with 500 μg ml^?1 ClO₂ for 5 min and subsequently heated at 60°C and 23% relative humidity for up to 48 h. Escherichia coli O157:H7 decreased by more than 4·8 log CFU g^?1 after 12 h dry‐heat treatment. The pathogen was inactivated after 48 h dry‐heat treatment, but the germination rate of treated seeds was substantially reduced from 91·2 ± 5·0% to 68·7 ± 12·3%. Conclusions: Escherichia coli O157:H7 on radish seeds can be effectively reduced by sequential treatments with ClO₂ and dry heat. To eliminate E. coli O157:H7 on radish seeds without decreasing the germination rate, partial drying of seeds at ambient temperature before dry‐heat treatment should be investigated, and conditions for drying and dry‐heat treatment should be optimized. Significance and Impact of the study: This study showed that sequential treatment with ClO₂ and dry‐heat was effective in inactivating large numbers of E. coli O157:H7 on radish seeds. These findings will be useful when developing sanitizing strategies for seeds without compromising germination rates.     

The chemical reactivity of tetraisobutyldialuminum towards ethers,nitrogen lewis bases,aluminumtrimethyl, borontrichloride and ethylene

Treatment of Al₂ⁱBu₄ with THF and Et₂O results in partial decomposition to afford Al metal while reaction with γ-picoline products a bis-adduct of limited stability. Reactions with AlMe₃ and BCl₃, separately, involves ligand exchange with accompanying disproportionation to yield Al metal. Dimethylamine induces disproportionation to afford AlⁱBu₃·HNMe₂ and an intemediate trialuminum species. The latter undergoes AlAl bond cleavage with formation of H₂, _iBu₂AlNMe₂, and [Me₂N- (ⁱBu)AlAlⁱBu₂]₂· Al₂iBu₄ eliminates Me₂CCH₂ in solution at 80 °C, and the catenated AlH intermediate reacts with ethylene to afford AlEt and AlCH₂CH₂CH₂CH₃ moieties.     

Facile synthesis of glycosylated Fmoc amino acid building blocks assisted by microwave irradiation

The synthesis of glycosylated Fmoc amino acids by reaction of mono- and disaccharide peracetates with Fmoc amino acids having free carboxyl groups was rapidly promoted by Lewis acids (SnCl₄, BF₃·Et₂O) under microwave irradiation. The products are useful building blocks for the synthesis of glycopeptides.     

A facile preparation of trehalose analogues: 1,1-thiodisaccharides

The synthesis of 1,1-thiodisaccharide trehalose analogues in good to excellent yields by a Lewis acid (BF₃·Et₂O)-catalysed coupling of sugar per-O-acetate with thiosugar is described. The reactivity of different sugar per-O-acetates and thiosugars is explored.     

Synthesis,structure, and thermal behavior of discrete Co(II), Ag(I), and Pd(II) complexes with 2,3-bis(2-pyridyl)quinoxaline. Insight into coordination modes

The reactions of 2,3-bis(2-pyridyl)quinoxaline (bpq) with CoCl₂·6H₂O, Ag(CH₃CN)₄BF₄, and PdCl₂(C₆H₅CN)₂ produce [CoCl₂(bpq)]₂·2CHCl₃, [Ag(bpq)CH₃CN]₂(BF₄)₂·2CH₃CN, and [PdCl₂(bpq)], respectively. All the products are discrete 1:1 (metal:bpq) adducts, where the chelation mode of the bpq is dependent upon the metal atoms. The structure of [CoCl₂(bpq)]₂·2CHCl₃ is a centrosymmetric Cl-bridged four-membered dimer, [Co₂Cl₂], in which the bpq is bonded to the cobalt(II) atom in an anisobidentate mode with the nitrogen donors of pyridine and pyrazine rings. For [Ag(bpq)CH₃CN]₂(BF₄)₂·2CH₃CN, each bpq ligand connects two tetrahedral silver(I) ions in a tridentate mode, resulting in a cationic cyclic dimer. The structure of [PdCl₂(bpq)] approximates to a molecular rocking chair with an isobidentate bpq through the nitrogen donors of 2-pyridyl rings. The compounds exhibit significant and characteristic relationships between the structures and their thermal properties. For [CoCl₂(bpq)]₂·2CHCl₃, the solvate chloroform molecules are safely contained up to 144°C, but drastically evaporate above this temperature. The striking feature of [Ag(bpq)CH₃CN]₂(BF₄)₂·2CH₃CN is that the skeletal cyclic dimer is basically retained after dissociation of the coordinated acetonitriles in the solid state.     

Studies on the synthesis and the antimicrobial and antioxidant activities of a novel class of fluorescein-based glycosides

Facile glycosylation of a fluorescein diol derivative with per-O-acetyl/benzoyl sugar derivatives using BF₃·Et₂O catalyst resulted in the formation of the expected glycosides in 54–66% yield. The biological screening of the glycosides against different microbes shows good inhibitory activity. The antioxidant activity of the fluorescein-based glycosides shows remarkable inhibition (IC₅₀ ∼80%).     

13C‐NMR chemical shift tensor and hydrogen‐bonded structure of glycine‐containing peptides in a single crystal

¹³C‐nmr chemical shift tensor components are reported for a ¹³C‐labeled Gly¹ amide carbonyl carbon of a glycylglycine (Gly¹Gly²) single crystal, a GlyGly · HNO₃ single crystal and a GlyGly · HCl · H₂O single crystal, for which the three‐dimensional crystal structures have already been determined by x‐ray diffraction. The tensor components were measured by changing the angle between the crystal plane and the applied magnetic field by using a goniometer designed in this work for use in conventional ¹³C cross‐polarization/magic angle spinning nmr probe. From these experimental data, the principal values of the ¹³C chemical shift tensor and its directions for the Gly¹ amide carbonyl carbon were determined. It was found that the ¹³C chemical shift tensor components (δ₁₁, δ₂₂, and δ₃₃) for the Gly¹ amide carbonyl carbon in GlyGly and GlyGly · HNO₃ with a >NH · · · OC< type of hydrogen bond depend on the hydrogen‐bond length and the directions of the δ₂₂ components of these peptides are along the hydrogen‐bonded >CO bond axis. In addition, the magnitude of the deviation from the bond axis depends on the hydrogen‐bond angle. Further, the experimental result for GlyGly · HCl · H₂O with a  O H · · ·OC< type of hydrogen bond was discussed. © 1999 John Wiley & Sons, Inc. Biopoly 50: 61–69, 1999     

The Hydronium Tetrafluoroborate Dimer in Nonpolar Media and its Proton NMR Spectrum

Hydronium tetrafluoroborate ion pairs, H₃O⁺·BF₄ ^- have been shown computationally to be unstable toward decomposition, in the absence of solvation or electrostatic interactions existing in crystals. As the proton NMR spectrum of a hydronium salt with the octanesulfonate-antimony pentachloride complex anion was reported in freon solution, we investigated the hypothesis that larger ionic clusters were present in the nonpolar solvent. It was found that the dimer (H₃O⁺·BF₄ ^-)₂ was stable at the MP2/6-31G* level. GIAO-B3LYP chemical shift calculations with the same basis set and also with the 6-31G**, 6-31++G**, 6-311++G**, dzvp, tzp, tz2p, and qz2p basis sets conducted on the hydronium fluoroborate dimer reproduce the main features of the experimental spectrum: the existence of two signals with a two-to-one intensity ratio and the more intense resonance at higher frequency (more deshielded). The alternative structures, of hydronium tetrafluoroborate ion pairs with one and with two hydrogen bonds between anion and cation, give calculated chemical shifts which are farther from the experimental values.     

Synthesis and luminescence properties of Ca3(BO3)2:Eu3+ via two‐step method

Novel Ca₂B₂O₅·H₂O:Eu³⁺ nanotubes, constructed with nanobelts, were prepared using a hydrothermal method. The Ca₃(BO₃)₂:Eu³⁺ nanobelts with a thickness of about 100 nm were made for the first time using a two‐step hydrothermal process with Ca₂B₂O₅·H₂O:Eu³⁺ as the precursor. The samples were characterized by energy dispersive X‐ray spectroscopy, X‐ray diffraction, Fourier transform infra‐red spectroscopy, thermogravimetry differential thermal analysis and scanning electron microscopy. The relationship between Ca₃(BO₃)₂:Eu³⁺ and Ca₂B₂O₅·H₂O:Eu³⁺ was also studied. Possible reaction and growth mechanisms for Ca₂B₂O₅·H₂O:Eu³⁺ and Ca₃(BO₃)₂:Eu³⁺ were proposed. Ca₃(BO₃)₂:Eu³⁺ preserved the basic microstructure unit of Ca₂B₂O₅·H₂O:Eu³⁺. Both Ca₂B₂O₅·H₂O:Eu³⁺ and Ca₃(BO₃)₂:Eu³⁺ exhibited red emissions centred at 614 nm, but the maximum excitation peaks for Ca₂B₂O₅·H₂O:Eu³⁺ and Ca₃(BO₃)₂:Eu³⁺ differed. Ca₃(BO₃)₂:Eu³⁺ exhibited higher photoluminescence intensity but a lower R/O value than Ca₂B₂O₅·H₂O:Eu³⁺.     

The electronic structure of the C2H4O···2HF tri-molecular heterocyclic hydrogen-bonded complex: a theoretical study

The geometries of three isomers of the C₂H₄O···2HF tri-molecular heterocyclic hydrogen-bonded complex were examined through B3LYP/aug-cc-pVDZ calculations. Analysis of structural parameters, determination of CHELPG (charge electrostatic potential grid) intermolecular charge transfer, interpretation of infrared stretching modes, and Bader’s atoms in molecules (AIM) theory calculations was carried out in order to characterize the hydrogen bonds in each isomer of the C₂H₄O···2HF complex. The most stable structure was determined through the identification of hydrogen bonds between C₂H₄O and HF, (O···H), as well as in the hydrofluoric acid dimer, (HF^D–R···HF^D). However, the existence of a tertiary interaction (F^λ···H^α) between the fluoride of the second hydrofluoric acid and the axial hydrogen atoms of C₂H₄O was decisive in the identification of the preferred configuration of the C₂H₄O···2HF system. Figure Geometries of three isomers of the C₂H₄O···2HF tri-molecular heterocyclic hydrogen-bonded complex     

Studies on the BF3·Et2O catalyzed Baeyer-Villiger reaction of spiroketalic steroidal ketones

During reactions of 23-oxosapogenins and the corresponding isomeric 22-oxo-23-spiroketals with MCPBA in the presence of BF₃·Et₂O, equilibration occurs between the ketones. The Baeyer-Villiger type oxidation is followed by fragmentation to the dinorcholanic lactones and 3-methylbutyrolactone. The mechanistic aspects of these reactions in the 25R and 25S series are discussed.