Synthetic Applications of Monofluoromethylsulfonium Salts

Monofluoromethylsulfonium salts are emerging reagents for the fluoromethylation and fluoromethylenation or fluoromethylene transfer. Using this type of reagent is a simple approach for the introduction of the fluoromethyl group into a wide range of nucleophiles using mild basic conditions. Recently, fluoromethylsulfonium salts have been demonstrated to act as a synthetic equivalent for the challenging fluoromethylene synthon. For instance, these reagents can be used for the direct synthesis of monofluoroepoxides and fluorocyclopropanes from activated alkenes via a sulfur fluoromethylide intermediate. Sulfonium salts are an alternative, easy-to-handle option to volatile and environmentally concerning freons for achieving monofluorinated compounds. This review focuses on synthetic application of these reagents known to date.

1 Introduction

2 Fluoromethylation of O-, N-, S-, P-, and C-Nucleophiles

3 Sulfonium Salts for Radical Monofluoromethylation of Alkenes

4 Sulfonium Salts for Fluoromethylene Transfer

5 Conclusions

(R)-α-Trifluoromethylalanine as a 19 F NMR Probe for the Monitoring of Protease Digestion of Peptides

Fluorinated non-natural amino acids are useful tools for improving the bioavailability of peptides but can also serve as fluorinated probes in ¹⁹ F NMR-based enzymatic assays. We report herein that the use of the non-natural α-quaternarized (R)-α-trifluoromethylalanine ((R)-α-TfmAla) provides convenient and accurate monitoring of trypsin proteolytic activity and increases resistance towards pepsin degradation.

Optimized Monofluoromethylsulfonium Reagents for Fluoromethylene-Transfer Chemistry

An investigation of the properties and reactivity of fluoromethylsulfonium salts resulted in the redesign of the reagents for fluoromethylene transfer chemistry. The model reaction, fluorocyclopropanation of nitrostyrene, turned out to be a suitable platform for the discovery of more streamlined fluoromethylene transfer reagents. The incorporation of halides on one aryl ring increased the reactivity, and 2,4-dimethyl substitution on the other aryl ring provided a balance between the reactivity/crystallinity of the reagent as well as the atom economy. The utility of new reagents was demonstrated by the development of an efficient fluorocyclopropanation protocol to access a range of monofluorinated cyclopropane derivatives.

19F NMR as a tool in chemical biology

We previously reviewed the use of 19F NMR in the broad field of chemical biology [Cobb, S. L.; Murphy, C. D. J. Fluorine Chem. 2009, 130, 132-140] and present here a summary of the literature from the last decade that has the technique as the central method of analysis. The topics covered include the synthesis of new fluorinated probes and their incorporation into macromolecules, the application of 19F NMR to monitor protein-protein interactions, protein-ligand interactions, physiologically relevant ions and in the structural analysis of proteins and nucleic acids. The continued relevance of the technique to investigate biosynthesis and biodegradation of fluorinated organic compounds is also described.

Trimethyllysine: From Carnitine Biosynthesis to Epigenetics

Trimethyllysine is an important post-translationally modified amino acid with functions in the carnitine biosynthesis and regulation of key epigenetic processes. Protein lysine methyltransferases and demethylases dynamically control protein lysine methylation, with each state of methylation changing the biophysical properties of lysine and the subsequent effect on protein function, in particular histone proteins and their central role in epigenetics. Epigenetic reader domain proteins can distinguish between different lysine methylation states and initiate downstream cellular processes upon recognition. Dysregulation of protein methylation is linked to various diseases, including cancer, inflammation, and genetic disorders. In this review, we cover biomolecular studies on the role of trimethyllysine in carnitine biosynthesis, different enzymatic reactions involved in the synthesis and removal of trimethyllysine, trimethyllysine recognition by reader proteins, and the role of trimethyllysine on the nucleosome assembly.

Fluorine NMR functional screening: from purified enzymes to human intact living cells

The substrate- or cofactor-based fluorine NMR screening, also known as n-FABS (n fluorine atoms for biochemical screening), represents a powerful method for performing a direct functional assay in the search of inhibitors or enhancers of an enzymatic reaction. Although it suffers from the intrinsic low sensitivity compared to other biophysical techniques usually applied in functional assays, it has some distinctive features that makes it appealing for tackling complex chemical and biological systems. Its strengths are represented by the easy set-up, robustness, flexibility, lack of signal interference and rich information content resulting in the identification of bona fide inhibitors and reliable determination of their inhibitory strength. The versatility of the n-FABS allows its application to either purified enzymes, cell lysates or intact living cells. The principles, along with theoretical, technical and practical aspects, of the methodology are discussed. Furthermore, several applications of the technique to pharmaceutical projects are presented.

Fluoromethylene Transfer from Diarylfluoromethylsulfonium Salts: Synthesis of Fluorinated Epoxides

Diarylfluoromethyl sulfonium salts are efficient fluoromethylene transfer reagents equivalent to fluorocarbene, which is difficult to access. This was demonstrated by the development of a monofluorinated Johnson-Corey-Chaykovsky reaction with ketones and aldehydes, delivering uncommon 2-unsubstituted fluoroepoxides. This is the first evidence for the feasibility of sulfur fluoromethylylide and its action as a reaction intermediate.

Substrate scope for trimethyllysine hydroxylase catalysis

Trimethyllysine hydroxylase (TMLH) is a non-haem Fe(ii) and 2-oxoglutarate dependent oxygenase that catalyses the C-3 hydroxylation of an unactivated C-H bond in l-trimethyllysine in the first step of carnitine biosynthesis. The examination of trimethyllysine analogues as substrates for human TMLH reveals that the enzyme does hydroxylate substrates other than natural l-trimethyllysine.

NMR studies of the non-haem Fe(II) and 2-oxoglutarate-dependent oxygenases

The non-haem Fe(II) and 2-oxoglutarate (2OG)-dependent oxygenases belong to a superfamily of structurally-related enzymes that play important biological roles in plants, microorganisms and animals. Structural, mechanistic and functional studies of 2OG oxygenases require efficient and effective biophysical tools. Nuclear magnetic resonance (NMR) spectroscopy is a useful tool to study this enzyme superfamily. It has been applied to obtain information about enzyme kinetics, identify and characterise 2OG oxygenase-catalysed oxidation products, elucidate the catalytic mechanism, monitor ligand binding and study protein dynamics. This review summarises the types of information that NMR spectroscopy can provide in the studies of 2OG oxygenases, highlights the advantages of the technique and describes its drawbacks.

Fluorinated trimethyllysine as a 19F NMR probe for trimethyllysine hydroxylase catalysis

Human trimethyllysine hydroxylase (TMLH)-catalysed C-3 hydroxylation of N^ε-(fluoromethyl)dimethyllysine can be monitored by ¹⁹F NMR spectroscopy.

Fluorine nuclear magnetic resonance-based assay in living mammalian cells

Nuclear magnetic resonance (NMR)-based screening has been recognized as a powerful approach for the identification and characterization of molecules interacting with pharmaceutical targets. Indeed, several NMR methods have been developed and successfully applied to many drug discovery projects. Whereas most of these approaches have targeted isolated biomolecular receptors, very few cases are reported with the screening performed in intact cells and cell extracts. Here we report the first successful application of the fluorine NMR-based assay n-FABS (n-fluorine atoms for biochemical screening) in living mammalian cells expressing the membrane protein fatty acid amide hydrolase (FAAH). This method allows the identification of both weak and potent inhibitors and the measurement of their potency in a physiological environment.

Comparison of the substrate selectivity and biochemical properties of human and bacterial γ-butyrobetaine hydroxylase

2-Oxoglutarate and iron dependent oxygenases have potential for the stereoselective hydroxylation of amino acids and related compounds. The biochemical and kinetic properties of recombinant γ-butyrobetaine hydroxylase from human and Pseudomonas sp. AK1 were compared. The results reveal differences between the two BBOXs, including in their stimulation by ascorbate. Despite their closely related sequences, the two enzymes also display different substrate selectivities, including for the production of (di)hydroxylated betaines, implying use of engineered BBOXs for biocatalytic purposes may be productive.

Oxygenase-catalyzed desymmetrization of N,N-dialkyl-piperidine-4-carboxylic acids

γ-Butyrobetaine hydroxylase (BBOX) is a 2-oxoglutarate dependent oxygenase that catalyzes the final hydroxylation step in the biosynthesis of carnitine. BBOX was shown to catalyze the oxidative desymmetrization of achiral N,N-dialkyl piperidine-4-carboxylates to give products with two or three stereogenic centers.