Synthesis and adrenergic activity of ring-fluorinated phenylephrines

Two New Fluorinated Phenol Derivatives Pyridine Schiff Bases: Synthesis, Spectral, Theoretical Characterization, Inclusion in Epichlorohydrin-β-Cyclodextrin Polymer, and Antifungal Effect

It has been reported that the structure of the Schiff bases is fundamental for their function in biomedical applications. Pyridine Schiff bases are characterized by the presence of a pyridine and a phenolic ring, connected by an azomethine group. In this case, the nitrogen present in the pyridine is responsible for antifungal effects, where the phenolic ring may be also participating in this bioactivity. In this study, we synthesized two new pyridine Schiff Bases: (E)-2-[(3-Amino-pyridin-4-ylimino)-methyl]-4,6-difluoro-phenol (F1) and (E)- 2-[(3-Amino-pyridin-4-ylimino)-methyl]-6-fluoro-phenol (F2), which only differ in the fluorine substitutions in the phenolic ring. We fully characterized both F1 and F2 by FTIR, UV-vis, ¹H; ¹³C; ¹⁹F-NMR, DEPT, HHCOSY, TOCSY, and cyclic voltammetry, as well as by computational studies (DFT), and NBO analysis. In addition, we assessed the antifungal activity of both F1 (two fluorine substitution at positions 4 and 6 in the phenolic ring) and F2 (one fluorine substitution at position 6 in the phenolic ring) against yeasts. We found that only F1 exerted a clear antifungal activity, showing that, for these kind of Schiff bases, the phenolic ring substitutions can modulate biological properties. In addition, we included F1 and F2 into in epichlorohydrin-β-cyclodextrin polymer (βCD), where the Schiff bases remained inside the βCD as determined by the k_i, TGA, DSC, and S_BET. We found that the inclusion in βCD improved the solubility in aqueous media and the antifungal activity of both F1 and F2, revealing antimicrobial effects normally hidden by the presence of common solvents (e.g., DMSO) with some cellular inhibitory activity. The study of structural prerequisites for antimicrobial activity, and the inclusion in polymers to improve solubility, is important for the design of new drugs.

Influence of Fluorination on the Conformational Properties and Hydrogen-Bond Acidity of Benzyl Alcohol Derivatives

The effect of fluorination on the conformational and hydrogen-bond (HB)-donating properties of a series of benzyl alcohols has been investigated experimentally by IR spectroscopy and theoretically with quantum chemical methods (ab initio (MP2) and DFT (MPWB1K)). It was found that o-fluorination generally resulted in an increase in the HB acidity of the hydroxyl group, whereas a decrease was observed upon o,o′-difluorination. Computational analysis showed that the conformational landscapes of the title compounds are strongly influenced by the presence of o-fluorine atoms. Intramolecular interaction descriptors based on AIM, NCI and NBO analyses reveal that, in addition to an intramolecular OH⋅⋅⋅F interaction, secondary CH⋅⋅⋅F and/or CH⋅⋅⋅O interactions also occur, contributing to the stabilisation of the various conformations, and influencing the overall HB properties of the alcohol group. The benzyl alcohol HB-donating capacity trends are properly described by an electrostatic potential based descriptor calculated at the MPWB1K/6-31+G(d,p) level of theory, provided solvation effects are taken into account for these flexible HB donors.

Synthesis and evaluation as biodegradable herbicides of halogenated analogs of L-meta-tyrosine

L-meta-tyrosine is an herbicidal nonprotein amino acid isolated some years ago from fine fescue grasses and characterized by its almost immediate microbial degradation in soil (half-life <24 h). Nine monohalogenated or dihalogenated analogs of this allelochemical have been obtained through a seven-step stereoselective synthesis from commercial halogenated phenols. Bioassays showed a large range of biological responses, from a growth root inhibition of lettuce seedling similar to that noted with m-tyrosine [2-amino-3-(2-chloro-5-hydroxyphenyl)propanoic acid or compound 8b] to an increase of the primary root growth concomitant with a delay of secondary root initiation [2-amino-3-[2-fluoro-5-hydroxy-3-(trifluoromethyl)phenyl]propanoic acid or compound 8h]. Compound 8b was slightly less degraded than m-tyrosine in the nonsterilized nutritive solution used for lettuce development, while the concentration of compound 8h remained unchanged for at least 2 weeks. These data indicate that it is possible to manipulate both biological properties and degradation of m-tyrosine by halogen addition.

Nucleophilic deoxyfluorination of catechols

Nucleophilic deoxyfluorinaiton of one of the two hydroxyl groups of catechols has been developed via the Umpolung concept. This method was successively applied to naturally occurring catechols, such as catechins and dopamine, to produce novel fluorinated analogues.

Metabolic stability of 6,7-dialkoxy-4-(2-, 3- and 4-[18F]fluoroanilino)quinazolines, potential EGFR imaging probes

Epidermal growth factor receptors (EGFR), upregulated in many tumor types, have been a target for therapeutic development and molecular imaging. The objective of this study was to evaluate the distribution and metabolic characteristics of fluorine-18 labeled anilinoquinazolines as potential imaging agents for EGFR tyrosine kinase expression. Fluorine-18 labeled fluoronitrobenzenes were prepared by reaction of potassium cryptand [(18)F]fluoride with 1,2- and 1,4-dinitrobenzenes, and 3-nitro-N,N,N-trimethylanilinium triflate in 5min. Decay-corrected radiochemical yields of [(18)F]fluoride incorporation into the nitro-aromatic compounds were 81±2%, 44±4% and 77±5% (n=3-5) for the 2-, 3- and 4-fluoro isomers, respectively. Sodium borohydride reduction to the corresponding [(18)F]fluoroanilines was achieved with greater than 80% conversion in 5min. Coupling of [(18)F]fluoroaniline-hydrochlorides to 6,7-dimethoxy-4-chloro-quinazoline gave the corresponding 6,7-dimethoxy-4-(2-, 3- and 4-[(18)F]fluoroanilino)quinazolines in 31±5%, 17±2% and 55±2% radiochemical yield, respectively, while coupling to the 6,7-diethoxy-4-chloro-quinazoline produced 6,7-diethoxy-4-(2-, 3- and 4-[(18)F]fluoroanilino)quinazolines in 19±6%, 9±3% and 36±6% radiochemical yield, respectively, in 90min to end of synthesis from [(18)F]fluoride. Biodistribution of 2- and 4-[(18)F]fluoroanilinoquinazolines was conducted in tumor-bearing mice (MDA-MB-435 and MDA-MB-468 xenografts). Low tumor uptake (<1% injected dose per gram (ID/g) of tissue up to 3h postinjection of the radiotracers) was observed. High bone uptake (5-15% ID/g) was noted with the 4-[(18)F]fluoroanilinoquinazolines. The metabolic stabilities of radiolabeled quinazolines were further evaluated by incubation with human female cryopreserved isolated hepatocytes. Rapid degeneration of the 4-fluoro-substituted compounds to baseline polar metabolites was observed by radio-TLC, whereas, the 2- and 3-[(18)F]fluoroaniline derivatives were significantly more stable, up to 2h, corroborating the in vivo biodistribution studies. para-Substituted [(18)F]fluoroanilines, a common structural motif in radiopharmaceuticals, are highly susceptible to metabolic degradation.

Synthesis of a series of caffeic acid phenethyl amide (CAPA) fluorinated derivatives: comparison of cytoprotective effects to caffeic acid phenethyl ester (CAPE)

A series of catechol ring-fluorinated derivatives of caffeic acid phenethyl amide (CAPA) were synthesized and screened for cytoprotective activity against H2O2 induced oxidative stress in human umbilical vein endothelial cells (HUVEC). CAPA and three fluorinated analogs were found to be significantly cytoprotective when compared to control, with no significant difference in cytoprotection between caffeic acid phenethyl ester (CAPE) and CAPA.

Structural basis of the selectivity of the beta(2)-adrenergic receptor for fluorinated catecholamines

The important and diverse biological functions of adrenergic receptors, a subclass of G protein-coupled receptors (GPCRs), have made the search for compounds that selectively stimulate or inhibit the activity of different adrenergic receptor subtypes an important area of medicinal chemistry. We previously synthesized 2-, 5-, and 6-fluoronorepinehprine (FNE) and 2-, 5-, and 6-fluoroepinephrine (FEPI) and found that 2FNE and 2FEPI were selective beta-adrenergic agonists and that 6FNE and 6FEPI were selective alpha-adrenergic agonists, while 5FNE and 5FEPI were unselective. Agonist potencies correlated well with receptor binding affinities. Here, through a combination of molecular modeling and site-directed mutagenesis, we have identified N293 in the beta(2)-adrenergic receptor as a crucial residue for the selectivity of the receptor for catecholamines fluorinated at different positions.

Metabolism of fluorine-containing drugs

This article reviews current knowledge of the metabolism of drugs that contain fluorine. The strategic value of fluorine substitution in drug design is discussed in terms of chemical structure and basic concepts in drug metabolism and drug toxicity.

Activation of α-1 adrenergic receptors modulates the control of left/right sidedness in rat embryos

Presomite stage rat embryos were cultured for 45-49 hr with medium containing various adrenergic agonists and antagonists. L-Norepinephrine but not D-norepinephrine (several orders of magnitude less potent than the L-isomer at alpha-1 adrenergic receptors) resulted in a dose-dependent increase of situs inversus similar to that found for phenylephrine, an alpha-1 adrenergic agonist. Prazosin, an alpha-1 adrenergic antagonist, inhibited phenylephrine-induced situs inversus in a dose-dependent manner. Neither dexmedetomidine, an alpha-2 adrenergic agonist, nor isoproterenol, a beta adrenergic agonist, caused situs inversus. These results provide pharmacological evidence that stimulation of alpha-1 but not of alpha-2 and beta adrenergic receptors modulates the control of left/right sidedness in rat embryos.

Oxidation of substituted 4-fluorobenzaldehydes: application to the no-carrier-added syntheses of 4-[18F]fluoroguaiacol and 4-[18F]fluorocatechol

The synthesis of 4-[18F]fluoroguaiacol (4-[18F]fluoro-2-methoxyphenol) has been achieved in no-carrier-added form starting from 2-methoxy-4-nitrobenzaldehyde, using nucleophilic aromatic substitution by [18F]fluoride followed by Baeyer-Villiger oxidation of the benzaldehyde to the phenol. Demethylation with boron tribromide gave 4-[18F]fluorocatechol (1,2-dihydroxy-4-[18F]fluorobenzene) with an overall yield of 18-28% (EOB) in less than 2 h synthesis time. The fluorine-18 labeled intermediates and products were identical to standards of 4-fluoroguaiacol and 4-fluorocatechol prepared by the same methods. This represents a new approach to the synthesis of fluorinated phenols in fluorine-19 and fluorine-18 forms.

Effect of ring fluorination of epinephrine on its cardiovascular adrenoceptor activities

Effects of fluorine (F) substitution on the 2- and 6-positions of the catechol ring of epinephrine (Epi) on its alpha- and beta-adrenoceptor agonist activities were studied in anesthetized dogs. Increments in heart rate and contractile force were used as measures of beta 1-adrenoceptor activity, while increases and decreases in blood pressure and decreases and increases in femoral blood flow were used as measures of alpha- and beta 2-adrenoceptor activities, respectively. F substitution on the 2- and 6-positions of the catechol ring yielded compounds with opposite receptor selectivities: 2-FEpi was a selective beta-adrenoceptor agonist with little agonist activity at alpha-adrenoceptors, while the 6-F analog was a selective a-adrenoceptor agonist with no significant beta-adrenoceptor effects. Of added significance, 2-FEpi was more potent than Epi as a beta 1-adrenoceptor agonist, while 6-F Epi was more potent than the parent compound as an alpha-adrenoceptor agonist. The possible mechanisms for the effects of ring fluorination on the adrenoceptor activities of Epi and other sympathomimetic amines are discussed.

Effects of ring fluorination on the adrenergic properties of phenylephrine

The adrenergic properties of 2-, 4- and 6-fluorophenylephrine (2-FPE, 4-FPE, 6-FPE) were compared to those of phenylephrine (PE). The order of affinities of these compounds for alpha 1-adrenoceptors as determined by displacement of [3H]prazosin and [3H]WB-4101 binding to brain membranes was the same as the order of potencies for eliciting two alpha 1-adrenergic metabolic responses in guinea-pig cerebral cortical synaptoneurosomes, namely the stimulation of phosphatidylinositol turnover and the potentiation of 2-chloroadenosine-induced accumulation of cyclic AMP. In all cases the order was 6-FPE greater than PE greater than 4-FPE greater than 2-FPE. The order of affinities for alpha 2-adrenoceptors as determined by displacement of binding of [3H]clonidine to brain membrane was 6-FPE greater than PE greater than or equal to 4-FPE = 2-FPE. In contrast, the order of potencies for inhibition of forskolin-stimulated adenylate cyclase activity in human platelet membranes via an alpha 2-adrenoceptor was 6-FPE approximately equal to PE greater than 4-FPE much greater than 2-FPE. The FPEs and PE were partial agonists compared to epinephrine in human platelets. The affinities of these compounds for beta-adrenoceptors as determined by displacement of binding of [3H]dihydroalprenolol to brain membranes are 2-FPE greater than PE greater than or equal to 4-FPE much greater than 6-FPE. The FPEs and PE had positive chronotropic and inotropic effects in isolated guinea-pig atria apparently through the activation of a beta-adrenoceptor, since pindolol blocked the response while prazosin did not. 6-FPE appeared less active than the other PEs in atria. In fat cell membranes, 2-FPE was more potent than PE in stimulating adenylate cyclase via a beta-adrenoceptor, while 4-FPE and 6-FPE were inactive. Both, 2-FPE and PE were partial agonists in fat cells compared to isoproterenol. Of the three FPEs, 6-FPE represents a more potent and more selective agonist for alpha-adrenoceptors compared to beta-adrenoceptors than PE, while 4-FPE and, in particular, 2-FPE are less potent and selective as alpha-agonists.