3-(4-Bromophenyl)-4-{[3-hydroxy-6-(hydroxymethyl)-4-oxo-4H-pyran-2-yl](m-tolyl)methyl}isoxazol-5(2H)-one

: In this communication, the electrochemically induced multicomponent transformation of 3-methylbenzaldehyde, 3-(4-bromophenyl)isoxazol-5(4 H )-one and kojic acid in n -PrOH in an undivided cell in the presence of sodium bromide was carefully investigated to give 3-(4-bromophenyl)-4-{[3-hydroxy-6-(hydroxymethyl)-4-oxo-4 H -pyran-2-yl]( m -tolyl)methyl}isoxa-zol-5(2 H )-one in good yield. The structure of the new compound was established by means of elemental analysis, mass-, nuclear magnetic resonance and infrared spectroscopy. Furthermore, its structure was determined and conﬁrmed by X-ray analysis. The synthesized compound is a promising compound for di ﬀ erent biomedical applications, and, in particular, for the regulation of inﬂammatory diseases, as shown by docking studies in this research.


Introduction
Among different strategies of drug discovery, the identification and application of "privileged structures or scaffolds" have gained special attention in the last decades [1,2]. These privileged scaffolds are constructed from a rigid heterocyclic system, which determine the orientation type of different functional substituents in order to be recognized by a target molecule [3].
Thus, diverse therapeutic activities of kojic acid and isoxazol-5(4H)-one derivatives explain the interest in the combination of both of these fragments.
Compound 4 was synthesized in 73% substance yield with 243% current efficiency. Taken into consideration the results on electrocatalytic assembling of carbonyl compounds and C-H acids [16], the following mechanism for the multicomponent transformation of 3-methylbenzaldehyde 1, 3-(4-bromophenyl)isoxazol-5(4H)-one 2 and kojic acid The first step of the electrochemically induced process of the deprotonation of n-propanol at the cathode leads to the formation of a n-propoxide anion with a release of hydrogen gas [16]. Then, the The first step of the electrochemically induced process of the deprotonation of n-propanol at the cathode leads to the formation of a n-propoxide anion with a release of hydrogen gas [16]. Then, the n-propoxide anion interaction with 3-(4-bromophenyl)isoxazol-5(4H)-one 2 provides the 3-(4-bromophenyl)isoxazol-5(4H)-one anion A formation. The following process is Knoevenagel condensation of the anion A and 3-methylbenzaldehyde 1 with the elimination of a hydroxide anion and the formation of adduct 5 [18]. The subsequent hydroxide-anion promoted Michael addition of kojic acid 3 to the electron deficient Knoevenagel adduct 5, which leads to the end compound 4 in the electrocatalytic chain process, namely, 3-(4-bromophenyl)-4-{[3-hydroxy-6-(hydroxymethyl)-4-oxo-4Hpyran-2-yl](m-tolyl)methyl}isoxazol-5(2H)-one 4 with the regeneration of the n-propoxide anion as the last step of the catalytic cycle. The catalytic chain process continues by the interaction of the n-propoxide anion with the next molecule of 3-(4-bromophenyl)isoxazol-5(4H)-one 2.

X-ray Diffraction Data
The structure of compound 4 is confirmed by X-ray crystallography, as shown in Figure 1. Thermal ellipsoids correspond to 50% probability. The crystal is a racemate, containing equal numbers of both enantiomers.

X-ray Diffraction Data
The structure of compound 4 is confirmed by X-ray crystallography, as shown in Figure 1. Thermal ellipsoids correspond to 50% probability. The crystal is a racemate, containing equal numbers of both enantiomers.

Docking Studies
During molecular docking studies, the binding modes of protein-ligand complexes were investigated using Lead Finder software (BioMolTech Corp) [19]. Lead Finder uses three specialized high accuracy scoring functions for binding energy prediction, rank-ordering of compounds in virtual screening experiments (with ordering most active to least active compound), and energy-ranking of docked ligand poses. This approach is semiempirical, molecular and mechanically functional and produces high accuracy predictions confirmed by numerous experiments [20].
The human C5a receptor was chosen for the study (PDB code: 6C1R) as it modulates inflammatory responses. It also modulates obesity, development and cancers [21][22][23]. The new synthesized compound 4 was subjected to the docking procedure.
Compound 4 showed good binding affinities of −8.63 kcal × mol −1 and a good virtual screening score of −10.25. Pose ranking also revealed good results. The best mode was ranked with a score of

Docking Studies
During molecular docking studies, the binding modes of protein-ligand complexes were investigated using Lead Finder software (BioMolTech Corp) [19]. Lead Finder uses three specialized high accuracy scoring functions for binding energy prediction, rank-ordering of compounds in virtual screening experiments (with ordering most active to least active compound), and energy-ranking of docked ligand poses. This approach is semiempirical, molecular and mechanically functional and produces high accuracy predictions confirmed by numerous experiments [20].
The human C5a receptor was chosen for the study (PDB code: 6C1R) as it modulates inflammatory responses. It also modulates obesity, development and cancers [21][22][23]. The new synthesized compound 4 was subjected to the docking procedure.
Compound 4 showed good binding affinities of −8.63 kcal × mol −1 and a good virtual screening score of −10.25. Pose ranking also revealed good results. The best mode was ranked with a score of −7.71. However, affinity calculated for W54011 (known C5a receptor antagonist) [24] was −8.68 kcal × mol −1 and the screening score was −9.70 for the best binding mode (−7.49). Figure 2 shows that compound 4 formed hydrogen interactions between the kojic fragment and THR303, ILE306 residues, as well as between the isoxazol fragment and CYS307, ALA218, ALA214 residues. One CH-π interaction was designed between 4-bromophenyl fragment and ALA214 residue. −7.71. However, affinity calculated for W54011 (known C5a receptor antagonist) [24] was −8.68 kcal × mol −1 and the screening score was −9.70 for the best binding mode (−7.49). Figure 2 shows that compound 4 formed hydrogen interactions between the kojic fragment and THR303, ILE306 residues, as well as between the isoxazol fragment and CYS307, ALA218, ALA214 residues. One CH-π interaction was designed between 4-bromophenyl fragment and ALA214 residue.
To sum up, compound 4 showed good results according to the LeadFinder virtual screening scoring function, pose ranking and calculated binding energy. Taking into consideration the above results, one can conclude that compound 4 is comparable to (or better than) the known C5a receptor antagonist-W54011.
All melting points were measured with Gallenkamp melting-point apparatus (London, UK) and were uncorrected. 1 H and 13 C NMR spectra were recorded in DMSO-d6 with a Bruker Avance II 300 spectrometer (Billerica, MA, USA) at ambient temperature. OH and NH signals were exchanged with D2O (it is present as an impurity in DMSO-d6). Chemical shift values are relative to Me4Si. IR spectrum was recorded with a Bruker ALPHA-T FT-IR spectrometer (Billerica, MA, USA) in KBr pellet. MS spectrum (EI = 70 eV) was obtained directly with a Kratos MS-30 spectrometer (Manchester, UK). For elemental analysis, 2400 Elemental Analyzer (Perkin Elmer Inc., Waltham, MA, USA) was used.
X-ray diffraction data were collected at 100K on a Bruker Quest D8 diffractometer (Billerica, MA, USA) equipped with a Photon-III area-detector (graphite monochromator, shutterless ϕand ω-scan technique), using Mo Kα-radiation (0.71073 Å). The intensity data were integrated by the SAINT program (version 8.38A) [27] and were corrected for absorption and decay using SADABS [28]. The structure was solved by direct methods using SHELXT [29] and refined on F 2 using SHELXL-2018 [30]. All nonhydrogen atoms were refined with individual anisotropic displacement parameters. Locations of hydroxy and amino H-atoms (H1, H2 and H4) were found from the electron density-difference map. The O-H distances were restrained to be 0.84(2) Å. These hydrogen atoms were refined with individual isotropic displacement parameters. All other hydrogen atoms were placed in ideally calculated positions and refined as riding atoms with relative isotropic displacement parameters. The SHELXTL program suite [27] was used for molecular graphics.
For docking simulation, Lead Finder (version 1.1.16.) by BioMolTech Corp. (Toronto, Ontario, Canada) [19] was used. The default program settings were used for estimation of binding affinity, virtual screening score and rank score. The human C5a receptor was retrieved from the RCSB protein bank (PDB code: 6C1R) [31]. W54011 was selected as the reference compound as a C5a To sum up, compound 4 showed good results according to the LeadFinder virtual screening scoring function, pose ranking and calculated binding energy. Taking into consideration the above results, one can conclude that compound 4 is comparable to (or better than) the known C5a receptor antagonist-W54011.
All melting points were measured with Gallenkamp melting-point apparatus (London, UK) and were uncorrected. 1 H and 13 C NMR spectra were recorded in DMSO-d 6 with a Bruker Avance II 300 spectrometer (Billerica, MA, USA) at ambient temperature. OH and NH signals were exchanged with D 2 O (it is present as an impurity in DMSO-d 6 ). Chemical shift values are relative to Me 4 Si. IR spectrum was recorded with a Bruker ALPHA-T FT-IR spectrometer (Billerica, MA, USA) in KBr pellet. MS spectrum (EI = 70 eV) was obtained directly with a Kratos MS-30 spectrometer (Manchester, UK). For elemental analysis, 2400 Elemental Analyzer (Perkin Elmer Inc., Waltham, MA, USA) was used.
X-ray diffraction data were collected at 100K on a Bruker Quest D8 diffractometer (Billerica, MA, USA) equipped with a Photon-III area-detector (graphite monochromator, shutterless ϕand ω-scan technique), using Mo K α -radiation (0.71073 Å). The intensity data were integrated by the SAINT program (version 8.38A) [27] and were corrected for absorption and decay using SADABS [28]. The structure was solved by direct methods using SHELXT [29] and refined on F 2 using SHELXL-2018 [30]. All nonhydrogen atoms were refined with individual anisotropic displacement parameters. Locations of hydroxy and amino H-atoms (H1, H2 and H4) were found from the electron density-difference map. The O-H distances were restrained to be 0.84(2) Å. These hydrogen atoms were refined with individual isotropic displacement parameters. All other hydrogen atoms were placed in ideally calculated positions and refined as riding atoms with relative isotropic displacement parameters. The SHELXTL program suite [27] was used for molecular graphics.
For docking simulation, Lead Finder (version 1.1.16.) by BioMolTech Corp. (Toronto, Ontario, Canada) [19] was used. The default program settings were used for estimation of binding affinity, virtual screening score and rank score. The human C5a receptor was retrieved from the RCSB protein bank (PDB code: 6C1R) [31]. W54011 was selected as the reference compound as a C5a antagonist [24]. The buildmodel tool in Lead Finder was used to treat 6C1R and W54011 before docking studies. Prior to docking studies, the 3D-structure of compound 4 was optimized and hydrogen atoms were set explicitly. The modes, rank scores, binding affinities and virtual screening scores are presented in Table  S8 ( A solution of 3-methylbenzaldehyde 1 (0.6 g, 5 mmol), 3-(4-bromophenyl)isoxazol-5(4H)-one 2 (1.2 g, 5 mmol), kojic acid 3 (0.71 g, 5 mmol) and sodium bromide (0.1 g, 1 mmol) in n-propanol (20 mL) was electrolyzed in an undivided cell equipped with a magnetic stirrer, a graphite anode and an iron cathode at 97 • C under a constant current density of 5 mA/cm 2 (I = 25 mA, electrodes square 5 cm 2 ) until the catalytic quantity of 0.3 F/mol of electricity passed. After the electrolysis was finished, the reaction mixture was concentrated to one fifth of its initial volume (ca. 4 mL) and chilled to 0 • C to crystallize the solid compound 4, which was then filtered out, twice rinsed with an ice-cold ethanol/water solution (3:1, 4 mL), and dried under reduced pressure.

Conclusions
The title compound, 3-(4-bromophenyl)-4-{[3-hydroxy-6-(hydroxymethyl)-4-oxo-4H-pyran-2yl](m-tolyl)methyl}isoxazol-5(2H)-one 4, was synthesized in good yield using the facile and efficient electrocatalytic approach with simple equipment and available starting compounds. The compound 4 was characterized by spectroscopic methods (NMR, IR, MS-EI) and elemental analysis. Its crystal structure was determined and confirmed by X-ray analysis. According to docking studies with Lead Finder software, the synthesized compound showed good results in virtual screening scoring, pose ranking and calculated binding energy to C5a receptor. When compared with W54011 (known C5a receptor antagonist, regulates inflammatory), compound 4 showed better results in the current study.