Fluorinated Quinine Alkaloids: Synthesis, X-ray Structure Analysis and Antimalarial Parasite Chemotherapy

Fluorinated Rings: Conformation and Application

The introduction of fluorine atoms into molecules and materials across many fields of academic and industrial research is now commonplace, owing to their unique properties. A particularly interesting feature is the impact of fluorine substitution on the relative orientation of a C-F bond when incorporated into organic molecules. In this Review, we will be discussing the conformational behavior of fluorinated aliphatic carbo- and heterocyclic systems. The conformational preference of each system is associated with various interactions introduced by fluorine substitution such as charge-dipole, dipole-dipole, and hyperconjugative interactions. The contribution of each interaction on the stabilization of the fluorinated alicyclic system, which manifests itself in low conformations, will be discussed in detail. The novelty of this feature will be demonstrated by presenting the most recent applications.

Fluoropolymers in biomedical applications: state-of-the-art and future perspectives

Biomedical applications of fluoropolymers in gene delivery, protein delivery, drug delivery, ¹⁹F MRI, PDT, anti-fouling, anti-bacterial, cell culture, and tissue engineering.

Fluorinated scaffolds for antimalarial drug discovery

INTRODUCTION

The unique physicochemical properties and chemical diversity of organofluorine compounds have remarkably contributed for their wide utility in the area of pharmaceuticals, materials and agrochemicals. The noteworthy characteristics of fluorine include high electron affinity, lipophilicity and bioavailability, extending the half-life of the drugs. The incorporation of fluorine substituents, particularly trifluoromethyl groups, into organic molecules has led to their high potency against various diseases, including malaria. Hence, organofluorinated molecules offer valuable avenues for the design of new drug candidates against malaria.

AREAS COVERED

In this review, the authors discuss the importance of fluorine substituents present in the chemical compounds, and their potential applications for antimalarial drug discovery.

EXPERT OPINION

Fluorinated molecules represent a reliable strategy to develop new antimalarial drugs. Fluorine or fluorinated groups have been identified as a promising precursor, and their presence in approximately twenty-five percent of approved drugs is notable. Selective fluorination of chemical entities has the potential to be applied not only to improve the activity profile against the malaria parasite, but could be extrapolated for favorable pharmacological applications. Hazardous reagents such as HF, F₂ and SF₄ used for fluorination, are not considered as safe, and therefore, this process remains challenging, particularly for the pharmaceutical industry.

A Cinchona Alkaloid Antibiotic That Appears To Target ATP Synthase in Streptococcus pneumoniae

Optochin, a cinchona alkaloid derivative discovered over 100 years ago, possesses highly selective antibacterial activity toward Streptococcus pneumoniae. Pneumococcal disease remains the leading source of bacterial pneumonia and meningitis worldwide. The structure-activity relationships of optochin were examined through modification to both the quinoline and quinuclidine subunits, which led to the identification of analogue 48 with substantially improved activity. Resistance and molecular modeling studies indicate that 48 likely binds to the c-ring of ATP synthase near the conserved glutamate 52 ion-binding site, while mechanistic studies demonstrated that 48 causes cytoplasmic acidification. Initial pharmacokinetic and drug metabolism analyses of optochin and 48 revealed limitations of these quinine analogues, which were rapidly cleared, resulting in poor in vivo exposure through hydroxylation pendants to the quinuclidine and O-dealkylation of the quinoline. Collectively, the results provide a foundation to advance 48 and highlight ATP synthase as a promising target for antibiotic development.

Cinchona Alkaloids-Derivatives and Applications

Major Cinchona alkaloids quinine, quinidine, cinchonine, and cinchonidine are available chiral natural compounds (chiral pool). Unlike many other natural products, these alkaloids are available in multiple diastereomeric forms which are separated on an industrial scale. The introduction discusses in short conformational equilibria, traditional separation scheme, biosynthesis, and de novo chemical syntheses. The second section concerns useful chemical applications of the alkaloids as chiral recognition agents and effective chiral catalysts. Besides the Sharpless ethers and quaternary ammonium salts (chiral PTC), the most successful bifunctional organocatalysts are based on 9-amino derivatives: thioureas and squaramides. The third section reports the main transformations of Cinchona alkaloids. This covers reactions of the 9-hydroxyl group with the retention or inversion of configuration. Specific Cinchona rearrangements enlarging [2.2.2]bicycle of quinuclidine to [3.2.2] products are connected to the 9-OH substitution. The syntheses of numerous esterification and etherification products are described, including many examples of bi-Cinchona alkaloid ethers. Further derivatives comprise 9-N-substituted compounds. The amino group is introduced via an azido function with the inversion of configuration at the stereogenic center C9. The 9-epi-amino-alkaloids provide imines, amides, imides, thioureas, and squaramides. The syntheses of 9-carbon-, 9-sulfur-, and 9-selenium-substituted derivatives are discussed. Oxidation of the hydroxyl group of any alkaloid gives ketones, which can be selectively reduced, reacted with Grignard reagents, or subjected to the Corey-Chaykovsky reaction. The alkaloids were also partially degraded by splitting C4'-C9 or N1-C8 bonds. In order to immobilize Cinchona alkaloids the transformations of the 3-vinyl group were often exploited. Finally, miscellaneous functionalizations of quinuclidine, quinoline, and examples of various metal complexes of the alkaloids are considered.

A Local-Desymmetrization-Based Divergent Synthesis of Quinine and Quinidine

Herein we report a novel synthetic entry to the legendary quinuclidine natural products quinine and quinidine. The developed strategy is based on the use of a symmetrical and nonstereogenic precursor to access quinine and quinidine through a "local-desymmetrization" approach, in stark contrast conceptually to the preparation of stereodefined disubstituted piperidines (or their acyclic precursors) as featured in all past syntheses. The developed strategy also provided quinine and quinidine derivatives that could not be readily obtained through previous total syntheses or by modification of the naturally occurring substances.

Can acyclic conformational control be achieved via a sulfur-fluorine gauche effect?

The gauche conformation of the 1,2-difluoroethane motif is known to involve stabilising hyperconjugative interactions between donor (bonding, σC-H) and acceptor (antibonding, σ*C-F) orbitals. This model rationalises the generic conformational preference of F-Cβ-Cα-X systems (φFCCX ≈ 60°), where X is an electron deficient substituent containing a Period 2 atom. Little is known about the corresponding Period 3 systems, such as sulfur and phosphorus, where multiple oxidation states are possible. Conformational analyses of β-fluorosulfides, -sulfoxides and -sulfones are disclosed here, thus extending the scope of the fluorine gauche effect to the 3rd Period (F-C-C-S(O) n ; φFCCS ≈ 60°). Synergy between experiment and computation has revealed that the gauche effect is only pronounced in structures bearing an electropositive vicinal sulfur atom (S+-O-, SO2).

Quinine conjugates and quinine analogues as potential antimalarial agents

Malaria is a tropical disease, prevalent in Southeast Asia and Africa, resulting in over half a million deaths annually; efforts to develop new antimalarial agents are therefore particularly important. Quinine continues to play a role in the fight against malaria, but quinoline derivatives are more widely used. Drugs based on the quinoline scaffold include chloroquine and primaquine, which are able to act against the blood and liver stages of the parasite's life cycle. The purpose of this review is to discuss reported biologically active compounds based on either the quinine or quinoline scaffold that may have enhanced antimalarial activity. The review emphasises hybrid molecules, and covers advances made in the last five years. The review is divided into three sections: modifications to the quinine scaffold, modifications to aminoquinolines and finally metal-containing antimalarial compounds.

Chemistry without bo(a)rders

Note from the Editor: Science, collegiality, travel without borders. Big smiles and warmth. Eternally youthful vigor. When I think of Tetsuo Nozoe, these are the images that immediately come to mind. When I think of the Nozoe Autograph Books, I also think of time. For 40 years, Nozoe carried with him his autograph books and collected his treasures which The Chemical Record is now publishing. Vladimir Prelog signed first when he was 47 and signed last when he was almost 80. Derek Barton signed first when he was 35 and signed last when he was almost 70. Time! I was fascinated by Eva Wille's recommendation that we publish an essay by Eva-Maria Tanzer, a young organic chemist with already a lifetime's experience in exploring chemical research in laboratories around the world. Tanzer brings to us a charming, even innocent perspective that embraces so much of Tetsuo Nozoe's ideals. We are privileged to include Tanzer's vision and her experiences in the collection of essays that accompany the Nozoe Autograph Books. Our contributors range in age and experiences, from the youthful Tanzer to the more mature (including myself!) as do the signatories in the autograph books. Tetsuo Nozoe would have beamed!--Jeffrey I. Seeman Guest Editor University of Richmond Richmond, Virginia 23173, USA E-mail: jseeman@richmond.edu.

Stereoselective fluorination alters the geometry of a cyclic peptide: exploration of backbone-fluorinated analogues of unguisin A

New methods for enhancing the efficiency of peptide cyclization, and for fine-tuning the conformations of cyclic peptides, are valuable from a drug development perspective. Herein stereoselective fluorination is investigated as a new strategy for achieving these goals. Four vicinal difluorinated analogues of the natural cyclic heptapeptide unguisin A have been efficiently synthesized. The analogues are found to adopt dramatically different secondary structures, controlled by the fluorine stereochemistry.

Modulating NHC catalysis with fluorine

Fluorination often confers a range of advantages in modulating the conformation and reactivity of small molecule organocatalysts. By strategically introducing fluorine substituents, as part of a β-fluoroamine motif, in a triazolium pre-catalyst, it was possible to modulate the behaviour of the corresponding N-heterocyclic carbene (NHC) with minimal steric alterations to the catalyst core. In this study, the effect of hydrogen to fluorine substitution was evaluated as part of a molecular editing study. X-ray crystallographic analyses of a number of derivatives are presented and the conformations are discussed. Upon deprotonation, the fluorinated triazolium salts generate catalytically active N-heterocyclic carbenes, which can then participate in the enantioselective Steglich rearrangement of oxazolyl carbonates to C-carboxyazlactones (e.r. up to 87.0:13.0).

Sequential deoxyfluorination approach for the synthesis of protected α,β,γ-trifluoro-δ-amino acids

Backbone-homologated amino acids have been synthesized, containing three vicinal fluorine atoms placed stereospecifically along the carbon chain. Different trifluoro stereoisomers are found to have contrasting conformations, consistent with known stereoelectronic effects associated with C-F bonds.

Identification of a new class of FtsZ inhibitors by structure-based design and in vitro screening

The Filamenting temperature-sensitive mutant Z (FtsZ), an essential GTPase in bacterial cell division, is highly conserved among Gram-positive and Gram-negative bacteria and thus considered an attractive target to treat antibiotic-resistant bacterial infections. In this study, a new class of FtsZ inhibitors bearing the pyrimidine-quinuclidine scaffold was identified from structure-based virtual screening of natural product libraries. Iterative rounds of in silico studies and biological evaluation established the preliminary structure-activity relationships of the new compounds. Potent FtsZ inhibitors with low micromolar IC₅₀ and antibacterial activity against S. aureus and E. coli were found. These findings support the use of virtual screening and structure-based design for the rational development of new antibacterial agents with innovative mechanisms of action.

Analysis of alkaloids from different chemical groups by different liquid chromatography methods

Alkaloids are biologically active compounds widely used as pharmaceuticals and synthesised as secondary methabolites in plants. Many of these compounds are strongly toxic. Therefore, they are often subject of scientific interests and analysis. Since alkaloids — basic compounds appear in aqueous solutions as ionized and unionized forms, they are difficult for chromatographic separation for peak tailing, poor systems efficiency, poor separation and poor column-to-column reproducibility. For this reason it is necessity searching of more suitable chromatographic systems for analysis of the compounds. In this article we present an overview on the separation of selected alkaloids from different chemical groups by liquid chromatography thus indicating the range of useful methods now available for alkaloid analysis. Different selectivity, system efficiency and peaks shape may be achieved in different LC methods separations by use of alternative stationary phases: silica, alumina, chemically bonded stationary phases, cation exchange phases, or by varying nonaqueous or aqueous mobile phase (containing different modifier, different buffers at different pH, ion-pairing or silanol blocker reagents). Developments in TLC (NP and RP systems), HPLC (NP, RP, HILIC, ion-exchange) are presented and the advantages of each method for alkaloids analysis are discussed.

Fluorine-directed β-galactosylation: chemical glycosylation development by molecular editing

Validation of the 2-fluoro substituent as an inert steering group to control chemical glycosylation is presented. A molecular editing study has revealed that the exceptional levels of diastereocontrol in glycosylation processes by using 2-fluoro-3,4,6-tri-O-benzyl glucopyranosyl trichloroacetimidate (TCA) scaffolds are a consequence of the 2R,3S,4S stereotriad. This study has also revealed that epimerization at C4, results in a substantial enhancement in β-selectivity (up to β/α 300:1).

Designing fluorinated cinchona alkaloids for enantioselective catalysis: controlling internal rotation by a fluorine-ammonium ion gauche effect (φ(NCCF))

The C9 position of cinchona alkaloids functions as a molecular hinge, with internal rotations around the C8-C9 (τ(1)) and C9-C4' (τ(2)) bonds giving rise to four low energy conformers (1; anti-closed, anti-open, syn-closed, and syn-open). By substituting the C9 carbinol centre by a configurationally defined fluorine substituent, a fluorine-ammonium ion gauche effect (σ(C-H) → σ(C-F)*; F(δ-)⋅⋅⋅N(+)) encodes for two out of the four possible conformers (2). This constitutes a partial solution to the long-standing problem of governing internal rotations in cinchonium-based catalysts relying solely on a fluorine conformational effect.

Fluorine conformational effects in organocatalysis: an emerging strategy for molecular design

Molecular design strategies that profit from the intrinsic stereoelectronic and electrostatic effects of fluorinated organic molecules have mainly been restricted to bio-organic chemistry. Indeed, many fluorine conformational effects remain academic curiosities with no immediate application. However, the renaissance of organocatalysis offers the possibility to exploit many of these well-described phenomena for molecular preorganization. In this minireview, we highlight examples of catalyst refinement by introduction of an aliphatic C-F bond which functions as a chemically inert steering group for conformational control.

Theoretical and X-ray crystallographic evidence of a fluorine-imine gauche effect: an addendum to Dunathan's stereoelectronic hypothesis

The preference of β-fluoroimines to adopt a gauche conformation has been studied by single-crystal X-ray diffraction analysis and DFT methods. Empirical and theoretical evidence for a preferential gauche arrangement around the NCCF torsion angle (φ) is presented ((E)-2-fluoro-N-(4-nitrobenzylidene)ethanamine: φ(NCCF) =70.0°). In the context of this study, the analysis of a pyridoxal-derived β-fluoroaldimine was performed, a species that is implicated in the inhibition of pyridoxal phosphate (PLP)-dependent enzymes by β-fluoroamine derivatives. The gauche preference of the internal aldimine (=NCH(2)CH(2)F) that can be rationalized by stereoelectronic arguments does not hold for the corresponding external system (N=CHCH(2)F) (E(min) when φ(NCCF) =120°). Moreover, the C-F bond is lengthened by more than 0.02 Å at φ(NCCF) =±90°, when it is exactly antiperiplanar to the conjugated imine. This activation of the C-F σ bond by an adjacent π system constitutes an addendum to Dunathan's stereoelectronic hypothesis.

The C-F bond as a conformational tool in organic and biological chemistry

Organofluorine compounds are widely used in many different applications, ranging from pharmaceuticals and agrochemicals to advanced materials and polymers. It has been recognised for many years that fluorine substitution can confer useful molecular properties such as enhanced stability and hydrophobicity. Another impact of fluorine substitution is to influence the conformations of organic molecules. The stereoselective introduction of fluorine atoms can therefore be exploited as a conformational tool for the synthesis of shape-controlled functional molecules. This review will begin by describing some general aspects of the C–F bond and the various conformational effects associated with C–F bonds (i.e. dipole–dipole interactions, charge–dipole interactions and hyperconjugation). Examples of functional molecules that exploit these conformational effects will then be presented, drawing from a diverse range of molecules including pharmaceuticals, organocatalysts, liquid crystals and peptides.