Kanamycin A: imine formation in aqueous solution

Imine formation in aqueous solution of kanamycin A with pyridoxal 5'‐phosphate and other aldehydes was studied by potentiometry, NMR spectroscopy and computational chemistry. It was found that imines are formed with yields near 100 % at pH 7 in equimolar reactant ratio. In order to identify the kanamycin amino groups involved in the reaction, a NMR spectroscopic study was conducted. The structures of possible imines formed between kanamycin and FURAN or PLP were optimized by molecular mechanics with the OPLS‐2005 force field. The ¹H NMR spectra were calculated at the DFT‐GIAO B3LYP/6‐31G(d) level of theory for all structures and compared with the experimentally observed spectra. From these results a probable structure of the imines was proposed. The results obtained in this work show that kanamycin has the ability to form imine derivatives in high yields due to its anion recognition properties. Copyright © 2012 John Wiley & Sons, Ltd.     

Ultrasound-assisted synthesis of unsymmetrical biaryls by Stille cross-coupling reactions

We describe herein an efficient method for the synthesis of unsymmetrically-substituted biphenyls using a sonochemical variation of the Stille coupling, whose results have also been compared with the conventional silent reaction. Ultrasound significantly enhances this useful organometallic transformation affording products in higher yields and in shorter reaction times than non-irradiated reactions. The scope has been explored with a selection of arylstannanes as precursors and, remarkably, no by-products resulting from homo-coupling could be detected.     

Efficient asymmetric hydrogenation of the C–C double bond of 2-methyl- and 2,3-dimethyl-N-phenylalkylmaleimides by Aspergillus fumigatus

Eight N-phenylalkylmaleimides (four 2-methyl-N-phenylalkylmaleimides and four 2,3-dimethyl-N-phenylalkylmaleimides with an alkyl chain (CH₂)_n (n = 1–4) between the imide N and the benzene ring) were subjected to biotransformation using the fungal strain Aspergillus fumigatus ATCC 26934. All compounds were reduced enantioselectively to their respective succinimides: (R)-2-methyl-N-phenylalkylsuccinimides and (2R,3R)-2,3-dimethyl-N-phenylalkylsuccinimides, with satisfactory conversion rates and high stereoisomeric excesses. NMR analysis using the chiral shift reagent Eu(hfc)₃ showed that enantiomeric excesses were >99%.     

Comparative effect of cyclodextrin nanocavities versus organic solvents on the fluorescence of carbamate and indole compounds

The effect of the addition of different amounts of organic solvents (S) on the fluorescence of aromatic compounds (C) and their inclusion complexes with β-cyclodextrin (βCD) and hydroxypropyl-β-cyclodextrin (HPCD) has been examined using steady-state measurements. Carbamate pesticides with different aromatic moiety, such as carbofuran (CF), promecarb (PC), carbaryl (CY) and bendiocarb (BC) were used, as well as indole derivatives with different polarity in their lateral chains, such as melatonin (M, neutral), 5-methoxytryptamine (MT, cation) and auxin (IA, anion). Their complexes in water show a fluorescence signal higher than that obtained for the free substrates in solvent:water mixtures (30%, v/v n-propanol or acetonitrile, and 50%, v/v methanol). The isofluorescent point (IF), the %IF and the F_85% are defined in order to evaluate the use of CD nanocavities as a non-polluting alternative for the analysis of the compounds analyzed.Apparent formation constants (K_AP, M⁻¹) for the complexes of C:HPCD at different solvent percentages were determined for CF and PC with methanol (MeOH), n-propanol (ProOH) and acetonitrile (ACN), and for indole compounds with ACN. A decrease in the K_AP values for the CF:HPCD (120–30) and PC:HPCD (2000–400) complexes occurs in accordance with the solvent affinities for CDs (MeOH < ACN < ProOH). Nevertheless, in the indolic series, the polar characteristics of MT, IA and M determine their behaviour in the presence of ACN. For the neutral substrate M, K_AP decreases with the increasing percentage of ACN (100–10). In contrast, for IA and MT (ionic substrates) K_AP increases (10–100).These results may be accounted for by two different mechanisms: the competition between C and S for the cavity of the receptor or the formation of ternary complexes C:S:CD with additional stabilization.     

How trifluoroacetone interacts with water

The rotational spectra of five isotopologues of the molecular adduct 1,1,1-trifluoroacetone-water have been assigned using pulsed-jet Fourier-transform microwave spectroscopy. All rotational transitions appear as doublets, due to the internal rotation of the methyl group. Analysis of the tunneling splittings allows one to determine accurately the height of the 3-fold barrier to internal rotation of the methyl group and its orientation, leading to V(3) = 3.29 kJ·mol(-1) and ∠(a,i) = 67.5°, respectively. The water molecule is linked to the keton molecule on the side of the methyl group through a O-H···O hydrogen bond and a C-H···O intermolecular contact, lying in the effective plane of symmetry of the complex.     

Interaction of surfactants with thickeners used in waterborne paints: a rheological study

Studies of the phase diagram and linear viscoelasticity of aqueous solutions of hydrophobically modified hydroxyethyl cellulose (HMHEC), a thickener used in water-based paints, and SDS reveal that SDS-HMHEC mixed micelles are formed that increase the number of hydrophobic junctions and enhance interpolymer association up to an [SDS]/[HMHEC] ratio. This fact produces a strong increase of viscoelasticity or a phase separation, depending on the [HMHEC]. At higher ratios the excess of micelles with predominant SDS isolates hydrophobes and disrupts the micellar network. Then, viscoelastic functions decrease and HMHEC behaves as a nonassociative polymer. TTAB and Brij30 also interact with HMHEC, but in a different way. No phase separation is observed with these surfactants. TTAB forms mixed micelles and new junction points in the same way as SDS. However, this surfactant does not stabilize the micelles as SDS does, presumably due to the different interaction between the OH from the cellulose and the charged groups. Results seem to indicate that Brij30 enters into the hydrophobic aggregates of HMHEC and stabilizes them, increasing relaxation time, but it does not form new junction points, since it forms quite big micelles.     

Kinetic Treatments for Catalyst Activation and Deactivation Processes based on Variable Time Normalization Analysis

Progress reaction profiles are affected by both catalyst activation and deactivation processes occurring alongside the main reaction. These processes complicate the kinetic analysis of reactions, often directing researchers toward incorrect conclusions. We report the application of two kinetic treatments, based on variable time normalization analysis, to reactions involving catalyst activation and deactivation processes. The first kinetic treatment allows the removal of induction periods or the effect of rate perturbations associated with catalyst deactivation from kinetic profiles when the quantity of active catalyst can be measured. The second treatment allows the estimation of the activation or deactivation profile of the catalyst when the order of the reactants for the main reaction is known. Both treatments facilitate kinetic analysis of reactions suffering catalyst activation or deactivation processes.     

Reactions of nido-1,2-(Cp*RuH)2B3H7 with RCCR′ (R, R′=H, Ph; Me, Me) to yield novel metallacarboranes

Addition of the internal alkyne, 2-butyne, to nido-1,2-(Cp*RuH)₂B₃H₇ (1) at ambient temperature produces nido-1,2-(Cp*Ru)₂(μ-H)(μ-BH₂)-4,5-Me₂-4,5-C₂B₂H₄ (2), nido-1,2-(Cp*RuH)₂-4,5-Me₂-4,5-C₂B₂H₄ (3), and nido-1,2-(Cp*RuH)₂-4-Et-4,5-C₂B₂H₅ (4), in parallel paths. On heating, 2, which contains a novel exo-polyhedral borane ligand, is converted into closo-1,2-(Cp*RuH)₂-4,5-Me₂-4,5-C₂B₃H₃ (5) and nido-1,6-(Cp*Ru)₂-4,5-Me₂-4,5-C₂B₂H₆ (6) the latter being a framework isomer of 3. Heating 2 with 2-butyne generates nido-1,2-(Cp*RuH)₂-3-{CMeCMeB(CMeCHMe)₂}-4,5-Me₂-4,5-C₂B₂H₃ (7) in which the exo-polyhedral borane is triply hydroborated to generate a boron bound ---CMeCMeB(CMeCHMe)₂ cluster substituent. Along with 3, 4, 5, 6, and 7, the reaction of 1 with 2-butyne at 85 °C gives closo-1,7-(Cp*Ru)₂-2,3,4,5-Me₄-6-(CHMeCH₂Me)-2,3,4,5-C₄B (8). Reaction of 1 with the terminal alkyne, phenylacetylene, at ambient temperature permits the isolation of nido-1,2-(Cp*Ru)₂(μ-H)(μ-CHCH₂Ph)B₃H₆ (9) and nido-1,2-(Cp*Ru)₂(μ-H)(μ-BH₂)-3-(CH₂)₂Ph-4-Ph-4,5-C₂B₂H₄ (11). The former contains a Ru---B edge-bridging alkylidene fragment generated by hydrometallation on the cluster framework whereas the latter contains an exo-polyhedral borane like that of 2. Thermolysis of 11 results in loss of hydrogen and the formation of closo-1,2-(Cp*RuH)₂-3-(CH₂)₂Ph-4-Ph-4,5-C₂B₃H₃ (12).