α-Amino-β-fluorocyclopropanecarboxylic acids as a new tool for drug development: Synthesis of glutamic acid analogs and agonist activity towards metabotropic glutamate receptor 4

Recent Advances in Monofluorinated Carbenes, Carbenoids, Ylides, and Related Species

The synthesis of monofluorinated compounds is of great interest because of the vast applications of organofluorine compounds. Recently, the introduction of monofluorocarbene synthons has emerged as an important strategy for the synthesis of fluorine-containing products. In contrast to direct fluorination, in which C-F bonds are formed, the use of monofluorinated carbenes and related reactive species involves C-C or C-X bond formation while delivering valuable fluorine atoms into the target structure. Owing to increased knowledge on carbon-carbon and carbon-heteroatom bond formations, monofluorinated carbenes have enormous potential for the synthesis of organofluorine compounds, which, in our opinion, has not yet been fully exploited. This review summarizes the recent advances in the synthetic applications of monofluorinated carbenes, carbenoids, ylides, and related species.

Tinker, Tailor, Soldier, Spy: The Diverse Roles That Fluorine Can Play within Amino Acid Side Chains

Side chain-fluorinated amino acids are useful tools in medicinal chemistry and protein science. In this review, we outline some general strategies for incorporating fluorine atom(s) into amino acid side chains and for elaborating such building blocks into more complex fluorinated peptides and proteins. We then describe the diverse benefits that fluorine can offer when located within amino acid side chains, including enabling 19F NMR and 18F PET imaging applications, enhancing pharmacokinetic properties, controlling molecular conformation, and optimizing target-binding.

Asymmetric Synthesis of Fluoro, Fluoromethyl, Difluoromethyl, and Trifluoromethylcyclopropanes

Fluorine-containing cyclopropanes are a subclass of cyclopropane derivatives that have generated considerable interest in medicinal chemistry for several decades. The replacement of a cyclopropane C-H or C-CH₃ bond with fluorine or a fluorinated group (such as CF₃ or CF₂H) can lead sometimes to synergistic effects in terms of biological activity and improved metabolic profile of a cyclopropane containing bioactive compound. In this context, the preparation of fluoro-, difluoromethyl-, or trifluoromethyl-cyclopropane is particularly attractive and important but quite challenging considering the unique electronic properties that result from the incorporation of a fluorine atom into a substrate or a reagent. In the past decade, we have sought to develop new routes for the stereoselective synthesis of these building blocks using the most reliable cyclopropanation methods and convenient and readily available starting materials. The challenge that had to be undertaken was how we could use the unique properties of the fluorine atom to improve upon the efficiency of a given process rather than shutting it down. This could be overcome by defining new substrate/reagent reactivity guidelines and carefully selecting whether the fluorinated group was introduced on the electrophilic or nucleophilic partner for a given reaction. In this Account, we describe our contributions in this area that take advantage of diazo-derived rhodium carbenes, zinc carbenoids, ring closure processes, and biocatalytic methods to access these important potential drug subunits. Our initial investigation relied on the development of a Michael-initiated ring closure reaction using the Reformatsky enolate derived from readily available ethyl dibromofluoroacetate and α,β-unsaturated electrophiles. The reaction proceeded extremely well but with modest to good diastereoselectivities with ester acrylates. Further extension to various fluorinated nucleophiles such as oxazolidinone based and DABCO ylides led to similar selectivities.In order to access enantioenriched fluorocyclopropanes, we then investigated the chiral dioxaborolane mediated zinc carbenoid based approaches using the fluoroiodomethylzinc carbenoid/allylic alcohol combination or the iodomethylzinc carbenoid/fluoroallylic alcohol combination. Quite surprisingly, both approaches were equally successful at providing the corresponding fluorocyclopropanes with excellent diastereo- and enantioselectivities.To broaden the scope of fluorinated cyclopropane building blocks that could be prepared with good enantiocontrol, we then investigated the rhodium-catalyzed cyclopropanation of fluoro-, difluoromethyl-, and trifluoromethyl-substituted alkenes with acceptor-acceptor and donor-acceptor diazo reagents. Depending on the substrate/reagent combination, Hashimoto's Rh₂((S)-TCPTTL)₄ or Davies' Rh₂((S)-BTPCP)₄ catalyst proved be the most efficient catalysts providing the cyclopropane derivatives with the highest enantioselectivities.More recently, a collaboration with Fasan's group led to the use of engineered myoglobins to catalyze the reaction of ethyl diazoacetate and difluoromethyl-substituted alkenes. This biocatalyzed process led to high turnover number and high enantioselectivities.Although our work has significantly increased the number of tools in the organic chemist's toolbox, continuous efforts in this area would be beneficial to the development of diastereo- and enantioselective approaches to allow the preparation of any elusive isomers of these valuable chiral building blocks.

Nickel-Catalyzed Cross-Electrophile Coupling of the Difluoromethyl Group for Fluorinated Cyclopropane Synthesis

Herein, we report a new strategy for fluorinated cyclopropane synthesis. Photocatalytic olefin difluoromethylation is coupled with a nickel-catalyzed intramolecular cross-electrophile coupling (XEC) reaction between a difluoromethyl moiety and a benzylic ether. To the best of our knowledge, this is the first example of a XEC reaction employing a difluoromethyl group as an electrophile. A plausible mechanism is highlighted, and DFT calculations are included to support the observed stereochemical outcome.

Synthesis of complex unnatural fluorine-containing amino acids

The area of fluorinated amino acid synthesis has seen rapid growth over the past decade. As reports of singly fluorinated natural amino acid derivatives have grown, researchers have turned their attention to develop methodology to access complex proteinogenic examples. A variety of reaction conditions have been employed in this area, exploiting new advances in the wider synthetic community such as photocatalysis and palladium cross-coupling. In addition, novel fluorinated functional groups have also been incorporated into amino acids, with SFX and perfluoro moieties now appearing with more frequency in the literature. This review focuses on synthetic methodology for accessing complex non-proteinogenic amino acids, along with amino acids containing multiple fluorine atoms such as CF3, SF5 and perfluoroaromatic groups.

Rhodium catalysed enantioselective synthesis of mono-(halo)-methyl-cyclopropanes

The catalytic asymmetric synthesis of mono-fluoro-, -chloro- and -bromomethyl-1,2-diaryl ester cyclopropanes is described.

General Catalytic Enantioselective Access to Monohalomethyl and Trifluoromethyl Cyclopropanes

An efficient catalytic enantioselective access to chiral functionalized trifluoromethyl cyclopropanes from two classes of diazo compounds and α-trifluoromethyl styrenes using Rh₂ ((S)-BTPCP)₄ as a catalyst is described. This method provides an efficient and practical strategy for the synthesis of highly functionalized CF₃ -cyclopropanes with excellent diastereoselectivities (up to 20:1) and enantioselectivities (up to 99 % ee). The depicted methodology represents, to date, the most efficient catalytic enantioselective method to access highly decorated chiral CF₃ -cyclopropanes. Extension to chiral monohalomethyl cyclopropanes in high ee is also reported.

Synthesis and studies on the mGluR agonist activity of FAP4 stereoisomers

The four stereoisomers of 1-amino-2-fluoro-2-(phosphonomethyl)cyclopropane-1-carboxylic acid (FAP4) were synthesized via diastereoselective Rh(II)-catalysed cyclopropanation of a phosphonylated fluoroalkene. Different isomers of FAP4 and the corresponding non-fluorinated analogs showed a similar pharmacological profile against the isoforms of metabotropic glutamate receptor (mGluR). Within the fluorinated series, (-)-(Z)-FAP4 and (-)-(E)-FAP4 demonstrated the highest agonist activity against mGlu4 (EC50 0.10 μM). Our results suggest that fluorocyclopropanes bearing an amino-acid function can be suitable for the development of potent conformationally restricted mGluR agonists.

Silver-catalyzed radical phosphonofluorination of unactivated alkenes

We report herein a mild and catalytic phosphonofluorination of unactivated alkenes. With catalysis by AgNO3, the condensation of various unactivated alkenes with diethyl phosphite and Selectfluor reagent in CH2Cl2/H2O/HOAc at 40 °C led to the efficient synthesis of β-fluorinated alkylphosphonates with good stereoselectivity and wide functional group compatibility. A mechanism involving silver-catalyzed oxidative generation of phosphonyl radicals and silver-assisted fluorine atom transfer is proposed.

Synthesis of mono-fluorinated functionalized cyclopropanes and aziridines using the α-fluorovinyl diphenyl sulfonium salt

The α-fluorovinyl diphenyl sulfonium salt 1 is attractive due to its high potential for the synthesis of mono-fluorinated cyclopropanes and aziridines as useful three-membered rings. The synthetically useful salt 1 is readily prepared from α-fluorovinyl phenyl sulfide and diphenyl iodonium salt in one step.

Efficient regio- and stereoselective access to novel fluorinated β-aminocyclohexanecarboxylates

A regio- and stereoselective method has been developed for the synthesis of novel fluorinated 2-aminocyclohexanecarboxylic acid derivatives with the fluorine attached to position 4 of the ring. The synthesis starts from either cis- or trans-β-aminocyclohex-4-enecarboxylic acids and involves regio- and stereoselective transformation of the ring C–C double bond through iodooxazine formation and hydroxylation, followed by hydroxy–fluorine or oxo–fluorine exchange.

Schiff bases attached L-glutamine and L-asparagine: first investigation on antimutagenic and antimicrobial analyses

This study was conducted to evaluate the antimutagenic and antimicrobial activities of Schiff bases attached L-glutamine and L-asparagine. Antibacterial activities of the compounds against S. aureus, Sh. dys. typ 7, L. monocytogenes 4b, E. coli, S. typhi H, S. epidermis, Br. abortus, M. luteus, B. cereus, P. putida, and antifungal activity against Candida albicans were studied. These compounds were investigated for antimutagenic properties against Aflatoxin Bı (AFBı) using micronuclei (MN) assay in human lymphocyte cell culture in vitro. The protective role of these compounds against AFBı-induced MN is probably related to its doses.