Tomas Lebl

Isolation and structural characterisation of two antibacterial free fatty acids from the marine diatom, Phaeodactylum tricornutum

One solution to the global crisis of antibiotic resistance is the discovery of novel antimicrobial compounds for clinical application. Marine organisms are an attractive and, as yet, relatively untapped resource of new natural products. Cell extracts from the marine diatom, Phaeodactylum tricornutum, have antibacterial activity and the fatty acid, eicosapentaenoic acid (EPA), has been identified as one compound responsible for this activity. During the isolation of EPA, it became apparent that the extracts contained further antibacterial compounds. The present study was undertaken to isolate these additional antibacterial factors using silica column chromatography and reverse-phase high-performance liquid chromatography. Two antibacterial fractions, each containing a pure compound, were isolated and their chemical structures were investigated by mass spectrometry and nuclear magnetic resonance spectroscopy. The antibacterial compounds were identified as the monounsaturated fatty acid (9Z)-hexadecenoic acid (palmitoleic acid; C16:1 n-7) and the relatively unusual polyunsaturated fatty acid (6Z, 9Z, 12Z)-hexadecatrienoic acid (HTA; C16:3 n-4). Both are active against Gram-positive bacteria with HTA further inhibitory to the growth of the Gram-negative marine pathogen, Listonella anguillarum. Palmitoleic acid is active at micro-molar concentrations, kills bacteria rapidly, and is highly active against multidrug-resistant Staphylococcus aureus. These free fatty acids warrant further investigation as a new potential therapy for drug-resistant infections.

The Activation Mechanism of Ru-Indenylidene Complexes in Olefin Metathesis

Olefin metathesis is a powerful tool for the formation of carbon-carbon double bonds. Several families of well-defined ruthenium (Ru) catalysts have been developed during the past 20 years; however, the reaction mechanism for all such complexes was assumed to be the same. In the present study, the initiation mechanism of Ru-indenylidene complexes was examined and compared with that of benzylidene counterparts. It was discovered that not all indenylidene complexes followed the same mechanism, highlighting the importance of steric and electronic properties of so-called spectator ligands, and that there is no single mechanism for the Ru-based olefin metathesis reaction. The experimental findings are supported quantitatively by DFT calculations.

Structural analysis of leader peptide binding enables leader-free cyanobactin processing

Regioselective modification of amino acids within the context of a peptide is common to a number of biosynthetic pathways and many such products have potential as therapeutics. The ATP dependent enzyme LynD heterocyclizes multiple cysteine residues to thiazolines within a peptide substrate. The enzyme requires the substrate to have conserved N-terminal leader for full activity. Catalysis is almost insensitive to immediately flanking residues in the substrate suggesting recognition occurs distant from the active site. Nucleotide and peptide substrate co-complex structures of LynD reveal the substrate leader peptide binds to and extends the β-sheet of a conserved domain of LynD, whilst catalysis is accomplished in another conserved domain. The spatial segregation of catalysis from recognition combines seemingly contradictory properties of regioselectivity and promiscuity; it appears to be a conserved strategy in other peptide modifying enzymes. A variant of LynD that efficiently processes substrates without a leader peptide has been engineered.

Isothiourea-mediated asymmetric Michael-lactonisation of trifluoromethylenones: a synthetic and mechanistic study

HBTM-2.1 promotes the catalytic asymmetric intermolecular Michael-lactonisation of arylacetic acids and trifluoromethylenones in the presence of pivaloyl chloride, giving C(6)-trifluoromethyldihydropyranones with high diastereo- and enantiocontrol (up to 95 : 5 dr and >99% ee) that are readily derivatised to diverse synthetic building blocks containing trifluoromethyl-stereogenicity. Kinetic studies indicate the reaction is first order with respect to both in situ formed mixed anhydride and catalyst concentration, with a primary kinetic isotope effect observed using α,α-di-deuterio 4-fluorophenylacetic acid, consistent with rate determining deprotonation of an intermediate acyl isothiouronium ion.

Catalytic asymmetric α-amination of carboxylic acids using isothioureas

HBTM-2.1 promotes the direct asymmetric α-amination of carboxylic acids with N-aryl-N-aroyldiazenes at low catalyst loadings (as low as 0.25 mol%), giving either 1,3,4-oxadiazin-6-ones or N-protected α-amino acid derivatives (upon ring opening) with exquisite enantiocontrol (typically ≥99% ee).

The Cyanobactin Heterocyclase Enzyme: A Processive Adenylase That Operates with a Defined Order of Reaction

This work was funded by the Leverhulme Trust (RPG-2012–504), BBSRC (BB/K015508/1) and the University of St Andrews infrastructure is supported by a Wellcome Trust Capital Award (086036).

Synthesis, Conformation and Biological Evaluation of the Enantiomers of 3-Fluoro-γ-Aminobutyric Acid ((R)- and (S)-3F-GABA): An Analogue of the Neurotransmitter GABA

γ‐Aminobutyric acid or GABA (1) is one of the major inhibitory amino acid neurotransmitters of the central nervous system. This article describes the first synthesis of both the (R)‐ and (S)‐ enantiomers of 3‐fluoro‐GABA (2, 3F‐GABA). DFT calculations were carried out in a continuum solvent model (PCM–B3LYP) to estimate the preferred conformations of 3F‐GABA in aqueous solution. NMR coupling constants were calculated for each conformer and were then used to simulate the NMR spectra to evaluate the solution conformation of 3F‐GABA. A preliminary evaluation of the 3F‐GABA enantiomers shows that they act similarly as agonists of cloned GABAA receptors; however, they behave quite differently in a whole animal (Xenopus laevis tadpole model).

Selective modification of the β–β linkage in DDQ-treated Kraft lignin analysed by 2D NMR spectroscopy

The depolymerisation of the biopolymer lignin has the potential to provide access to a range of high value and commodity chemicals. However, research in this increasingly important area of green chemistry is hindered by the lack of analytical methods. The key challenge in using NMR is the throughput that can be achieved without the need for high field spectrometers fitted with cryoprobes. Here, we report the use of a relatively fast 2D HSQC NMR experiment performed on a 500 MHz spectrometer fitted with a BBFO+ probe to obtain high quality spectra. The use of the developed protocol to study the selective modification of the β–β linkage in Kraft lignin is also reported.

Asymmetric pericyclic cascade approach to spirocyclic oxindoles.

The reaction of chiral N-arylnitrones with carbocyclic alkylarylketenes generates spirocyclic oxindoles in good yields and with excellent levels of enantioselectivity (90-99% ee) via a pericyclic cascade process.

Isothiourea-Mediated Stereoselective C-Acylation of Silyl Ketene Acetals

Isothiourea DHPB promotes the diastereoselective C-acylation of silyl ketene acetals with anhydrides or benzoyl fluoride, giving 3-acyl-3-aryl or 3-acyl-3-alkylfuranones in excellent yields and stereoselectivities (up to 99:1 dr).