Synthesis of 2,2-difluoro-1,3-diketone and 2,2-difluoro-1,3-ketoester derivatives using fluorine gas.

Solutions of 1,3-diketones and 1,3-ketoester derivatives react with fluorine to give the corresponding 2,2-difluoro-1,3-dicarbonyl derivatives in the presence of quinuclidine. Quinuclidine reacts with fluorine in situ to generate a fluoride ion that facilitates limiting enolization processes, and an electrophilic N-F fluorinating agent that is reactive towards neutral enol species.

19 F†NMR Spectroscopy Tagging and Paramagnetic Relaxation Enhancement-Based Conformation Analysis of Intrinsically Disordered Protein Complexes.

The combination of 19 F NMR spectroscopy tagging and paramagnetic relaxation enhancement (PRE) NMR spectroscopy experiments was evaluated as a versatile method to probe protein-protein interactions and conformational changes of intrinsically disordered proteins upon complex formation. The feasibility of the approach is illustrated with an application to the Myc-Max protein complex; this is an oncogenic transcription factor that binds enhancer box DNA fragments. The single cysteine residue of Myc was tagged with highly fluorinated [19 F]3,5-bis(trifluoromethyl)benzyl bromide. Structural dynamics of the protein complex were monitored through intermolecular PREs between 19 F-Myc and paramagnetic (1-oxyl-2,2,5,5-tetramethyl-Δ3-pyrroline-3-methyl)methanethiosulfonate (MTSL)-tagged) Max. The 19 F R2 relaxation rates obtained with three differently MTSL-tagged Max mutants revealed novel insights into the differential structural dynamics of Myc-Max bound to DNA and the tumour suppressor breast cancer antigen 1. Given its ease of implementation, fruitful applications of this strategy to structural biology and inhibitor screening can be envisaged.

Kinetics of Electrophilic Fluorination of Steroids and Epimerisation of Fluorosteroids.

Fluorinated steroids, which are synthesised by electrophilic fluorination, form a significant proportion of marketed pharmaceuticals. To gain quantitative information on fluorination at the 6-position of steroids, kinetics studies were conducted on enol ester derivatives of progesterone, testosterone, cholestenone and hydrocortisone with a series of electrophilic N-F reagents. The stereoselectivities of fluorination reactions of progesterone enol acetate and the kinetic effects of additives, including methanol and water, were investigated. The kinetics of epimerisation of 6ß-fluoroprogesterone to the more pharmacologically active 6α-fluoroprogesterone isomer in HCl/acetic acid solutions are detailed.

Lewis acid promoted fluorine-alkoxy group exchange reactions for the synthesis of 5-alkoxy-4,4-difluoroisoxazoline systems.

Fluorine-alkoxy group exchange reactions of fluorinated isoxazoline derivatives promoted by Lewis acids to give various 5-alkoxylated 4,4-difluoroisoxazolines via SN1 type processes in good to excellent yields are reported. Sterically demanding phenol substrates such as 2,6-diphenylphenol gave novel aryl substituted products via electrophilic aromatic substitution.

2,2,2-Trifluoroethanol as a solvent to control nucleophilic peptide arylation.

The SNAr arylation of peptides with perfluoroaromatics provides a route by which to install a useful chemical handle that enables both 19F-NMR analysis and further chemical modification. However, chemo-selective arylation in peptides containing multiple nucleophilic side chains currently presents a challenge to the field. Herein, we demonstrate that employing 2,2,2-trifluoroethanol (TFE) as a solvent in peptide SNAr reactions significantly improves nucleophile-selectivity when compared to N,N'-dimethylformamide (DMF).

The application of perfluoroheteroaromatic reagents in the preparation of modified peptide systems.

The perfluoroheteroaromatic reagent pentafluoropyridine has proved to be a highly reactive electrophile, undergoing SNAr arylation reactions in the presence of a range of nucleophilic peptide side chains (i.e. cysteine, tyrosine, serine and lysine) under mild conditions. Moreover, we have shown how one-step peptide-modification using perfluoroheteroaromatics can deliver enhanced proteolytic stability in pharmaceutically-relevant peptides such as oxytocin.

Biotransformation of fluorophenyl pyridine carboxylic acids by the model fungus Cunninghamella elegans.

1. Fluorine plays a key role in the design of new drugs and recent FDA approvals included two fluorinated drugs, tedizolid phosphate and vorapaxar, both of which contain the fluorophenyl pyridyl moiety. 2. To investigate the likely phase-I (oxidative) metabolic fate of this group, various fluorinated phenyl pyridine carboxylic acids were incubated with the fungus Cunninghamella elegans, which is an established model of mammalian drug metabolism. 3. 19F NMR spectroscopy established the degree of biotransformation, which varied depending on the position of fluorine substitution, and gas chromatography-mass spectrometry (GC-MS) identified alcohols and hydroxylated carboxylic acids as metabolites. The hydroxylated metabolites were further structurally characterised by nuclear magnetic resonance spectroscopy (NMR), which demonstrated that hydroxylation occurred on the 4' position; fluorine in that position blocked the hydroxylation. 4. The fluorophenyl pyridine carboxylic acids were not biotransformed by rat liver microsomes and this was a consequence of inhibitory action, and thus, the fungal model was crucial in obtaining metabolites to establish the mechanism of catabolism.

Evaluation of fluorinated biphenyl ether pro-drug scaffolds employing the chemical-microbial approach.

Incorporation of fluorine in a drug can dramatically affect its metabolism and methods to assess the effect of fluorine substitution on drug metabolism are required for effective drug design. Employing a previously developed chemical-microbial method the metabolism of a series of fluorinated biphenyl ethers was determined. The substrates were synthesized via Ullmann-type condensation reactions between bromotoluene and fluorophenol. The ethers were incubated with the fungus Cunninghamella elegans, which oxidises xenobiotics in an analogous fashion to mammals, generating a number of hydroxylated biphenyl ethers and acids. The propensity of the fluorinated ring to be hydroxylated depended upon the position of the fluorine atom, and the oxidation of the methyl group was observed when it was meta to the oxygen. The experiments demonstrate the applicability of the method to rapidly determine the effect of fluorine substitution on CYP-catalysed biotransformation of pro-drug molecules.

Fluorotetrahydroquinolines from diethyl 2-fluoromalonate ester.

A short series of fluorotetrahydroquinolines was synthesised in two steps from diethyl fluoromalonate and appropriate ortho-nitrobenzyl bromide precursors.

Syntheses of fluorooxindole and 2-fluoro-2-arylacetic acid derivatives from diethyl 2-fluoromalonate ester.

Diethyl 2-fluoromalonate ester is utilised as a building block for the synthesis of 2-fluoro-2-arylacetic acid and fluorooxindole derivatives by a strategy involving nucleophilic aromatic substitution reactions with ortho-fluoronitrobenzene substrates followed by decarboxylation, esterification and reductive cyclisation processes.

A convenient chemical-microbial method for developing fluorinated pharmaceuticals.

A significant proportion of pharmaceuticals are fluorinated and selecting the site of fluorine incorporation can be an important beneficial part a drug development process. Here we describe initial experiments aimed at the development of a general method of selecting optimum sites on pro-drug molecules for fluorination, so that metabolic stability may be improved. Several model biphenyl derivatives were transformed by the fungus Cunninghamella elegans and the bacterium Streptomyces griseus, both of which contain cytochromes P450 that mimic oxidation processes in vivo, so that the site of oxidation could be determined. Subsequently, fluorinated biphenyl derivatives were synthesised using appropriate Suzuki-Miyaura coupling reactions, positioning the fluorine atom at the pre-determined site of microbial oxidation; the fluorinated biphenyl derivatives were incubated with the microorganisms and the degree of oxidation assessed. Biphenyl-4-carboxylic acid was transformed completely to 4'-hydroxybiphenyl-4-carboxylic acid by C. elegans but, in contrast, the 4'-fluoro-analogue remained untransformed exemplifying the microbial oxidation - chemical fluorination concept. 2'-Fluoro- and 3'-fluoro-biphenyl-4-carboxylic acid were also transformed, but more slowly than the non-fluorinated biphenyl carboxylic acid derivative. Thus, it is possible to design compounds in an iterative fashion with a longer metabolic half-life by identifying the sites that are most easily oxidised by in vitro methods and subsequent fluorination without recourse to extensive animal studies.

Continuous gas/liquid-liquid/liquid flow synthesis of 4-fluoropyrazole derivatives by selective direct fluorination.

4-Fluoropyrazole systems may be prepared by a single, sequential telescoped two-step continuous gas/liquid-liquid/liquid flow process from diketone, fluorine gas and hydrazine starting materials.

Palladium-catalyzed C-F activation of polyfluoronitrobenzene derivatives in Suzuki-Miyaura coupling reactions.

Highly fluorinated nitrobenzene derivatives are suitable substrates for palladium-catalyzed C-F bond arylation using readily available palladium catalysts under both conventional heating and microwave conditions. Arylation occurs ortho to the nitro group offering a synthetic route to polyfluorinated 2-arylnitrobenzene systems. The regiochemistry of the arylation reactions suggests that there is a significant directing interaction between the nitro group and the incoming nucleophilic palladium catalyst which is facilitated by the presence of several fluorine atoms attached the ring. Investigations into the regioselectivity and reactivity of several tetrafluoro- and trifluoronitrobenzene derivatives provides further evidence for the highly nucleophilic character of the oxidative addition step in contrast to the concerted mechanism of more conventional Suzuki-Miyaura coupling reactions involving aryl iodides and bromides.

9,10-Dioxa-1,2-diaza-anthracene derivatives from tetrafluoropyridazine.

Reaction of tetrafluoropyridazine with catechol gives a tricyclic 9,10-dioxa-1,2-diaza-anthracene system by a sequential nucleophilic aromatic substitution ring annelation process, further extending the use of perfluoroheteroaromatic derivatives for the synthesis of unusual polyfunctional heterocyclic architectures. The tricyclic scaffold reacts with amines and sodium ethoxide providing a short series of functional 9,10-dioxa-1,2-diaza-anthracene systems.

Fluorocyanoesters as Additives for Lithium-Ion Battery Electrolytes.

A range of methyl 2-fluorocyanoester derivatives were synthesized from dimethyl 2-fluoromalonate ester, and their efficacy as additives in lithium-ion battery (LIB) electrolytes was determined. The role played by the 2-fluorocyanoester additives on battery performance was explored by linear sweep cyclic voltammetry, NMR, GCMS, and XPS techniques. For all fluorocyanoester additives studied, initial reduction of the carbonyl group occurs which is then followed by formation of the corresponding radical anion. Possible degradation routes arising from loss of fluoride ion, loss of methyl radicals, and cleavage of the αß carbon-carbon bond were observed, and all affect battery performance. Electrode protection upon addition of fluorocyanoesters to the electrolyte is the main contribution to the improvement of battery stability, but improvements on the electrode protection are somewhat offset by free radical processes initiated at the anode. Longer alkyl-chain fluorocyanoesters showed the best LIB improvement with effective cathode protection.

Polysubstituted pyridazinones from sequential nucleophilic substitution reactions of tetrafluoropyridazine.

4,5,6-trifluoropyridazin-3(2H)-one can be used as a scaffold for the synthesis of various 4,5- and 4,6-disubstituted and ring-fused pyridazinone systems by sequential nucleophilic aromatic substitution processes. Although the regioselectivity of nucleophilic substitution can be affected by the nature of the nucleophile and the substituent attached to the pyridazinone ring, a variety of polyfunctional systems can be readily accessed by sequential nucleophilic substitution methodology which may have applications in the drug discovery arena. For example, reaction of 4,5,6-trifluoropyridazin-3(2H)-one with nitrogen nucleophiles leads to a mixture of aminated products arising from substitution of fluorine located at the 4- and 5-positions. The ratio of isomers obtained depends on the nucleophile where the 4-isomer is the major product for reaction with primary and secondary amines such as butylamine, morpholine, and aniline derivatives. Subsequent reaction of representative 4-aminated products gave 4,5-disubstituted systems and ring fused derivatives may be formed by reaction of 4,5,6-trifluoropyridazin-3(2H)-one or 4-substituted systems with N,N'-dimethylethylenediamine.

Enolization rates control mono- versus di-fluorination of 1,3-dicarbonyl derivatives.

Fluorine-containing 1,3-dicarbonyl derivatives are essential building blocks for drug discovery and manufacture. To understand the factors that determine selectivity between mono- and di-fluorination of 1,3-dicarbonyl systems, we have performed kinetic studies of keto-enol tautomerism and fluorination processes. Photoketonization of 1,3-diaryl-1,3-dicarbonyl derivatives and their 2-fluoro analogues is coupled with relaxation kinetics to determine enolization rates. Reaction additives such as water accelerate enolization processes, especially of 2-fluoro-1,3-dicarbonyl systems. Kinetic studies of enol fluorination with Selectfluor™ and NFSI reveal the quantitative effects of 2-fluorination upon enol nucleophilicity towards reagents of markedly different electrophilicity. Our findings have important implications for the synthesis of α,α-difluoroketonic compounds, providing valuable quantitative information to aid in the design of fluorination and difluorination reactions.

Perhalogenated pyrimidine scaffolds. Reactions of 5-chloro-2,4,6-trifluoropyrimidine with nitrogen centred nucleophiles.

BACKGROUND: Highly functionalised pyrimidine derivatives are of great importance to the life-science industries and there exists a need for efficient synthetic methodology that allows the synthesis of polysubstituted pyrimidine derivatives that are regioselective in all stages to meet the demands of RAS techniques for applications in parallel synthesis. 5-Chloro-2,4,6-trifluoropyrimidine may be used as a scaffold for the synthesis of polyfunctional pyrimidine systems if sequential nucleophilic aromatic substitution processes are regioselective. RESULTS: Use of 5-chloro-2,4,6-trifluoropyrimidine as a core scaffold for the synthesis of functionalised pyrimidine systems is assessed in reactions with a small range of nitrogen centred nucleophiles. Mixtures of products arising from nucleophilic aromatic substitution processes are formed, reflecting the activating effect of ring nitrogen and the steric influences of the chlorine atom. CONCLUSIONS: 5-Chloro-2,4,6-trifluoropyrimidine is not an ideal scaffold for analogue synthesis or for multiple substitution processes because purification must be performed to remove the 2-substituted regioisomer from the mixture before further reactions can be carried out. However, 4-amino derivatives can be isolated in acceptable yields using this methodology.

A quantitative reactivity scale for electrophilic fluorinating reagents.

Electrophilic N-F fluorination agents underpin the introduction of fluorine in aliphatic systems across drug and academic research. The choice of N-F reagent is currently determined through empirical experimentation in the absence of quantitative values for electrophilicities. Here we report an experimentally-determined kinetic reactivity scale for ten N-F fluorinating reagents, including Selectfluor™, NFSI, Synfluor™ and several N-fluoropyridinium salts, in CH3CN. The reactivity scale, which covers eight orders of magnitude, employs para-substituted 1,3-diaryl-1,3-dicarbonyl derivatives to measure relative and absolute rate constants. The para-substituted 1,3-diaryl-1,3-dicarbonyl scaffold delivers a convenient, sensitive spectrophotometric reporter of reactivity that also led to the discovery of a unique form of tautomeric polymorphism.