Alkoxyamines Designed as Potential Drugs against Plasmodium and Schistosoma Parasites

Hybrid molecules based on an emodin scaffold. Synthesis and activity against SARS-CoV-2 and Plasmodium

Since the Covid-19 epidemic, it has been clear that the availability of small and affordable drugs that are able to efficiently control viral infections in humans is still a challenge in medicinal chemistry. The synthesis and biological activities of a series of hybrid molecules that combine an emodin moiety and other structural moieties expected to act as possible synergistic pharmacophores in a single molecule were studied. Emodin has been reported to block the entry of the SARS-CoV-2 virus into human cells and might also inhibit cytokine production, resulting in the reduction of pulmonary injury induced by SARS-CoV-2. The pharmacophore associated with emodin was either a polyamine residue (emodin-PA series), a choice driven by the fact that a natural alkyl PA like spermine and spermidine play regulatory roles in immune cell functions, or a diphenylmethylpiperazine derivative of the norchlorcyclizine series (emoxyzine series). In fact, diphenylmethylpiperazine antagonists of the H1 histamine receptor display activity against several viruses by multiple interrelated mechanisms. In the emoxyzine series, the most potent drug against SARS-CoV-2 was (R)-emoxyzine-2, with an EC50 value = 1.9 μM, which is in the same range as that of the reference drug remdesivir. However, the selectivity index was rather low, indicating that the dissociation of antiviral potency and cytotoxicity remains a challenge. In addition, since emodin was also reported to be a relatively high-affinity inhibitor of the virulence regulator FIKK kinase from the malaria parasite Plasmodium vivax, the antimalarial activity of the synthesized hybrid compounds has been evaluated. However, these molecules cannot efficiently compete with the currently used antimalarial drugs.

Cascade strategy for triggered radical release by magnetic nanoparticles grafted with thermosensitive alkoxyamine

The design of smart nanoplatforms presenting well-definite structures able to achieve controlled cascade action remotely triggered by external stimuli presents a great challenge. We report here a new nanosystem consisting of magnetic iron oxide nanoparticles covalently grafted with a thermosensitive radical initiator alkoxyamine, able to provide controlled and localized release of free radicals triggered by an alternating current (ac) magnetic field. These nanoparticles exhibit a high intrinsic loss power of 4.73 nHm² kg^-1 providing rapid heating of their surface under the action of an ac field, inducing the homolysis of alkoxyamine C-ON bond and then the oxygen-independent formation of radicals. This latter was demonstrated by electronic paramagnetic resonance spectroscopy, and the kinetics of homolysis has been investigated allowing a comparison of the temperature of alkoxyamine's homolysis with the one measured during the magnetothermia process.

Spectroscopic and quantum mechanical study of a scavenger molecule: N,N-diethylhydroxylamine

We report a combination of quantum mechanical calculations and a range of spectroscopic measurements in the gas phase of N,N-diethylhydroxylamine, an important scavenger compound. Three conformers were observed by pulsed jet Fourier transform microwave spectroscopy in the 6.5-18.5 GHz frequency range. They are characterized by the hydroxyl hydrogen atom being in trans orientation with respect to the bisector of the CNC angle while the side alkyl chains can be both trans (global minimum, C_s symmetry, A = 7608.1078(4), B = 2020.2988(2) and C = 1760.5423(2) MHz) or one trans and the other gauche (second energy minimum, A = 5302.896(1), B = 2395.9822(4) and C = 1804.8567(3) MHz) or gauche' (third energy minimum, A = 5960.8025(6), B = 2273.6627(4) and C = 1975.8074(4) MHz). For the global minimum, the ¹³C_α,¹³C_β and ¹⁵N isotopologues were observed in natural abundance, allowing for an accurate partial structure determination. Moreover, several lines were detected by free jet absorption millimeter wave spectroscopy in the 59.6-74.4 GHz spectral range. The electron binding energies of the highest occupied molecular orbital and the next-to-highest occupied molecular orbital, determined by photoelectron spectroscopy, are 8.95 and 10.76 eV, respectively. Supporting calculations evidence that, (i) upon ionization of the HOMO, the molecular structure changes from an amine to an N-oxoammonium arrangement and (ii) the 0-0 of the HOMO-1 photoionization is 10.46 eV. The K-shell binding energies, determined by X-ray photoelectron spectroscopy, are 290.42 eV (C_β), 291.45 eV (C_α), 405.98 eV (N) and 538.75 eV (O). The Fourier transform near infrared spectrum is reported and a tentative assignment is proposed. The equilibrium wavenumber (ω̃ = 3811 cm^-1) and the anharmonicity constant (ω̃χ = -87.5 cm^-1) of the hydroxyl stretching mode were estimated using a quadratic model.

Design of Anti-infectious Agents from Lawsone in a Three-Component Reaction with Aldehydes and Isocyanides

The first effective synthetic approach to naphthofuroquinones via a reaction involving lawsone, various aldehydes, and three isocyanides under microwave irradiation afforded derivatives in moderate to good yields. In addition, for less-reactive aldehydes, two naphtho-enaminodione quinones were obtained for the first time, as result of condensation between lawsone and isocyanides. X-ray structure determination for 9 and 2D-NMR spectra of 28 confirmed the obtained structures. All compounds were evaluated for their anti-infectious activities against Plasmodium falciparum, Leishmania donovani, and Mycobacterium tuberculosis. Among the naphthofuroquinone series, 17 exhibited comparatively the best activity against P. falciparum (IC₅₀ = 2.5 μM) and M. tuberculosis (MIC = 9 μM) with better (P. falciparum) or equivalent (M. tuberculosis) values to already-known naphthofuroquinone compounds. Among the two naphtho-enaminodione quinones, 28 exhibited a moderate activity against P. falciparum with a good selectivity index (SI > 36) while also a very high potency against L. donovani (IC₅₀ = 3.5 μM and SI > 28), rendering it very competitive to the reference drug miltefosine. All compounds were studied through molecular modeling on their potential targets for P. falciparum, Pfbc1, and PfDHODH, where 17 showed the most favorable interactions.

Homolysis/mesolysis of alkoxyamines activated by chemical oxidation and photochemical-triggered radical reactions at room temperature

Instantaneous and spontaneous room temperature C–ON bond mesolysis of alkoxyamines triggered by chemical oxidation.

Smart Alkoxyamines: A New Tool for Smart Applications

In 1986, Rizzardo et al. discovered the nitroxide-mediated polymerization which relies on the reversibility of homolysis of the C-ON bond of alkoxyamine R¹R²NOR³, a unique property of these molecules. This discovery has generated a tremendous endeavor in the field of polymer chemistry. Alkoxyamines have been used as initiators/controllers for nitroxide-mediated polymerization. Moreover, photoexcitable alkoxyamines that dissociate under light at different wavelengths have also been developed for polymer chemistry. Over the past few years, alkoxyamines have started to be used in materials sciences. In many cases (e.g., self-healing polymers), the development of smart materials requires the use of smart building blocks, that is, molecules or systems whose properties and/or structures change upon external stimuli. Alkoxyamines exhibit a unique property: reversible homolysis (i.e., homolysis of the C-ON bond into alkyl R³• and nitroxyl R¹R²NO• radicals and reformation via the coupling of these two species). Until now, this property has been controlled only by changes in temperatures or by light irradiation. Chemical and/or biochemical control of the homolysis event would open new gates for the application of these molecules in different fields such as biology and medicine. Thus, the concept of smart alkoxyamines is discussed and exemplified via the activation of alkoxyamines using chemical or/and biochemical changes amplifying the polar, steric, and stabilization effects. In situ activation is also discussed. It is shown that (i) increasing the electron-withdrawing properties of the alkyl fragment weakens the C-ON bond and thus favors homolysis but is opposite for the nitroxyl fragment; (ii) increasing the steric hindrance on the nonactive site affords dramatic conformation changes which weaken the C-ON bond; and (iii) increasing the stabilization of the released alkyl radical weakens the C-ON bond. Solvent effects and intramolecular hydrogen bonding are also discussed. Reactions used to highlight our purpose are either reversible or nonreversible and used under conditions that are as mild as possible (temperatures below 40 °C and atmospheric pressure). For example, a several (thousands of millions of) millions of orders of magnitude enhancement of the homolysis rate constant is observed upon enzymatic hydrolysis at 37 °C, meaning that a shift from a stable alkoxyamine (t_1/2 = 42 000 milleniums) to a highly labile alkoxyamine (t_max = 1500 s for 35% conversion) is achieved. Applications of this concept are discussed for safe NMP initiators and for theranostic agents.