4-Trifluoromethoxy proline: synthesis of stereoisomers and lipophilicity study.

All four stereoisomers of 4-CF3O-proline have been synthesized through a fluorodesulfurization approach using the corresponding 4-hydroxyprolines as starting materials. The investigation of their lipophilicity characteristics and comparison with those of other 4-substituted proline analogs demonstrated a similar impact of CF3 and CF3O groups on log D.

Synthesis, radiosynthesis and biochemical evaluation of fluorinated analogues of sphingosine-1-phosphate receptor 3 specific antagonists using PET.

Sphingosine-1-phosphate and its receptors (S1PRs) are involved in several diseases such as auto immunity, inflammation and cardiovascular disorders. The S1P analogue fingolimod (Gilenya®) is currently in use for the treatment of relapsing multiple sclerosis. S1PRs are also promising targets for clinical molecular imaging in vivo. The organ distribution of individual S1PRs can be potentially achieved by using S1PR subtype-specific (radiolabeled) chemical probes. Here, we report our efforts on synthesis and in vivo potency determination of new ligands for the S1P receptor 3 (S1P3) based on the S1P3 antagonist TY-52156 and in validation of a potential imaging tracer in vivo using Positron Emission Tomography (PET) after 18F-labelling. A p-fluorophenyl derivative exhibited excellent S1P3 antagonist activity in vitro, good serum stability, and medium lipophilicity. In vivo biodistribution experiments using 18F-PET exhibited significant uptake in the myocardium suggesting potential applications in cardiac imaging.

Selective Synthesis of Monofluorinated Compounds Applying Amine/HF Reagents.

For nucleophilic monofluorination, amine/HF reagents such as Et3 N⋅3HF, Pyr⋅9HF (Olah's reagent) and similar combinations belong to the most frequently used fluoride sources, whereupon the selectivity of these reagents can be very different depending of its acidity, the nucleophilicity of the fluoride equivalent, and the structure of the particular substrate. These reagents can be used safely in ordinary chemistry laboratories for nucleophilic substitution reactions by fluoride at sp3 -hybridized carbon centers. For ring opening reactions of epoxides, the regio- and stereoselectivity is very much depending of the nature of the epoxide and the acidity of the HF reagent favoring either SN 1 or SN 2 type reactions. Similarly, the outcome of halofluorination and similar reactions with sulfur or seleno electrophiles can be controlled by the particular combination of the electrophile and the fluoride source. Examples for the application of these reaction types for the synthesis of fluorine-containing analogues of natural products or biologically relevant compounds are in the focus of this personal account.

Influence of N-arylsulfonamido d-valine N-substituents on the selectivity and potency of matrix metalloproteinase inhibitors.

To develop matrix metalloproteinase inhibitors (MMPIs) for both therapy and medicinal imaging by fluorescence-based techniques or positron-emission tomography (PET), a small library of eighteen N-substituted N-arylsulfonamido d-valines were synthesized and their potency to inhibit two gelatinases (MMP-2, and MMP-9), two collagenases (MMP-8, and MMP-13) and macrophage elastase (MMP-12) was determined in a Structure-Activity-Relation study with ({4-[3-(5-methylthiophen-2-yl)-1,2,4-oxadiazol-5-yl]phenyl}sulfonyl)-d-valine (1) as a lead. All compounds were shown to be more potent MMP-2/-9 inhibitors (nanomolar range) compared to other tested MMPs. This is a remarkable result considering that a carboxylic acid group is the zinc binding moiety. The compound with a terminal fluoropropyltriazole group at the furan ring (P1' substituent) was only four times less potent in inhibiting MMP-2 activity than the lead compound 1, making this compound a promising probe for PET application (after using a prosthetic group approach to introduce fluorine-18). Compounds with a TEG spacer and a terminal azide or even a fluorescein moiety at the sulfonylamide N atom (P2' substituent) were almost as active as the lead structure 1, making the latter derivative a suitable fluorescence imaging tool.

Impact of ß-perfluoroalkyl substitution of proline on the proteolytic stability of its peptide derivatives.

A series of all stereoisomers of ß-CF3 or ß-C2F5 substituted prolines and their dipeptide derivatives were synthesized. Mouse plasma stability assay was carried out to study the impact of fluoroalkyl substituents on the proteolytic stability of proline-derived peptides. The effect of the (R)-/(S)-configuration at the C-2 atom in combination with electronic and steric effects imposed by fluoroalkyl groups was addressed to rationalize the difference in the half-life stability of diastereomeric ß-CF3-Pro-Gly and ß-C2F5-Pro-Gly derivatives and compared to those of parent (S)-Pro-Gly and (R)-Pro-Gly dipeptides. The steric effect was predominant when the ß-CF3 or ß-C2F5 group was placed properly to create a spatial interference within the pockets of proteases, thereby protecting the substances from degradation (e.g., for cis-isomeric derivatives). Otherwise, a smaller electronic effect accelerating proteolysis was in charge (i.e., for the (2S,3S) isomers).

Synthesis and [3,3]-sigmatropic rearrangements of 5-(pentafluorosulfanyl)-pent-3-en-2-ol, its homologues, and trifluoromethyl analogues.

The synthesis of aliphatic (pentafluoro-λ6-sulfanyl)(SF5)-substituted compounds is more challenging than that of the related CF3-substituted analogues. Previous investigations of [3,3]-sigmatropic rearrangements of γ-SF5-substituted allylic alcohols failed to yield 3-SF5-substituted carboxylic acid derivatives. Herein, we present the synthesis of a series of 1-SF5-alk-1-en-3-ols and our efforts to apply them in Johnson-Claisen, ester enolate-Claisen, and Ireland-Claisen rearrangements. Unfortunately, these reactions failed to include the 1-SF5-substituted 1,2-double bond, although successful reactions of analogous CF3-allylic alcohols were reported. Further experiments revealed that bulkiness rather than electronic properties of the SF5 group prevented [3,3]-sigmatropic rearrangements. Indeed, the introduction of a competing second vinyl group into the system (1-SF5-penta-1,4-dien-3-ol) confirmed that a Johnson-Claisen rearrangement was successful (92% yield of methyl 7-SF5-hepta-4,6-dienoate) with incorporation of the unsubstituted 4,5-double bond while the 1-SF5-substituted 1,2-double bond remained unchanged. Efforts to apply 1-(SF5CF2)-substituted 1,2-double bond systems, which are similar to CF3 analogues in steric requirements, in Johnson-Claisen or ester enolate-Claisen rearrangements failed because of the instability of the SF5CF2 substituent under various reaction conditions. On the other hand, when the SF5 group was separated from the reaction center by a CH2 group instead (5-SF5-pent-3-en-2-ol), Johnson-Claisen rearrangements using six orthoesters provided the target 2-substituted 3-(CH2SF5)-hex-4-enoates in 55-76% yields as ∼1 : 1 mixtures of diastereomers. As an example to demonstrate the utility of these products, methyl 3-(CH2SF5)-hex-4-enoate was reduced, and the formed alcohol was oxidized to the aldehyde. Finally, initial experiments showed that the synthetic sequence developed for SF5 compounds is also applicable for analogous CF3-substituted allylic alcohols (5-CF3-pent-3-en-2-ol) and their Johnson-Claisen rearrangement.

Optimizing the Biodistribution of Radiofluorinated Barbiturate Tracers for Matrix Metalloproteinase Imaging by Introduction of Fluorescent Dyes as Pharmacokinetic Modulators.

Dysregulated expression or activation of matrix metalloproteinases (MMPs) is observed in many kinds of life-threatening diseases. Therefore, MMP imaging-for example, with radiolabeled MMP inhibitors (MMPIs)-potentially represents a valuable tool for clinical diagnostics using noninvasive single photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. Despite numerous preclinical imaging approaches, translation to a clinical setting has not yet been successful. We introduce and oppose three potential radiofluorinated MMP-targeted imaging probes, modified by the introduction of pentamethine cyanine (Cy5) dyes and therefore containing both radio- as well as fluorescent label with respect to their capability to assess MMP activity in vivo by means of scintigraphic (PET) and/or fluorescent (NIRF) imaging. New hybrid MMPI tracer candidates, structurally based on radiofluorinated pyrimidine-2,4,6-triones (barbiturates) from previous approaches, were synthesized by convenient two-step syntheses. In the first step, Cy5 dyes, varying in the number of sulfonate groups (nSO3- = 1, 2, or 4) and bearing an additional "clickable" alkyne moiety, were coupled to the barbiturate MMPI by amide formation. In the second step, the [18F]fluoride radiolabel was introduced into the resulting Cy5 dye conjugates by "radio-click" chemistry. Biodistribution studies of these hybrid tracer candidates were assessed and compared in C57BL/6 mice by PET as well as fluorescence imaging. MMP activity was imaged in a MMP-positive mouse model of irritant contact dermatitis (ICD) by PET and sequential fluorescence reflectance imaging (FRI), respectively. In vivo data were validated by scintillation counting, gelatin zymography, and MMP-histology. Three new potential hybrid MMP imaging probes were prepared, differing essentially in the number of sulfonate groups, introduced by Cy5 dye components. Although the hydrophilicity of these compounds was substantially increased, 10a (nSO3- = 1) and 10b (nSO3- = 2) were still rapidly eliminated via unfavorable hepatobiliary pathways, as observed in earlier approaches. Only 11 (nSO3- = 4) showed delayed in vivo clearance and a shift towards higher renal elimination. In the chosen mouse model of ICD, only 11 (nSO3- = 4) significantly accumulated in the inflamed mouse ear, which could be precisely visualized by means of PET and FRI.

Fluorinated 2-Arylcyclopropan-1-amines - A new class of sigma receptor ligands.

Stereoisomeric 2-aryl-2-fluoro-cyclopropan-1-amines have been discovered as a new class of σ receptor ligands showing different selectivity for the two subtypes of the receptor. Generally, compounds substituted in 4-position are much more active than corresponding 3-substituted isomers. trans-2-Fluoro-2-(4-methoxyphenyl)cyclopropan-1-amine (19a) was the most potent (Ki = 4.8 nM) σ1 receptor ligand, while cis-2-fluoro-2-(4-trifluoromethylphenyl)cyclopropan-1-amine (20b) was the most potent (Ki = 95 nM) σ2 receptor ligand.

New in Vivo Compatible Matrix Metalloproteinase (MMP)-2 and MMP-9 Inhibitors.

Matrix metalloproteinases (MMPs) are emerging as pivotal fine-tuners of cell function in tissue homeostasis and in various pathologies, in particular inflammation. In vivo monitoring of the activity of specific MMPs, therefore, provides high potential for assessing disease progression and tissue function, and manipulation of MMP activity in tissues and whole organisms may further provide a mode of controlling pathological processes. We describe here the synthesis of novel fluorinated and nonfluorinated analogues of a secondary sulfonamide-based lead structure, compound 2, and test their efficacy as in vivo inhibitors and tracers of the gelatinases, MMP-2 and MMP-9. Using a murine neuroinflammatory model, we show that compound 2 is a highly effective in vivo inhibitor of both MMP-2 and MMP-9 activity with little or no adverse effects even after long-term daily oral administration. A fluorescein-labeled derivative compound 17 shows direct binding to activated gelatinases surrounding inflammatory cuffs in the neuroinflammation model and to pancreatic ß-cells in the islets of Langerhans, colocalizing with MMP-2 and MMP-9 activity as detected using in situ zymography techniques. These results demonstrate that compound 2 derivatives have potential as in vivo imaging tools and for future development for specific MMP-2 versus MMP-9 probes. Our chemical modifications mainly target the residues directed toward the S1' and S2' pockets and, thereby, provide new information on the structure-activity relationships of this inhibitor type.

Syn-selective silicon Mukaiyama-type aldol reactions of (pentafluoro-λ6-sulfanyl)acetic acid esters with aldehydes.

Aldol reactions belong to the most frequently used C-C bond forming transformations utilized particularly for the construction of complex structures. The selectivity of these reactions depends on the geometry of the intermediate enolates. Here, we have reacted octyl pentafluoro-λ6-sulfanylacetate with substituted benzaldehydes and acetaldehyde under the conditions of the silicon-mediated Mukaiyama aldol reaction. The transformations proceeded with high diastereoselectivity. In case of benzaldehydes with electron-withdrawing substituents in the para-position, syn-α-SF5-ß-hydroxyalkanoic acid esters were produced. The reaction was also successful with meta-substituted benzaldehydes and o-fluorobenzaldehyde. In contrast, p-methyl-, p-methoxy-, and p-ethoxybenzaldehydes led selectively to aldol condensation products with (E)-configured double bonds in 30-40% yields. In preliminary experiments with an SF5-substituted acetic acid morpholide and p-nitrobenzaldehyde, a low amount of an aldol product was formed under similar conditions.

Synthesis of α-(Pentafluorosulfanyl)- and α-(Trifluoromethyl)-Substituted Carboxylic Acid Derivatives by Ireland-Claisen Rearrangement.

Earlier studies have shown that [3,3]-sigmatropic rearrangements of allyl esters are useful for the construction of fluorine-containing carboxylic acid derivatives. This paper describes the synthesis of 3-aryl-pent-4-enoic acid derivatives bearing either a pentafluorosulfanyl (SF5) or a trifluoromethyl (CF3) substituent in the 2-position by treatment of corresponding SF5- or CF3-acetates of p-substituted cinnamyl alcohols with triethylamine followed by trimethylsilyl triflate (TMSOTf). This Ireland-Claisen rearrangement delivered approximate 1:1 mixtures of syn/anti diastereoisomers due to tiny differences (<0.5 kcal/mol) both in the energy of (Z)/(E)-isomeric ester enolates and in the alternative Zimmerman-Traxler transition states of model compounds as shown by DFT calculations. Acidic reaction conditions have to be avoided since addition of the reagents in opposite sequence (first TMSOTf then Et3N) led to oligomerization of the cinnamyl SF5- and CF3-acetates. Treatment of the corresponding regioisomeric 1-phenyl-prop-2-en-1-yl acetates under the latter conditions resulted in [1,3]-sigmatropic rearrangement and subsequent oligomerization of the intermediately formed cinnamyl esters. When Et3N was added first followed by TMSOTf, no further reaction of the formed ester was detected.

Cyclobutyl-Containing Rigid Analogues of Threonine: Synthesis and Physical Chemical Properties.

Hitherto unknown cis- and trans-1-amino-3-hydroxy-3-methylcyclobutanecarboxylic acids were synthesized in multigram scale. The obtained compounds can be considered as achiral conformationally restricted analogues of threonine with fixed spatial orientation of functional groups. pKa values are noticeably different for both amino acids. According to the X-ray data the cyclobutane rings in both compounds are almost planar (the corresponding torsion angles are below 7°).

Fluorinated tranylcypromine analogues as inhibitors of lysine-specific demethylase 1 (LSD1, KDM1A).

We report a series of tranylcypromine analogues containing a fluorine in the cyclopropyl ring. A number of compounds with additional m- or p-substitution of the aryl ring were micromolar inhibitors of the LSD1 enzyme. In cellular assays, the compounds inhibited the proliferation of acute myeloid leukemia cell lines. Increased levels of the biomarkers H3K4me2 and CD86 were consistent with LSD1 target engagement.

Synthesis and physical chemical properties of 2-amino-4-(trifluoromethoxy)butanoic acid - a CF3O-containing analogue of natural lipophilic amino acids.

2-Amino-2-(trifluoromethoxy)butanoic acid (O-trifluoromethyl homoserine) was synthesized as a racemate and in both enantiomeric forms. The measured pKa and log D values establish the compound as a promising analogue of natural aliphatic amino acids.

Synthesis and evaluation of a [18F]BODIPY-labeled caspase-inhibitor.

BODIPYs (boron dipyrromethenes) are fluorescent dyes which show high stability and quantum yields. They feature the possibility of selective 18F-fluorination at the boron-core. Attached to a bioactive molecule and labeled with [18F]fluorine, the resulting compounds are promising tracers for multimodal imaging in vivo and can be used for PET and fluorescence imaging. A BODIPY containing a phenyl and a hydroxy substituent on boron was synthesized and characterized. Fluorinated and hydroxy substituted dyes were coupled to an isatin-based caspase inhibitor via cycloaddition and the resulting compounds were evaluated in vitro in caspase inhibition assays. The metabolic stability and the formed metabolites were investigated by incubation with mouse liver microsomes and LC-MS analysis. Subsequently the fluorophores were labeled with [18F]fluorine and an in vivo biodistribution study using dynamic PET was performed.

C3 -Symmetric Tricyclo[2.2.1.02,6 ]heptane-3,5,7-triol.

A straightforward access to a hitherto unknown C3 -symmetric tricyclic triol both in racemic and enantiopure forms has been developed. Treatment of 7-tert-butoxynorbornadiene with peroxycarboxylic acids provided mixtures of C1 - and C3 -symmetric 3,5,7-triacyloxynortricyclenes via transannular π-cyclization and replacement of the tert-butoxy group. By refluxing in formic acid, the C1 -symmetric esters were converted to the C3 -symmetric formate. Hydrolysis gave diastereoisomeric triols, which were separated by recrystallization. Enantiomer resolution via diastereoisomeric tri(O-methylmandelates) delivered the target triols on a gram scale. The pure enantiomers are useful as core units of dopants for liquid crystals.

Synthesis of SF5CF2-Containing Enones and Instability of This Group in Specific Chemical Environments and Reaction Conditions.

The chemistry of the SF5CF2 moiety has been scarcely investigated. In this report, we present synthetic pathways to a variety of SF5CF2-substituted compounds starting from vinyl ethers and SF5CF2C(O)Cl. In specific chemical environments and under particular reaction conditions, the SF5CF2 moiety is unstable in downstream products resulting in the elimination of the SF5(-) anion and its decomposition to SF4 and F(-). Surprisingly, the formed F(-) can attack the intermediate difluorovinyl moiety to form trifluoromethyl substituted products. This appears to happen when an intermediate neighboring group participation involving a double bond is possible. Under slightly different conditions, the reaction stops at the stage of a difluorovinyl compound.

Synthesis of α,ω-polyfluorinated α-amino acid derivatives and δ,δ-difluoronorvaline.

Intending to synthesize ω,ω-difluoroalkyl amino acid derivatives by oxidative desulfurization-fluorination reactions of suitable arylthio-2-phthalimido butanoates and pentanoates, in addition to small amounts of the target products, mainly α,ω-polyfluorinated amino acid derivatives were formed by additional sulfur-assisted α-fluorination. This novel structural motif was verified spectroscopically as well as by X-ray analysis. A plausible mechanism of formation is suggested. Using a different approach, δ,δ-difluoronorvaline hydrochloride was synthesized with at least 36% enantiomeric excess via deoxofluorination of the corresponding aldehyde.

Fluorinated matrix metalloproteinases inhibitors--Phosphonate based potential probes for positron emission tomography.

Fluorine-containing inhibitors of matrix metalloproteinases (MMPs) can serve as lead structures for the development of (18)F-labeled radioligands. These compounds might be useful as non-invasive imaging probes to characterize pathologies associated with increased MMP activity. Results with a series of fluorinated analogs of a known biphenyl sulfonamide inhibitor have shown that fluorine can be incorporated into two different positions of the molecular scaffold without significant loss of potency in the nanomolar range. Additionally, the potential of a hitherto unknown fluorinated tertiary sulfonamide as MMP inhibitor has been demonstrated.

Asymmetric Desymmetrization via Metal-Free C-F Bond Activation: Synthesis of 3,5-Diaryl-5-fluoromethyloxazolidin-2-ones with Quaternary Carbon Centers.

We disclose the first asymmetric activation of a non-activated aliphatic C-F bond in which a conceptually new desymmetrization of 1,3-difluorides by silicon-induced selective C-F bond scission is a key step. The combination of a cinchona alkaloid based chiral ammonium bifluoride catalyst and N,O-bis(trimethylsilyl)acetoamide (BSA) as the silicon reagent enabled the efficient catalytic cycle of asymmetric Csp3 -F bond cleavage under mild conditions with high enantioselectivities. The ortho effect of the aryl group at the prostereogenic center is remarkable. This concept was applied for the asymmetric synthesis of promising agrochemical compounds, 3,5-diaryl-5-fluoromethyloxazolidin-2-ones bearing a quaternary carbon center.