The solubility of drug-like compounds has been described as one of the main problems causing the failure of new chemical entities in medicinal chemistry programmes. This paper describes our efforts to overcome lack of solubility and to produce a novel template protocol for forming salts of poorly soluble compounds: we formed a series of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one amine salts by using ultrasound irradiation. The poorly water-soluble molecule 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one was recently described as an inhibitor of the Mycobacterium tuberculosis orotate phosphoribosyltransferase enzyme. Salts of this molecule were formed from a mixture of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one with primary and secondary amines [dimethylamine, diethylamine, dipropylamine, ethanolamine, 2-amino-2-methyl-1,3-propanediol, D-threoninol, pyrrolidine and piperidine] after ultrasound irradiation, producing 88-98% yields. The main achievements of this method were a significant reduction in reaction times and the use of a renewable solvent as a reaction medium, with the compounds obtained after undergoing irradiation for 8 min using ethanol as the solvent.
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Key words: ultrasound, pyridin-2(1H)-ones, salt, solubility
Solubility is a key physicochemical property that needs to be controlled during drug development processes [1,2]. The difficulty of solubilising compounds in aqueous conditions contributes to low dissolution rates that culminate in low bioavailability and weak in vivo pharmacological activity. Approximately one-third of the newly synthesised compounds in medicinal chemistry laboratories have an aqueous solubility of less than 10 Î¼g/mL . More alarming still is that 80-90% of drug candidates in the R&D pipeline could fail because of solubility problems . A major contributor to this problem arose specifically when combinatorial chemistry and high-throughput screening became popular in drug discovery and medicinal chemistry approaches in the mid-1990s . These programmes produced new drug-like molecules that were screened for their biological activity using highly polar solvents, such as dimethyl sulphoxide (DMSO) and polyethyleneglycol (PEG), and high-throughput screening approaches . Whereas this rapid screening technology identifies molecules that bind strongly to drug targets, the use of DMSO and PEG as solvents results in these drug-like molecules having exceptionally low aqueous solubilities . In the past few years, one of the most reliable methods used by medicinal chemists to improve in vitro activity has been the incorporation of properly positioned lipophilic groups . While this approach contributed to an increased entropy of binding, resulting from the favourable solvation entropy associated with hydrophobic groups , the properties of these new chemical entities were shifted towards higher molecular weights with a decreased aqueous solubilities .
Aqueous solubility, together with membrane permeability, is a critical aspect of the oral bioavailability of new drug-like compounds . In accordance with the Biopharmaceutical Classification System , solubility enhancement can improve the oral bioavailability of substances that are classified in class 2 (poorly soluble/permeable) and class 4 (poorly soluble/poorly permeable). Particularly for class 2 substances, improved solubility has been used as part of the strategy for oral bioavailability enhancement .
New, solid forms of active pharmaceutical ingredients have been designed in order to address problems related to solubility and other important properties such as bioavailability, dissolution rate, or stability . Making solid forms generally includes searching for single components of active pharmaceutical ingredients as polymorphs  and multicomponents forms from these active ingredients, which include solvates , cocrystals,  and mainly, salts . It is estimated that 50% of all drug molecules are administered as organic salts . This multicomponent approach improves the aqueous solubility of poorly soluble substances because, in general, ionised species have a greater aqueous solubility owing to the dipolar interactions of the ions with water. The selected salt ion can significantly overcome the suboptimal physicochemical or biopharmaceutical properties of a drug, influencing the drug's pharmacokinetics . As a result, the time course of the drug's pharmacodynamic and toxicological effects may undergo be modified or modulated. Thus, it has now been accepted by regulatory bodies in the US that different solid forms (new salts, for example) of a drug are effectively different drugs, which can, in principle, be protected by patents [14-15]. European regulators, however, currently view them as a single drug.
Recently, we reported the inhibitory activity of pyrimidin-2(1H)-ones based on Mycobacterium tuberculosis orotate phosphoribosyltransferase . Among the tested compounds, the 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one obtained showed 15 % inhibition of enzymatic activity at 100 ÂµM . As part of our ongoing programme, we are interested in developing more efficient methods for the preparation of pharmacologically active compounds as alternatives for tuberculosis treatment. Our aim has been to investigate alternative reaction media and energy sources in order to accomplish the desired chemical transformations in short periods with a minimum of by-products and waste generation, as well as to eliminate the use of conventional organic solvents from methodologies already reported. Recently, the energy provided by ultrasound has been utilised to accelerate a number of synthetically useful reactions . The use of this form of energy has been reported to show a remarkable rate of enhancement and a dramatic reduction in reaction times .
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Because of the importance of the salification process (i.e., salt formation) of pharmacologically active compounds and the need for alternative methodologies to accomplish such important transformations, we report, in this paper, on the salts formed from 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one using ultrasound irradiation. In addition, a selected example was performed using conventional thermal heating to provide a comparison with our ultrasound protocol.
Results and Discussion
The salification of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1 using different primary and secondary amines 2 was conducted in the presence of ethanol as a solvent in a molar ratio of 1:1 under ultrasonic irradiation (Scheme 1). After irradiation, the 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one salts 3a-h were obtained in 88-98% yields (isolated compounds). The salt 3a was also prepared from the salt formation of pyridin-2(1H)-one 1 with dimethylamine 2a under conventional thermal heating. The reactions were carried out in refluxing ethanol for 3 h to afford a similar product yield (84%) as that obtained under ultrasonic irradiation. The method for forming 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one salts 3 under ultrasonic irradiation offers advantages over conventional thermal heating, including faster reaction rates, and higher purity with similar yields. This protocol included also the use of a renewable solvent. Because the solvents form a major part of the waste from organic synthetic chemistry protocols, the use of green solvents such as ethanol is widely desired . The main goal of using ultrasound was to decrease the reaction time in comparison with conventional methods. Whereas conventional methods required agitation for 3 h in refluxing ethanol, the products were obtained in 8 min with ultrasound irradiation. When compared with methods from the literature, the protocol using ultrasound irradiation showed a significant decrease in reaction times; for example, the salt formation of piroxicam with ethanolamines has been obtained after agitation in dichloromethane for 24 h . It is noteworthy that piroxicam showed a hydroxyl group endowed with a weak-acid character attached at the 2H-1,2-benzothiazine-1,1-dioxide system, similar to 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1. In both compounds, the negative charge was present on this group. The acid character of the hydroxyl group present at the 4-position of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1 can be, in part, explained by the presence of a nitro group attached at the 3-position of the heterocyclic ring. Nitro electron-withdrawing effect stabilises the conjugated base, making the hydrogen more acidic. Therefore, the negative charge on the oxygen at the 4-position can be expected to interact with the positively charged nitrogen from amines 2. It is also important to mention that compound 1 can exist in an aromatic tautomeric form, with a second hydroxyl group on the heterocyclic structure. This arrangement could support the formation of bi-salts, with two molecules of amine as counterions; however, as determined by spectrometric methods, these species were not obtained under these reaction conditions.
All 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one salts 3 showed lower melting points than compound 1 (247-249Â°C). In general, the melting points of salts are expected to be lower than those of the corresponding parent compound owing to the lower crystalline lattice energy of salts [20a, 21]. In accordance with the literature, we did not observe any trend in the decreased melting points of salt forms of 1 related to the counterion changes from amines 2.
The structures of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one salts 3 were determined by 1H NMR, IR spectroscopy, and mass spectrometry. 4-Hydroxy-6-methyl-3-nitropyridin-2(1H)-one amine salts 3 showed sets of 1H NMR data that corresponded to the proposed structures. Using 1H NMR, compounds 3a-h showed the methyl hydrogen (6-CH3) at a range of Î´ 1.92-1.99 and vinylic hydrogen (H-5) at a range of Î´ 5.23-5.41. These chemical shifts only slightly diverge from 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1 on the methyl and vinylic hydrogens in the compound that was obtained at Î´ 2.18 ppm and Î´ 5.85 ppm, respectively, using the same conditions used to obtain the spectroscopy data. In addition, 13C NMR spectra of the selected salt, 3h, showed chemical shifts compatible with the proposed structure. An anion formed on the oxygen attached at the 4-position of pyridin-2(1H)-one, based on a chemical shift difference of the C-4 from salt 3h, which was obtained at 170.5 ppm, whereas in compound 1, the same carbon was observed at ~162 ppm . This deshielding effect on C-4 can be attributed to the negative charge on the oxygen atom, which is shared to a large extent with the pyrrolidinium group via intermolecular interactions.
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The analysis of FTIR data from salt structures 3a-h showed an obvious, and important, difference between the spectrum of 1 and those of its salts 3a-h. Whereas the O-H and N-H stretching regions of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1 showed a strong signal at 2818 cmâˆ’1, the salts 3a-c showed shifted and broadened peaks in the same region. The broadened peaks observed in this region were also indicative of a strong intermolecular interaction between 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1 and the amines 2.
Finally, high resolution mass spectra showed the corresponding [M + H]+ ions of free bases and of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one within a Â± 0.4 - 1.4 ppm error. Moreover, although in very low relative abundance, the [M + H]+ ions of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one amine salts 3 were obtained within a 0.4 - 1.65 ppm error.
We have reported herein the preparation of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one amine salts by a highly efficient and environmentally friendly protocol using ultrasonic irradiation. The simplicity, the use of a renewable solvent as a reaction medium, the satisfactory yields (88-98%), and the short reaction times (8 min) make this method highly attractive. Studies on the use of this template protocol for obtaining other pharmacologically active compounds in salt form and the use this methodology in scale-up processes are in progress.
Apparatus and analysis
All common reactants and solvents were used as obtained from commercial suppliers without further purification. The reactions were carried out with a standard probe (25 mm) connected to a 1500 Watt Sonics Vibra-cell ultrasonic processor (Newtown, Connecticut, USA) equipped with integrated temperature control. The device operates at a frequency of 20 KHz, and the amplitude was set to 20% of the maximum power output. All melting points were measured using a Microquímica MQAPF-302 apparatus. 1H NMR spectra were acquired on an Anasazi EFT-60 spectrometer (1H at 60.13 MHz) at 303 K in 5 mm sample tubes. The selected 13C NMR spectra were obtained on a DPX 400 spectrometer (13C at 100.63 MHz) at 298 K in 5 mm sample tubes (Federal University of Santa Maria, Brazil). DMSO-d6 was used as solvent, and TMS was used as internal standard in both spectrometers. High resolution mass spectra were obtained for all compounds on a LTQ Orbitrap Discovery mass spectrometer (Thermo Fisher Scientific). This hybrid system combines the LTQ XL linear ion trap mass spectrometer and an Orbitrap mass analyser. The experiments were performed via direct infusion of sample in MeOH (flow: 5 Î¼L/min) in positive-ion mode using electrospray ionisation (ESI). Elemental composition calculations were executed using the specific tool included in the Qual Browser module of Xcalibur (Thermo Fisher Scientific, release 2.0.7) software. Fourier transform infrared (FTIR) spectra were recorded using a universal attenuated total reflectance (UATR) attachment on a PerkinElmer Spectrum 100 spectrometer in the wavenumber range of 650-4000 cm-1 with a resolution of 4 cm-1.
General procedure for the synthesis of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one salts 3a-h under ultrasound irradiation
In a 25 mL beaker, the 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1 (1.0 mmol, 0.170 g) and amine 2 (1.0 mmol) were mixed with EtOH (15 mL). The reaction mixtures were then sonicated with an ultrasonic probe for 20 min (the time required for the total solubilisation of the reactants) (Scheme 1). After sonication, the reaction temperature was increased to 67-70Â°C for 5-6 min. The sample was then cooled to room temperature, and the solvent was evaporated under reduced pressure. The residue obtained was dissolved in water (10 mL), filtered (Millipore 0.22 Î¼m), and lyophilised. When necessary, the products were washed with cold hexane, affording analytically pure compounds. The synthesised compounds showed high water solubility.
General procedure for the synthesis of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one dimethylammonium salt (3a) under conventional thermal heating
A mixture of 4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1 (1.0 mmol, 0.170 g) and dimethylamine 2a (1.0 mmol, 0.102 g) was stirred under reflux in ethanol (15 mL) for 3 h. After the mixture was cooled to room temperature, the solvent was evaporated under reduced pressure. The residue obtained was dissolved in water (10 mL), filtered (Millipore 0.22 Î¼m), and lyophilised. Finally, the product was washed with cold hexane, affording the salt 3a with an 84% yield.
4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one dimethylammonium salt (3a) Yield: 0.193 g (90%); M.p. 70-72 Â°C; 1H NMR (60 MHz, DMSO-d6): d 1.99 (s, 3H, CH3-6), 2.56 (s, 6H, N(CH3)2), 5.41 (s, 1H, H-5); FT-IR (UATR, cm-1): 2970, 2800, 2468, 1628, 1556, 1464, 1404, 1349, 1275, 1026, 855, 837, 794; HRMS (ESI) calcd for C6H6N2O4 . C2H7N+H: 216.0979. Found 216.0978 (M + H)+ (-0.46 ppm).
4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one diethylammonium salt (3b) Yield: 0.214 g (88%); M.p. 183-185 Â°C (Dec); 1H NMR (60 MHz, DMSO-d6): d 1.15 (t, 6H, 2CH3), 1.98 (s, 3H, CH3-6), 2.92 (q, 4H, 2CH2), 5.41 (s, 1H, H-5); FT-IR (UATR, cm-1): 2984, 2778, 2502, 1600, 1537, 1471, 1398, 1356, 1281, 1180, 1091, 1062, 839, 816, 800, 787; HRMS (ESI) calcd for C6H6N2O4 . C4H11N+H: 244.1292. Found 244.1289 (M + H)+ (-1.23 ppm).
4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one dipropylammonium salt (3c) Yield: 0.257 g (95%); M.p. 67-69 Â°C; 1H NMR (60 MHz, DMSO-d6): d 0.81 (t, 6H, 2CH3), 1.53 (quin, 4H, 2CH2), 1.94 (s, 3H, CH3-6), 2.84 (t, 4H, 2CH2), 5.29 (s, 1H, H-5); FT-IR (UATR, cm-1): 2967, 2815, 2541, 2440, 1613, 1519, 1456, 1398, 1340, 1268, 1180, 1092, 1044, 821, 786, 755; HRMS (ESI) calcd for C6H6N2O4 . C6H15N+H: 272.1605. Found 272.1604 (M + H)+ (0.37 ppm).
4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one ethanolaminium salt (3d) Yield: 0.214 g (93%); M.p. 212-214 Â°C (Dec); 1H NMR (60 MHz, DMSO-d6): d 1.92 (s, 3H, CH3-6), 2.85 (t, 2H, CH2), 3.59 (t, 2H, CH2), 5.23 (s, 1H, H-5), 7.48 (br, 3H); FT-IR (UATR, cm-1): 3401, 2894, 2663, 1618, 1508, 1445, 1410, 1357, 1291, 1261, 1062, 1013, 840, 791; HRMS (ESI) calcd for C6H6N2O4 . C2H7NO+H: 232.0928. Found 232.0929 (M + H)+ (0.43 ppm).
4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one 1,3-dihydroxy-2-methylpropan-2-aminium salt (3e) Yield: 0.244 g (89%); semi-solid; 1H NMR (60 MHz, DMSO-d6): d 1.11 (s, 3H, CH3), 1.93 (s, 3H, CH3-6), 3.43 (s, 4H, 2CH2), 5.27 (s, 1H, H-5), 5.75 (br, 3H); FT-IR (UATR, cm-1): 3168, 2940, 1609, 1513, 1399, 1344, 1268, 1038, 824, 789; HRMS (ESI) calcd for C6H6N2O4 . C4H11NO2+H: 276.1190. Found 276.1189 (M + H)+ (-0.37 ppm).
4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one (2S,3S)-1,3-dihydroxybutan-2-aminium salt (3f) Yield: 0.253 g (92%); M.p. 39-41 Â°C; 1H NMR (60 MHz, DMSO-d6): d 1.14 (d, 3H, CH3), 1.95 (s, 3H, CH3-6), 2.83 (q, 1H, CH), 3.53-3.89 (m, 3H, CH; CH2), 5.34 (s, 1H, H-5); FT-IR (UATR, cm-1): 3173, 2914, 1608, 1508, 1401, 1344, 1265, 1177, 1101, 1036, 926, 826, 787; HRMS (ESI) calcd for C6H6N2O4 . C4H11NO2+H: 276.1190. Found 276.1188 (M + H)+ (-0.72 ppm).
4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one pyrrolidinium salt (3g) Yield: 0.234 g (97%); M.p. 143-145 Â°C; 1H NMR (60 MHz, DMSO-d6): d 1.72-1.90 (m, 4H, 2CH2), 1.98 (s, 3H, CH3-6), 2.99-3.26 (m, 4H, 2CH2), 5.36 (s, 1H, H-5); FT-IR (UATR, cm-1): 2985, 2773, 1625, 1536, 1400, 1354, 1269, 1180, 1095, 1036, 819, 789; HRMS (ESI) calcd for C6H6N2O4 . C4H9N +H: 242.1135. Found 242.1131 (M + H)+ (-1.65 ppm).
4-hydroxy-6-methyl-3-nitropyridin-2(1H)-one piperidinium salt (3h) Yield: 0.250 g (98%); M.p. 73-75 Â°C; 1H NMR (60 MHz, DMSO-d6): d 1.60 (br, 6H, 3CH2), 1.97 (s, 3H, CH3-6), 3.01 (br, 4H, 2CH2), 5.37 (s, 1H, H-5); 13C NMR (100 MHz, DMSO-d6): d 18.4 (CH3-6), 21.7 (CH2), 22.1 (2CH2), 43.8 (2CH2), 104.6 (C-5), 126.2 (C-3), 144.5 (C-6), 158.8 (C-2), 170.5 (C-4); FT-IR (UATR, cm-1): 3000, 2947, 2860, 2515, 1608, 1511, 1417, 1381, 1251, 1162, 1081, 1030, 945, 866, 789; HRMS (ESI) calcd for C6H6N2O4 . C5H11N +H: 256.1292. Found 256.1294 (M + H)+ (0.78 ppm).
This work was supported by funds from the National Institute of Science and Technology on Tuberculosis (INCT-TB), Decit/SCTIE/MS-MCT-CNPq-FNDCT-CAPES (Brazil) to D. S. Santos and L. A. Basso. L. A. Basso and D. S. Santos also acknowledge financial support awarded by FAPERGS-CNPq-PRONEX-2009. L. A. Basso (CNPq, 520182/99-5) and D. S. Santos (CNPq, 304051/1975-06) are Research Career Awardees of the National Research Council of Brazil (CNPq). The postdoctoral fellowship from CAPES (P. Machado) and scientific initiation fellowship from CNPq (E. Ritter) are also acknowledged.
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 Integrated Spectral Database System of Organic Compounds. (Data were obtained from the National Institute of Advanced Industrial Science and Technology (Japan) via SciFinderÂ®).
Scheme 1. Conditions: (i) EtOH, ))), 67-70Â°C, 20 min.