Current State of Microflow Trifluoromethylation Reactions.

The trifluoromethyl group is a powerful structural motif in drugs and polymers; thus, developing trifluoromethylation reactions is an important area of research in organic chemistry. Over the past few decades, significant progress has been made in developing new methods for the trifluoromethylation of organic molecules, ranging from nucleophilic and electrophilic approaches to transition-metal catalysis, photocatalysis, and electrolytic reactions. While these reactions were initially developed in batch systems, more recent microflow versions are highly attractive for industrial applications owing to their scalability, safety, and time efficiency. In this review, we discuss the current state of microflow trifluoromethylation. Approaches for microflow trifluoromethylation based on different trifluoromethylation reagents are described, including continuous flow, flow photochemical, microfluidic electrochemical reactions, and large-scale microflow reactions.

Regio- and Z-Selective Alkyne Hydroamination and Hydrophenoxylation using Tetrafluoro-λ6 -Sulfanyl Alkynes under Superbasic, Naked Anion Conditions.

Alkyne hydroamination is an effective approach for the production of enamines and enamine-containing N-heterocycles. However, stereoselectivity control is a considerable challenge in this reaction because of the electronic repulsion between an incoming nitrogen lone pair and the alkyne π-system. Herein, we propose a methodology involving ß-regio- and Z-selective alkyne hydroamination by using tetrafluoro-λ6 -sulfanyl (SF4 ) alkynes under superbasic, naked anion conditions. The reaction is compatible with a wide variety of N-heterocycles, including indoles, carbazoles, pyrazoles, and imidazoles, and selectively furnishes SF4 -linked Z-vinyl enamines with ß-regioselectively. Moreover, the method can be extended to the ß- and Z-controlled, base-mediated alkyne hydrophenoxylation with phenols to provide SF4 -linked Z-vinyl ethers in high yields. As the SF4 unit has attracted attention as a bioisostere for alkynes, p-benzenes, bicyclo[1.1.1]pentyl (BCP) groups, and cubanes in medicinal chemistry, this chemistry represents an effective approach to creating novel drug candidates incorporating SF4 -containing molecules.

Synthesis of triarylmethanes by silyl radical-mediated cross-coupling of aryl fluorides and arylmethanes.

Although the cross-couplings of aryl halides with diarylmethanes are mostly achieved by transition-metal catalysis, aryl fluorides are rarely used as coupling partners owing to the high inertness of C-F bonds. Herein, we describe the efficient silylboronate-mediated cross-coupling reaction of aryl fluorides with arylalkanes under transition-metal-free, room-temperature conditions. The combination of silylboronate and KO t Bu is critical for driving a radical process via the cleavage of C-F and C-H bonds in two appropriate coupling precursors, resulting in a cross-coupling product. This practical cross-coupling protocol is applicable to a wide variety of aryl fluorides with a C(sp2)-F bond. This method can be extended to other coupling partners with a C(sp3)-H bond, including diarylmethanes, diarylethanes, and monoarylalkanes. Many di- and triarylalkanes with tertiary or quaternary carbon centers can be obtained easily in moderate to high yields. We believe that the developed silylboronate-mediated cross-coupling method is a valuable contribution to C-F and C-H activation chemistry.

Elemental Sulfur-Mediated Transformation of Carboxylic Acids to Acyl Fluorides by Electrophilic Fluorinating Reagent, Selectfluor.

The elemental sulfur-mediated synthesis of acyl fluorides from carboxylic acids is achieved using Selectfluor. A broad range of acyl fluorides are accessible from carboxylic acids while avoiding the formation of acid anhydrides. 19F NMR spectra suggest that S8-fluoro-sulfonium cation A and neutral S8-difluoride A' generated in situ are the reactive species in this deoxyfluorination reaction.

Unusual Photoisomerization Pathway in a Near-Infrared Light Absorbing Enzymerhodopsin.

Microbial and animal rhodopsins possess retinal chromophores which capture light and normally photoisomerize from all-trans to 13-cis and from 11-cis to all-trans-retinal, respectively. Here, we show that a near-infrared light-absorbing enzymerhodopsin from Obelidium mucronatum (OmNeoR) contains the all-trans form in the dark but isomerizes into the 7-cis form upon illumination. The photoproduct (λmax = 372 nm; P372) possesses a deprotonated Schiff base, and the system exhibits a bistable nature. The photochemistry of OmNeoR was arrested at <270 K, indicating the presence of a potential barrier in the excited state. Formation of P372 is accompanied by protonation changes of protonated carboxylic acids and peptide backbone changes of an α-helix. Photoisomerization from the all-trans to 7-cis retinal conformation rarely occurs in any solvent and protein environments; thus, the present study reports on a novel photochemistry mediated by a microbial rhodopsin, leading from the all-trans to 7-cis form selectively.

KHMDS/Triglyme Cryptate as an Alternative to Phosphazene Base in Stereodivergent Pentafluoroethylation of N-Sulfinylimines Using HFC-125.

A protocol for the stereodivergent pentafluoroethylation of N-sulfinylimines using HFC-125 with KHMDS/triglyme has been developed. Both diastereomers of the pentafluoroethylated amines can be selectively synthesized based on the presence or absence of triglyme. This additive-controlled protocol allows the KHMDS/triglyme cryptate to be a straightforward and cheap alternative to previously reported base-controlled stereodivergent trifluoromethylation using potassium hexamethyldisilazide (KHMDS) versus P4-tBu.

Synthesis of Pyridine-SF4-Isoxazolines Using the Functionality of trans-Tetrafluoro-λ6-sulfanyl Rodlike Linkers.

The tetrafluoro-λ6-sulfanyl (SF4) moiety has been relatively undeveloped since its discovery in the 1970s. In this study, we synthesized pyridine-SF4-isoxazolines, in which the two heterocycles are connected by a rodlike trans-SF4 linker, via the regioselective 1,3-dipolar cycloaddition of pyridine-SF4-alkynes and nitrones in the presence of triethylamine. SF4 linkers are a viable alternative to para-substituted benzenes, alkynes, and bicyclo[1.1.1]pentyl derivatives in drug design, and pyridine-SF4-isoxazolines have potential applications in drug development.

Regioselective Synthesis of Pyridine-SF4-Methyl Ketones via Hydration of Pyridine-SF4-Alkynes.

Herein, we report the metal-free hydration of pyridine-SF4-alkynes, under acidic conditions and with reaction durations ranging from 5 min to 10 h at room temperature, to synthesize pyridine-SF4-methyl ketones in yields of 59-93%. Further, we demonstrate the synthetic applications of the synthesized pyridine-SF4-methyl ketones, such as chlorination, NaBH4 reduction, Baeyer-Villiger oxidation, and the generation of enol-triflates. These compounds hold promise as useful building blocks in the syntheses of a wide range of SF4-containing drug candidates.

Study of the Structure-Activity Relationship of an Anti-Dormant Mycobacterial Substance 3-(Phenethylamino)Demethyl(oxy)aaptamine to Create a Probe Molecule for Detecting Its Target Protein.

The current tuberculosis treatment regimen is long and complex, and its failure leads to relapse and emergence of drug resistance. One of the major reasons underlying the extended chemotherapeutic regimen is the ability of Mycobacterium tuberculosis to attain a dormant state. Therefore, the identification of new lead compounds with chemical structures different from those of conventional anti-tuberculosis drugs is essential. The compound 3-(phenethylamino)demethyl(oxy)aaptamine (PDOA, 1), isolated from marine sponge of Aaptos sp., is known as an anti-dormant mycobacterial substance, and has been reported to be effective against the drug resistant strains of M. tuberculosis. However, its target protein still remains unclear. This study aims to clarify the structure-activity relationship of 1 using 15 synthetic analogues, in order to prepare a probe molecule for detecting the target protein of 1. We succeeded in creating the compound 15 with a photoaffinity group that retained antimicrobial activity, which proved to be a suitable probe molecule for identifying the target protein of 1.

Synthesis of enantioenriched 2,2-disubstituted pyrrolidines via sequential asymmetric allylic alkylation and ring contraction.

The synthesis of a variety of enantioenriched 2,2-disubstituted pyrrolidines is described. A stereogenic quaternary center is first formed utilizing an asymmetric allylic alkylation reaction of a benzyloxy imide, which can then be reduced to a chiral hydroxamic acid. This compound can then undergo a thermal "Spino" ring contraction to afford a carbamate protected 2,2-disubstituted pyrrolidine stereospecifically. These pyrrolidines can be further advanced to enantioenriched indolizidine compounds. This reaction sequence allows access to new molecules that could be useful in the development of pharmaceutical agents.

Pentafluoroethylation of Carbonyl Compounds Using HFC-125 in a Flow Microreactor System.

The protocol of micro-flow nucleophilic pentafluoroethylation using pentafluoroethane (HC2F5, HFC-125), a nontoxic, inexpensive, and commercially available greenhouse gas, is described. The micro-flow pentafluoroethylation by HFC-125 proceeded smoothly at room temperature or at -10 °C in DMF or toluene in the presence of a potassium base, namely, t-BuOK or KHMDS. A broad range of ketones, aldehydes, and chalcones with various substituted benzene rings were successfully converted to the corresponding pentafluoroethyl carbinols instantly with good to high yields.

Synthesis of Difluoromethanesulfinate Esters by the Difluoromethanesulfinylation of Alcohols.

Herein, we report the first synthesis of difluoromethanesulfinate esters via the direct difluoromethanesulfinylation of alcohols with HCF2SO2Na/Ph2P(O)Cl. Primary, secondary, and tertiary alcohols were converted to the corresponding difluoromethanesulfinate esters in good to excellent yields under mild conditions. The late-stage functionalization of complexed biologically active natural products was also demonstrated. The method was extended to the trifluoromethanesulfinylation of alcohols using CF3SO2Na in the presence of a catalytic amount of Me3SiCl to provide trifluoromethanesulfinate esters.

Vibrational analysis of acetylcholine binding to the M2 receptor.

The M2 muscarinic acetylcholine receptor (M2R) is a prototypical G protein-coupled receptor (GPCR) that responds to acetylcholine (ACh) and mediates various cellular responses in the nervous system. We recently established Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy for ligand binding to M2R reconstituted in lipid membranes, paving the way to understand the mechanism in atomic detail. However, the obtained difference FTIR spectra upon ligand binding contained ligand, protein, lipid, and water signals, so a vibrational assignment was needed for a thorough understanding. In the present study, we compared difference FTIR spectra between unlabeled and 2-13C labeled ACh, and assigned the bands at 1741 and 1246 cm-1 as the C[double bond, length as m-dash]O and C-O stretches of ACh, respectively. The C[double bond, length as m-dash]O stretch of ACh in M2R is close to that in aqueous solution (1736 cm-1), and much lower in frequency than the free C[double bond, length as m-dash]O stretch (1778-1794 cm-1), indicating a strong hydrogen bond, which probably formed with N4046.52. We propose that a water molecule bridges ACh and N4046.52. The other ACh terminal is positively charged, and it interacts with negatively charged D1033.32. The present study revealed that D1033.32 is deprotonated (negatively charged) in both ACh-bound and free states, a suggested mechanism to stabilize the negative charge of D1033.32 in the free M2R.

Synthesis of Chiral gem-Difluoromethylene Compounds by Enantioselective Ethoxycarbonyldifluoromethylation of MBH Fluorides via Silicon-Assisted C-F Bond Activation.

The enantioselective ethoxycarbonyldifluoromethylation of Morita-Baylis-Hillman (MBH) fluorides with Me3SiCF2CO2Et under organocatalysis is described. Moderately functionalized chiral gem-difluoromethylene compounds with a stereogenic "C-CF2-C*" unit were synthesized in high yields with high enantioselectivities. The initial C-F bond activation is assisted by the silicon atom via a dual SN2'-SN2' stepwise pathway. Dynamic kinetic resolution of the MBH-fluorides explained the high yields and high ee's of the products. The method was extended to the enantioselective introduction of "Het-CF2" units.

3-(Phenethylamino)demethyl(oxy)aaptamine as an anti-dormant mycobacterial substance: Isolation, evaluation and total synthesis.

3-(Phenethylamino)demethyl(oxy)aaptamine (1) was re-discovered from the marine sponge of Aaptos sp. as an anti-dormant mycobacterial substance through the bioassay-guided separation. Compound 1 showed potent anti-microbial activity against Mycobacterium bovis BCG with a minimum inhibitory concentration of 0.75 µg/mL under both aerobic conditions and hypoxic conditions inducing dormant state. Compound 1 was also effective against pathogenic M. tuberculosis strains including clinical multidrug-resistant strains. Furthermore, the successful total syntheses of 1 and its analog 3-aminodemethyl(oxy)aaptamine (2) afford sufficient quantities for further biological studies.

Contribution of Organofluorine Compounds to Pharmaceuticals.

Inspired by the success of fluorinated corticosteroids in the 1950s and fluoroquinolones in the 1980s, fluorine-containing pharmaceuticals, which are also known as fluoro-pharmaceuticals, have been attracting attention for more than half of a century. Presently, about 20% of the commercial pharmaceuticals are fluoro-pharmaceuticals. In this mini-review, we analyze the prevalence of fluoro-pharmaceuticals in the market and categorize them into several groups based on the chemotype of the fluoro-functional groups, their therapeutic purpose, and the presence of heterocycles and/or chirality to highlight the structural motifs, patterns, and promising trends in fluorine-based drug design. Our database contains 340 fluoro-pharmaceuticals, from the first fluoro-pharmaceutical, Florinef, to the latest fluoro-pharmaceuticals registered in 2019 and drugs that have been withdrawn. The names and chemical structures of all the 340 fluorinated drugs discussed are provided in the Supporting Information.

Studies of Halogen Bonding Induced by Pentafluorosulfanyl Aryl Iodides: A Potential Group of Halogen Bond Donors in a Rational Drug Design.

The activation of halogen bonding by the substitution of the pentafluoro-λ6-sulfanyl (SF5) group was studied using a series of SF5-substituted iodobenzenes. The simulated electrostatic potential values of SF5-substituted iodobenzenes, the ab initio molecular orbital calculations of intermolecular interactions of SF5-substituted iodobenzenes with pyridine, and the 13C-NMR titration experiments of SF5-substituted iodobenzenes in the presence of pyridine or tetra (n-butyl) ammonium chloride (TBAC) indicated the obvious activation of halogen bonding, although this was highly dependent on the position of SF5-substitution on the benzene ring. It was found that 3,5-bis-SF5-iodobenzene was the most effective halogen bond donor, followed by o-SF5-substituted iodobenzene, while the m- and p-SF5 substitutions did not activate the halogen bonding of iodobenzenes. The similar ortho-effect was also confirmed by studies using a series of nitro (NO2)-substituted iodobenzenes. These observations are in good agreement with the corresponding Mulliken charge of iodine. The 2:1 halogen bonding complex of 3,5-bis-SF5-iodobenzene and 1,4-diazabicyclo[2.2.2]octane (DABCO) was also confirmed. Since SF5-containing compounds have emerged as promising novel pharmaceutical and agrochemical candidates, the 3,5-bis-SF5-iodobenzene unit may be an attractive fragment of rational drug design capable of halogen bonding with biomolecules.

Target Identification of the Marine Natural Products Dictyoceratin-A and -C as Selective Growth Inhibitors in Cancer Cells Adapted to Hypoxic Environments.

Hypoxia-adapted cancer cells in tumors contribute to the pathological progression of cancer. The marine spongean sesquiterpene phenols dictyoceratin-A (1) and -C (2) have been shown to induce hypoxia-selective growth inhibition in cultured cancer cells and exhibit in vivo antitumor effects. These compounds inhibit the accumulation of hypoxia-inducible factor-1α (HIF-1α), which is a drug target in hypoxia-adapted cancer cells, under hypoxic conditions. However, the target molecules of compounds 1 and 2, which are responsible for decreasing HIF-1α expression under hypoxic conditions, remain unclear. In this study, we synthesized probe molecules for compounds 1 and 2 to identify their target molecules and found that both compounds bind to RNA polymerase II-associated protein 3 (RPAP3), which is a component of the R2TP/Prefoldin-like (PEDL) complex. In addition, RPAP3-knockdown cells showed a phenotype similar to that of compound-treated cells.

Asymmetric Electrophilic Difluoromethylthiolation of Indanone-Based ß-Keto Esters Using Difluoromethanesulfonyl Hypervalent Iodonium Ylides.

The first electrophilic diastereoselective direct introduction of the difluoromethylthio group is described. We used a chiral auxiliary-based approach to illustrate the versatility of our recently developed difluoromethanesulfonyl hypervalent iodonium ylide reagents for the difluoromethylthiolation of indanone-based ß-keto esters. Chiral SCF2H-featuring compounds were obtained in up to 93% ee value.

Design and synthesis of galactose-conjugated fluorinated and non-fluorinated proline oligomers: towards antifreeze molecules.

Galactose-conjugated fluorinated and non-fluorinated proline oligomers that exhibit an α-helical structure with hydrophilic and lipophilic parts were designed as potential antifreeze molecules. These galactose-proline oligomers were synthesized and their physical properties were evaluated. Interestingly, the non-fluorinated galactose-proline oligomers showed in contrast to the fluorinated analogues weak antifreeze activity. The difference in antifreeze activity should be attributed to the fluorine gauche effect, which should induce a conformation in fluorinated prolines that is different from that of natural proline. The results obtained in this study thus suggest that the 3D conformation of the galactose-conjugated fluorinated and non-fluorinated proline oligomers is very important for their anti-freezing properties.