Synthesis of selectively fluorinated cyclohexanes : the observation of phenonium rearrangements during deoxyfluorination reactions on cyclohexane rings with a vicinal phenyl substituent

A study focused on the synthesis of a derivative of all-cis phenyl-2,3,5,6-tetrafluorocyclohexane, incorporating a methyl group at the benzylic position, found that the fluorination reactions of diepoxide and diol precursors, were susceptible to rearrangement and unexpected products. The origin of these rearranged products can be rationalised by aryl migrations occurring via phenonium ion intermediates, in adventitious pathways occurring during deoxofluorination reactions with hydrogen fluoride reagents.PostprintPeer reviewe

Multicomponent reactions of methyl substituted all-cis tetrafluorocyclohexane aldehydes

The authors thank the EPSRC for supporting this work and DO’H is grateful for a Royal Society Wolfson Research Merit Award.This paper reports the preparation of methyl substituted all-cis tetrafluorocyclohexanes prepared from a Birch reduction of benzoic acid, worked up with a methyl iodide quench. The resultant methylcyclohexadiene carboxylic acid was reduced to the alcohol and protected. The cyclohexadienyl ring was then epoxidised and the C-O bonds sequentially converted through deoxyfluorination reactions to two sets of isomers of all-cis tetrafluorocyclohexane isomers. Deprotection of the benzylic ether and then oxidation gave aldehydes which were then used in Ugi and Passerini multicomponent reactions, allowing this facially polarised cyclohexane to be incorporated into peptidic structural motifs.Publisher PDFPeer reviewe

Polar alicyclic rings: synthesis and structure of all cis-1,2,3,4-tetrafluorocyclopentane

The authors are grateful to the Chinese Scholarship Council for Studentship support (ZF), to EPSRC for financial support and to the EPSRC National Mass Spectrometry Service at Swansea for analytical support. DO’H thanks the Royal Society for a Wolfson Research Merit Award.The all-cis isomer of 1,2,3,4-tetrafluorocyclopentane is prepared and characterised by NMR and X-ray crystallography and the experimental structure compared with theory. The structure has a similarly high polarity to all-cis tetrafluorocyclohexanes despite the increased conformational flexibility of a cyclopentane.PostprintPeer reviewe

Fluorinated liquid crystals: evaluation of selectively fluorinated facially polarised cyclohexyl motifs for liquid crystal applications

The authors thank the European Research Council for and Advanced Grant and DO’H acknowledges the Royal Society for a Wolfson Merit Award.This paper explores the synthesis of a series of prototype negative dielectric liquid crystalline (LC) compounds which contain fluorinated cyclohexane motifs. The series are analogues and differ only in that they contain between one to four fluorine atoms. The stereochemistry is such that all of the fluorines/C-F bonds are on one face of the cyclohexane ring. This follows from the recent recognition that cyclohexanes with fluorines orientated in one direction, perpendicular to the ring have a strong molecular dipole, a characteristic that might be an advantage in the design of negative dielectric materials. However it is found that the increased polarity, particularly with two or more oriented C-F bonds, leads to higher melting points and poorer solubility in test matrix formulations, relative to hydrocarbon liquid crystals. This arises due to electrostatic intermolecular interactions between the polarised cyclohexyl and aryl rings. The study highlights that in order to take advantage of these polar cyclohexane motifs for liquid crystal design, appropriate scaffolds are required which are free of aromatic rings and contain peripheral solubilising motifs.PostprintPostprintPeer reviewe

Fluorine induced pseudo-anomeric effects in methoxycyclohexanes through electrostatic 1,3-diaxial interactions

Authors thank FAPESP, CONFAP and The UK Academies for a São Paulo International Research Collaboration (FAPESP #2019/05028-7). FAPESP is also gratefully acknowledge for an undergraduate fellowship to BAP (#2019/03855-3), and a Young Research Award to RAC (#2018/03910-1). The Chinese Scholarship Council (CSC) is thanked for a Studentship (CY). Finally CENAPAD-SP, CESUP and SDumont are acknowledged for computational resources used in theory calculations.We report counter‐intuitive axial preferences in non‐stereochemically biased, selectively fluorinated methoxycyclohexanes. These pseudo‐anomeric effects are apparent when electronegative CF2 groups are placed at the C‐2, C‐4 and C‐6 positions of the cyclohexane ring to render the C‐3/5 axial hydrogen atoms electropositive. The electrostatic interaction between these axial hydrogen atoms and the ‐OMe oxygen is stabilising. The effect is explored using high‐level ab initio and DFT calculations in the framework of NBO, QTAIM and NCI analysis across a range of derivatives, and experimentally (19F{1H}‐NMR at −80 °C) for some illustrative examples. The effect is significant in energy terms for a weak interaction, and illustrates a new stereoelectronic aspect attributed to selective fluorine substitution in organic chemistry.Publisher PDFPeer reviewe

Synthesis of organic liquid crystal containing selectively fluorinated cyclopropanes

Authors thank the Chinese Scholarship Council for a studentship (ZF) and the EPSRC Mass Spectroscopy Service at Swansea for analysis.This paper describes the synthesis of a series of organic liquid crystals (LC) containing selectively fluorinated cyclopropanes at their termini. The syntheses used difluorocarbene additions to olefin precursors, an approach which proved straightforward such that these liquid crystal candidates could be efficiently prepared. Their physical and thermodynamic properties were evaluated and depending on individual structures, they either displayed positive or negative dielectric anisotropy. The study gives some guidance into effective structure property relationships for the design of LCs containing selectively fluorinated cyclopropane motifs.Publisher PDFPeer reviewe

Synthesis of aryl α,α-difluoroethyl thioethers a novel structure motif in organic chemistry, and extending to aryl α,α-difluoro oxyethers

The authors thank the Marie Curie ITN programme for Funding (AR).A method for the preparation of aryl α,α-difluoroethyl thioethers (ArSCF2CH3) is reported and the synthesis approach is extended to aryl alpha,alpha-difluoroethyl oxygen ethers. Selected building blocks are further elaborated in cross-coupling reactions and are incorporated into analogues of established trifluoromethyl ether drugs. Conformations are explored and log P studies of these motifs indicate that they are significantly more polar than their trifluoromethyl ether analogues rendering them attractive for bioactives discovery.PostprintPostprintPeer reviewe

Fluorine-containing substituents : metabolism of the α,α-difluoroethyl thioether motif

The Marie Curie-ITN Programme (FLUOR-21: Synthesis structure and function of fluorinated systems) is thanked for a Studentship (AR) and financial support.We report the metabolism of the recently introduced α,α-difluoroethyl thioether motif to explore further its potential as a substituent for bioactives discovery chemistry. Incubation of two aryl–SCF2CH3 ethers with the model yeast organism Cunninghamella elegans, indicates that the sulfur of the thioether is rapidly converted to the corresponding sulfoxide, and then significantly more slowly to the sulfone. When the substrate was (p-OMe)PhSCF2CH3, then the resultant (demethylated) phenol sulfoxide had an enantiomeric excess of 60%, and when the substrate was the β-substituted-SCF2CH3 naphthalene, then the enantiomeric excess of the resultant sulfoxide was 54%. There was no evidence of defluorination, unlike the corresponding oxygen ether (p-OMe)PhOCF2CH3, which was converted to the (demethylated) phenol acetate ester during C. elegans incubation. We conclude that the aryl–S–CF2CH3 motif is metabolised in a similar manner to aryl–SCF3, a motif that is being widely explored in discovery chemistry. It is however, significantly less lipophilic than aryl-SCF3 which may offer a practical advantage in tuning overall pharmacokinetic profiles of molecules in development.Publisher PDFPeer reviewe

Incorporation of [2H1]-(1R,2R)- and [2H1]-(1S,2R)-glycerols into the antibiotic nucleocidin in Streptomyces calvus

The authors thank the Chinese Scholarship Council for Ph.D Studentship support (to XF).Deuterium incorporations from [2H1]-(1R,2R) and [2H1]-(1S,2R) glycerols into the fluorine containing antibiotic nucleocidin, in Streptomyces calvus indicate that one deuterium atom is incorporated at the C-5′ site of nucleocidin from each of these isotopomers of glycerol. Two deuteriums become incorporated at C-5′ of nucleocidin after a feeding experiment with [2H5]-glycerol. These observations indicate that there is no obligate oxidation of the pro-R hydroxymethyl group of glycerol as it progresses through the pentose phosphate pathway and becomes incorporated into the fluorinated antibiotic.PostprintPeer reviewe

Unexpected triaxial preferences in some all-syn 1,3,5-trifluorocyclohexanes

Authors thank the Chinese Scholarship Council and FAPESP for studentships (BAP, #2021/09716-5) a Young Researcher Award (RAC, #2018/03910-1) and FAEPEX for a fellowship (RAC, #2479/21).Theory and solution NMR indicate that all-syn 1,3,5-trifluorocyclohexane 5 adopts the expected tri-equatorial conformation, however in the solid state the more polar triaxial conformation is observed. This and the favoured conformations of substituted (Me, OMe, NH(CO)Me, NHBoc) derivatives of 5 are investigated to explore triaxial C–F preferences.Publisher PDFPeer reviewe