Fluorination methods in drug discovery

Synthesis of Aryl Triflones through the Trifluoromethanesulfonylation of Benzynes

The direct synthesis of aryl triflones, that is, trifluoromethanesulfonyl arenes, was achieved through the trifluoromethanesulfonylation of benzynes. The trifluoromethanesulfonyl group, one of the fluorinated functional groups, is a highly electron-negative and mild lipophilic substituent. Aryl triflones have high potential in the synthesis of bioactive compounds and specialty materials. The treatment of 2-(trimethylsilyl)aryl trifluoromethanesulfonates with cesium fluoride in the presence of 15-crown-5 generated benzynes, which reacted with sodium trifluoromethanesulfinate followed by protonation with tBuOH under heating conditions, provided aryl triflones in moderated to good yields. Both symmetrical and unsymmetrical triflones were nicely accessed under the same reaction conditions. Interestingly, the trifluoromethanesulfonylation of unsymmetrical benzyne precursors proceeded smoothly to furnish corresponding aryl triflones in good yields with good to high regioselectivities. The balance of polarization of electric charge as well as steric hindrance of the benzyne intermediates are central factors to control the outcome of regioselectivity.

O-Difluorodeuteromethylation of phenols using difluorocarbene precursors and deuterium oxide

A convenient synthesis of difluorodeuteromethyl aryl ethers using difluorocarbene precursors and deuterium oxide is described.

A photocatalytic decarboxylative/defluorinative [4 + 3] annulation of o-hydroxyphenylacetic acids and trifluoromethyl alkenes: synthesis of fluorinated dihydrobenzoxepines

A photocatalytic decarboxylative/defluorinative [4 + 3] annulation of o-hydroxyphenylacetic acids and trifluoromethyl alkenes was developed. The reaction led to fluorinated dihydrobenzoxepines through two consecutive C–F substitutions in a CF₃ group.

Nucleophilic fluorination facilitated by a CsF–CaF2 packed bed reactor in continuous flow

A simple to prepare, dry and handle packed bed reactor carrying CsF on CaF₂, towards nucleophilic fluorinations in continuous flow, is reported.

Potassium fluoride activation for the nucleophilic fluorination reaction using 18-crown-6, [2.2.2]-cryptand, pentaethylene glycol and comparison with the new hydro-crown scaffold: a theoretical analysis

Ion-pair binding in the hydro-crown ether leads to more effective catalytic cycle for nucleophilic fluorination.

The synthesis of the 2,3-difluorobutan-1,4-diol diastereomers

The diastereoselective synthesis of fluorinated building blocks that contain chiral fluorine substituents is of interest. Here we describe optimisation efforts in the synthesis of anti-2,3-difluorobutane-1,4-diol, as well as the synthesis of the corresponding syn-diastereomer. Both targets were synthesised using an epoxide opening strategy.

Regioselective decarboxylative addition of malonic acid and its mono(thio)esters to 4-trifluoromethylpyrimidin-2(1H)-ones

Background: Due to the high reactivity towards various C-nucleophiles, trifluoromethylketimines are known to be useful reagents for the synthesis of α-trifluoromethylated amine derivatives. However, decarboxylative reactions with malonic acid and its mono(thio)esters have been poorly investigated so far despite the potential to become a convenient route to β-trifluoromethyl-β-amino acid derivatives and to their partially saturated heterocyclic analogues. Results: In this paper we show that 4-trifluoromethylpyrimidin-2(1H)-ones, unique heterocyclic ketimines, react with malonic acid under organic base catalysis to regioselectively provide either Michael- or Mannich-type decarboxylative addition products depending on solvent polarity. Malonic mono(thio)esters give exclusively Michael-type products. The two regioisomeric products can be converted into saturated (2-oxohexahydropyrimidin-4-yl)acetic acid derivatives by mild hydrogenation of the endocyclic C=C double bond in the presence of Pd/C as catalyst. The cis-stereoisomers selectively formed upon reduction of the Michael-type products were structurally determined by X-ray diffraction. As a result of this study, a number of novel acetic acid derivatives containing trifluoromethylated, partially or fully saturated 2-oxopyrimidine cores were prepared and characterized as promising building blocks. Conclusions: Regio- and stereoselective protocols have been developed for the synthesis of novel isomeric 4(6)-trifluoromethylated 1,2,3,4-tetrahydro- and perhydro-(2-oxopyrimidin-4-yl)acetic acid derivatives.

Preparation of imidazo[1,2-a]-N-heterocyclic derivatives with gem-difluorinated side chains

Using an aerobic oxidative coupling, different new imidazo[1,2-a]-N-heterocycles with gem-difluroroalkyl side chains have been prepared in fair yields by the reaction of gem-difluoroenones with aminopyridines, -pyrimidines and -pyridazines. Condensed heterocycles of this type play an important role as key core structures of various bioactive compounds. Further, starting with a chloroimidazopyridazine derivative, Pd-catalyzed coupling reactions as well as nucleophilic substitutions have been performed successfully in order to increase the molecular diversity.

Visible-Light Photocatalytic Decarboxylation of α,β-Unsaturated Carboxylic Acids: Facile Access to Stereoselective Difluoromethylated Styrenes in Batch and Flow

The development of synthetic methodologies which provide access to both stereoisomers of α,β-disubstituted olefins is a challenging undertaking. Herein, we describe the development of an operationally simple and stereoselective synthesis of difluoromethylated styrenes via a visible-light photocatalytic decarboxylation strategy using fac-Ir(ppy)₃ as the photocatalyst. Meta- and para-substituted cinnamic acids provide the expected E-isomer. In contrast, ortho-substituted cinnamic acids yield selectively the less stable Z-product, whereas the E-isomer can be obtained via continuous-flow processing through accurate control of the reaction time. Furthermore, our protocol is amenable to the decarboxylative difluoromethylation of aryl propiolic acids.

Silver-promoted (radio)fluorination of unsaturated carbamates via a radical process

The intramolecular fluorocyclization of unsaturated carbamates is described here using a hypervalent iodine reagent in the presence of a silver catalyst. Both (hetero)aryl-substituted olefins and acrylamides can be utilized as effective substrates. Preliminary mechanistic investigations suggest that the reaction proceeds via a cyclization/1,2-(hetero)aryl migration/fluorination cascade involving an unusual radical process. Furthermore, starting from no-carrier-added [¹⁸F]TBAF, a simple one-pot, two-step cascade method was developed for the generation of ¹⁸F-labeled heterocycles with high radiochemical purity.

Design of potent fluoro-substituted chalcones as antimicrobial agents

Owing to ever-increasing bacterial and fungal drug resistance, we attempted to develop novel antitubercular and antimicrobial agents. For this purpose, we developed some new fluorine-substituted chalcone analogs (3, 4, 9–15, and 20–23) using a structure–activity relationship approach. Target compounds were evaluated for their antitubercular efficacy against Mycobacterium tuberculosis H37Rv and antimicrobial activity against five common pathogenic bacterial and three common fungal strains. Three derivatives (3, 9, and 10) displayed significant antitubercular activity with IC₅₀ values of ≤16,760. Compounds derived from trimethoxy substituent scaffolds with monofluoro substitution on the B ring of the chalcone structure exhibited superior inhibition activity compared to corresponding hydroxy analogs. In terms of antimicrobial activity, most compounds (3, 9, 12–14, and 23) exhibited moderate to potent activity against the bacteria, and the antifungal activities of compounds 3, 13, 15, 20, and 22 were comparable to those of reference drugs ampicillin and fluconazole.

Generation of Magnesium Pentafluoropropen-2-olate from Hexafluoroisopropanol and Synthesis of 2,2,4,4,4-Pentafluoro-3,3-dihydroxyketones

2,2,4,4,4-Pentafluoro-3,3-dihydroxyketones are valuable precursors to difluoroenolates following fragmentation during the release of trifluoroacetate; however, there are few synthetic strategies to prepare this unique class of compound. We addressed this issue and report a mild, two-step synthesis of 2,2,4,4,4-pentafluoro-3,3-dihydroxyketones from aldehydes. Specifically, aldehydes are treated with pentafluoropropen-2-olate, generated from a new fragmentation of hexafluoroisopropanol with a mixed Mg/Li amide, to give pentafluoroalcohols. A subsequent oxidation with Dess-Martin periodinane provides the targets in good isolated yields.

Nickel-catalyzed selective C-5 fluorination of 8-aminoquinolines with NFSI

The first nickel-catalyzed selective C-5 fluorination of 8-aminoquinoline derivatives was achieved using NFSI as the “F” source.

Catalyst free synthesis of α-fluoro-β-hydroxy ketones/α-fluoro-ynols via electrophilic fluorination of tertiary propargyl alcohols using Selectfluor™ (F-TEDA-BF4)

A facile access to α-fluoro-β-hydroxyketones via electrophilic fluorination.

A facile approach for the trifluoromethylthiolation of methylenecyclopropanes

Trifluoromethylthiolation of methylenecyclopropanes (MCPs) has been developed by using AgSCF₃/Na₂S₂O₈as a trifluoromethylthiolation source (SCF₃) to give trifluoromethylthiolated 1,2-dihydronaphthalene derivatives in moderate to good yields.

Diastereoselective [3 + 2] cycloaddition of 3-ylideneoxindoles with in situ generated CF2HCHN2: syntheses of CF2H-containing spirooxindoles

Structurally diverse CF₂H-containing spirooxindoles were constructed through a [3 + 2] cycloaddition of isatin-derived 3-ylideneoxindoles with in situ generated CF₂HCHN₂.

Visible-Light-Driven, Radical-Triggered Tandem Cyclization of o-Hydroxyaryl Enaminones: Facile Access to 3-CF2 /CF3-Containing Chromones

A practical and straightforward synthetic route to construct a variety of 3-CF₂/CF₃-containing chromones via photoredox catalysis was developed. This novel protocol features a visible-light-induced radical-triggered tandem cyclization.