Tentative explanation for the kinetic compensation effect in doped catalysts

The kinetic parameter compensation effect displayed by heterogeneous catalysts may occur when the global reaction is a combination of competing reactions which show the same mechanism and take place on different groups of active centres, each group showing different activation energy and pre-exponential factor values. In this study, a simple procedure is described for employing apparent kinetic data (lnA and E _a) that show a compensation effect in order to calculate the activation energies and pre-exponential factors (proportional to the population densities) of active centres that present equal reaction mechanisms in dual site catalysts. The procedure was used to reproduce apparent kinetic data taken from the literature and obtained from experiments with doped-catalysts prone to dual-site catalytic behaviour. The fittings obtained in all cases were very good. The population density of active centres with $E_{a_2 }$ activation energy showed constant growth with the increase in doping agent content, whereas that of the active centres originally present in the undoped catalysts $\left( {E_{a_1 } } \right)$ showed varying trends.     

Contribution to the total synthesis of caribenolide I

Stereoselective synthesis of C₁₃-C₂₉ fragment of caribenolide I, a potent antitumour macrolide isolated from a marine dinoflagellate Amphidinium sp. is described. The key step relies on a highly diastereoselective C-glycosylation, using a bulky chiral oxazolidin-2-thione, to control the absolute configuration of C₂₁. The C₂₄ and C₂₅ stereogenic centres were controlled by the enantioselective vinylogous Mukayiama aldol reaction, whereas C₁₄ and C₁₆ stereogenic centres were built from a chiral bis-epoxide.     

Asymmetric synthesis of dimethyl-1,2-bis-(diphenylphosphino)-1,2-ethanedicarboxylate by means of a chiral palladium template promoted hydrophosphination reaction

An optically pure C₂-symmetrical diphosphine ligand containing two ester functional groups at the two chiral carbon stereogenic centres was prepared efficiently from the asymmetric hydrophosphination reaction between diphenylphosphine and dimethyl acetylenedicarboxylate in the presence of an organopalladium(II) complex derived from (S)-N,N-dimethyl-1-(1-naphthyl)ethylamine.     

The use of 1,2-O-isopropylidene-α-d-xylofuranose as a chiral auxiliary in asymmetric cyclopropanation reactions

The stereoselective synthesis of cyclopropylmethylidene acetals derived from 1,2-O-isopropylidene-d-xylofuranose, as a chiral auxiliary, is described. The Simmons–Smith cyclopropanation reaction of the corresponding alkenylidene derivatives with CH₂I₂/ZnEt₂, in different reaction conditions, took place with high stereoselectivity. The diastereomeric excess in each case depended on the solvent and the temperature used in the reaction. The absolute configuration of the new stereogenic centres formed was determined by acid hydrolysis of the cyclopropane moiety of the chiral auxiliary, which was also recovered.     

Synthesis of substituted 4-(1H-indol-6-yl)-1H-indazoles as potential PDK1 inhibitors

The development of a preparative route to a series of novel 4-(1H-indol-6-yl)-1H-indazole compounds as potential PDK1 inhibitors is described. The synthetic strategy centres on the late-stage Suzuki cross-coupling of N-unprotected indazole and indole fragments. The use of a monoligated palladium catalyst system was found to be highly beneficial in the cross-coupling reaction. The indazole and indole fragments were constructed by diazotisation/cyclisation and S_NAr/reductive cyclisation sequences, respectively.     

Reactivity of polyhydroxyalkyl-heterocycles towards Lewis acids

We report the reactivity of different polyhydroxyalkyl-heterocycles towards ceric ammonium nitrate (CAN) and ferric chloride. The behaviour of 2-methyl-5-(tetritol-1-yl)-pyrroles and -furans is different towards CAN oxidation. Pyrroles afford 2,5-diformylheterocycles, while furans give access to 1,4-dicarbonyl compounds containing three stereogenic centres. Ferric chloride promotes an intramolecular C-arylation reaction on O-benzylated-polyhydroxyalkyl furans, yielding an isochroman moiety, which is the basic skeleton of a variety of natural products.     

Electron spin polarization in a three-electron spin system. An application to bacterial photosynthesis

Calculations on a system consisting of three electron spins and one nuclear spin are presented and their implications for bacterial photosynthesis discussed. Comparison with experimental measurements of electron spin polarization in pre-reduced photosynthetic reaction centres leads to conclusion that the exchange interaction within the primary radical pair is positive and less than 0.8 mT when the g values of the photoinduced radicals are taken to be those measured for the isolated radical species.     

Stereoselective cascade reactions for construction of polyfunctionalised octahydroquinolines via [2C+2C+1C,1N] cyclisation

A conceptually unifying approach for a highly enantio- and diastereoselective synthesis of polyfunctionalised octrahydroquinolines incorporating three contiguous chiral centres is reported. The synthesis involves diphenylprolinol silyl ether-catalysed Michael addition of 1,3-cyclohexanedione to nitroalkenes followed by potassium carbonate-promoted aza-Henry reaction with N-tosyl aldimines, intramolecular hemiaminalisation and dehydration reaction cascade in a one-pot operation.     

Magnetic field effects on the recombination of radical ions in reaction centers of photosynthetic bacteria

The amount of triplet products formed upon recombination of the bacteriochlorophyll dimer cation and the bacteriopheophytin anion in modified bacterial reaction centres can be manipulated by external magnetic fields. Making use of this effect, two methods provide information about structural and dynamic properties of the reaction centre. The two methods differ in that one (MARY) uses low static magnetic fields (0 ? B₀ < 1 kG) whereas the other (RYDMR) uses microwaves and high static fields (B₀ > 1 kG). As is shown here the two methods are equivalent from a theoretical point of view, the interpretation of experimental spectra being equally involved in both cases.     

Synthesis of N-benzyl-des-D-ring lamellarin K via an acyl-Claisen/Paal-Knorr approach

Lamellarin K is a complex pyrrole natural product and member of the lamellarin family – a group of natural products known for their potent biological activities, such as, antiproliferative activity and inhibition of P-gp mediated drug efflux pumps. We herein describe the synthesis of the N-benzyl-des-D ring analogue of lamellarin K using a route that centres on an acyl-Claisen reaction to eventually prepare a highly-functionalised 1-aryl-4-methyl-1,4-diketone. Paal-Knorr pyrrole formation using this diketone undergoes auto-oxidation to give a fully-substituted 5-formyl pyrrole which was converted into the natural lactone B ring. Antiproliferative testing of the N-benzyl-des-D ring analogue gave an IC₅₀ of 2.63 μM against the MDA-MB-231 breast cancer cell line.     

Stereoselective epoxidation of alkenylidene acetals derived from carbohydrates with d-allo,d-altro,d-galacto,d-gluco and d-xylo configurations

The synthesis of 2,3-epoxypropylidene acetals of sugar derivatives from N-acetyl-2-amino-2-deoxy-d-allopyranose, d-altropyranose, d-galactopyranose, N-acetyl-d-glucosamine, d-glucofuranose and d-xylofuranose is described. The epoxidation with m-CPBA of the corresponding alkenylidene derivatives took place with different stereoselectivities depending upon the sugar configuration, the protecting group of the hydroxyl groups of the sugar, and the substitution of the unsaturated system. The analysis of the ring-opening reaction of these oxiranes by hydrogenolysis enabled the assignation of their configuration at the new stereogenic centres.     

Anionic polymerization of ϵ-caprolactam-lix effect of the ratio of reacting components,of the medium and of the ring size on the initial stage of the anionic polymerization of lactams

Effects of the concentrations and ratio of reacting components and of temperature on the kinetics of reaction of N-propionyllactam (I) with potassium salt of lactam (KL) were studied for derivatives of ϵ-caprolactam, 8-octanelactam and 12-dodecanelactam. For ϵ-caprolactam, the initial rates of propagation, acylation of open-chain amide groups and condensation of growth centres were estimated. At the ratio of the starting components [I]₀/[KL]₀ = 0.5-3, the participation of the polymerization reaction is constant, amounting to ca 45% of the overall consumption of 1. The condensation reaction is 14–36% of the total consumption of I; its initial rate passes through a maximum at [I]₀/[KL]₀ = 2. With increasing permittivity of the medium, the total rate of consumption of I increases; in two media with the same bulk permittivity, however, the rates may differ by as much as one order of magnitude.     

Towards EPC-syntheses of the structural class of cochleamycins and macquarimicins. Part 1: EPC-synthesis of the hydrindene subunit of the cochleamycins

A racemic as well as an EPC-synthesis of the cis-hydrindene subunit of the cochleamycins, physiologically active microbial secondary metabolites, are reported. The five stereogenic centres of this subunit are introduced in high stereoselectivity in a short sequence by intermolecular Diels-Alder reaction, stereoselective methylation and hydride reduction. Cyclisation via nucleophilic addition, acidic fragmentation, regioselective Shapiro reaction and inversion of a secondary alcohol are the further key steps of these syntheses.     

Surface reactions of singlet excitons in solid films of 8-hydroxyquinoline aluminium (Alq3)

Fluorescence excitation (FE) spectra of Al(III) 8-hydroxyquinoline (Alq₃) solid films in contact with quartz, metal and some red-emitting dyes were measured and related to the FE spectrum recorded from the uncovered film surface. Their profiles indicate that singlet excitons are quenched by the surface and near-surface energy acceptors. The quenching efficiency (δ) dependence on penetration depth (l_a) of the exciting light is a result of both diffusion of singlet excitons towards the surface and their reactions with exponential distribution of quenching centres in the films. It is possible to fit the calculated δ(l_a) curves to the experimental data if values of the diffusion length of singlet excitons l_d=30±15 nm or the penetration depth of the quenching centres from the surface into the bulk of the order of 10²–10³ nm are used. Possible origin of quenching centres and the quenching effect on quantum electroluminescence yield of organic light-emitting diodes based on Alq₃ emitter are briefly discussed.     

Efficient stereoselective synthesis of enantiopure cis- and trans-1,2,4-trisubstituted piperidines

Enantiomerically pure (2R,4S)- and (2R,4R)-2-[(S)-1,2-dibenzyloxyethyl]-4-[2-(diphenylmethoxy)ethyl]-1-[(S)-1-phenylethyl]piperidines cis-1 and trans-1 have been synthesised from N-[(S)-1-phenylethyl]-(S)-2,3-di-O-benzylglyceraldimine in six steps in 31% and 18% overall yields, respectively. The efficiency of the synthetic strategy developed for the synthesis of these compounds relies on: (a) the totally diastereoselective tandem Mannich–Michael reaction between Danishefsky’s diene and the starting glyceraldimine, (b) the high yielding Wadsworth–Emmons reaction of the 4-piperidone intermediate and (c) the diastereodivergent reduction of the exocyclic C–C double bond at C₄ of the piperidine ring. These transformations led to 1,2,4-trisubstituted piperidines with two new stereogenic centres with excellent stereoselectivity.     

Synthesis of condensed tetrahydroimidazo[1,2-a]quinazoline-1,5-dione derivatives

Heating of N-{2-[(R-amino)carbonyl]phenyl}prolinamides in triethyl orthoformate solution was found to give 6-R-5,6,6a,8,9,10,10a,11-octahydropyrrolo[1′,2′:3,4]imidazo[1,2-a]quinazoline-5,11-diones. Similar reaction of N-{2-[(R-amino)carbonyl]phenyl}thiazolidine-4-carboxamides afforded 6-R-5,6,6a,10,10a,11-hexahydrothiazolo[3′,4′:3,4]imidazo[1,2-a]quinazoline-5,11-diones. The relative configuration of C-6a and C-10a centres of the tetracyclic compounds obtained was assigned as trans on the basis of X-ray crystallographic study.     

A study of the mechanism of hydrolysis of nitrophenyl esters catalyzed by polyethyleneimine containing benzyl groups

The mechanism of hydrolysis of n-nitrophenyl acetate (NPA), butyrate (NPB), caprylate (NPC), and o-methoxycinnamate (NPOMC) catalysed by benzyl-containing polyethyleneimines of linear and branched structures was investigated in aqueous media. The reaction seems to proceed via a general basic mechanism of catalysis and does not involve acylation of the catalyst. Benzyldiethylamine is an analogue of the active centres in polymers with pK_a = 8 · 35 ± 0 · 1, localized in the polymer globules at sites of higher hydrophobity.The reaction has a three-step mechanism involving binding of the substrate to an active centre (to give Michaelis sorption complex), substrate conversion and desorption of products. For each step, rate constants were determined. The effect of polymer (K₂/K_m)/K_II increases from NPA to NPC; in the latter case, it is of order 10⁵.     

Spectroscopy of the complexation of polybutadienyl- and polypentadienyllithium by ditertiary bases—II. 1,2-Dipiperidinoethane and N,N,N′,N′-tetraethylethylenediamine

The influence of 1,2-dipiperidinoethane (DPE) and tetraethylethylenediamine (TEEDA) on the butadienyl- and pentadienyllithium active centres has been examined by NMR and electronic spectroscopy. The effect of DPE on the butadienyllithium species differs from that of TEEDA in that the magnitudes of the ¹H shifts are considerably smaller at any given [base]:[lithium] ratio. The trans conformation was found to be the preferred structure of the active centres in the presence of diamines. The electronic spectra of these dienyllithium active centres in the presence of these bases exhibited new peaks with γ_max at 323–325 nm where the maximum influence of either base was seen at [base]:[lithium] ratios of ca two.     

ESR studies of heterogeneous radiation processes in a B2O3 + H2O system

The accumulation of radiation-induced defects and of H-atoms on γ-irradiation of a B₂O₃ + H₂O system, at 77K, at different concentrations of water molecules have been investigated by ESR. The radiation-chemical yields of paramagnetic centres and of H-atoms have been determined from appropriate kinetic curves. It has been established that increasing the concentration of water molecules in the system results in a lowering of the yield of paramagnetic centres and an increase in the H-atom concentration. On the basis of the results obtained a mechanism has been suggested for heterogeneous radiation processes in this system at 77 K.     

Enantioselective synthesis of new oxazolidinylthiazolidines as enzyme inhibitors

The synthesis of new oxazolidinylthiazolidines bicycles, oxygen analogues of bisthiazolidines, also known as metallo-β-lactamase inhibitors is described. The reaction of β-aminoalcohols and 2,5-dihydroxy-1,4-dithiane led to oxazolidinylthiazolidines and/or dithioazabicycles as the main products. The distribution pattern depends mainly on the aminoalcohol substituents. In a one-pot reaction, four new bonds are formed in good yields and with high atom efficiency. When the oxazolidinylthiazolidines are formed, two stereogenic centres are generated with high enantiospecificity. The reaction mechanism is discussed based on crystallographic data and interconversion studies. Two oxazolidinylthiazolidines were evaluated as inhibitors of the potent lactamase NDM-1 and compound 4f displayed competitive inhibition with K_i = 1.6 ± 0.6 μM.