X-ray Molecular Structure of ({[(1 E )-3-(1 H -Imidazol-1-yl)-1-phenylpropylidene]amino}oxy)(3,4,5-trimethoxyphenyl)- methanone: A Potential Anti- Candida Agent

Purpose: To elucidate the solid-state conformation as well as the imine double bond configuration of a potential anti-Candida agent ({[(1E)-3-(1H-imidazol-1-yl)-1-phenylpropylidene]amino}oxy)(3,4,5-trimethoxyphenyl)methanone. Methods: Acetophenone was used as a starting material to prepare the target oximino ester in a four-step reaction sequence. Nuclear magnetic resonance ( 1 H-NMR and 13 C-NMR) and mass spectrometry were used to confirm the chemical structure of the synthesized compounds. Thereafter, x-ray crystallography was performed on single crystals of the target compound. The solid-state conformation of the target molecule and the (E)-configuration of its imine double bond were determined via the investigation of its single crystal x-ray molecular structure. Results: The titled compound crystallized in the triclinic space group P-1 with a = 11.0719 (7) Å, b = 14.6602 (9) Å, c = 14.8530 (9) Å, α = 67.205 (4)°, β = 80.388 (5) º, γ = 70.100 (5)°, V = 2088.2 (2) Å 3 , and Z = 4. Individual molecules were packed in the crystal by three weak non-classical intermolecular hydrogen interactions, including C9A—H9AA•••O3A, C9B—H9BA•••O3B, C18B—H18C•••O2A and C20B—H20B•••O4B. Conclusion: The results of the single crystal x-ray molecular structure of the titled anti-Candida agent unequivocally confirmed its (E)-configuration.


INTRODUCTION
A substantial increase in fungal infections has been observed over the past few decades.Invasive and nosocomial fungal infections are primarily caused by Candida and Aspergillus species, where Candida albicans is responsible for the majority of invasive and superficial Candida infections.
The azole antifungal agents constitute a major antifungal class and contain an azole pharmacophoric moiety.In spite of the fact that there is a growing list of new antifungal agents, the treatment of fungal infections remains unsatisfactory in many cases.Furthermore, the use of many antifungal drugs is often complicated by clinical obstacles, including a suboptimal spectrum of activity, hazardous drugdrug interactions, toxicity, and limited bioavailability [1][2][3][4][5][6].Consequently, the search for novel antifungal agents is urgently needed for clinical therapy.

EXPERIMENTAL General
Melting points were determined on a Gallenkamp melting point apparatus and are uncorrected.NMR spectra were carried out on a Bruker NMR spectrometer operating at 500 MHz for

Preparation of 3-(1H-imidazol-1-yl)-1-phenylpropan-1-one (2)
A catalytic amount of concentrated hydrochloric acid (0.1 mL) was added to a mixture of acetophenone (2.4 g, 20 mmol), dimethylamine hydrochloride (2.2 g, 27 mmol) and paraform-aldehyde (0.81 g, 9 mmol).The reaction mixture was refluxed in absolute ethanol (5 mL) for two hours.Acetone (20 mL) was added to the cooled reaction mixture in order to precipitate the Mannich base hydrochloride 1.A solution containing compound 1 (3.7 g, 17.4 mmol) and imidazole (2.4 g, 34.8 mmol) in water (10 mL) was refluxed for five hours.Cooling the reaction mixture led to precipitation of the ketone 2 which was filtered off to give 2.7 g (77%) of 2 mp 368-370 K [7].Ketone 2 was used in the next step without any further purification, and its chemical structure was confirmed via

Crystal structure determination
The slow evaporation of the isopropanol solution containing pure oximino ester 4 afforded its colorless block single crystals.A single crystal with dimensions of 0.32 × 0.23 × 0.05 mm was selected for x-ray diffraction analysis.Data were collected on a Bruker APEX-II CCD area diffractometer equipped with graphite monochromatic CuKa radiation (λ = 1.54178Å) at 296 (2) K.

Synthetic reactions
Oximino ester 4 was obtained as shown in Scheme 1.Briefly, acetophenone was subjected to a Mannich reaction followed by substitution of the resulting Mannich base hydrochloride 1 with imidazole to furnish the pivotal ketone 2. Subsequently, the ketone function in compound 2 was transformed to an oxime via reaction with hydroxylamine hydrochloride in the presence of potassium hydroxide to yield the oxime 3. Esterification of the hydroxyl moiety in compound 3 with 3,4,5-trimethoxybenzoic acid in the presence of the coupling agent EDCI.HCl and DMAP provided the target oximino ester 4.

Crystal structure of the target oximino ester 4
Cell refinement and data reduction were performed using Bruker SAINT [9].SHELXS-97 [10] was used to solve and refine the structure.The final refinement was performed using the full-matrix least-squares techniques with anisotropic thermal data for non-hydrogen atoms on F2.All hydrogen atoms were placed in calculated positions and constrained to ride on their parent atoms.Multi-scan absorption correction was applied using SADABS software [9].The crystallographic data and refinement information are summarized in Table 1, and the selected bond lengths and angles are listed in Table 2.The labeled displacement ellipsoid plot is shown in Figure 1, in which the minor disordered component has been omitted for clarity.Figure 2 depicts the packing of the molecules in the crystal structure.The configuration of the anti-Candida oximino ester 4 was confirmed via x-ray crystallography approach as an analytical tool to unequivocally assign its structure.In this manner, the imine group in the target compound 4 was assigned an (E)-configuration.4), has been obtained in this study.The assigned (E)configuration of the imine moiety in the target oximino ester 4 was confirmed via single-crystal x-ray crystallography as a decisive analytical tool.Compound 4 may be a promising new anti-Candida lead agent bearing an imidazole pharmacophore.

Figure 2 :
Figure 2: Crystal packing showing intermolecular C-H•••O hydrogen bonds as dashed lines along the c axis.DISCUSSIONThe chemical structures of the synthesized compounds were confirmed via spectral analysis including1 H NMR, 13 C NMR, and mass spectral data.The target oximino ester 4 has exhibited potential in vitro anti-Candida activity and has been evaluated using clinical isolates of C. albicans and C. tropicalis.It has a MIC value of 0.3053 µmol/mL against both C. albicans and C. tropicalis and is approximately 5-fold more potent than the gold standard antifungal agent fluconazole (MIC ˃1.6325 µmol/mL) [8].

Table 1 :
Crystallographic data and refinement information.