Novel electrochemically-mediated peptide dethiylation in processes relevant to native chemical ligation.

Here we explore electrochemical dethiylation in processes relevant to Native Chemical Ligation (NCL). NCL's reliance on the redox active amino acid cysteine and ß-mercaptamine derivatives suggests a potential role for electrochemistry. We show that the application of a 1 V potential to platinum electrodes in 0.15 M TCEP solution is sufficient to convert Cys to Ala in cyclic peptides, and to cleave the 2-mercapto-2-phenethyl class of acyl transfer auxiliary.

Mechanistic studies of reactive oxygen species mediated electrochemical radical reactions of alkyl iodides.

Mechanistic studies of a reactive oxygen species mediated electrochemical radical reaction of alkyl iodides are described. Hydroxyl radicals and ozone are identified to be the active species involved in the formation of alkyl radicals under mildly reducing potential (-1.0 V vs. Ag QRE) in buffered acidic conditions (pH 3.6).

Electrochemical radical reactions of alkyl iodides: a highly efficient, clean, green alternative to tin reagents.

An electrochemical 'redox-relay' system has been developed which allows the generation of C-centered radicals. Intermolecular 'tin-like' radical reactions can subsequently be conducted under the most benign of conditions. The yields and efficiency of the processes are competitive and even superior in most cases to comparable conditions with tributyltin hydride. The use of air and electricity as the promotor (instead of a tin or other reagent) combined with the aqueous reaction media make this a clean and 'green' alternative to these classic C-C bond forming processes.

Exploring the Relationship between BODIPY Structure and Spectroscopic Properties to Design Fluorophores for Bioimaging.

Designing chromophores for biological applications requires a fundamental understanding of how the chemical structure of a chromophore influences its photophysical properties. We here describe the synthesis of a library of BODIPY dyes, exploring diversity at various positions around the BODIPY core. The results show that the nature and position of substituents have a dramatic effect on the spectroscopic properties. Substituting in a heavy atom or adjusting the size and orientation of a conjugated system provides a means of altering the spectroscopic profiles with high precision. The insight from the structure-activity relationship was applied to devise a new BODIPY dye with rationally designed photochemical properties including absorption towards the near-infrared region. The dye also exhibited switch-on fluorescence to enable visualisation of cells with high signal-to-noise ratio without washing-out of unbound dye. The BODIPY-based probe is non-cytotoxic and compatible with staining procedures including cell fixation and immunofluorescence microscopy.

Carboxylic Acid Functionalization at the Meso-Position of the Bodipy Core and Its Influence on Photovoltaic Performance.

Two bodipy dyes with different carboxylic acids on the meso-position of the bodipy core were prepared and used to sensitize TiO2 photoelectrodes. On the basis of spectroscopic characterization, the photoelectrodes were used to fabricate photoelectrochemical cells (PECs) for solar light harvesting. Photovoltaic measurements showed that both bodipy dyes successfully sensitized PECs with short-circuit current densities (JSC) two-fold higher compared to the control. The increase in generated current was attributed to the gain in spectral absorbance due to the presence of bodipy. Finally, the influence of co-sensitization of bodipy and N719 dye was also investigated and photovoltaic device performance discussed.

Investigations into the mechanism of copper-mediated Glaser-Hay couplings using electrochemical techniques.

The mechanism of the copper mediated C-C bond forming reaction known as Glaser-Hay coupling (alkyne dimerization) has been investigated using electrochemical techniques. Applying an oxidative potential to a copper or copper-coated graphite electrode in the presence of the organic base DABCO results in the dimerization of phenylacetylene in good yield. Further mechanistic investigation has shown that this reaction medium results in the assembly of a dinuclear Cu(i) complex which, although previously reported, has never been shown to have catalytic properties for C-C bond formation. The complex is reminiscent of that proposed in the Bohlmann model for the Glaser-Hay reaction and as such lends weight to this proposed mechanism above the alternative proposed mononuclear catalytic cycle.

Electrochemical synthesis of copper(i) acetylides via simultaneous copper ion and catalytic base electrogeneration for use in click chemistry.

We report an efficient and sustainable electrochemical synthesis of copper(i) acetylides using simultaneous copper oxidation and Hofmann elimination of quaternary ammonium salts. The electrochemically-generated base was also regenerated electrochemically, making it catalytic. A 'Click test' (CuAAC reaction) was performed to assess product purity and an electrochemically-promoted, one-pot CuAAC reaction was performed, which serves as a promising initial demonstration of this approach in a pharmaceutically-relevant reaction.

Structure and function of L-threonine-3-dehydrogenase from the parasitic protozoan Trypanosoma brucei revealed by X-ray crystallography and geometric simulations.

Two of the world's most neglected tropical diseases, human African trypanosomiasis (HAT) and Chagas disease, are caused by protozoan parasites of the genus Trypanosoma. These organisms possess specialized metabolic pathways, frequently distinct from those in humans, which have potential to be exploited as novel drug targets. This study elucidates the structure and function of L-threonine-3-dehydrogenase (TDH) from T. brucei, the causative pathogen of HAT. TDH is a key enzyme in the metabolism of L-threonine, and an inhibitor of TDH has been shown to have trypanocidal activity in the procyclic form of T. brucei. TDH is a nonfunctional pseudogene in humans, suggesting that it may be possible to rationally design safe and specific therapies for trypanosomiasis by targeting this parasite enzyme. As an initial step, the TDH gene from T. brucei was expressed and the three-dimensional structure of the enzyme was solved by X-ray crystallography. In multiple crystallographic structures, T. brucei TDH is revealed to be a dimeric short-chain dehydrogenase that displays a considerable degree of conformational variation in its ligand-binding regions. Geometric simulations of the structure have provided insight into the dynamic behaviour of this enzyme. Furthermore, structures of TDH bound to its natural substrates and known inhibitors have been determined, giving an indication of the mechanism of catalysis of the enzyme. Collectively, these results provide vital details for future drug design to target TDH or related enzymes.

A novel sulfonamide non-classical carbenoid: a mechanistic study for the synthesis of enediynes.

Alkynyl sulfonamides undergo sequential 1,4- then 1,2-addition/rearrangement with lithium acetylides to yield enediynes in the absence of any promoters or catalysts. Mechanistic investigations suggest that the reaction proceeds via 1,4-conjugate addition of the nucleophile to the unsaturated system to give a key alkenyl lithium species which is stabilised by an intramolecular coordination effect by a sulfonamide oxygen atom. This species can be considered a vinylidene carbenoid given the carbon atom bears both an anion (as a vinyllithium) and a leaving group (the sulfonamide). The intramolecular coordination effect serves to stabilise the vinyllithium but activates the sulfonamide motif towards nucleophilic attack by a second mole of acetylide. The resulting species can then undergo rearrangement to yield the enediyne framework in a single operation with concomitant loss of aminosulfinate.

The chemistry of ynol and thioynol ethers.

Alkynyl ethers and alkynyl thioethers ('ynol ethers' and 'thioynol ethers') are appealing building-blocks in synthetic chemistry due to their ease of manipulation and predictable reactivity. Until recently however, their potential has remained underexploited due to difficulties in preparation and isolation. Although recent advances in synthetic chemistry have highlighted various applications for ynol ethers, the equivalent thioynol examples have been rather less exploited despite a unique and fascinating reactivity profile. Although superficially the chemistry of alkynyl ethers and their sulfide counterparts are similar, close examination of their chemistry reveals important differences which can be exploited by the synthetic chemist. This review will examine the preparation of both classes of compound and examine their reactivity to highlight their powerful synthetic applications. Particular focus will be made of thiynol ethers whose chemistry exhibits some fascinating differences compared to their oxygen counterparts and have immense untapped potential for synthetic chemistry.

An improved transition-metal-free synthesis of aryl alkynyl sulfides via substitution of a halide at an sp-centre.

A simple high-yielding preparation of aryl alkynyl sulfides is presented. The reaction of a chloroacetylene with a thiolate salt in the presence of an amine mediator (dimethylamine or N,N'-dimethylethylenediamine) yields the alkynyl sulfides in excellent yields. The alkynyl chloride is easily prepared from the parent alkyne.

Transition-metal-free synthesis of ynol ethers and thioynol ethers via displacement at sp centers: a revised mechanistic pathway.

We present here valuable extensions to our previous work in preparing highly functionalized, heteroatom-substituted alkynes via displacement at an sp center. Our results show that a wide range of ynol ethers can be prepared by the same methodology and that the same protocol can be applied to the synthesis of synthetically useful thioynol ethers. We also present new observations that have led us to revise our original hypothesis in favor of a pathway involving radical intermediates.

Observations on transition metal free biaryl coupling: potassium tert-butoxide alone promotes the reaction without diamine or phenanthroline catalysts.

Biaryl coupling (often labelled 'C-H activation') of aromatic systems can be achieved by potassium tert-butoxide alone in the absence of any amine or bipyridine catalyst (1,10-phenanthroline or N,N'-dimethylethylenediamine being the most common), previously reported to be essential. Various mechanistic studies and observations are presented which suggest that when 1,10-phenanthroline is employed as the catalyst, the alkoxide is destroyed almost immediately.

Transition-metal-free synthesis of aryl-substituted tert-butyl ynol ethers through addition/elimination substitution at an sp centre.

Nucleophilic attack of an alkoxide on an alkynyl sulfonamide leads to displacement of the sulfonamide at the sp centre and isolation of the ynol ether in good yield in a single operation. The mechanistic pathway has been probed by the use of coordinating additives, (13)C-labelling experiments and ab initio calculations, which indicated that an addition/elimination mechanism is in operation.

A versatile synthesis of 2,4-substituted oxazoles.

A variety of five-membered ring oxazoles have been synthesised with complete regiocontrol and without the requirement for ring oxidation via a reaction sequence based on a vinyl sulfonamide template.

Trichlorophenol (TCP) sulfonate esters: a selective alternative to pentafluorophenol (PFP) esters and sulfonyl chlorides for the preparation of sulfonamides.

2,4,6-Trichlorophenol (TCP) sulfonate esters undergo effective aminolysis under conventional heating and microwave irradiation; the reactivity of these species is such that chemoselective transformations of PFP sulfonate esters can be achieved.

New synthesis of beta-sultams from pentafluorophenyl sulfonates.

A stereoselective one-pot synthesis of substituted 1,2-thiazetidine 1,1-dioxides (beta-sultams) has been achieved from heterocyclic pentafluorophenyl (PFP) sulfonates. Mild N-O bond cleavage of isoxazolidines followed by intramolecular cyclization of the amine onto the PFP demonstrates the potential utility for using the PFP sulfonate as a valuable precursor to sulfonamides. [reaction: see text].

Inhibition of dimethylarginine dimethylaminohydrolase (DDAH) and arginine deiminase (ADI) by pentafluorophenyl (PFP) sulfonates.

A range of pentafluorophenyl (PFP) sulfonate esters derived from the reaction of PFP vinyl sulfonate and various nitrones are shown to have significant inhibitory activity against the bacterial enzymes DDAH and ADI.