Veselin Maslak

Assembly of Amphiphilic Baskets into Stimuli-Responsive Vesicles. Developing a Strategy for the Detection of Organophosphorus Chemical Nerve Agents

We designed basket 1 to comprise a C3-symmetric hydrophobic cage (477 Å(3)) at its southern edge and three polar ammonium caps at the northern edge. This amphiphilic molecule was observed to assemble into large unilamellar vesicles (350 nm, TEM) in water and thereby entrap dimethyl phenylphosphonate (184 Å(3)) in its cavity (K(app) = (1.97 ± 0.02) × 10(3) M(-1)). The entrapment of the organophosphonate, akin to soman in size (186 Å(3)), triggers the transformation of the vesicular material into nanoparticles (100 nm, TEM). Stimuli-responsive vesicles, containing baskets of type 1 in their bilayer membrane, are unique assemblies and important for obtaining novel sensing devices.

Highly efficient Michael-type addition of acetaldehyde to β-nitrostyrenes by whole resting cells of Escherichia coli expressing 4-oxalocrotonate tautomerase.

A novel whole cell system based on recombinantly expressed 4-oxalocrotonate tautomerase (4-OT) was developed and shown to be an effective biocatalyst for the asymmetric Michael addition of acetaldehyde to β-nitrostyrenes. Optimal ratio of substrates (2mM β-nitrostyrenes and 20mM acetaldehyde) and biocatalyst of 5 g of cell dry weight of biocatalyst per liter was determined. Through further bioprocess improvement by sequential addition of substrate 10mM nitrostyrene biotransformation was achieved within 150 min. Excellent enantioselectivity (>99% ee) and product yields of up to 60% were obtained with β-nitrostyrene substrate. The biotransformation product, 4-nitro-3-phenyl-butanal, was isolated from aqueous media and further transformed into the corresponding amino alcohol. The biocatalyst exhibited lower reaction rates with p-Cl-, o-Cl- and p-F-β-nitrostyrenes with product yields of 38%, 51%, 31% and ee values of 84%, 88% and 94% respectively. The importance of the terminal proline of 4-OT was confirmed by two proline enriched variants and homology modeling.

Titanium tetrachloride promoted reaction of silyl ketene acetals with epoxides : a new method for the synthesis of γ-butanolides

Abstract A new method for the synthesis of γ-butanolides is described. The titanium tetrachloride promoted reaction of silyl ketene acetals with epoxides, followed by acidic work-up, affords the corresponding butyrolactones in 44–83% yield.

Improved Flavin-Based Catalytic Photooxidation of Alcohols through Intersystem Crossing Rate Enhancement

The triplet excited-state formation efficiency in a flavin derivative was increased by the introduction of iodine into the molecular framework. The transient absorption measurements showed that the intersystem crossing rate was 1.1 × 10(10) s(-1), significantly faster than in the parent flavin compound. Furthermore, the photocatalytic efficiency of iodoflavin was evaluated using the oxidation of benzyl alcohol as a model reaction. The benzaldehyde product yields were higher when iodoflavin was used as a photocatalyst, showing that the increased triplet yield directly translates into improved photocatalysis. The iodoflavin catalyst also allowed the use of higher substrate concentrations (since the undesired electron transfer from singlet excited state was minimized), which is expected to improve the practical aspects of photocatalysis by flavins.

A Useful Synthetic Equivalent of a Hydroxyacetone Enolate

Indium promoted allylation of carbonyl compounds with 4-(bromomethyl)-1,3-dioxol-2-one diastereoselectively affords anti-α,β-dihydroxyketones, protected as enol carbonates. These initial products can be deprotected to free dihydroxyketones or transformed under mild conditions into the corresponding cyclic carbonates, which constitutes a useful approach to hydroxyacetone aldols.

A simple and convenient synthesis of tautomeric (6 or 2)-hydroxy-4-methyl-(2 or 6)-oxo-1-(substituted phenyl)-(1,2 or 1,6)-dihydropyridine-3-carbonitriles

A simple and convenient synthesis of tautomeric (6 or 2)-hydroxy-4-methyl-(2 or 6)-oxo-1-(substituted phenyl)-(1,2 or 1,6)-dihydropyridine-3-carbonitriles from ethyl acetoacetate and 2-cyano-N-(substituted phenyl)ethanamides using microwave-assisted chemistry is presented. The structure of the obtained product was confirmed by the use of FT-IR, NMR, UV, and MS techniques. The presence of tautomeric forms (6-hydroxy-4-methyl-2-oxo-1-(substituted phenyl)-1,2-dihydropyridine-3-carbonitrile and 2-hydroxy-4-methyl-6-oxo-1-(substituted phenyl)-1,6-dihydropyridine-3-carbonitrile) and the state of equilibrium of the obtained product in DMSO-d6 was studied by 1H and 13C NMR spectroscopy, as well as B3LYP/6-311++G(d,p) and GIAO/WP04/aug-cc-pVDZ theoretical calculations.Graphical abstract

Electrochemical, theoretical, and morphological studies of antioxidant fullerosteroids

Four fullerosteroidal conjugates, previously confirmed to express two- to threefold better antioxidant activity in vitro than C60, were subjected to additional studies, including electrochemical, theoretical, and morphological examination. All tested compounds underwent reversible, diffusion-controlled reductions. A notable influence of the solvent properties on the reduction potential, the level of aggregation, and the lowest unoccupied molecular orbital (LUMO) energy was observed. Theoretical calculations indicated that the energy gain obtained by an intermediate formation, together with compounds’ polarizability, polarity, and lipophilicity contributed to the radical quenching capacity. Very large supramolecular aggregates of all fullerosteroidal esters with no hierarchical arrangement were observed in precipitated samples, while solvent induced self-assembling led to round nanoplates, which further arranged to flower-shaped hierarchically ordered architectures or uniformly distributed discoid particles. As in electrochemical studies, fine tuning of the aggregation level was achieved by the solvent.Graphical abstract

Chemoselective biocatalytic reduction of conjugated nitroalkenes: New application for an Escherichia coli BL21(DE3) expression strain

Chemoselective reduction of activated carbon-carbon double bond in conjugated nitroalkenes was achieved using Escherichia coli BL21(DE3) whole cells. Nine different substrates have been used furnishing the reduced products in moderate to good yields. 1-Nitro-4-phenyl-1,3-butadiene and (2-nitro-1-propenyl)benzene were successfully biotransformed with corresponding product yields of 54% and 45% respectively. Using this simple and environmentally friendly system 2-(2-nitropropyl)pyridine and 2-(2-nitropropyl)naphthalene were synthesized and characterized for the first time. High substrate conversion efficiency was coupled with low enantioselectivity, however 29% enantiomeric excess was detected in the case of 2-(2-nitropropyl)pyridine. It was shown that electronic properties of the aromatic ring, which affected polarity of the double bond, were not highly influential factors in the reduction process, but the presence of the nitro functionality was essential for the reaction to proceed. 1-Phenyl-4-nitro-1,3-butadiene could not be biotransformed by whole cells of Pseudomonas putida KT2440 or Bacillus subtilis 168 while it was successfully reduced by E. coli DH5α but with lower efficiency in comparison to E. coli BL21(DE3). Knockout mutant affected in nemA gene coding for N-ethylmaleimide reductase (BL21ΔnemA) could still catalyze bioreductions suggesting multiple active reductases within E. coli BL21(DE3) biocatalyst. The described biocatalytic reduction of substituted nitroalkenes provides an efficient route for the preparation of the corresponding nitroalkanes and introduces the new application of the strain traditionally utilized for recombinant protein expression.

Fulleropyrrolidine molecular dumbbells act as multi-electron-acceptor triads. Spectroscopic, electrochemical, computational and morphological characterizations

We synthesized three dumbbell-like compounds 2a–c, each containing two C60 groups at the periphery and pyromellitic diimide (PMDI) in the middle, and examined their electronic as well as assembly characteristics with both experimental and computational methods. Cyclic voltammetry (CV) measurements revealed that each of three electron-accepting (AAA) triads could accommodate up to eight electrons. Computational studies (density functional theory, DFT) of 2a–c at PBEPBE/6-311G(d,p) level of theory, with B3LYP/6-31G(d) optimized geometries, revealed that HOMO–LUMO energy gaps are similar to those of the model compound [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Compounds 2a–c were also found to assemble into vesicles and nanoparticles on the copper grid (100–300 nm, TEM), while giving more sizeable aggregates after a deposition on the glass (SEM, >5 μm). Understanding the packing of 2a–c on various solid substrates, as well as the assembly characteristics in general, is important for tuning the properties and fabrication of electronic/optical devices. On the basis of the results of conformational analysis (MM and DFT calculations), we deduced that different alkyl spacers in 2a–c ought to play a role in π–π interactions between the aromatic components of the triad to guide the packing and therefore morphology of the material.

Synthesis of γ-nitroaldehydes containing quaternary carbon in the α-position using a 4-oxalocrotonate tautomerase whole-cell biocatalyst

Synthesis of γ-nitroaldehydes from branched chain aldehydes and a range of α,β-unsaturated nitroalkenes was achieved by a whole-cell biocatalytic reaction using 4-oxalocrotonate tautomerase as catalyst. Under mild conditions, cyclic and acyclic branched aldehydes were converted into synthetically valuable quaternary carbon containing γ-nitroaldehydes. The yields of the desired products were influenced by reaction condition parameters such as organic solvent, temperature and pH. The whole-cell biocatalytic approach to the generation of α,α-substituted γ-nitroaldehydes was compared to the organocatalytic approach involving the lithium salt of phenylalanine as a catalyst. As the resulting γ-nitroaldehydes exhibited moderate antifungal activity and mild in vitro cytotoxicity against human fibroblasts (0.2–0.4 mM) they could further be examined as potentially useful pharmaceutical synthons.