Synthesis of new optically active propargylic fluorides and application to the enantioselective synthesis of monofluorinated analogues of fatty acid metabolites

A new approach to obtain optically active unsaturated or polyunsaturated systems with a single fluorine atom in an allylic or propargylic position is reported. Central to this strategy is the high regio- and stereocontrol observed during the fluorination of propargylic alcohols allowing a short and efficient synthesis of 1. Further, simple functional group transformations gave the enals 2 and 3. These three key intermediates were used for the preparation of optically active monofluorinated analogues of fatty acid metabolites.     

Stereoselective Synthesis of Fluorinated Galactopyranosides as Potential Molecular Probes for Galactophilic Proteins: Assessment of Monofluorogalactoside–LecA Interactions

The replacement of hydroxyl groups by fluorine atoms on hexopyranoside scaffolds may allow access to invaluable tools for studying various biochemical processes. As part of ongoing activities toward the preparation of fluorinated carbohydrates, a systematic investigation involving the synthesis and biological evaluation of a series of mono- and polyfluorinated galactopyranosides is described. Various monofluorogalactopyranosides, a trifluorinated, and a tetrafluorinated galactopyranoside have been prepared using a Chiron approach. Given the scarcity of these compounds in the literature, in addition to their synthesis, their biological profiles were evaluated. Firstly, the fluorinated compounds were investigated as antiproliferative agents using normal human and mouse cells in comparison with cancerous cells. Most of the fluorinated compounds showed no antiproliferative activity. Secondly, these carbohydrate probes were used as potential inhibitors of galactophilic lectins. The first transverse relaxation-optimized spectroscopy (TROSY) NMR experiments were performed on these interactions, examining chemical shift perturbations of the backbone resonances of LecA, a virulence factor from Pseudomonas aeruginosa. Moreover, taking advantage of the fluorine atom, the ¹⁹F NMR resonances of the monofluorogalactopyranosides were directly monitored in the presence and absence of LecA to assess ligand binding. Lastly, these results were corroborated with the binding potencies of the monofluorinated galactopyranoside derivatives by isothermal titration calorimetry experiments. Analogues with fluorine atoms at C-3 and C-4 showed weaker affinities with LecA as compared to those with the fluorine atom at C-2 or C-6. This research has focused on the chemical synthesis of “drug-like” low-molecular-weight inhibitors that circumvent drawbacks typically associated with natural oligosaccharides.     

Organocatalytic strategy for hydrophenolation of gem-difluoroalkenes

Gem-difluoroalkenes are an easily accessed fluorinated functional group, and a useful intermediate for elaborating into more complex fluorinated compounds. Currently, most functionalization reactions of gem-difluoroalkenes, with or without a transition metal-based catalyst system, involve the addition or removal of a fluorine atom to generate trifluorinated or monofluorinated products, respectively. In contrast, we present a complementary “fluorine-retentive” reaction that exploits an organocatalytic strategy to add phenols across gem-difluoroalkenes to deliver β,β-difluorophenethyl arylethers. The products are produced in good to moderate yields and selectivities, thus providing a range of compounds that are underrepresented in the synthetic and medicinal chemistry literature.     

Nematic mesophase enhanced via lateral monofluorine substitution on benzoxazole-liquid crystals

Fluorine is widely used as a lateral substituent to modify the physical properties of liquid crystals. Here, laterally monofluorinated compounds, 2-(4?-alkoxy-2-fluorobiphenyl-4-yl)-benzoxazole derivatives (nPPF(2)Bx) bearing different substituents (H, CH₃, NO₂, coded as nPPF(2)BH, nPPF(2)BM and nPPF(2)BN, respectively) at 5-position, were synthesised and characterised. It is interesting to note that these only display enantiotropic nematic mesophases with mesophase ranges of 12–28°C and 13–45°C on heating and cooling for nPPF(2)BH, 46–97°C and 62–120°C for nPPF(2)BM and 82–108°C and 87–113°C for nPPF(2)BN, which are very different from the corresponding monofluorine-substituted analogue (compounds I) with enantiotropic smectic or smectic/nematic mesophases. The enhanced nematic mesophase is attributed to the reduced π–π interaction/conjugation resulting from the twisted structure of the molecule caused by the introduction of a fluorine atom into the inter-ring of the biphenyl unit. These results suggest that modification of the monofluorine substituent position is an effective method to improve the nematic mesophase in benzoxazole-liquid crystals.     

Synthesis of monofluorinated isofagomine analogues and evaluation as glycosidase inhibitors

The straightforward synthesis of monofluorinated isofagomine analogues 1-3 was described. The synthetic strategy featured that the chiral carbon center bearing fluorine atom was constructed stereoselectively via silicon-induced Reformatskii-Claisen rearrangement of allyl bromofluoroacetate. These compounds were tested for inhibition of five glycosidases. The 3S,4R,5R isomer 3 has been found to be a potent inhibitor against β-glucosidase from almonds with K_i value of 11.9 μM.     

Fragmentation of carbohydrate anomeric alkoxyl radicals: new synthesis of chiral 1-fluoro-1-halo-1-iodoalditols

A new general methodology for the synthesis of 1,1,1-trihaloalditols by starting from 1,5-anhydro-2-deoxy-hex-1-enitol derivatives (glycals) is described. The halogens are introduced sequentially in each of the three different steps of the process. The fluorine is introduced in the first step by electrophilic fluorination of the starting glycal; next, hydroxyhalogenation of the resulting vinyl fluoride allows the addition of any halogen (F, Cl, Br or I) at will, and finally, an iodine atom is inserted through an alkoxyl radical fragmentation reaction. This methodology allows the preparation of diverse types of 1,1,1-trihalogenated compounds (R--CF(2)I, R--CFI(2), R--CFClI and R--CFBrI) under mild conditions compatible with sensitive substituents. In some cases, the diastereomeric mixtures generated from R--CFClI and R--CFBrI can be chromatographically separated, and their configuration determined by X-ray crystallographic analysis. The synthetic usefulness of these compounds has been preliminarily assessed by examining the reactivity of the fluorinated radical generated by rupture of the C--I bond.     

电化学氟化的新进展

电化学氟化是利用电极反应将氟直接引入有机或无机物的一种重要方法。有机物的电化学氟化方法有两种: 一是Simons于1941年发明的, 用镍作阳极, 在无水氟化氢溶液中, 电解制备全氟化合物的方法。此方法在近年来有不少改进。另一是1970年Rozhkov提出的, 以有机溶剂(如含Et3N.3HF的MeCN)为介质, 在铂阳极上,氧化得到单氟化产物的方法。本文全面地综述了这两种方法, 并着重于最新民发展。     

Comprehensive structural studies of 2',3'-difluorinated nucleosides: comparison of theory, solution, and solid state

The conformations of three 2',3'-difluoro uridine nucleosides were studied by X-ray crystallography, NMR spectroscopy, and ab initio calculations in an attempt to define the roles that the two vicinal fluorine atoms play in the puckering preferences of the furanose ring. Two of the compounds examined contained fluorine atoms in either the arabino or xylo dispositions at C2' and C3' of a 2',3'-dideoxyuridine system. The third compound also incorporated fluorine atoms in the xylo configuration on the furanose ring but was substituted with a 6-azauracil base in place of uracil. A battery of NMR experiments in D 2O solution was used to identify conformational preferences primarily from coupling constant and NOE data. Both (1)H and (19)F NMR data were used to ascertain the preferred sugar pucker of the furanose ring through the use of the program PSEUROT. Compound-dependent parameters used in the PSEUROT calculations were newly derived from complete sets of conformations calculated from high-level ab initio methods. The solution and theoretical data were compared to the conformations of each molecule in the solid state. It was shown that both gauche and antiperiplanar effects may be operative to maintain a pseudodiaxial arrangement of the C2' and C3' vicinal fluorine atoms. These data, along with previously reported data by us and others concerning monofluorinated nucleoside conformations, were used to propose a model of how fluorine influences different aspects of nucleoside conformations.     

Catalytic enantioselective fluorination of oxindoles

We have developed a highly efficient catalytic enantioselective fluorination of oxindole derivatives. In the presence of a catalytic amount of chiral Pd complex 2 (2.5 mol %), various substrates, including aryl- and alkyl-substituted oxindoles, were fluorinated in a highly enantioselective manner (up to 96% ee). In addition, when R was a hydrogen atom, enantioselective fluorination followed by solvolysis gave a monofluorinated ester with up to 93% ee. To our knowledge, this is the first example of catalytic enantioselective fluorination of oxindoles.     

Electrolytic partial fluorination of organic compounds. Part 51: Electrochemical fluorination of 1-methylpyrroles having electron-withdrawing groups at the 2-position: Effect of supporting fluoride salts on the fluorinated product selectivity

Anodic fluorination of 1-methylpyrroles having electron-withdrawing groups at the 2-position was comparatively studied using Et₃N•3HF and Et₃N•2HF as a supporting electrolyte and a fluorine source. The use of Et₃N•3HF gave trifluorinated products predominantly or selectively depending on solvents used while the use of Et₃N•2HF provided the corresponding monofluorinated pyrrole derivatives in considerable amount along with the trifluorinated products regardless of the solvents. This is the first successful example of selective anodic fluorination of pyrroles. The effects of supporting electrolyte on product selectivity are discussed.     

Synthesis of 1-substituted 3-nitroquinolin-4(1H)-ones

A versatile synthetic method for preparing 1-substituted 3-nitroquinolin-4(1H)-ones from corresponding 2-fluoro-α-nitroacetophenones is demonstrated by the synthesis of 6,7-difluoro derivatives 7a-c . The method involves sequential treatment of the starting nitroacetophenone with triethyl orthoformate and the appropriate amine, followed by a nucleophilic cyclization reaction under mild conditions. The C-7 fluorine atom of 7 can be displaced by cyclic amines. Substituted 6-fluoro-7-(4-methyl-1-piperazinyl)-3-nitroquinolin-4(1H)-ones 8a-c were prepared in this way.     

Liquid-phase fluorination of aromatic compounds by elemental fluorine

The fluorination of aromatic compounds (benzene, toluene, phenol and benzoic acid) by elemental fluorine diluted with nitrogen has been investigated in various solvents (Freon 11, chloroform, methanol, trifluoroacetic acid, 2,2,2-trifluoroethanol, water) in order to define the influence of the experimental conditions on the reaction. Experiments have been carried out by varying the temperature, the substrate concentration in solution, the molar ratio of fluorine to substrate, and the concentration of fluorine in the fluorine/nitrogen mixture. In all cases, the effects on the yield of fluorinated products were studied. Monofluorinated compounds were mainly found in the reaction mixture, the isomers formed being in accord with the mechanism for electrophilic substitution. The highest yield of monofluorinated products was obtained with polar solvents and the following order was found: CFCl₃ < CHCl₃ < CH₃OH < CF₃CH₂OH < CF₃COOH. Interesting results were also found using particular additives (for instance, KOH or C₄F₉SO₃Na in methanol) or water as the solvent. A direct relationship was observed between the yield of monofluorinated compounds and the molar ratio of fluorine to substrate, which has to be less than one in order to obtain high yields. In contrast, low selectivity, expressed as the yield ratio of ortho to para (or meta) isomers, was found.     

Heats of formation of krypton fluorides and stability predictions for KrF4 and KrF6 from high level electronic structure calculations

Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for KrF+, KrF-, KrF2, KrF3+, KrF4, KrF5+, and KrF6 from coupled-cluster theory (CCSD(T)) calculations with effective core potential correlation-consistent basis sets for krypton. To achieve near chemical accuracy (+/-1 kcal/mol), three corrections were added to the complete basis set binding energies based on frozen core coupled-cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, and a correction for first-order atomic spin-orbit effects. Vibrational zero point energies were computed at the coupled-cluster level of theory. The calculated value for the heat of formation of KrF2 is in excellent agreement with the experimental value. Contrary to the analogous xenon fluorides, KrF2, KrF4, and KrF6 are predicted to be thermodynamically unstable with respect to loss of F2. An analysis of the energetics of KrF4 and KrF6 with respect to fluorine atom loss together with calculations of the transition states for the intramolecular loss of F2 show that fluorine atom loss is the limiting factor determining the kinetic stabilities of these molecules. Whereas KrF4 possesses a marginal energy barrier of 10 kcal/mol toward fluorine atom loss and might be stable at moderately low temperatures, the corresponding barrier in KrF6 is only 0.9 kcal/mol, suggesting that it could exist only at very low temperatures. Although the simultaneous reactions of either two or four fluorine atoms with KrF2 to give KrF4 or KrF6, respectively, are exothermic, they do not represent feasible synthetic approaches because the attack of the fluorine ligands of KrF2 by the fluorine atoms, resulting in F2 abstraction, is thermodynamically favored over oxidative fluorination of the krypton central atom. Therefore, KrF6 could exist only at very low temperatures, and even the preparation of KrF4 will be extremely difficult.     

QTAIM charge-charge flux-dipole flux interpretation of electronegativity and potential models of the fluorochloromethane mean dipole moment derivatives

Infrared fundamental vibrational intensities and quantum theory atoms in molecules (QTAIM) charge-charge flux-dipole flux (CCFDF) contributions to the polar tensors of the fluorochloromethanes have been calculated at the QCISD/cc-pVTZ level. A root-mean-square error of 20.0 km mol(-1) has been found compared to an experimental error estimate of 14.4 and 21.1 km mol(-1) for MP2/6-311++G(3d,3p) results. The errors in the QCISD polar tensor elements and mean dipole moment derivatives are 0.059 e when compared with the experimental values. Both theoretical levels provide results showing that the dynamical charge and dipole fluxes provide significant contributions to the mean dipole moment derivatives and tend to be of opposite signs canceling one another. Although the experimental mean dipole moment derivative values suggest that all the fluorochloromethane molecules have electronic structures consistent with a simple electronegativity model with transferable atomic charges for their terminal atoms, the QTAIM/CCFDF models confirm this only for the fluoromethanes. Whereas the fluorine atom does not suffer a saturation effect in its capacity to drain electronic charge from carbon atoms that are attached to other fluorine and chlorine atoms, the zero flux electronic charge of the chlorine atom depends on the number and kind of the other substituent atoms. Both the QTAIM carbon charges (r = 0.990) and mean dipole moment derivatives (r = 0.996) are found to obey Siegbahn's potential model for carbon 1s electron ionization energies at the QCISD/cc-pVTZ level. The latter is a consequence of the carbon mean derivatives obeying the electronegativity model and not necessarily to their similarities with atomic charges. Atomic dipole contributions to the neighboring atom electrostatic potentials of the fluorochloromethanes are found to be of comparable size to the atomic charge contributions and increase the accuracy of Siegbahn's model for the QTAIM charge model results. Substitution effects of the hydrogen, fluorine, and chlorine atoms on the charge and dipole flux QTAIM contributions are found to be additive for the mean dipole derivatives of the fluorochloromethanes.     

Rearrangement routes to selectively fluorinated compounds

Fluoroallylic alcohols are easy to synthesise by a variety of routes using one or two carbon fluorinated building blocks. Sigmatropic rearrangement then transforms these intermediates into species in which the fluorine atom (or atoms) is borne on an sp³-hybridised carbon. Alternatively, allylic alcohols can be transformed into fluorinated vinyl ether derivatives; rearrangement then affords products in which the fluorine atoms occupy a different and complementary location with respect to a carbonyl function.     

AIM Study on the Reaction of CH2SH Radical with Fluorine Atom

The reaction of CH2SH radical with fluorine atom was studied at the levels of B3LYP/6-311G(d,p) and MP2(Full)/6-311G(d,p). The computational results show that the reaction has three channels and proceeds by the addition of fluorine atoms on carbon or sulfur sites of CH2SH, forming initial intermediates. The calculated results show that the channel in which fluorine attaches to the carbon atom to form CH2S and HF, is the most likely reaction pathway. Topological analysis of electron density was carried out for the three channels. The change trends of the chemical bonds on the reaction paths were discussed. The energy transition states and the structure transition regions (states) of the three channels were found. The calculated results show that the structure transition regions are broad in unobvious exothermic reactions or unobvious endothermic reactions, and are narrow in obvious exothermic reactions or obvious endothermic reactions.     

Effect of fluoro substituents and central atom nature on chiral derivatives of bisdiphenylborates and isoelectronic structures

A DFT study of the geometrical, electronic, and energetic parameters of homo- and heterochiral complexes of fluoro derivatives of diphenylborate and isoelectronic structures where the central atom has been substituted by carbon and nitrogen has been carried out. The results have allowed correlation of the relative energies (hetero- minus homochiral complex) versus the presence of fluorine in the different positions on the aromatic rings and the presence of different central atoms. Other correlations between geometrical parameters and the relative energy have also been found. Some of the predictions have been confirmed with the calculation of new derivatives.     

Selective electrophilic monofluorination on rings of various sizes using elemental fluorine

Elemental fluorine substitutes tertiary unactivated hydrogens in an electrophilic mode. This unorthodox substitution depends on the atomic charge density, on the hydrogen atom and on the p-orbital contribution on the CH bond. This is demonstrated by reacting F₂ with tertiary CH bonds located on rings of various sizes, producing the corresponding tertiary fluorine derivatives.     

Gaseous SF(3)(+): An efficient electrophilic monofluorinating agent for five-membered heteroaromatic compounds

Reactions of gaseous SF(3)(+) ions with furan, thiophene, pyrrole, and several of their alkyl derivatives were performed via MS(2) experiments and found to occur readily both by electron abstraction and F(+) transfer. Then, by performing MS(3) experiments, the F(+) transfer products-the protonated monofluorinated molecules-were mass-selected and deprotonated by a second reaction with a stronger base. F(+) transfer from gaseous SF(3)(+) followed by deprotonation promotes therefore C-H by C-F replacement in five-membered heteroaromatic compounds and the efficient gas-phase synthesis of their neutral monofluorinated derivatives.     

Comparison by mass spectrometry of ring cleavage in barbituric acid and 5-fluorobarbituric acid derivatives

The mass spectrometric behaviour of barbituric acid and 1,3-dimethylbarbituric acid was compared with that of the corresponding 5,5-difluoro derivatives and of some 5-fluoro-5-alkyl derivatives in order to study the influence of fluorine in the fragmentation processes. This investigation, performed using both electron impact ionization and positive- and negative-ion fast atom bombardment, evidences well the role of fluorine in barbiturate ring bond cleavages.