Regioselective and Stereoselective Synthesis of Parthenolide Analogs by Acyl Nitroso-Ene Reaction and Their Biological Evaluation against Mycobacterium tuberculosis

Historically, natural products have played a major role in the development of antibiotics. Their complex chemical structures and high polarity give them advantages in the drug discovery process. In the broad range of natural products, sesquiterpene lactones are interesting compounds because of their diverse biological activities, their high-polarity, and sp3-carbon-rich chemical structures. Parthenolide (PTL) is a natural compound isolated from Tanacetum parthenium, of the family of germacranolide-type sesquiterpene lactones. In recent years, parthenolide has been studied for its anti-inflammatory, antimigraine, and anticancer properties. Recently, PTL has shown antibacterial activities, especially against Gram-positive bacteria. However, few studies are available on the potential antitubercular activities of parthenolide and its analogs. It has been demonstrated that parthenolide’s biological effects are linked to the reactivity of α-exo-methylene-γ-butyrolactone, which reacts with cysteine in targeted proteins via a Michael addition. In this work, we describe the ene reaction of acylnitroso intermediates with parthenolide leading to the regioselective and stereoselective synthesis of new derivatives and their biological evaluation. The addition of hydroxycarbamates and hydroxyureas led to original analogs with higher polarity and solubility than parthenolide. Through this synthetic route, the Michael acceptor motif was preserved and is thus believed to be involved in the selective activity against Mycobacterium tuberculosis.


Introduction
Natural products (NPs) and their derivatives have always been an important source of new medicines.Natural products often have complex chemical structures and can exhibit a wide range of biological activities, making them valuable starting points in the drug discovery process.Compared with conventional synthetic molecules, NPs are sometimes also more polar, which is an advantage in the discovery of new targets [1].In the vast range of NPs, sesquiterpene lactones are compounds of interest for their structural complexity and diversity.Parthenolide (PTL) is a germacranolide-type sesquiterpene lactone isolated from Tanacetum parthenium [2].For decades, PTL has been known for its anti-inflammatory and antimigraine effects [3].The wide range of biological activities shown by PTL is mainly attributed to the α-exo-methylene-γ-butyrolactone moiety present in its structure [4,5].In fact, α-exo-methylene is able to react with nucleophilic cysteines from biomolecules through a Michael addition (Figure 1) [6][7][8] to form a covalent bond.Through this mechanism, PTL inhibits irreversibly the NF-κB pathway, which is involved in anti-inflammatory activity, in the regulation of cell proliferation, and in antiapoptotic mechanisms [9].Parthenolide reacts with thiols from glutathione, leading to the depletion of GSH and consequent activation of the ROS cascade [10,11].Precisely for all these targets, parthenolide is well-known as an anticancer agent.Many studies have been carried out through the chemical modification of PTL, mainly focusing on α-exo-methylene, including (i) a Michael addition with primary and secondary amines and thiol groups with increasing solubility [12][13][14][15] and (ii) Heck coupling with aromatic iodo-compounds [16].All these studies have shown important advances in the anticancer properties of parthenolide [11,17].More recently, interest in PTL as an antibacterial agent has grown.Hakkinen et al. studied the activity against Gram-positive and Gram-negative bacteria, and PTL showed higher activity against Staphylococcus aureus (S. aureus) than against Gram-negative bacteria, such as Pseudomonas aeruginosa and Escherichia coli [18,19].Few studies have investigated the activity against Mycobacterium tuberculosis (M.tuberculosis) [20][21][22].PTL displayed a minimum inhibitory concentration (MIC) of 16 µg/mL against M. tuberculosis strains, but no further investigation has been carried out on the structureactivity relationship or target identification [22].Hence, we planned the synthesis of original parthenolide-based derivatives using an acylnitroso-ene reaction, which were evaluated as antibacterial agents and, more precisely, as anti-tuberculosis drugs.With this work, we optimize a regioselective and stereoselective synthetic route for original analogs.Through an acyl nitroso-ene modification, the Michael acceptor motif is preserved.Moreover, owing to the introduction of the hydroxylamine moiety on the main core, the new semisynthetic analogs displayed increased solubility compared to that of the parent compound.

Chemistry
C-7-hydroxycarbamate/urea-parthenolide analogs were synthesized from commercially available parthenolide via a reaction with hydroxylamine-functionalized building blocks (Scheme 1).First, the optimization was performed, starting from the conditions previously described by Frazier et al. [23].The critical step in the acylnitroso-ene reaction is the oxidation of hydroxylamine-containing compounds to nitroso derivatives prior to the reaction with parthenolide.Through this mechanism, PTL inhibits irreversibly the NF-κB pathway, which is involved in anti-inflammatory activity, in the regulation of cell proliferation, and in antiapoptotic mechanisms [9].Parthenolide reacts with thiols from glutathione, leading to the depletion of GSH and consequent activation of the ROS cascade [10,11].Precisely for all these targets, parthenolide is well-known as an anticancer agent.Many studies have been carried out through the chemical modification of PTL, mainly focusing on α-exo-methylene, including (i) a Michael addition with primary and secondary amines and thiol groups with increasing solubility [12][13][14][15] and (ii) Heck coupling with aromatic iodo-compounds [16].All these studies have shown important advances in the anticancer properties of parthenolide [11,17].More recently, interest in PTL as an antibacterial agent has grown.Hakkinen et al. studied the activity against Gram-positive and Gram-negative bacteria, and PTL showed higher activity against Staphylococcus aureus (S. aureus) than against Gram-negative bacteria, such as Pseudomonas aeruginosa and Escherichia coli [18,19].Few studies have investigated the activity against Mycobacterium tuberculosis (M.tuberculosis) [20][21][22].PTL displayed a minimum inhibitory concentration (MIC) of 16 µg/mL against M. tuberculosis strains, but no further investigation has been carried out on the structure-activity relationship or target identification [22].Hence, we planned the synthesis of original parthenolide-based derivatives using an acylnitroso-ene reaction, which were evaluated as antibacterial agents and, more precisely, as anti-tuberculosis drugs.With this work, we optimize a regioselective and stereoselective synthetic route for original analogs.Through an acyl nitroso-ene modification, the Michael acceptor motif is preserved.Moreover, owing to the introduction of the hydroxylamine moiety on the main core, the new semisynthetic analogs displayed increased solubility compared to that of the parent compound.

Chemistry
C-7-hydroxycarbamate/urea-parthenolide analogs were synthesized from commercially available parthenolide via a reaction with hydroxylamine-functionalized building blocks (Scheme 1).First, the optimization was performed, starting from the conditions previously described by Frazier et al. [23].The critical step in the acylnitroso-ene reaction is the oxidation of hydroxylamine-containing compounds to nitroso derivatives prior to the reaction with parthenolide.
Therefore, an optimization step was necessary to identify the best conditions and the best oxidative system.Different reaction times, equivalents, and oxidant agents were investigated in a model reaction with 1-hydroxy-3-phenyl-urea (2a).All the studied conditions are available in the supplementary information.After the optimization, oxygen was chosen as oxidizing agent, and the reaction was carried out in the presence of a catalyst system of CuCl (0.05 eq.), pyridine (0.125 eq.), and THF as the solvent (Scheme 2).
Therefore, an optimization step was necessary to identify the best conditions and the best oxidative system.Different reaction times, equivalents, and oxidant agents were investigated in a model reaction with 1-hydroxy-3-phenyl-urea (2a).All the studied conditions are available in the supplementary information.After the optimization, oxygen was chosen as oxidizing agent, and the reaction was carried out in the presence of a catalyst system of CuCl (0.05 eq.), pyridine (0.125 eq.), and THF as the solvent (Scheme 2).To expand the scope of this reaction, two series of compounds were synthesized based on hydroxylamine-containing functions: hydroxyurea and hydroxycarbamate, either purchased or synthesized.Urea (2a-h) and carbamate (3a-k) precursors were obtained from isocyanates and chloroformates, respectively (Scheme 3A).Unfortunately, the Therefore, an optimization step was necessary to identify the best conditions and the best oxidative system.Different reaction times, equivalents, and oxidant agents were investigated in a model reaction with 1-hydroxy-3-phenyl-urea (2a).All the studied conditions are available in the supplementary information.After the optimization, oxygen was chosen as oxidizing agent, and the reaction was carried out in the presence of a catalyst system of CuCl (0.05 eq.), pyridine (0.125 eq.), and THF as the solvent (Scheme 2).To expand the scope of this reaction, two series of compounds were synthesized based on hydroxylamine-containing functions: hydroxyurea and hydroxycarbamate, either purchased or synthesized.Urea (2a-h) and carbamate (3a-k) precursors were obtained from isocyanates and chloroformates, respectively (Scheme 3A).Unfortunately, the To expand the scope of this reaction, two series of compounds were synthesized based on hydroxylamine-containing functions: hydroxyurea and hydroxycarbamate, either purchased or synthesized.Urea (2a-h) and carbamate (3a-k) precursors were obtained from isocyanates and chloroformates, respectively (Scheme 3A).Unfortunately, the introduction of more hydrophilic moieties was challenging owing to the high polarity of the corresponding ureas.In fact, after the work-up, urea precursors were isolated as a mixture of the expected compound, salts, and HCl residues, which prevented the formation of nitroso-ene compounds.On the other hand, ureas bearing aromatic substituents were easily accessed.Hydroxycarbamates with polar and less-polar groups were easily obtained in good to high yields.Parthenolide reacted with a variety of substrates, from some polar alkyl groups to more lipophilic aromatic substituents (Scheme 3B) under the optimized conditions in Scheme 2. As a result, nineteen original compounds were obtained in yields ranging from 25 to 91%.
Moreover, it must be noted that the reaction between the parthenolide and hydroxyurea led to the formation of two compounds (Scheme 4).In fact, it was observed that after 48 h, a mixture of the parthenolide, hydroxyurea derivative (9), and side compound corresponding to the oxime derivative (23) was obtained.Compound 23 was then isolated, and a biological assessment was carried out.Surprisingly, the removal of the O 2 supply resulted in the formation of hydroxyurea (9) in a very good yield, and just traces of oxime were observed.
Moreover, it must be noted that the reaction between the parthenolide and hydroxyurea led to the formation of two compounds (Scheme 4).In fact, it was observed that after 48 h, a mixture of the parthenolide, hydroxyurea derivative (9), and side compound corresponding to the oxime derivative (23) was obtained.Compound 23 was then isolated, and a biological assessment was carried out.Surprisingly, the removal of the O2 supply   Scheme 4. Nitroso-ene reaction between parthenolide and hydroxyurea.Two compounds were obtained in the presence of O2: the oxime derivative (23) and hydroxyurea derivative (9).Moreover, the conversion was not total.Compounds 23 and 9 were isolated after purification under reversephase conditions (C-18 column).
Conditions under air were tested for this reaction, and it was observed that O2 enhanced the acylnitroso-ene reaction, and no formation of oxidized compounds was observed (entry E, Table S1 in Supplementary Materials).To the best of our knowledge, this is the first report of a nitroso-ene reaction with parthenolide under mild conditions and with high stereoselectivity and regioselectivity to give twenty semisynthetic analogs in moderate to good yields.Scheme 4. Nitroso-ene reaction between parthenolide and hydroxyurea.Two compounds were obtained in the presence of O 2 : the oxime derivative (23) and hydroxyurea derivative (9).Moreover, the conversion was not total.Compounds 23 and 9 were isolated after purification under reversephase conditions (C-18 column).
Conditions under air were tested for this reaction, and it was observed that O 2 enhanced the acylnitroso-ene reaction, and no formation of oxidized compounds was observed (entry E, Table S1 in Supplementary Materials).To the best of our knowledge, this is the first report of a nitroso-ene reaction with parthenolide under mild conditions and with high stereoselectivity and regioselectivity to give twenty semisynthetic analogs in moderate to good yields.

Proposed Reaction Mechanism
Numerous hypotheses have been postulated for the mechanism of the nitroso-ene reaction, but it has not been totally elucidated yet [24][25][26][27][28].The ene reaction of nitrosocarbonyl derivatives follows Markovnikov's rule with the addition of the electron-deficient enophile (24) to the less-substituted carbon of the alkene (1).According to previous studies, we hereby suggest that the reaction takes place through an intermediary aziridinium state (25, Scheme 5), which can explain the regioselectivity and stereoselectivity.In an oxidant environment, O 2 leads to oxidation from Cu (I) to Cu (II), then Cu (II) oxidizes the hydroxylamine derivative (24, hydroxyurea or hydroxycarbamate) to nitrosocarbonyl species.The combination of CuCl, pyridine, and THF promotes the generation of the transient state of nitroso species in situ [29].Afterward, a concerted mechanism takes place: the electrophilic nitrogen of 24 attacks the ene species, parthenolide, and, vice versa, the ene attacks the enophile (24) leading to the aziridium intermediate (25).The intermediate (25), which is unstable, evolves to the final compound by H-abstraction from the more-substituted and less-crowded end, leading to high regioselectivity.The less-hindered Re-face attack from the nitroso species is favored, thus conferring high stereoselectivity.

X-ray Crystallographic and NOESY Data
In support of our hypothesis, the configuration of the new chiral carbon (C-7) was determined by a NOESY experiment with compound 14.MOE modelling enabled the determination of four possible structures: two for each configuration R or S (images are available in Supplementary Materials).For the R configuration, both models showed a possible spatial interaction between proton 3 and proton 7 (depicted in green, Figure 2A).On the contrary, for the S configuration, the two models showed that proton 7 is in spatial proximity to methyl group 15 (depicted in red, Figure 2B).NOESY spectra determined correlations between H3/H7 and H2/H15, suggesting the R configuration for carbon 7.In addition, obtaining the crystallographic structure of 14 enabled us to confirm the exact configuration (Figure 2C).

X-ray Crystallographic and NOESY Data
In support of our hypothesis, the configuration of the new chiral carbon (C-7) was determined by a NOESY experiment with compound 14.MOE modelling enabled the determination of four possible structures: two for each configuration R or S (images are available in Supplementary Materials).For the R configuration, both models showed a possible spatial interaction between proton 3 and proton 7 (depicted in green, Figure 2A).On the contrary, for the S configuration, the two models showed that proton 7 is in spatial proximity to methyl group 15 (depicted in red, Figure 2B).NOESY spectra determined correlations between H3/H7 and H2/H15, suggesting the R configuration for carbon 7.In addition, obtaining the crystallographic structure of 14 enabled us to confirm the exact configuration (Figure 2C).able in Supplementary Materials).For the R configuration, both models showed a possible spatial interaction between proton 3 and proton 7 (depicted in green, Figure 2A).On the contrary, for the S configuration, the two models showed that proton 7 is in spatial proximity to methyl group 15 (depicted in red, Figure 2B).NOESY spectra determined correlations between H3/H7 and H2/H15, suggesting the R configuration for carbon 7.In addition, obtaining the crystallographic structure of 14 enabled us to confirm the exact configuration (Figure 2C).

Biological Evaluation
All the synthesized compounds together with parthenolide were tested against the virulent strain M. tuberculosis H37Rv and S. aureus (Tables 1 and 2).A resazurin microtiter assay (REMA) and a GFP fluorescent assay were performed for M. tuberculosis, and a REMA assay was performed for S. aureus.The compounds were dissolved in DMSO and tested at eight different concentrations (from 300 µM to 2.3 µM).After a period of incubation (7 days for M. tuberculosis and 5 h for S. aureus), resazurin was added, and the fluorescence was measured.Rifampicin and DMSO were used as positive and negative controls, respectively.The results were expressed as MIC 90 (REMA assay) or MIC 98 (GFP assay) values, meaning that the drug concentration inhibits 90% or 98% of the bacterial growth.All the nitroso-ene derivatives showed antibacterial activity against M. tuberculosis.Instead, no activity (MIC 90 ca.300 µM) was observed against S. aureus.It was observed that bulky groups enhance the activity against M. tuberculosis.In fact, compounds 9 and 23 with a hydroxyurea and an oxime moiety, respectively, were found to be inactive (MIC 90 ca.150 µM).In accordance with this observation, aromatic substituents were well-tolerated for both series.However, the urea derivatives showed greater inhibitory activity against M. tuberculosis strains.The introduction of small substituents, such as a methoxy group (compound 7, Table 1) and a fluorine atom (compound 8, Table 1), to the para position of the benzene ring enhances the inhibitory activity with a MIC 90 value of 19 µM (REMA assay).Compared with the parent compound, the most active compounds are in the same range of activity (MIC 90 is between 9.4 µM and 19 µM).However, advantageously, all the nitroso-ene derivatives are more polar and more water soluble than PTL.The urea derivatives (compounds 8 and 9) showed from 1.3-to 1.5-fold higher solubility in PBS compared to parthenolide, respectively.Table 1.Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxyureas (ND is the abbreviation for "not determined").
group (compound 7, Table 1) and a fluorine atom (compound 8, Table 1), to the para position of the benzene ring enhances the inhibitory activity with a MIC90 value of 19 µM (REMA assay).Compared with the parent compound, the most active compounds are in the same range of activity (MIC90 is between 9.4 µM and 19 µM).However, advantageously, all the nitroso-ene derivatives are more polar and more water soluble than PTL.The urea derivatives (compounds 8 and 9) showed from 1.3-to 1.5-fold higher solubility in PBS compared to parthenolide, respectively.1) and a fluorine atom (compound 8, Table 1), to the para pos tion of the benzene ring enhances the inhibitory activity with a MIC90 value of 19 µM (REMA assay).Compared with the parent compound, the most active compounds are i the same range of activity (MIC90 is between 9.4 µM and 19 µM).However, advanta geously, all the nitroso-ene derivatives are more polar and more water soluble than PTL The urea derivatives (compounds 8 and 9) showed from 1.3-to 1.5-fold higher solubilit in PBS compared to parthenolide, respectively.1) and a fluorine atom (compound 8, Table 1), to the para posi-tion of the benzene ring enhances the inhibitory activity with a MIC90 value of 19 µM (REMA assay).Compared with the parent compound, the most active compounds are in the same range of activity (MIC90 is between 9.4 µM and 19 µM).However, advantageously, all the nitroso-ene derivatives are more polar and more water soluble than PTL.
The urea derivatives (compounds 8 and 9) showed from 1.3-to 1.5-fold higher solubility in PBS compared to parthenolide, respectively.As described in the introduction (Section 1), parthenolide reacts with biological nucleophiles through a Michael addition, and it results in potent anticancer agents.Given their unspecific electrophilicity, all the synthesized compounds were also tested for their cytotoxicity activities in Balb3T3 cell lines.The aromatic derivatives showed increased cytotoxicities compared with that of the parent compound.However, the specific activity against M. tuberculosis does not seem to be related to cytotoxicity, given the absence of activity against S. aureus.At this stage of the investigation, we can only suggest that the Michael acceptor is involved in the activity.In line with our hypothesis, no activity against either M. tuberculosis or S. aureus was found for Dihydro-PTL (26, Table 2).Extensive studies are needed to discover a possible target and/or perhaps a mechanism of action.As described in the introduction (Section 1), parthenolide reacts with biological nucleophiles through a Michael addition, and it results in potent anticancer agents.Given their unspecific electrophilicity, all the synthesized compounds were also tested for their As described in the introduction (Section 1), parthenolide reacts with biological nucleophiles through a Michael addition, and it results in potent anticancer agents.Given their unspecific electrophilicity, all the synthesized compounds were also tested for their cytotoxicity activities in Balb3T3 cell lines.The aromatic derivatives showed increased cy- As described in the introduction (Section 1), parthenolide reacts with biological nucleophiles through a Michael addition, and it results in potent anticancer agents.Given their unspecific electrophilicity, all the synthesized compounds were also tested for their cytotoxicity activities in Balb3T3 cell lines.The aromatic derivatives showed increased cytotoxicities compared with that of the parent compound.However, the specific activity against M. tuberculosis does not seem to be related to cytotoxicity, given the absence of As described in the introduction (Section 1), parthenolide reacts with biological nucleophiles through a Michael addition, and it results in potent anticancer agents.Given their unspecific electrophilicity, all the synthesized compounds were also tested for their cytotoxicity activities in Balb3T3 cell lines.The aromatic derivatives showed increased cytotoxicities compared with that of the parent compound.However, the specific activity against M. tuberculosis does not seem to be related to cytotoxicity, given the absence of activity against S. aureus.At this stage of the investigation, we can only suggest that the

Chemistry
The reagents and solvents for synthesis, analysis, and purification were purchased from commercial suppliers (Fisher Scientific (France), Sigma Aldrich (France), Key Organics (UK), and Fluorochem (UK and Ireland)) and used without further purification.The progress of all the reactions was routinely monitored by thin-layer chromatography (TLC) and/or by high-performance liquid chromatography-mass spectrometry (HPLC-MS).TLC was performed using Merck ® commercial aluminum sheets coated with silica gel 60 F254.Visualization was achieved by fluorescence quenching under UV light at 254 nm and 215 nm or by staining with potassium permanganate.Purifications were performed by flash chromatography and reverse chromatography.Flash chromatography purifications were conducted in columns prepacked with Reveleris ® flash cartridges, Buchi, France (40 µm, Büchi ® FlashPure, Villebon sur Yvette, France or 15-40 µM, Macherey-Nagel ® Chromabond ® , Hoerdt, France) under pressure with an Interchim Puriflash ® 430 instrument (Montluçon, France).The products were detected by UV absorption at 254 nm and by ELSD.Reverse chromatographies were conducted using Combiflash ® C18 Rf200 in 4g, 12g, and C18 columns (Serlabo Technologies, EntraiguessurlaSorgues, France).The products were detected by UV absorption at 215 nm and 254 nm.UPLC-MS analysis was performed on an LC-MS Waters ACQUITY UPLC I-Class system (Guyancourt, France) equipped with a UPLC I BIN SOL MGR solvent manager, a UPLC I SMP MGR-FTN sample manager, an ACQUITY UPLC I-Class eλ PDA photodiode array detector (210-400 nm), and an ACQUITY QDa (Performance) mass detector (full scan ESI+/− in the range 30-1250).A Waters Acquity BEH C18 column (1.7 µm particle size; dimensions: 50 mm × 2.1 mm) was used for UPLC analysis (Guyancourt, France).The injection volume was 0.5 µL.For a 5 min analysis, the elution was carried out with a gradient starting at 98% H 2 O and 5 mM ammonium formate (pH 3.8) and reaching 98% CH 3 CN and 5 mM ammonium formate (pH 3.8; 5% aqueous) over 3.5 min at a flow rate of 600 µL/min.For a 30 min analysis, the elution was carried out with a gradient starting at 98% H 2 O and 5 mM ammonium formate (pH 3.8) and reaching 98% CH 3 CN and 5 mM ammonium formate (pH 3.8; 5% aqueous) over 25 min at a flow rate of 600 µL/min.Purity (%) was determined by reverse-phase UPLC, using UV detection (at 215 and 254 nm).HRMS analyses were performed on an LCT Premier XE Micromass (Guyancourt, France), using a Waters XBridge BEH C18 column with a 3.5 µm particle size and dimensions of 50 mm × 4.6 mm.A gradient starting from 98% H 2 O and 5 mM ammonium formate (pH 3.8) and reaching 100% CH 3 CN and 5 mM ammonium formate (pH 3.8; 5% aqueous) within 3 min at a flow rate of 2 mL/min was used.NMR spectra were recorded on a Bruker ® Avance-300 spectrometer (Wissembourg, France).The results were calibrated to signals from the solvent as an internal reference (e.g., 7.26 (residual CDCl 3 ) and 77.16 (CDCl 3 ) ppm and 2.50 (residual DMSO d 6 ) and 39.52 (DMSO d 6 ) ppm for 1 H and 13 C NMR spectra, respectively.)Chemical shifts (δ) are in parts per million (ppm) downfield from tetramethylsilane (TMS).The assignments were made using one-dimensional (1D) 1 H and 13 C spectra and two-dimensional (2D) HSQC-DEPT, COSY, and HMBC spectra.NMR coupling constants (J) are reported in hertz (Hz), and splitting patterns are indicated as follows: s (singlet); brs (broad singlet); d (doublet); dd (doublet of doublet); ddd (double of doublet of doublet); dt (doublet of triplet); t (triplet); td (triplet of doublet); q (quartet); m (multiplet).

General Procedure for the Synthesis of Hydroxyurea Derivatives (2a-h)
In a round-bottom flask, 1.0 eq. of isocyanate was diluted in DCM.Then, 6.0 eq. of hydroxylamine hydrochloride and 6.0 eq. of sodium hydroxide in water were added at 0 • C, and the solution was stirred for 3 h at room temperature.The aqueous layer was extracted once with DCM.The dichloromethane phase was discarded.Then, the aqueous phase was acidified with HCl (1 M) and extracted three times with ethyl acetate.The organic layer was dried over MgSO 4 and concentrated under reduced pressure to give a crude residue.Purification by flash chromatography afforded the desired compound.

General Procedure for the Synthesis of Hydroxycarbamate Derivatives (3a-k)
In a round-bottom flask, 1.0 eq. of chloroformate was added to a stirred solution of hydroxylamine hydrochloride (2.50 eq.) and sodium hydroxide (3 eq.) in water at room temperature.After 4 h, the mixture was acidified with HCl (1 M) and then extracted three times with ethyl acetate.The organic layer was dried over MgSO 4 and concentrated under reduced pressure to give a crude residue.Purification by flash chromatography afforded the desired compound.
1,3-benzodioxol-5-yl N-hydroxycarbamate (3i) was obtained as a white powder after purification by flash chromatography in column 15g, with a gradient from 100% dichloromethane to 70/30 dichloromethane/ethyl acetate (75 mg, 51%). 1  Tetrahydropyran-4-yl N-hydroxycarbamate (3k) was obtained as a white powder, without further purification (100 mg, 95%). 1  In a double-neck flask, CuCl (0.05 eq., 1 mg) and pyridine (0.125 eq., 2 µL) were added to the parthenolide (50 mg) and hydroxylamine derivative (1.1 eq.).Then, 2.2 mL of THF was added.The reaction mixture was stirred at room temperature under O 2 (in a balloon).The reaction was monitored using TLC.After 24 h or 48 h, the reaction was stopped with a solution of EDTA (0.5 M, pH 7.0), diluted with ethyl acetate, and stirred until the color no longer persisted in the organic layer.Then, the water phase was extracted three times withe ethyl acetate.The organic phase was dried over MgSO 4 and concentrated under reduced pressure to give a crude residue.Purification by flash chromatography afforded the desired compound.

Determination of MICs
The minimal inhibitory concentrations (MICs) of the compounds against M. tuberculosis were determined using both GFP and resazurin (REMA) as a readout of the bacterial viability.Briefly, a mid-log phase of the H37Rv::pJKD6 cultures was diluted to an OD600 of 0.001 and distributed in a 96-well plate (100 µL) in the presence of serial dilutions of the compounds of interest.Following 7 days of incubation (at 37 • C), bacterial growth was measured based on GFP fluorescence (Ex.at 485 nm; Em. at 510 nm) using an EnSight fluorescence plate reader (Perkin Elmer).Subsequently, 10 µL of resazurin (0.1% w/v) was added to the same bacteria and incubated (for 18 h at 37 • C), and the bacterial metabolic conversion of resazurin to resorufin was measured based on fluorescence (Ex.at 560 nm; Em. at 590 nm) using the same plate reader.For the GFP fluorescence readout, the data were background-corrected (without a bacterial control) and expressed as a percentage of the GFP signal compared to that for the untreated bacteria, with MIC GFP defined as the concentration that results in less than 2% of the initial GFP signal.For the resazurin (REMA) fluorescence readout, the data were background-corrected (without a bacterial control) and expressed as a percentage of the REMA signal compared to that for the untreated bacteria, with MIC REMA defined as the concentration that results in less than 10% of the initial REMA signal.
The MICs of the compounds against S. aureus were determined using only the resazurin (REMA)-based viability readout.The experiment was performed as for M. tuberculosis, with the following changes: the bacteria were grown in the presence of the compounds for only 5 h (not 7 days) and were co-incubated with resazurin for 1 h before the fluorescence reading and analysis.

X-ray Structural Determination
Single-crystal X-ray crystallographic analyses were conducted on compound 14.The diffraction data were obtained using a combination of phi-and omega-scans on a Bruker Apex DUO diffractometer equipped with a Photon 3 hybrid pixel area detector mounted on a four-circle D8 goniometer.Cu Kα radiation (λ = 1.54178Å) was obtained using an ImuS Incoatec Cu microfocus sealed tube.The data solution was found using SHELXT [33] and refined using SHELXL [34], as implemented in the Olex 2 crystallographic suite for small molecules [35].The X-ray crystallographic data for compound 14 have been deposited at the Cambridge Crystallographic Data Centre, under the reference number CCDC-2226330, and can be obtained freely at https://www.ccdc.cam.ac.uk/structures/ (accessed on 11 December 2023).

Solubility Determination
The kinetic solubility was determined using LC-MS/MS analysis in PBS at pH 7.4.Briefly, a 10 mM 100% DMSO solution of the compound was diluted 50-fold either in PBS at pH 7.4 (in triplicate) or in DMSO in polypropylene tubes (n = 3 for PBS and n = 6 for DMSO).The tubes were gently shaken for 24 h at 21 • C.Then, the three PBS tubes and three of the six DMSO tubes were centrifuged for 5 min at 4000 rpm and filtered over 0.45 µm filters (Millex-LH Millipore).Then, 40 µL of each solution was diluted in 160 µL of MeCN and 40 µL of Milli-Q water and transferred to matrix tubes for the LC-MS/MS analysis.The solubility was determined according to the following formula: Solubility (µM) = (AUC (filtered PBS)/AUC (not filtered DMSO)) × 200.The test was validated if (AUC (not filtered DMSO) − AUC (filtered DMSO))/AUC (not filtered DMSO) ≤ 10%.

Cytotoxicity
The cytotoxicity of the compounds to BALB/3T3 cells was determined using live imaging following both Hoechst 33342 and propidium iodide staining.Briefly, BALB/3T3 cells were seeded in a 384-well plate and 24 h later, the compounds were added (0, 12.5, 25, 50, and 100 µM) to the culture medium, as well as Hoechst 33342 and propidium iodide (PI).Then, 24 h and 48 h after the compounds were added, live imaging was performed using

Figure 1 .
Figure 1.Biological reactivity of parthenolide in the presence of a cysteine residue in a protein attacking the β-carbon of the Michael acceptor.

Figure 1 .
Figure 1.Biological reactivity of parthenolide in the presence of a cysteine residue in a protein attacking the β-carbon of the Michael acceptor.

22 Scheme 5 .
Scheme 5. Proposed mechanism of reaction to explain regioselectivity and stereoselectivity.

Scheme 5 .
Scheme 5. Proposed mechanism of reaction to explain regioselectivity and stereoselectivity.

Figure 2 .
Figure 2. NOESY correlations of compound 14.Possible correlations were identified, using MOE (images in Supplementary Materials), between proton 3 and proton 7 (in green, A) corresponding

Figure 2 .
Figure 2. NOESY correlations of compound 14.Possible correlations were identified, using MOE (images in Supplementary Materials), between proton 3 and proton 7 (in green, A) corresponding to R configuration and between proton 7 and methyl group 15 (in red, B) corresponding to S configuration.(C) ORTEP drawing of compound 14.

Table 1 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxyureas (ND is the abbreviation for "not determined").

Table 1 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, an cytotoxicities of synthesized hydroxyureas (ND is the abbreviation for "not determined").5 group (compound 7, Table

Table 1 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxyureas (ND is the abbreviation for "not determined").Int.J. Mol.Sci.2023, 24, x FOR PEER REVIEW

Table 2 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxycarbamates (ND is the abbreviation for "not determined").Int.J. Mol.Sci.2023, 24, x FOR PEER REVIEW

Table 2 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxycarbamates (ND is the abbreviation for "not determined").

Table 2 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxycarbamates (ND is the abbreviation for "not determined").

Table 2 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxycarbamates (ND is the abbreviation for "not determined").

Table 2 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxycarbamates (ND is the abbreviation for "not determined").

Table 2 .
Summary of antibacterial activities against M. tuberculosis and S. aureus, solubilities, and cytotoxicities of synthesized hydroxycarbamates (ND is the abbreviation for "not determined").