Quassinoids from Picrasma crenata

From woods of Picrasma crenata, a new stereoisomer dihydronorneoquassin was obtained together with others well knowns dihydronorneoquassin, parain, alpha-neoquassin, beta-neoquassin and quassin. The structures were determined by spectroscopic data and chemical evidence.     

Acid/extraction treatment of bivalves for organotin speciation

A method is described for leaching of nanogram amounts of monobutyltin (MBT), dibutyltin (DBT), tributyltin (TBT), monophenyltin (MPT), diphenyltin (DPT), triphenyltin (TPT), and monomethyltin (MMT) from bivalves. The procedure is based on soaking the samples in a water-hydrogen bromide mixture (1:1) with magnetic stirring for 30 min followed by extraction with a 0.04% (w/v) tropolone solution in dichloromethane for 2 h. Organotins are determined by GC-FPD after clean-up through a Florisil column and fat hydrolysis by 1mol/L NaOH, followed by derivatization through Grignard pentylation. The method has been applied for an investigation of the occurrence of organotins in bivalves in south-west Spain. Received: 3 September 1994 / Revised: 2 October 1996 / Accepted: 7 November 1996     

A Protein‐Resistant Matrix for Electrochemical Based Recognition Assays

In this work, two well known polymers, carboxymethyl dextran and poly(ethyleneglycol), are used for easily producing a platform for electrochemical affinity assays, avoiding nonspecific adsorption of proteins. In this way, modified electrode surfaces able to bind a recognition agent are constructed through simple and reliable reactions.     

How bridging ligands and neighbouring groups tune the gold-gold bond strength

Gold-gold interactions in small polynuclear complexes are analysed using extended Hückel calculations. They are influenced by the nature of the ligand donor atoms, by the bridging ligands, but most by the formal oxidation state of the metal. Au---Au bonds are much stronger in complexes of Au(II) and Au(III), but a weak interaction between two d¹⁰ centres exists for Au(I) complexes, owing to mixing of the s and p orbitals with the d orbitals. Phosphines induce stronger metal-metal bonds when coordinated trans to the Au---Au bond in [Au(II)[(CH₂)₂PPh₂]L]₂ (Ph = phenyl), but have the opposite effect when bonded orthogonally to the metal-metal axis in Au(I) binuclear species. When two gold atoms are bridged by a single carbon atom, belonging either to mesityl (Mes = 2,4,6-Me₃C₆H₂) or CR₂, the former produces stronger Au(I)---Au(I) interactions, reflected in shorter distances. Formal oxidation states are proposed for the gold atoms in two mixed-valence clusters, [Au4(C₆F₅)₂((PPh₃)_{2CH})2(PPh3)2](ClO4)2 and [{(2,4,6-C6F3H2)Au(CH2PPh2CH2)2Au{in2}-Au(CH₂PPh₂CH₂)₂Au](ClO₄)₂. The results suggest a higher oxidation state for the outer gold atoms, in both the T-shaped tetranuclear cluster and the Au₆ linear chain.     

Convenient synthesis of nucleoside and isonucleoside analogues

[reaction: see text]. A very simple methodology to stereoselectively achieve tricyclic isonucleosides (nucleobase = thymine, uracil, and 5-fluoruracil) and 3'-C-branched nucleosides (nucleobase = theophylline) was performed by means of a DBU-mediated addition process using a readily available 2-bromo sugar. The mechanism for these transformations implies the loss of both substituents at C-2 and C-3 on the sugar moiety, and although it seems that DBU is probably involved, its involvement has not yet been ascertained. Cytosine did not react under these conditions.     

Synthesis and Characterization of Heterodinuclear IrCo, RuCo, IrNi, and RuNi Complexes Containing Pyrazolate and Pyrazolylborate Ligands

Treatment of the metallo ligands [ML(pz)(2)(Hpz)] (pz = pyrazolate; L = C(5)Me(5), M = Ir (1); L = mesitylene, M = Ru (3)) with [M'Cl{HB(3-i-Pr-4-Br-pz)(3)}] (M' = Co (4), Ni (5)) yields heterodinuclear complexes of formula [LM(&mgr;-pz)(2)(&mgr;-Cl)M'{HB(3-i-Pr-4-Br-pz)(3)}] (L = C(5)Me(5); M = Ir; M' = Co (6), Ni (7). L = mesitylene; M = Ru; M' = Co (8)). The related complex [Ru(eta(6)-p-cymene)(pz)(2)(Hpz)] (2) reacts with equimolar amounts of 4 or 5 to give mixtures of the corresponding bis(&mgr;-pyrazolato) &mgr;-chloro complexes [(eta(6)-p-cymene)Ru(&mgr;-pz)(2)(&mgr;-Cl)M'{HB(3-i-Pr-4-Br-pz)(3)}] (M' = Co (9), Ni (10)) and the triply pyrazolato-bridged complexes [(eta(6)-p-cymene)Ru(&mgr;-pz)(3)M'{HB(3-i-Pr-4-Br-pz)(3)}] (M' = Co (11), Ni (12)). Complex 1 reacts with 5 in the presence of KOH to give the IrNi complex [(eta(5)-C(5)Me(5))Ir(&mgr;-pz)(3)Ni{HB(3-i-Pr-4-Br-pz)(3)}] (13) whereas its reaction with 4 and KOH rendered the bis(&mgr;-pyrazolato) &mgr;-hydroxo complex [(eta(5)-C(5)Me(5))Ir(&mgr;-pz)(2)(&mgr;-OH)Co{HB(3-i-Pr-4-Br-pz)(3)}] (14). The molecular structure of the heterobridged IrCo complex (6) and those of the homobridged RuNi (12) and IrNi (13) complexes have been determined by X-ray analyses. Compound 6 crystallizes in the monoclinic space group P2(1)/n, with a = 10.146(5) ?, b = 18.435(4) ?, c = 22.187(13) ?, beta = 97.28(4) degrees, and Z = 4. Complex 12 is monoclinic, space group P2(1), with a = 10.1169(7) ?, b = 21.692(2) ?, c = 11.419(1) ?, beta = 112.179(7) degrees, and Z = 2. Compound 13 crystallizes in the monoclinic space group Cc, with a = 13.695(2) ?, b = 27.929(6) ?, c = 13.329(2) ?, beta = 94.11(4) degrees, and Z = 4. All the neutral complexes 6, 12, and 13 consist of linear M.M'.B backbones with two (6) or three (12, 13) pyrazolate ligands bridging the dimetallic M.M' units and three substituted 3-i-Pr-4-Br-pz groups joining M' to the boron atoms. The presence in the proximity of the first-row metal M' of the three space-demanding isopropyl substituents of the pyrazolate groups induces a significant trigonal distortion of the octahedral symmetry, yielding clearly different M'-N bond distances on both sides of the ideal octahedral coordination sphere of these metals.     

Quantitative characterization of the global electrophilicity pattern of some reagents involved in 1,3-dipolar cycloaddition reactions

The global electrophilicity power, ω, of a series of dipoles and dipolarophiles commonly used in 1,3-dipolar cycloadditions may be conveniently classified within a unique relative scale. The effects of chemical substitution on the electrophilicity of molecules have been evaluated using a representative set of electron-withdrawing and electron-releasing groups for a series of dipoles including nitrone, nitrile oxide and azide derivatives. The absolute scale of electrophilicity is used to rationalize the chemical reactivity of these species as compared to the static reactivity pattern of the reagents involved in the Diels-Alder reactions.     

A pyrimidine thiolate Rh(I) complex: structure, bonding and one-dimensional interactions in solid and in solution

The reaction of [Rh(micro-Cl)(COD)]2 with 4,6-dimethyl-pyrimidinethiolate (Me2-pymt) and subsequent substitution of COD by CO yields [Rh(Me2-pymt)(CO)2]. The stacking pattern found in this compound is in contradiction with previously studied comparable square-planar complexes of type d8-[M(chelate)(monodentate)2] in which each ligand has different pi-acidic character. A theoretical study of the intermolecular interactions and conformation of the title compound has been carried out, combining semi-empirical band calculations on the real chains and ab initio(MP2 level) calculations on a model dimer. The combination of electronic and steric effects determines the rotation of the successive monomers and the deviation from linearity of the one-dimensional stacks. Its behaviour in solution is also special, developing a blue colour and forming micelles, when adding water to acetone solutions.     

Use of modified rice husks as a natural solid adsorbent of trace metals: characterisation and development of an on-line preconcentration system for cadmium and lead determination by FAAS

The use of rice husks as an alternative adsorbent in an on-line preconcentration system for Cd (II) and Pb (II) determination by flame atomic absorption spectrometry (FAAS) is described. The potential of rice husks as a natural adsorbent was evaluated as a material modified with 0.75 mol l⁻¹ NaOH solution and in the unmodified form. For this task, several techniques such as spectroscopy and thermogravimetry were used for elucidation of possible functional groups responsible for the uptake of Cd (II) and Pb (II). Furthermore, based on adsorption studies and adsorption isotherms applied to the Langmüir model, it was possible to verify that modified rice husks present a higher adsorption capacity for both metals. After establishing this material as a promising natural adsorbent, it was used for on-line preconcentration of Cd (II) and Pb (II) metals. The multivariate optimisation of chemical and flow variables was performed by using a full factorial design (2⁴) including the following factors: preconcentration time, preconcentration flow rate, concentration and volume of eluent. The optimum pH values used for on-line preconcentration were taken from prior univariate experiments. Under optimised conditions for Cd (II) determination (4 min of preconcentration at a 6 ml min⁻¹ preconcentration flow rate, in which comprises 24 ml of preconcentration volume, 200 μl elution volume and 1.0 mol l⁻¹ HNO₃ solution as eluent), the system achieved a detection limit of 1.14 μg l⁻¹ and an enrichment factor of 72.4. Similar conditions were used for Pb (II) determination (4 min of preconcentration, 6 ml min⁻¹ preconcentration flow rate, 300 μl elution volume and 1.0 mol l⁻¹ HNO₃ solution as eluent) from which a detection limit of 14.1 μg l⁻¹ and enrichment factor of 46.0 were achieved. Also, rice husks have been shown to be a homogeneous and stable adsorbent in which more than 100 preconcentration/elution cycles provide a relative standard deviation (RSD) of less than 6.0% on the analytical signal. The satisfactory accuracy of the method developed was obtained by using spiked water samples (mineral water and lake water) and spiked red wine samples. These values were confirmed by electrothermal atomic absorption spectrometry (ETAAS). The certified reference material [pig kidney (CRM 186)] and the reference material [beech leaves (CRM 100)] were also used.     

Solid-phase synthesis of the cyclic lipononadepsipeptide [N-Mst(Ser1), d-Ser4, L-Thr6, L-Asp8, L-Thr9]syringotoxin

An optimized solid-phase strategy for the preparation of the cyclic lipononadepsipeptide [N-Mst(L-Ser1), D-Ser4, L-Thr6, L-Asp8, L-Thr9]syringotoxin is reported. The strategy is based on the use of a mild orthogonal protection scheme and the incorporation of the nonproteinogenic amino acid (Z)-Dhb into the peptide chain as the dipeptide Fmoc-Thr(tBu)-(Z)-Dhb-OH. The didehydrodipeptide was synthesized by a water-soluble carbodiimide-induced beta-elimination of a protected dipeptide containing a residue of Thr with its free hydroxy side chain unprotected.