Synthesis, Crystal Structure and DFT Calculation of 2-Methoxyimino Phenylacetate Derivatives Containing 1,3,4-Oxadiazole Ring①

REN Hui-Dong LI Kng-Ming YI Yng-Jie WANG Mn YE Jio② HU Ai-Xi②CHEN Yun OU Xio-Ming LI Jin-Ming

a (College of Chemistry and Chemical Engineering, Hunan Uniνersity, Changsha 410082, China)、 b (National Pesticide Creation Engineering Technology Research Center, Hunan Research Institute of Chemical Industry Co., Ltd., Changsha 410007, China)

ABSTRACT Four novel 2-methoxyimino phenylacetate derivatives containing 1,3,4-oxadiazole ring were designed and synthesized from the key intermediate of Trifloxystrobin or Azoxystrobin via intermediate derivatization and active structure splicing. The chemical structures of the target compounds were confirmed by 1H NMR, 13C NMR and elemental analysis. The crystal structure of methyl (E)-2-(methoxyimino)-2-(2-(((5-((4-methoxyphenoxy)methyl)-1,3,4-oxadiazol-2-yl)thio)methyl)phenyl)acetate (A1) was determined by single-crystal X-ray diffraction. Compound A1 belongs to triclinic system, space group P1 with two molecules in each unit cell. The benzene ring plane C(2)-C(3)-C(4)-C(5)-C(6)-C(7) and oxazole ring plane are nearly parallel with the dihedral angle of 6.4°. The benzene ring plane C(12)-C(13)-C(14)-C(15)-C(16)-C(17) and oxazole ring plane are not perpendicular with the dihedral angle of 49.4°. The crystal of compound A1 is stabilized by π-π stacking interactions. The fungicidal activities of the target compounds against four plant pathogenic fungi in vitro were tested, and some of them had good activities. The DFT calculation was carried out to study the structure-activity relationship of the title derivatives using Gasian 09 and Multiwfn 3.6.

Keywords: 2-methoxyimino phenylacetate derivatives, synthesis, crystal structure, fungicidal activities, DFT calculation; DOI: 10.14102/j.cnki.0254-5861.2011-3292

1 INTRODUCTION

The research of heterocyclic compounds is the trend of modern pesticide development. As an important member of nitrogen-containing heterocyclic compounds, oxadiazole derivatives have a wide range of applications in chemistry and biology. They not only have excellent insecticidal[1-3],bactericidal[4-6], herbicidal[7-10], anticancer[11]and antiinflammatory[12]activities, but also can be used as electron transport materials[13,14]. In recent years, it has been found that phenoxy-1,3,4-oxadiazole ring is a mature bioactive fragment with good antibacterial activity and bactericidal universal type[15-19].

In addition, 2-methoxyiminobenzoacetate fungicides are one of the main fungicides at present. Among them,TrifloxystrobinandAzoxystrobinare widely used in fruits and vegetables because of their unique mechanism of action,high efficiency, broad spectrum and low toxicity. Therefore,the development ofTrifloxystrobinorAzoxystrobinanalogues with independent intellectual property rights will have a large market space in the future.

Recently, we have designed and synthesized four novel 2-methoxyimino-phenylacetate derivatives (A) containing 1,3,4-oxadiazole ring by introducing the bioactive fragment of 1,3,4-oxadiazole ring into the side chain of compound (B),which is the key intermediate for the synthesis ofTrifloxystrobinorAzoxystrobin. The chemical structures of the target compounds were confirmed by1H NMR,13C NMR and elemental analysis. The crystal structure of methyl(E)-2-(methoxyimino)-2-(2-(((5-((4-methoxyphenoxy)methyl)-1,3,4-oxadiazol-2-yl)thio)methyl)phenyl) acetate (A1)was determined by single-crystal X-ray diffraction. In addition, the fungicidal activities of the target derivatives against four phytopathogenic fungi were tested, and the structure-activity relationship was analyzed combining with the DFT calculation, which established a guiding significance for subsequent development. The synthetic route of the title derivatives is depicted in Scheme 1.

Scheme 1. Synthetic route of 2-methoxyimino phenylacetate derivatives containing 1,3,4-oxadiazole ring

2 EXPERIMENTAL

2. 1 Materials and methods

All the reagents and solvents were purchased from commercial sources and used without further purification.Melting points (℃) were determined on an X-4 electrothermal digital melting point apparatus and uncorrected.1H NMR and13C NMR spectra were recorded on a Bruker advanced instrument with TMS as the internal standard at 400 MHz (chemical shifts (δ) in ppm). The Bruker AXS SMART 1000 CCD diffractometer was used for crystal structure determination. The VARIO EL III elemental analyzer provides the elemental analysis data of compounds.The data of fungicidal activity in vitro provided by Hunan Research Institute of Chemical Industry Co, Ltd and National pesticide Creation Engineering Technology Research. The course of the reactions was monitored by TLC, and analytical TLC was performed on silica gel GF254.

2. 2 General procedure

2. 2. 1 Synthesis of intermediates B and C

Intermediate B was synthesized according to the method in reference[20-22].O-methyl acetophenone was selected as raw material, and B was prepared via oxidation,esterification, oximation and bromination with overall yield of 55.7%.

Intermediate C was synthesized according to the method in reference[23]. The corresponding substituted phenol was selected as the raw material, which reacted with chloroacetic acid, then esterified, and reacted with hydrazine hydrate to obtain corresponding aryloxyacetylhydrazine, and finally cyclized with carbon disulfide to obtain C.

2. 2. 2 Syntheses of the target compounds A1～A4

1.0 mmol compound C, 1.1 mmol potassium carbonate, 5 mL acetonitrile, 0.6 mmol potassium iodide and 1.05 mmol compound B were added in batches under room temperature with stirring, and then the reaction mixture was heated to reflux for 3 hours. Compounds A1～A4 were obtained by column chromatography using petroleum ether/ethyl acetate(ν/ν= 10:1) as eluent.

Methyl (E)-2-(methoxyimino)-2-(2-(((5-((4-methoxyphenoxy)methyl)-1,3,4-oxadiazol-2-yl)thio)methyl)phenyl)acetate (A1). Yield: 58.3%, white solid, m.p. 73～75 ℃.1H NMR (400 MHz, CDCl3)δ: 7.60～7.12 (m, 4H, PhH), 6.93(d,J= 9.1 Hz, 2H, PhH 2,6-H), 6.83 (d,J= 9.1 Hz, 2H, PhH 3,5-H), 5.14 (s, 2H, OCH2), 4.35 (s, 2H, SCH2), 4.07 (s, 3H,NOCH3), 3.89 (s, 3H, COOCH3), 3.76 (s, 3H, OCH3);13C NMR (100 MHz, CDCl3)δ: 165.57, 163.60, 163.21, 154.93,151.63, 149.02, 133.79, 130.58, 130.21, 129.86, 128.66,128.13, 116.22, 114.81, 63.92, 60.76, 55.69, 53.16, 34.68.Anal. Calcd. (%) for C21H21N3O6S: C, 56.88; H, 4.77; N,9.48. Found (%): C, 56.88; H, 4.78; N, 9.46.

Methyl (E)-2-(methoxyimino)-2-(2-(((5-((p-tolyloxy)methyl)-1,3,4-oxadiazol-2-yl)thio)methyl)phenyl)acetate (A2).Yield: 74.0%, white solid, m.p. 82～84 ℃.1H NMR (400 MHz, CDCl3)δ: 7.59～7.15 (m, 4H, PhH), 7.10 (d,J= 8.2 Hz, 2H, C6H23,5-H), 6.88(d,J= 8.2 Hz, 2H, C6H22,6-H),5.17(s, 2H, OCH2), 4.35(s, 2H, SCH2), 4.07 (s, 3H, NOCH3),3.89(s, 3H, OCH3), 2.29(s, 3H, CH3).13C NMR (100 MHz,CDCl3)δ: 165.58, 163.56, 163.22, 155.44, 149.02, 133.79,131.62, 130.58, 130.21, 130.14, 129.86, 128.66, 128.13,114.78, 63.92, 60.02, 53.17, 34.68, 20.50. Anal. Calcd. (%)for C21H21N3O5S: C, 59.02; H, 4.92; N, 9.84. Found (%): C,58.98; H, 4.93; N, 9.86.

Methyl (E)-2-(2-(((5-((4-chlorophenoxy)methyl)-1,3,4-oxadiazol-2-yl)thio)methyl)phenyl)-2-(methoxyimino)acetate(A3). Yield: 58.3%, yellow solid, m.p. 92～94 ℃.1H NMR(400 MHz, CDCl3)δ: 7.59～7.15(m, 4H, Ar-H), 7.26(d,J=9.0 Hz, 2H, C6H43,5-2H), 6.92 (d,J= 9.0 Hz, 2H, C6H42,6-2H), 5.17 (s, 2H, OCH2), 4.35 (s, 2H, SCH2), 4.06 (s, 3H,CH3), 3.88 (s, 3H, CH3);13C NMR (100 MHz, CDCl3)δ:165.35, 164.12, 163.25, 150.67, 149.03, 134.67, 133.79,130.74, 130.59, 130.22, 129.90, 128.76, 128.22, 127.42,125.51, 124.99, 63.96, 53.20, 34.84, 20.37. Anal. Calcd. (%).for C20H19N3O5S: C, 58.10; H, 4.63; N, 10.16. Found: C,58.08; H, 4.64; N, 10.17.

Methyl(E)-2-(methoxyimino)-2-(2-(((5-((4-nitrophenoxy)methyl)-1,3,4-oxadiazol-2-yl)thio)methyl)phenyl)acetate (A4). Yield: 73.9%, viscous liquid.1H NMR (400 MHz, CDCl3)δ: 8.22 (d,J= 9.2 Hz, 2H, C6H43,5-H),7.60～7.14(m, 4H, PhH), 7.09 (d,J= 9.2 Hz, 2H, C6H42,6-H), 5.30 (s, 2H, OCH2), 4.35 (s, 2H, SCH2), 4.06 (s, 3H,NOCH3), 3.89 (s, 3H, OCH3);13C NMR (100 MHz, CDCl3)δ: 166.16, 163.21, 162.21, 162.05, 148.98, 142.64, 133.63,130.58, 130.18, 129.87, 128.72, 128.22, 126.03, 114.92,63.94, 59.92, 53.18, 34.73. Anal. Calcd. (%) for C20H19N3O5S: C, 58.10; H, 4.63; N, 10.16. Found: C, 58.08;H, 4.64; N, 10.17.

2. 3 X-ray structure determination

The representative compound A1 was dissolved in EtOH and kept for self-volatilization. The colorless crystals suitable for X-ray structure determination were obtained after about 6 days with dimensions of 0.12mm × 0.11mm ×0.1mm. X-ray intensity data were measured on the BRUKER SMCRT 1000 CCD diffractometer, and collected at 273.15 K by using a graphite-monochromatic with MoKαradiation (λ= 0.71073 Å) in anω-φscan mode. Out of the total 9597 reflections collected in the range of 2.36≤θ≤28.33°, 4931 were independent (Rint= 0.0361) and 283 were considered to be observed (I＞ 2σ(I)) and used in the succeeding refinement. Corrections for incident and empirical adsorption adjustment were applied with SADABS[24], and the structure was solved with SHELXS-97[25]and expanded by difference Fourier techniques. All non-hydrogen atoms were refined anisotropically and all H atoms were located at the theoretical positions. The structure was refined by full-matrix least-squares techniques onF2with SHELXL-97.The final refinement converged atR= 0.0669,wR= 0.1266(w= 1/[σ2(Fo2) + (0.0449P)2+ 0.7314P], whereP= (Fo2+2Fc2)/3),S= 1.038, (Δρ)max= 0.814 and (Δρ)min= -0.293 e/Å3.

2. 4 Fungicidal bioassay in vitro

The fungicidal activities of compounds A1～A4 have been determined using the mycelium growth rate method andPhytophthora capsici(PC),Alternaria alternate(AA),Gibberella zeae(GZ) andBotrytis cinerea(BC) were selected as fungicidal targets in vitro. According to NY/T1156.2-2006[26]as the biological activity test standard,the test compound was prepared into 500 mg/L drug solution,and 2 mL drug solution was added into 38 mL PDA to prepare the final concentration of 25 mg/L drug containing medium plate. Then about 6.5 mm mycelium was taken and transferred to the drug containing medium. After treatment,the drug containing medium was cultured in a constant temperature biochemical incubator at 28 ℃ for 96 hours, and the growth inhibition rate was calculated by measuring the diameter of the colony.

2. 5 DFT calculation

The ground state geometries of A1～A4 were computed by using density functional theory (DFT) with Gaussian 09 package[27]and all the calculations were performed at the Becke-Lee-Parr hybrid exchange correlation three-parameter(B3LYP) functional[28]level with standard 6-31G(d,p)[29]basis set. Electrostatic potential (ESP) on molecular van der Waals surface was calculated by Multiwfn 3.6 based on B3LYP/6-31G (d,p) density[30,31]and the surface area was plotted by VMD 1.9.3 program[32].

3 RESULTS AND DISCUSSION

3. 1 Synthesis and characterization

Intermediate B reacted with intermediate C via Williamson etherification to give A1～A4 in 58.3%～74.0%yields and the structure of compounds was determined by1H NMR,13C NMR and elemental analysis.

From1H NMR data, the methyl peak on -OCH3is located atδ3.88～3.90 ppm; The methyl peak of -NOCH3is found atδ4.07～4.09 ppm; The methyl group on -SCH2- appears at theδ4.35～4.40 ppm and a single peak with a peak area of 2 nearδ5.20 ppm is assigned to -OCH2-. It is consistent with the peak rule[33,34].

In the13C NMR of the target compounds, there are four carbons in the high field, which exactly corresponds to the four high field alkyl carbons of the target compounds. There is a response atδ160 ppm, which further confirms that there is a carbonyl structure in the structure. The imine carbon(-C=N-) nearδ133.74 ppm further indicates that the structure contains oxime methyl ether structure. The peak positions of other carbons are consistent with the peak regularity.

The element analysis result of A1～A4 is agreement with the calculated value.

3. 2 Crystal structure

Compound A1 was selected as the representative of this kind of derivatives for crystal structure analysis. Some representative bond lengths and bond angles are listed in Table 1, and selected crystal torsion angles are listed in Table 2.The crystal structure and packing diagram of A1 are illustrated in Figs. 1 and 2, respectively. Compound A1 belongs to triclinic system, space groupP1, and each unit cell contains two molecules. The unit cell parameter isa=7.6716(16) Å,α= 88.615(4)°,b= 8.3065(12) Å,β= 81.736(5)°,c= 17.482(4) Å,γ= 65.629(5)°,Z= 2,V= 1003.4(3) × 103Å3,Dc= 1.468 g/cm3,F(000) = 464,μ= 0.207 mm-1and 283 observable points (I＞ 2σ(I)). The final deviation factor of observable point finishing isR= 0.0669,wR= 0.1266,S=1.038, (Δρ)max= 0.814 and (Δρ)min= -0.293 e/Å3.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

Table 2. Selected Torsion Angles (Å)

As shown in Table 1, all of the bond angles and bond lengths are in normal ranges. The N(1)-C(9) (1.279(3) Å)and N(3)-C(18) (1.292(2) Å) of 1,3,4-oxadiazole ring were close to the normal C=N (1.27 Å)[35], while the O(5)-C(19)(1.218(3) Å) and O(3)-C(9) (1.366(3) Å) are shorter than the normal O(1)-C(1) (1.424(3) Å) because of the conjugation effect. The angle of C(10)-S(1)-C(11) is 97.1° due to theSatom belonging tosp3hybridization and containing two pairs of lone pairs of electrons. The torsion angle of C(21)-O(6)-N(3)-C(18) is -168.8(2)° (Table 2), which means that the methoxyimino group is inE-type configuration.

As outlined in Fig. 1, there are three planes in the molecule. The benzene ring plane C(2)-C(3)-C(4)-C(5)-C(6)-C(7) and oxazole ring plane are nearly parallel with the dihedral angle of 6.4°. The dihedral angle of benzene ring plane C(12)-C(13)-C(14)-C(15)-C(16)-C(17) and oxazole ring plane is 49.4°, which indicated that the two planes are not perpendicular.

The cell packing diagram (Fig. 2) shows no hydrogen bonds in the unit cell. The crystal of compound A1 is stabilized byπ-πstacking interactions. The molecules are packed in a head-to-tail arrangement showingπ-πstacking interactions. The distance between the centroid of the benzene ring plane C(2)-C(3)-C(4)-C(5)-C(6)-C(7) and the oxazole ring plane is 3.673 Å, and that from the centroid of the oxazole ring to the plane of the benzene ring plane C(2)-C(3)-C(4)-C(5)-C(6)-C(7) is 3.513 Å, with the angle between them to be 1.493°. The values of the distance are also in accordance withπ-πstacking[36].

Fig. 1. Crystal structure of compound A1 with atom labels

Fig. 2. Unit cell packing diagram of compounds A1

3. 3 Fungicidal activity

In this experiment, the key intermediate ofTrifloxystrobinwas used to design new compounds. In order to investigate the effect of the introduced fragment on fungicidal activity,Trifloxystrobinwas selected as the positive control. The fungicidal activities of A1～A4 are listed in Table 3.

Compounds A1 ～A4 exhibited weak to moderate inhibitory activity against the four plant fungus. As the electron-withdrawing ability of the substituents (from A1 to A4) increases, the inhibitory activity againstP. capsiciincreases significantly, and A4 (R = 4-NO2) is the most potent compound; While the same regular is exhibited forG.zeae, the electron withdrawing substituent (A3 R = 4-Cl; A4 R = 4-NO2) has better fungicidal activity; ForA. alternataandB. cinerea, however, the inhibitory activity is better when R is weak electron donating substituents (A2 R =4-CH3). A1 with stronger electron donating substituent(4-OCH3) exhibits very weak fungicidal activity against all the four fungus.

Table 3. Chemical Structures and Antifungal Activities in Vitro of the Target Compounds

3. 4 DFT calculation

According to the frontier sub-orbital theory, the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) can give priority to and receive electrons respectively[37]. These two molecular orbitals are important quantum chemical parameters that can affect biological activity[38,39]. Density functional theory is adopted. The theoretical Becke three-parameter mixed functional method (B3LYP) calculates the FMO energy data for compounds A1～A4 and the schematic diagram of the energy level orbital and the energy level difference are shown in Fig. 3.

Fig. 3. LUMO, HOMO and energy gap of compound A1～A4

According to the FMO energy data and the diagram of energy level orbital and energy level difference, compounds A1 and A2 have the same characteristics. LUMO orbitals are mainly distributed in methyl methoxyiminoacetate, while HOMO orbitals are mainly distributed in the benzene ring connected with oxazole ring through -CH2O-, which is caused by the fact that the substituents are electron donating groups.

In contrast, compounds A3 and A4 have different electron withdrawing groups. The LUMO orbitals of A3 are mainly concentrated in the methyl methoxyiminophenyl acetate part,while the HOMO orbitals are concentrated in the oxazole ring connected through -CH2O-. The LUMO orbital of compound A4 is mainly distributed on the benzene ring connected to the oxazole ring through -CH2O-, and the HOMO orbital is concentrated on -SCH2- connected to the oxazole ring.

The molecular van der Waals surface electrostatic potential (ESP) is the key to studying and predicting the interaction between molecules[40,41]. Fig. 4 shows the surface electrostatic potential of molecular van der Waals calculated by Multiwn 3.6, and the electrostatic potential diagram drawn by VMD program. The molecular size, structure and charge density of A1～A4 are also obtained.

Fig. 4. ESP-mapped molecular vdW surface of compound A1～A4

It can be seen from the electrostatic potential diagram that the negative charge of A1 and A2 is mainly concentrated on theOatom of the oxazole ring and the benzene ring -OCH2-,and the positive charge is concentrated around the electron-donating substituent. The negative charge of A3 is mainly concentrated on the N-N bond of oxazole ring and methyl methoxyiminoacetate, while the positive charge is concentrated on the plane of C(2)-C(3)-C(4)-C(5)-C(6)-C(7)benzene ring. The negative charge of A4 is concentrated on the twoOatoms of NO2, and the positive charge is concentrated on the link bond -OCH2-.

Combined with the analysis of fungicidal activity data, it is found that forP. capsici,A. alternate,G. zeaeandB.cinereathe spatial structure of substituents has the greatest influence on the activity, and the longer substituents were not conducive to the inhibitory activity against the four fungi.ForP. capsici, the more negative charge concentrated on the substituent, the better the fungicidal activity; ForB. cinerea,the more negative charge concentrated on the link -OCH2-,the better the fungicidal activity. This provides a guiding significance for the subsequent development of methyl methoxyiminophenyl acetate derivatives with stronger fungicidal activity.

4 CONCLUSION

In this paper, two broad-spectrum fungicidal active groups of 2-methoxyiminophenylacetate and oxadiazole ring were spliced together, and four 2-methoxy ethylene groups containing 1,3,4-oxadiazole ring were designed and synthesized, yielding 58.3%～74.0%. The structure of the target compound was characterized, and the single-crystal X-ray diffraction analysis of compound A1 showed that it belongs to the triclinic crystal system with space groupP1.The C(18)=N(3) in the molecule is in theE-type configuration,and each unit cell contains two molecules, and the entire crystal is stabilized byπ-πstacking. The in vitro fungicidal activity test found that at a concentration of 25 mg/L, the title compounds have a certain inhibitory effect onP. capsici,A.alternate,G. zeaeandB.cinerea. By combining the fungicidal activity data with the DFT calculation results of the configuration of compounds A1 ～A4, it has guiding significance for the research of new 2-methoxyiminophenylacetate fungicides.