Synthesis of the (α,α-difluoroalkyl)phosphonate analogue of phosphoserine

Rapid Entry into Biologically Relevant α,α-Difluoroalkylphosphonates Bearing Allyl Protection-Deblocking under Ru(II)/(IV)-Catalysis

A convenient synthetic route to α,α-difluoroalkylphosphonates is described. Structurally diverse aldehydes are condensed with LiF2CP(O)(OCH2CH═CH2)2. The resultant alcohols are captured as the pentafluorophenyl thionocarbonates and efficiently deoxygenated with HSnBu3, BEt3, and O2, and then smoothly deblocked with CpRu(IV)(π-allyl)quinoline-2-carboxylate (1-2 mol %) in methanol as an allyl cation scavenger. These mild deprotection conditions provide access to free α,α-difluoroalkylphosphonates in nearly quantitative yield. This methodology is used to rapidly construct new bis-α,α-difluoroalkyl phosphonate inhibitors of PTPIB (protein phosphotyrosine phosphatase-1B).

Development of Biologically Active Compounds on the Basis of Phosphonic and Phosphinic Acid Functionalities

Phosphonic and phosphinic acids, especially α-heteroatom-substituted ones, possess unique structural and physical features which enable them to act as hydrotically stable analogs to biological phosphates in biological processes. They also act as mimetics in the transition state of the protease-induced hydrolysis of dipeptides. The first half of this review focuses on selected new synthetic methods developed by our research group for the stereoselective synthesis of α-heteroatom-substituted phosphonic and phosphinic acid derivatives, including modified nucleotide analogs and phosphinyl dipeptide isosteres. In the latter half, this review summarizes the utility of difluoromethylenephosphonic acids and phosphonic acid esters in the development of enzyme inhibitors against protein tyrosine phosphatases, sphingomyelinases, purine nucleoside phosphorylases and thrombin. The enzyme inhibitors developed were used as probes to elucidate signal transductions and the mechanisms of enzyme actions. The findings of the studies are briefly described.

Advances in Fmoc solid-phase peptide synthesis

Today, Fmoc SPPS is the method of choice for peptide synthesis. Very-high-quality Fmoc building blocks are available at low cost because of the economies of scale arising from current multiton production of therapeutic peptides by Fmoc SPPS. Many modified derivatives are commercially available as Fmoc building blocks, making synthetic access to a broad range of peptide derivatives straightforward. The number of synthetic peptides entering clinical trials has grown continuously over the last decade, and recent advances in the Fmoc SPPS technology are a response to the growing demand from medicinal chemistry and pharmacology. Improvements are being continually reported for peptide quality, synthesis time and novel synthetic targets. Topical peptide research has contributed to a continuous improvement and expansion of Fmoc SPPS applications.

Synthesis of a phosphoserine mimetic prodrug with potent 14-3-3 protein inhibitory activity

Many protein-protein interactions in cells are mediated by functional domains that recognize and bind to motifs containing phosphorylated serine and threonine residues. To create small molecules that inhibit such interactions, we developed methodology for the synthesis of a prodrug that generates a phosphoserine peptidomimetic in cells. For this study, we synthesized a small molecule inhibitor of 14-3-3 proteins that incorporates a nonhydrolyzable difluoromethylenephosphoserine prodrug moiety. The prodrug is cytotoxic at low micromolar concentrations when applied to cancer cells and induces caspase activation resulting in apoptosis. The prodrug reverses the 14-3-3-mediated inhibition of FOXO3a resulting from its phosphorylation by Akt1 in a concentration-dependent manner that correlates well with its ability to inhibit cell growth. This methodology can be applied to target a variety of proteins containing phosphoserine and other phosphoamino acid binding domains.

Introduction of fluorine and fluorine-containing functional groups

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

Selective synthesis of α-fluoro-β-keto- and α-fluoro-β-aminophosphonates via electrophilic fluorination by selectfluor

A series of α-mono- and α,α-difluoro-β-ketophosphonates were synthesized in moderate to good yields with excellent selectivities via electrophilic fluorination by Selectfluor. Subsequently, synthetic potential of the obtained α-monofluoro-β-ketophosphonates was demonstrated by their application in synthesis of α-monofluoro-β-aminophosphonates, useful building blocks in the preparation of phosphapeptides.

The alpha,alpha-difluorinated phosphonate L-pSer-analogue: an accessible chemical tool for studying kinase-dependent signal transduction

This overview focuses on the (alpha,alpha-difluoromethylene)phosphonate mimic of phosphoserine (pCF(2)Ser) and its application to the study of kinase-mediated signal transduction-pathways of great interest to drug development. The most versatile modes of access to these chemical biological tools are discussed, organized by method of PCF(2)-C bond formation. The pCF(2)-Ser mimic may be site-specifically incorporated into peptides (SPPS) and proteins (expressed protein ligation). This isopolar, dianionic pSer mimic results in a "constitutive phosphorylation" phenotype and is seen to support native protein-protein interactions that depend on serine phosphorylation. Signal transduction pathways studied with this chemical biological approach include the regulation of p53 tumor suppressor protein activity and of melatonin production. Given these successes, the future is bright for the use of such "teflon phospho-amino acid mimics" to map kinase-based signaling pathways.

A convenient synthesis of the C-1-phosphonate analogue of UDP-GlcNAc and its evaluation as an inhibitor of O-linked GlcNAc transferase (OGT)

The C-1-phosphonate analogue of UDP-GlcNAc has been synthesized using an alpha-configured C-1-aldehyde as a key intermediate. Addition of the anion of diethyl phosphate to the aldehyde produced the hydroxyphosphonate. The configuration of this key intermediate was determined by X-ray crystallography. Deoxygenation, coupling of the resulting phosphonic acid with UMP and deprotection gave the target molecule as a di-sodium salt. This analogue had no detectable activity as an inhibitor of (OGT).

Syntheses of ω-Hydroxy-α,α-difluoromethylphosphonates by Oxacycle Ring-Opening Reactions

[reaction: see text] Oxacycle ring-opening reactions from a non-HCFC-based source of phosphonodifluoromethyl carbanion 1 are reported. This straightforward strategy opens access to a variety of primary and secondary omega-hydroxy-alpha,alpha-difluoromethylphosphonates via one step. The syntheses of a glycerol monophosphate analogue and precursors to nucleoside phosphorylase inhibitors are described using this method.

Synthesis of fluorine-containing bioisosteres corresponding to phosphoamino acids and dipeptide units

It has been shown that fluorinated analogues of naturally occurring biological active compounds including amino acids often exhibit unique physiological activity. Among wide varieties of fluorine-containing amino acids, nonhydrolyzable phosphoamino acids possessing a substituent of the difluoromethylene (CF(2)) unit for the phosphoryl ester oxygen are of value in the medicinal and biological fields. We have engaged in the synthesis of these classes of nonhydrolyzable phosphoamino acids corresponding to pTyr 3, pSer 4, and pThr 5 with their incorporation into peptides using newly developed deprotecting procedures. In this article, stereoselective synthesis of the CF(2)-substituted pThr mimetics and development of a two-step deprotecting methodology for the nonhydrolyzable analogues are reviewed. In the course of the above synthetic study, we found that gamma,gamma-difluoro-alpha,beta-enoates were reduced to gamma-fluoro-beta,gamma-enoates by organocopper reagents and then applied to the synthesis of (Z)-fluoroalkene dipeptide isosteres, which have served as potential dipeptide mimetics having structural as well as electrostatic similarity to the parent peptide bonds. Furthermore, mechanistic investigation of the organocopper-mediated reduction led us to development of a SmI(2)-mediated approach toward the synthesis of the fluoroalkene isosteres.

Synthesis of thiazolines linked to a difluoromethylphosphonate diester via dithioester chemistry

[reaction: see text] A two-step, high-yielding synthesis of delta(2)-thiazolines containing a difluoromethylphosphonate diester moiety has been devised using a building block approach. Racemic or chiral beta-amino alcohols and diols were coupled with methyl difluoro(diethoxyphosphono)dithioacetate to give predominantly the corresponding beta-hydroxythioamides, which were then cyclized to provide a series of novel substituted delta(2)-thiazolines.

C-Difluoromethylene-containing, C-trifluoromethyl and C-perfluoroalkyl carbohydrates. Synthesis by carbohydrate transformation or building block methods

The synthetic methods for preparing carbohydrates bearing a C-branched substituent of the type CF2-Y, with Y = F, Y = CnF(2n + 1) or Y = a carbon-attached or heteroatom-attached nonfluorinated residues, are reviewed. Both direct introduction of C-branched fluorinated substituents (direct trifluoromethylation, perfluoroalkylation or difluoromethylenation) and building block methods from fluorinated synthons are considered.

Chemoenzymatic synthesis of iminocyclitol derivatives: a useful library strategy for the development of selective fucosyltransfer enzymes inhibitors

A chemoenzymatic strategy has been developed for the synthesis of libraries of iminocyclitol derivatives for the discovery of new and selective fucosidase inhibitors.

Solid-phase synthesis of tyrosyl H-phosphonopeptides and methylphosphonopeptides

Phosphopeptides are a useful tool for the investigation of phosphorylation as a reversible posttranslational modification. There is a growing interest in using mimics of phosphoamino acids involved in phosphorylation in order to study the enzymes concerned in these processes. These mimics should contain a non-hydrolysable or isoelectrically modified phosphate moiety to be used as a specific inhibitor of phosphatases and kinases. We introduce sold-phase synthesis of H- and methylphosphonopeptides as a new class of mimics of phosphotyrosyl peptides. The peptides were synthesized on solid phase using the standard fluorenyl-methyloxycarbonyl (Fmoc) strategy. Tyrosine residues were incorporated as allyl-protected derivatives, which were selectively deprotected on the resin by treatment with Pd(PPh3)4. The peptide resin carrying the side-chain unprotected tyrosine of the model peptide Gly-Gly-Tyr-Ala was phosphonylated with di-tert-butyl-N,N-diethyl-phosphoramidite in the presence of 1H-tetrazole, yielding H-phosphonopeptides after trifluoroacetic acid (TFA) cleavage. Alternatively, phosphonylation of the unprotected tyrosine with O-tert-butyl-N,N-diethyl-P-methylphosphonamidite catalysed by 1H-tetrazole and followed by oxidation led to the methyphosphonopeptides after TFA cleavage. We obtained both the H-phosphonopeptides and the methylphosphonopeptides of the tetrapeptide in high yields and purities above 90%, according to reversed-phase high-performance liquid chromatography (RP-HPLC). To investigate the general applicability of our new methodology, we synthesized phosphonopeptides up to 13 amino acids long, corresponding to recognition sequences of tyrosine kinases. After cleavage and deprotection, all phosphonopeptides were obtained in high yields and purities of about 90%, as shown by mass spectrometry. The only by-product found was the unmodified peptide.

Solid-phase synthesis of H- and methylphosphonopeptides

We introduce solid-phase syntheses of H- and methylphosphonopeptides, giving access for the first time to a new class of mimics for o-phosphoamino acids. The model peptides H-GlyGlyXaaAla-OH (Xaa = Ser, Thr) were synthesized on a solid-phase using Fmoc/tBu strategy and HBTU/HOBt activation by incorporation of hydroxyl-protected serine and threonine. As selectively cleavable hydroxyl-protecting groups we used triphenylmethyl and tert-butyldimethylsilyl for both amino acids, as described in the literature. All peptides were phosphitilated with O, O-di-tert-butyl-N,N-diethylphosphoramidite and yielded H-phosphonopeptides after trifluoroacetic acid cleavage. Alternatively we phosphonylated the peptides with O-tert-butyl-N,N-diethyl-P-methylphosphonamidite, which was synthesized by a two-step one-pot procedure starting from commercially available chemicals. All H- and methylphosphonopeptides were obtained in high purities and yields, as shown by reversed-phase high-performance liquid chromatography and anion-exchange chromatography. The phosphonopeptides were characterized by 1H and 31P NMR. We confirmed their molecular masses by electrospray mass spectrometry and analyzed their fragmentation schemes, which seemed to be characteristic for each class of analogues. The H-phosphonopeptides lost phosphonic acid (H3PO3, 82 mass units) and the methylphosphonopeptides lost methylphosphonic acid (MeH2PO3, 96 mass units). Both H- and methylphosphonopeptides represent a new and simply accessible class of mimics for phosphopeptides. Compared with the corresponding phosphopeptides all phosphonopeptides were synthesized in higher yields and purities (> 80%).