Medicinal Chemistry of Fluorinated Cyclic and Acyclic Nucleoside Phosphonates

Expanding the Scope of Group Transfer Radical Reaction: Toward the Synthesis of Fluorinated Nucleoside Analogues Incorporating Difluorophosphonylated Allylic Ether Moieties

Scope and limitations of the group transfer radical reaction of diisopropyl iododifluoromethylphosphonate onto carbohydrates and nucleosides are described. This key step allowed us to explore the synthesis of new fluorinated nucleoside analogues containing a difluorophosphonylated allylic ether moiety onto the 2'-position, in purine and pyrimidine series (B = A, C, G, T, U). Indeed, two unprecedented chemical approaches involving a late introduction of either the nucleobase or the fluorinated moiety are discussed.

Synthesis of LAVR-289, a new [(Z)-3-(acetoxymethyl)-4-(2,4-diaminopyrimidin-6-yl)oxy-but-2-enyl]phosphonic acid prodrug with pronounced antiviral activity against DNA viruses

New acyclic pyrimidine nucleoside phosphonate prodrugs with a 4-(2,4-diaminopyrimidin-6-yl)oxy-but-2-enyl]phosphonic acid skeleton (O-DAPy nucleobase) were prepared through a convergent synthesis by olefin cross-metathesis as the key step. Several acyclic nucleoside 4-(2,4-diaminopyrimidin-6-yl)oxy-but-2-enyl]phosphonic acid prodrug exhibited in vitro antiviral activity in submicromolar or nanomolar range against varicella zoster virus (VZV), human cytomegalovirus (HCMV), human herpes virus type 1 (HSV-1) and type 2 (HSV-2), and vaccinia virus (VV), with good selective index (SI). Among them, the analogue 9c (LAVR-289) proved markedly inhibitory against VZV wild-type (TK+) (EC50 0.0035 μM, SI 740) and for thymidine kinase VZV deficient strains (EC50 0.018 μM, SI 145), with a low morphological toxicity in cell culture at 100 μM and acceptable cytostatic activity resulting in excellent selectivity. Compound 9c exhibited antiviral activity against HCMV (EC50 0.021 μM) and VV (EC50 0.050 μM), as well as against HSV-1 (TK-) (EC50 0.0085 μM). Finally, LAVR-289 (9c) deserves further (pre)clinical investigations as a potent candidate broad-spectrum anti-herpesvirus drug.

Design, synthesis, and evaluation of 4'-phosphonomethoxy pyrimidine ribonucleosides as potential anti-influenza agents

Influenza viruses belong to the Orthomyxoviridae family and cause acute respiratory distress in humans. The developed drug resistance toward existing drugs and the emergence of viral mutants that can escape vaccines mandate the search for novel antiviral drugs. Herein, the synthesis of epimeric 4'-methyl-4'-phosphonomethoxy [4'-C-Me-4'-C-(O-CH₂ P═O)] pyrimidine ribonucleosides, their phosphonothioate [4'-C-Me-4'-C-(O-CH₂ P═S)] derivatives, and their evaluation against an RNA viral panel are described. Selective formation of the α- l-lyxo epimer, [4'-C-(α)-Me-4'-C-(β)-(O-CH₂ -P(═O)(OEt)₂ )] over the β- d-ribo epimer [4'-C-(β)-Me-4'-C-(α)-(O-CH₂ -P(═O)(OEt)₂ )] was explained by DFT equilibrium geometry optimizations studies. Pyrimidine nucleosides having the [4'-C-(α)-Me-4'-C-(β)-(O-CH₂ -P(═O)(OEt)₂ )] framework showed specific activity against influenza A virus. Significant anti-influenza virus A (H1N1 California/07/2009 isolate) was observed with the 4'-C-(α)-Me-4'-C-(β)-O-CH₂ -P(═O)(OEt)₂ -uridine derivative 1 (EC₅₀  = 4.56 mM, SI₅₀  > 56), 4-ethoxy-2-oxo-1(2H)-pyrimidin-1-yl derivative 3 (EC₅₀  = 5.44 mM, SI₅₀  > 43) and the cytidine derivative 2 (EC₅₀  = 0.81 mM, SI₅₀  > 13), respectively. The corresponding thiophosphonates 4'-C-(α)-Me-4'-C-(β)-(O-CH₂ -P( S)(OEt)₂ ) and thionopyrimidine nucleosides were devoid of any antiviral activity. This study shows that the 4'-C-(α)-Me-4'-(β)-O-CH₂ -P(═O)(OEt)₂ ribonucleoside can be further optimized to provide potent antiviral agents.

Design of organyl phosphate-based pro-drugs: comparative analysis of the antibiotic action of alkyl protecting groups with different degree of fluorination

Background. Molecular structures combining a phosphorus-containing counterpart and non-polar radicals are employed in design of pro-drugs as structural and functional groups necessary for transportation of drugs through cellular barriers. It is assumed that the carrier itself does not exhibit biological activity. However, the “organic phosphate – alkyl radical” complex may possess its own metabolic and pharmacological properties even in the absence of a drug moiety.The aim. To study the effect of fluorinated alkyl phosphates on the growth of bacterial test cultures in an agar medium and to identify conjugated metabolic markers using UV/visible spectroscopy.Materials and methods. The effect of six organyl phosphates on the growth of five types of bacteria under aerobic conditions was evaluated by the method of wells in an agar medium. For solutions containing cell metabolites of Pseudomonas aeruginosa, the absorption spectra were recorded at 250–280 nm. The principal component analysis (PCA) was used for multivariate comparative analysis of the spectra. Results. The studied organyl phosphates bearing the ethyl and propyl radicals are potential temporary carriers of the drug moiety, since they are capable of penetrating through cellular barriers. However, the fluorinated compounds exhibit bactericidal properties, the degree of which depends on the arrangement of fluorine atoms in the radical. The most active compounds are those exhaustively halogenated at the terminal carbon atom of the ethyl radical (-СН2-СF3), while non-fluorinated organyl phosphate is the least active. UV/visible spectra of P. aeruginosa cultivation products, according to PCA data, contain patterns reflecting the metabolic effects mediated by these structural features of the radicals.Conclusion. In terms of practical application of the studied compounds, the activity of a proantibiotic based on organyl phosphate with a non-fluorinated ethyl(propyl) radical will be determined only by the specificity of the drug moiety. Exactly the same molecule, but exhaustively fluorinated at the terminal carbon atom of the alkyl radical, is likely to be characterized by lower specificity and higher activity under the additive (or synergistic) action of metabolically active groups.

A Unified Strategy to Fluorinated Nucleoside Analogues Via an Electrophilic Manifold

Herein, we present a strategy for the preparation of 3'-fluorinated nucleoside analogues via the aminocatalytic, electrophilic fluorination of readily accessible and bench-stable 2'-ketonucleosides. Initially developed to facilitate the manufacture of 3'-fluoroguanosine (3'-FG)─a substructure of anticancer therapeutic MK-1454─this strategy has been extended to the synthesis of a variety of 3'-fluoronucleosides. Finally, we demonstrate the utility of the 2'-ketonucleoside synthon as a platform for further diversification and suggest that this methodology should be broadly applicable to the discovery of novel nucleoside analogues.

Fluorinated Nucleosides: Synthesis, Modulation in Conformation and Therapeutic Application

Over the last twenty years, fluorination on nucleoside has established itself as the most promising tool to use to get biologically active compounds that could sustain the clinical trial by affecting the pharmacodynamics and pharmacokinetic properties. Due to fluorine's inherent unique properties and its judicious introduction into the molecule, makes the corresponding nucleoside metabolically very stable, lipophilic, and opens a new site of intermolecular binding. Fluorination on various nucleosides has been extensively studied as a result, a series of fluorinated nucleosides come up for different therapeutic uses which are either approved by the FDA or under the advanced stage of the clinical trial. Here in this review, we are summarizing the latest development in the chemistry of fluorination on nucleoside that led to varieties of new analogs like carbocyclic, acyclic, and conformationally biased nucleoside and their biological properties, the influence of fluorine on conformation, oligonucleotide stability, and their use in therapeutics.

Phosphonates and Phosphonate Prodrugs in Medicinal Chemistry: Past Successes and Future Prospects

Compounds with a phosphonate group, i.e., -P(O)(OH)₂ group attached directly to the molecule via a P-C bond serve as suitable non-hydrolyzable phosphate mimics in various biomedical applications. In principle, they often inhibit enzymes utilizing various phosphates as substrates. In this review we focus mainly on biologically active phosphonates that originated from our institute (Institute of Organic Chemistry and Biochemistry in Prague); i.e., acyclic nucleoside phosphonates (ANPs, e.g., adefovir, tenofovir, and cidofovir) and derivatives of non-nucleoside phosphonates such as 2-(phosphonomethyl) pentanedioic acid (2-PMPA). Principal strategies of their syntheses and modifications to prodrugs is reported. Besides clinically used ANP antivirals, a special attention is paid to new biologically active molecules with respect to emerging infections and arising resistance of many pathogens against standard treatments. These new structures include 2,4-diamino-6-[2-(phosphonomethoxy)ethoxy]pyrimidines or so-called "open-ring" derivatives, acyclic nucleoside phosphonates with 5-azacytosine as a base moiety, side-chain fluorinated ANPs, aza/deazapurine ANPs. When transformed into an appropriate prodrug by derivatizing their charged functionalities, all these compounds show promising potential to become drug candidates for the treatment of viral infections. ANP prodrugs with suitable pharmacokinetics include amino acid phosphoramidates, pivaloyloxymethyl (POM) and isopropoxycarbonyloxymethyl (POC) esters, alkyl and alkoxyalkyl esters, salicylic esters, (methyl-2-oxo-1,3-dioxol-4-yl) methyl (ODOL) esters and peptidomimetic prodrugs. We also focus on the story of cytostatics related to 9-[2-(phosphonomethoxy)ethyl]guanine and its prodrugs which eventually led to development of the veterinary drug rabacfosadine. Various new ANP structures are also currently investigated as antiparasitics, especially antimalarial agents e.g., guanine and hypoxanthine derivatives with 2-(phosphonoethoxy)ethyl moiety, their thia-analogues and N-branched derivatives. In addition to ANPs and their analogs, we also describe prodrugs of 2-(phosphonomethyl)pentanedioic acid (2-PMPA), a potent inhibitor of the enzyme glutamate carboxypeptidase II (GCPII), also known as prostate-specific membrane antigen (PSMA). Glutamate carboxypeptidase II inhibitors, including 2-PMPA have been found efficacious in various preclinical models of neurological disorders which are caused by glutamatergic excitotoxicity. Unfortunately its highly polar character and hence low bioavailability severely limits its potential for clinical use. To overcome this problem, various prodrug strategies have been used to mask carboxylates and/or phosphonate functionalities with pivaloyloxymethyl, POC, ODOL and alkyl esters. Chemistry and biological characterization led to identification of prodrugs with 44-80 fold greater oral bioavailability (tetra-ODOL-2-PMPA).

C1'-Branched acyclic nucleoside phosphonates mimicking adenosine monophosphate: Potent inhibitors of Trypanosoma brucei adenine phosphoribosyltransferase

Some pathogens, including parasites of the genus Trypanosoma causing Human and Animal African Trypanosomiases, cannot synthesize purines de novo and they entirely rely on the purine salvage pathway (PSP) for their nucleotide generation. Thus, their PSP enzymes are considered as promising drug targets, sparsely explored so far. Recently, a significant role of acyclic nucleoside phosphonates (ANPs) as inhibitors of key enzymes of PSP, namely of 6-oxopurine phosphoribosyltransferases (PRTs), has been discovered. Herein, we designed and synthesized two series of new ANPs branched at the C1' position as mimics of adenosine monophosphate. The novel ANPs efficaciously inhibited Trypanosoma brucei adenine PRT (TbrAPRT1) activity in vitro and it was shown that the configuration on the C1' chiral centre strongly influenced their activity: the (R)-enantiomers proved to be more potent compared to the (S)-enantiomers. Two ANPs, with K_i values of 0.39 μM and 0.57 μM, represent the most potent TbrAPRT1 inhibitors reported to date and they are an important tool to further study purine metabolism in various parasites.

Diverse synthetic approaches towards C1'-branched acyclic nucleoside phosphonates

Acyclic nucleoside phosphonates (ANPs) represent a significant class of antiviral, anticancer, and antiprotozoal compounds. It is therefore highly desirable to have diverse synthetic routes leading towards these molecules. In the past, many structural modifications were explored, but surprisingly, the field of C1'-branched ANPs has been neglected with only a handful of articles reporting their synthesis. Herein we describe and compare five convenient approaches leading to key synthetic 6-chloropurine ANPs bearing the 9-phosphonomethoxyethyl (PME) moiety branched at the C1' position. These intermediates can be further vastly diversified into target C1'-branched ANPs bearing either natural or unnatural nucleobases. The importance of C1'-branched ANPs is emphasized by their analogy with C1'-substituted cyclic nucleotides (such as remdesivir, a broad-spectrum antiviral agent) and evaluation of their biological activity (e.g. antiviral, antineoplastic, and antiprotozoal) will be a tempting subject of further research.

Design, Synthesis, and Activity Evaluation of Novel Acyclic Nucleosides as Potential Anticancer Agents In Vitro and In Vivo

In the present work, 103 novel acyclic nucleosides were designed, synthesized, and evaluated for their anticancer activities in vitro and in vivo. The structure-activity relationship (SAR) studies revealed that most target compounds inhibited the growth of colon cancer cells in vitro, of which 3-(6-chloro-9H-purin-9-yl)dodecan-1-ol (9b) exhibited the most potent effect against the HCT-116 and SW480 cells with IC50 values of 0.89 and 1.15 μM, respectively. Furthermore, all of the (R)-configured acyclic nucleoside derivatives displayed more potent anticancer activity compared to their (S)-counterparts. Mechanistic studies revealed that compound 9b triggered apoptosis in the cancer cell lines via depolarization of the mitochondrial membrane and effectively inhibited colony formation. Importantly, compound 9b inhibited the growth of the SW480 xenograft in a mouse model with low systemic toxicity. These results indicated that acyclic nucleoside compounds are viable as potent and effective anticancer agents, and compound 9b may serve as a promising lead compound that merits further attention in future anticancer drug discovery.

Overview of Biologically Active Nucleoside Phosphonates

The use of the phosphonate motif featuring a carbon-phosphorous bond as bioisosteric replacement of the labile P–O bond is widely recognized as an attractive structural concept in different areas of medicinal chemistry, since it addresses the very fundamental principles of enzymatic stability and minimized metabolic activation. This review discusses the most influential successes in drug design with special emphasis on nucleoside phosphonates and their prodrugs as antiviral and cancer treatment agents. A description of structurally related analogs able to interfere with the transmission of other infectious diseases caused by pathogens like bacteria and parasites will then follow. Finally, molecules acting as agonists/antagonists of P2X and P2Y receptors along with nucleotidase inhibitors will also be covered. This review aims to guide readers through the fundamentals of nucleoside phosphonate therapeutics in order to inspire the future design of molecules to target infections that are refractory to currently available therapeutic options.

Design, synthesis and antiviral evaluation of novel acyclic phosphonate nucleotide analogs with triazolo[4,5-b]pyridine, imidazo[4,5-b]pyridine and imidazo[4,5-b]pyridin-2(3H)-one systems

A new series of phosphonylated triazolo[4,5-b]pyridine (1-deaza-8-azapurine), imidazo[4,5-b]pyridine (1-deazapurine) and imidazo[4,5-b]pyridin-2(3H)-one (1-deazapurin-8-one) were synthesized from 2-chloro-3-nitropyridine and selected diethyl ɷ-aminoalkylphosphonates followed by reduction of the nitro group and cyclization. In the final step O,O-diethylphosphonates were transformed into the corresponding phosphonic acids. All synthesized compounds were evaluated in vitro for inhibitory activity against a broad variety of DNA and RNA viruses and their cytotoxic potencies were also established. Compound 12f showed marginal activity against cytomegalovirus Davis strain (EC₅₀ = 76.47 μM) in human embryonic lung (HEL) cells while compounds 10g (EC₅₀ = 52.53 μM) and 12l (EC₅₀ = 61.70 μM) were minimally active against the varicella-zoster virus Oka strain in HEL cells. Compounds under investigation were not cytotoxic at the maximum concentration evaluated (100 µM).

Synthesis of fluorinated acyclic nucleoside phosphonates with 5-azacytosine base moiety

With respect to the strong antiviral activity of (S)-1-[3-hydroxy-2-(phosphonomethoxy)propyl]-5-azacytosine various types of its side chain fluorinated analogues were prepared. The title compound, (S)-1-[3-fluoro-2-(phosphonomethoxy)propyl]-5-azacytosine (FPMP-5-azaC) was synthesised by the condensation reaction of (S)-2-[(diisopropoxyphosphoryl)methoxy)-3-fluoropropyl p-toluenesulfonate with a sodium salt of 5-azacytosine followed by separation of appropriate N¹ and O² regioisomers and ester hydrolysis. Transformations of FPMP-5-azaC to its 5,6-dihydro-5-azacytosine counterpart, amino acid phosphoramidate prodrugs and systems with an annelated five-membered imidazole ring, i.e. imidazo [1,2-a][1,3,5]triazine derivatives were also carried out. 1-(2-Phosphonomethoxy-3,3,3-trifluoropropyl)-5-azacytosine was prepared from 5-azacytosine and trifluoromethyloxirane to form 1-(3,3,3-trifluoro-2-hydroxypropyl)-5-azacytosine which was treated with diisopropyl bromomethanephosphonate followed by deprotection of esters. Antiviral activity of all newly prepared compounds was studied. FPMP-5-azaC diisopropyl ester inhibited the replication of herpes viruses with EC₅₀ values that were about three times higher than that of the reference anti-HCMV drug ganciclovir without displaying cytotoxicity.

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5-Azacytosine acyclic nucleoside phosphonates fluorinated in the aliphatic side chain have been synthesized.

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Stabilized analogues with 5,6-dihydro arrangement and/or with an annelated five-membered ring were also prepared.

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Diisopropyl phosphonate esters were converted to amino acid amidate prodrugs.

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In most cases, antiviral activity of the compounds was only marginal.

Synthesis of an α-phosphono-α,α-difluoroacetamide analogue of the diphosphoinositol pentakisphosphate 5-InsP7

Diphosphoinositol phosphates (PP-InsPs) are an evolutionarily ancient group of signalling molecules that are essential to cellular and organismal homeostasis. As the detailed mechanisms of PP-InsP signalling begin to emerge, synthetic analogues of PP-InsPs containing stabilised mimics of the labile diphosphate group can provide valuable investigational tools. We synthesised 5-PCF2Am-InsP5 (1), a novel fluorinated phosphonate analogue of 5-PP-InsP5, and obtained an X-ray crystal structure of 1 in complex with diphosphoinositol pentakisphosphate kinase 2 (PPIP5K2). 5-PCF2Am-InsP5 binds to the kinase domain of PPIP5K2 in a similar orientation to that of the natural substrate 5-PP-InsP5 and the PCF2Am structure can mimic many aspects of the diphosphate group in 5-PP-InsP5. We propose that 1, the structural and electronic properties of which are in some ways complementary to those of existing phosphonoacetate and methylenebisphosphonate analogues of 5-PP-InsP5, may be a useful addition to the expanding array of chemical tools for the investigation of signalling by PP-InsPs. The PCF2Am group may also deserve attention for wider application as a diphosphate mimic.

Cyclization Reaction of Donor-Acceptor Oxiranes with N,N'-Disubstituted Thioureas: A Domino Process to trans-Dihydropyrimidines

An unprecedented cyclization reaction of donor-acceptor oxiranes with N,N'-disubstituted thioureas to construct trans-dihydropyrimidines is presented. Preliminary reaction mechanism studies demonstrated that the reaction underwent sequential cycloaddition/amine ester exchange/oxygen-sulfur exchange/desulfuration/Michael addition process. A wide range of trans-dihydropyrimidines were produced with high yields up to 94% by using this method.

Orchestrated approaches using pure shift NMR: Extraction of spectral parameters, ultra-high resolution, and sensitivity enhancement

The limited chemical shift range of protons and pairwise interaction among all the abundant nuclear spins of a molecule makes ¹ H spectrum too complicated. As a consequence, the straightforward analysis and the accurate extraction of their interaction strengths from the ¹ H spectrum of a complex spin system are formidably difficult or often impossible. This problem persists in the determination of scalar couplings be it between two abundant homonuclear spins or between ¹ H and an abundant heteronuclear spin (viz., ¹⁹ F and ³¹ P). Such problems are encountered in many situations where the determination of homonuclear and heteronuclear couplings is challenging. The several pure shift based one-dimensional and two-dimensional NMR strategies recently developed in our laboratory for the straightforward extraction of homonuclear and heteronuclear interaction parameters in diverse situations are discussed. Initially, the unique application of pure shift technique that paves the way for easy and straightforward extraction of magnitudes of heteronuclear couplings, namely, ⁿ J_HX (where X stands for ¹⁹ F, ³¹ P, etc.), is discussed. Subsequently, several pure shift edited one-dimensional and two-dimensional NMR strategies that are developed for the direct extraction of homonuclear and heteronuclear couplings and for achieving ultra-high-resolved ¹ H spectra with complete eradication of zero frequency peaks and the evolution of unwanted couplings. The enhancement in the sensitivity has also been achieved in the slice-selective pure shift experiments by the rapid acquisition of proton spectrum where the polarization from the adjacent protons is transferred to the selectively excited proton.

Highly convergent synthesis and antiviral activity of (E)-but-2-enyl nucleoside phosphonoamidates

Several hitherto unknown (E)-but-2-enyl nucleoside phosphonoamidate analogs (ANPs) were prepared directed with nitrogen reagents by cross-metathesis in water-under ultrasound irradiation. Two diastereoisomers were formally identified by X-ray diffraction. These compounds were evaluated against a large spectrum of DNA and RNA viruses. Among them, the phosphonoamidate thymine analogue 19 emerged as the best prodrug against varicella-zoster virus (VZV) with EC₅₀ values of 0.33 and 0.39 μM for wild-type and thymidine kinase deficient strains, respectively, and a selectivity index ≥200 μM. This breakthrough approach paves the way for new purine and pyrimidine (E)-but-2-enyl phosphonoamidate analogs.

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Phosphonoamidate prodrugs acyclic nucleosides were synthesized by convergent approach.

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Metathesis reaction in water was used between pyrimidic bases and a new phosphonoamidate synthons.

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EC₅₀ values of any molecules were in (sub)micromolar range against DNA viruses.

Expanding the Antiviral Spectrum of 3-Fluoro-2-(phosphonomethoxy)propyl Acyclic Nucleoside Phosphonates: Diamyl Aspartate Amidate Prodrugs

Acyclic nucleosides containing a 3-fluoro-2-(phosphonomethoxy)propyl (FPMP) side chain are known to be moderately potent antihuman immunodeficiency virus (HIV) agents, while being completely devoid of antiviral activity against a wide range of DNA viruses. The derivatization of the phosphonic acid functionality of FPMPs with a diamyl aspartate phenoxyamidate group led to a novel generation of compounds that not only demonstrate drastically improved antiretroviral potency but also are characterized by an expanded spectrum of activity that also covers hepatitis B and herpes viruses. The best compound, the (S)-FPMPA amidate prodrug, exerts anti-HIV-1 activity in TZM-bl and peripheral blood mononuclear cells at low nanomolar concentrations and displays excellent potency against hepatitis B virus (HBV) and varicella-zoster virus (VZV). This prodrug is stable in acid and human plasma media, but it is efficiently processed in human liver microsomes with a half-life of 2 min. The (R) isomeric guanine derivative emerged as a selectively active anti-HIV and anti-HBV inhibitor, while being nontoxic to human hepatoblastoma cells. Notably, the pyrimidine containing prodrug (S)-Asp-FPMPC is the only congener within this series to demonstrate micromolar antihuman cytomegalovirus (HCMV) potency.

Synthesis of 5,5-difluoro-5-phosphono-pent-2-en-1-yl nucleosides as potential antiviral agents

A series of hitherto unknown acyclic 5,5-difluoro-5-phosphono-pent-2-en-1-yl-pyrimidines, -purines and -(1,2,4)-triazolo-3-carboxamide were successfully synthesized from (E)-1-bromo-5-diethoxyphosphoryl-5,5-difluoro-pent-2-ene in a stereoselective manner.

Phosphinodifluoroalkylation of alkynes using P(O)H compounds and ethyl difluoroiodoacetate

The first phosphinodifluoroalkylation of alkynes is achieved and the simultaneous addition of both difluoromethylene and phosphinoyl groups across the alkynes with high regio- and stereoselectivity is of great significance.

Synthesis and biological properties of prodrugs of (S)-3-(adenin-9-yl)-2-(phosphonomethoxy)propanoic acid

The lack of antiviral activity of recently described (S)-3-(adenin-9-yl)-2-(phosphonomethoxy)propanoic acid, or (S)-CPMEA in brief, has been speculated to possibly be due to the increased hydrophilicity of the molecule and, thus, by its limited cellular permeability. Efficient syntheses of novel lipophilic prodrugs of (S)-CPMEA masking either the carboxylic group or preferably both the phosphonate and carboxylic moieties, have been developed in order to increase bioavailability of the parent compound. Two prodrugs of (S)-CPMEA, namely phosphonate bis-amidate 15 and phenyloxy amidate 16, exhibited pan-genotypic anti-HCV activity at submicromolar concentrations.

The synthesis of heteroleptic phosphines

This perspective presents an overview of modern synthetic approaches to heteroleptic phosphines.