Probing the mechanism of Prins cyclisations and application to the synthesis of 4-hydroxytetrahydropyrans

Trapping intermediates on the Prins cyclisation pathway with carbon-based nucleophiles has given further insight into factors affecting the acid-mediated reactions of homoallylic alcohols with aldehydes, enabling the design of efficient syntheses of 4-hydroxy-2,6-disubstituted tetrahydropyrans.     

Different transition-state structures for the reactions of beta-lactams and analogous beta-sultams with serine beta-lactamases

Beta-sultams are the sulfonyl analogues of beta-lactams, and N-acyl beta-sultams are novel inactivators of the class C beta-lactamase of Enterobacter cloacae P99. They sulfonylate the active site serine residue to form a sulfonate ester which subsequently undergoes C-O bond fission and formation of a dehydroalanine residue by elimination of the sulfonate anion as shown by electrospray ionization mass spectroscopy. The analogous N-acyl beta-lactams are substrates for beta-lactamase and undergo enzyme-catalyzed hydrolysis presumably by the normal acylation-deacylation process. The rates of acylation of the enzyme by the beta-lactams, measured by the second-order rate constant for hydrolysis, kcat/K(m), and those of sulfonylation by the beta-sultams, measured by the second-order rate constant for inactivation, k(i), both show a similar pH dependence to that exhibited by the beta-lactamase-catalyzed hydrolysis of beta-lactam antibiotics. Electron-withdrawing groups in the aryl residue of the leaving group of N-aroyl beta-lactams increase the rate of alkaline hydrolysis and give a Bronsted beta(lg) of -0.55, indicative of a late transition state for rate-limiting formation of the tetrahedral intermediate. Interestingly, the corresponding Bronsted beta(lg) for the beta-lactamase-catalyzed hydrolysis of the same substrates is -0.06, indicative of an earlier transition state for the enzyme-catalyzed reaction. By contrast, although the Bronsted beta(lg) for the alkaline hydrolysis of N-aroyl beta-sultams is -0.73, similar to that for the beta-lactams, that for the sulfonylation of beta-lactamase by these compounds is -1.46, compatible with significant amide anion expulsion/S-N fission in the transition state. In this case, the enzyme reaction displays a later transition state compared with hydroxide-ion-catalyzed hydrolysis of the beta-sultam.     

Cd(II) binding by particulate low-rank coals in aqueous media: sorption characteristics and NICA-Donnan models

An experimental investigation of Cd(II) sorption onto two Australian coals was carried out in 0.1 M NaNO3 at 298.2 K. The initial concentration of Cd(II) was varied from 0.133 to 2.000 mmol/g in a series of batch adsorption experiments with an initial coal concentration of 3.75 g/L of Loy Yang (brown) or of Collie (sub-bituminous) coals in the p[H+] range 2-8. Adsorption edges were typical of metal ion adsorption onto negatively charged organic substrates, starting at p[H+] approximately 3 and increasing with increasing pH. The largest measured Cd(II) uptake capacities from these experiments were of 1.2 mmol/g for Loy Yang and 0.7 mmol/g for Collie coals. This difference is ascribed to the larger concentrations of carboxyl groups in Loy Yang coal (2.78 mmol/g) compared to Collie coal (1.34 mmol/g). An adsorption isotherm for Loy Yang coal at p[H+] 6 was collected up to a surface loading of 1.7 mmol/g of adsorbed Cd(II). These experiments also revealed a release of about 1.5-1.6 protons per adsorbed Cd(II). Zeta potentials of Loy Yang coal suspensions were not affected by Cd(II) adsorption, suggesting that the coal particles efficiently neutralize the charge of Cd(II). Collie coal, on the other hand, exhibited a zeta potential increase that may indicate a modification of the surface potentials of the coal particles. Cd(II) uptake data obtained from both batch experiments and proton balance data have been combined with p[H+] stat data for the same experimentally covered Cd(II)/coal ratios to model adsorption using the NICA-Donnan model. The modeling results suggest that both coals possess identical affinities and reaction stoichiometries. Loy Yang coal, however, possessed a narrower distribution of affinities.     

Quantum theoretical physics is statistical and relativistic. II

The vanishing divergence of 4-velocity, crucial to our theoretical basis for a classical statistical quantum mechanics, is investigated further. We demonstrate that this property of 4-velocity is of purely relativistic—kinematic origin. So, unlike our treatment in the initial work, we need not constrain the environment to that of electromagnetism; a superior argument results by the analog of a relativistic particle's world line with fixed curves in a Euclidean 3-space, such that ·v=0 iff (v)²=–1!     

Self-asssembly of a molecular M8L12 cube having S6 symmetry

Reaction of the bis-bidentate ligand L1, having two bidentate pyrazolyl-pyridine termini, with Co(II) or Zn(II) results in formation of the complexes [M8(L1)12]X16 (X = perchlorate or tetrafluoroborate); [Zn8(L1)2](ClO4)16 has been structurally characterised and is a cube with a metal ion at each corner, a bridging ligand along each edge, and an anion in the central cavity.     

Simulation of organic monolayers as templates for the nucleation of calcite crystals

Living organisms can control the size, shape, and structure of minerals. Attempts to reproduce this biological control in the laboratory often use Langmuir monolayers of long-chain carboxylic acids. We use large-scale molecular dynamics simulations to calculate the interfacial energies of calcite crystals grown on stearic (octadecanoic) acid monolayers. In light of these simulations we discuss the argument that the orientation of the growing mineral is controlled by the organic substrate acting as a template which the mineral must fit in order to grow.     

Stereoselective synthesis of 4-hydroxy-2,3,6-trisubstituted tetrahydropyrans

[reaction: see text] Reaction of homoallylic alcohols with aldehydes in the presence of TFA gives, after hydrolysis of the ester, 4-hydroxy-2,3,6-trisubstituted tetrahydropyrans with the creation of three new stereocenters in a single-pot process. By varying the aldehyde component, a variety of functionalized side chains are installed at C-2. The utility of this approach is extended to the enantioselective synthesis of tetrahydropyrans with >99% ee.     

The d2/d3 alkyne redox pair [WF2(PhC identical to CPh)Tp']z (z = +1 or 0): missing links in a 'redox family tree'

The d2/d3 redox pair [WF2(PhC identical to CPh)Tp']z [z = +1 or 0, Tp' = hydrotris(3,5-dimethylpyrazolyl)borate] is the missing link in a 'redox family tree' relating the d6 tricarbonyls [M(CO)3L]- to the d2 trihalides [MX3L] (M = Mo or W, L = Cp or Tp') by a series of stepwise reactions involving sequential one-electron oxidation followed by ligand substitution.