Design,synthesis,and herbicidal activity of novel 2-(arylamino)-5-methyl-4-methylene-7-(methylthio)-4 H -pyrido[4,3- d ][1,3]oxazine-8-carbonitrile derivatives

Abstract

A series of 5-methyl-4-methylene-7-methylthio-2-arylmino-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile derivatives were synthesized via tandem aza-Wittig and annulation reactions with {N-[3-acetyl-5-cyano-2-methyl-6-(methylthio)pyridin-4-yl]imino}triphenylphosphorane and aryl isocyanate in dry dichloromethane. Their structures were clearly confirmed by infrared, ¹H NMR, ¹³C NMR, high-resolution mass spectrometry, and X-ray single-crystal diffraction. All newly synthesized compounds were screened for herbicidal activities against monocotyledonous and dicotyledonous plants. The results indicated that the target compound 2-[(4-methoxyphenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile showed 100% inhibition rate to grain sorghum (monocotyledonous) at the concentration of 100 mg L⁻¹.

Introduction

As one of the nitrogen- and oxygen-containing fused heterocycles, pyridooxazine derivatives occupy an important place in medicinal chemistry owing to their notable pharmacological and biological activities such as thermochromic agents, ¹ antinociceptives, ² antivirals, ³ antibacterials, ⁴ antimalarials, ⁵ anticancer agents, ⁶ tissue factor inhibitors, ⁷ and scavenger agents of carbohydrates. ⁸ In addition, the pyridooxazine derivative pazufloxacin mesilate (PZFX) is a novel quinolone antimicrobial agent for injection. ⁹ In recent years, the synthesis and biological activity of these derivatives have become a hot topic in biological and pharmacological studies.

Among the pyridooxazine derivatives, most of the synthetic studies are focused on pyrido[2,3-b][1,4]oxazines,^10,11 pyrido[2,1-b][1,3]oxazines, ¹² pyrido[3,2-b][1,4] oxazines,^13,14 pyrido[4,3-b][1,4]oxazines, ¹⁵ and pyrido[3,2-b][1,2]oxazines. ¹⁶ However, several reports are available on the synthesis of pyrido[4,3-d][1,3]oxazine derivatives. Hansen synthesized a series of pyrido[4,3-d][1,3]oxazines by the reaction of 4-amino-3-picolinic acid with substituted benzoyl chloride. ¹⁷ Raslanc and colleagues ¹⁸ reported a new preparative approach, utilizing tandem aza-Wittig and annulation reactions with the corresponding iminophosphoranes, aromatic isocyanate, and acid chloride to give two stable pyrido[4,3-d][1,3]oxazine compounds. Therefore, to discover novel pyrido[4,3-d][1,3]oxazine compounds with excellent biological activity is of greater theoretical and application value.

The aza-Wittig reaction has been improved to be a versatile and powerful technique for synthesizing five- to eight-membered N-heterocycles, including some active natural compounds.^19,20 Some N-containing compounds, which are not easily prepared by other methods, have been synthesized via the tandem aza-Wittig reactions of functionalized iminophosphoranes under mild conditions. ²¹ The tandem aza-Wittig reactions of functionalized iminophosphoranes were also applied in the synthesis of many fused N-heterocycles such as pyrazolopyrimidine, ²² triazolopyrimidine, ²³ thienopyrimidine, ²⁴ pyridopyrimidine, ²⁵ imidazolopyrimidine, ²⁶ and furopyrimidine derivatives. ²⁷ Recently, we have become interested in the synthesis of new bioactive fused heterocycles, such as pyrido[4,3-d][1,3]oxazines from iminophosphoranes, with the aim of evaluating their herbicidal activities. In this paper, an effective method for the preparation of a novel series of pyrido[4,3-d][1,3]oxazine derivatives from the {N-[3-acetyl-5-cyano-2-methyl-6-(methylthio)pyridin-4-yl]imino}triphenylphosphorane is described. The subsequent herbicidal testing indicated that many target compounds possessed herbicidal activity against the root and stalk of radish (Raphanus sativus), grain sorghum (Sorghum bicolor), cucumber (Cucumis sativus), purple medic (Medicago sativa), rape (Brassica napus), and barnyard grass (Echinochloa crusgalli).

Results and discussion

Synthesis and spectroscopic characterization

The synthetic route of the novel pyrido[4,3-d][1,3]oxazine derivatives is illustrated in Scheme 1. 2-methyl-3-acetyl-4-amino-5-cyan-6-methylthio-pyridine 1, which was prepared according to Mo et al., ²⁸ reacted with triphenylphosphine, hexachloroethane, and triethylamine to produce {N-[3-acetyl-5-cyano-2-methyl-6-(methylthio)pyridin-4-yl]imino}triphenylphosphorane 2 conveniently. Intermediate 2 reacted with aryl isocyanate to afford carbodiimide 3. Then, carbodiimide 3 readily underwent heterocyclization at room temperature to give the corresponding target compound 4 (Table 1) in satisfactory yields.

Scheme 1.

The synthetic route of the compound 4.

Table 1.

Structures of products 4a–k.

Compound	Ar
4a	-C6H5
4b	4-CH3-C6H4
4c	3,4-2Cl-C6H3
4d	4-Cl-C6H4
4e	2-CF3-C6H4
4f	3-Cl-C6H4
4g	4-OCH3-C6H4
4h	-CH2-C6H5
4i	3,5-2CH3-C6H3
4j	4-CF3-C6H4
4k	4-F-C6H4

The title compounds 4 were investigated by considering the data of infrared (IR), ¹H NMR, ¹³C NMR, high-resolution mass spectrometry (HRMS) and X-ray single-crystal diffraction. For example, the IR spectrum of 2-[(4-methoxyphenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4g) showed three absorption peaks at 3271, 2928, and 2224 cm^–1 assignable to amino, methylene, and cyano functions, respectively. For its ¹H NMR spectrum, the singlet signals at 3.75, 2.57, and 2.55 ppm were assigned to the methyl of OCH₃, SCH₃, and CH₃, respectively. The signals at 5.02 and 4.94 ppm belonged to the protons of methylene. The singlet peak of amino proton was at 10.42 ppm. The ¹³C NMR peaks at 12.8, 26.3, and 55.7 ppm were attributed to the carbon of SCH₃, CH₃, and OCH₃, respectively. The HRMS displayed the expected molecular at m/z: 353.1067 [M + H]⁺. In addition, the structure of compound 4g was explicitly assigned by X-ray crystallography analysis (Figure 1 and Table 2).

Figure 1.

ORTEP diagram of 4g with 30% probability.

Table 2.

X-ray crystallography analysis information for 4g.

Characteristics	Values
Identification code	CCDC 1888919
Empirical formula	C18H16N4O2S
Formula weight	352.41
Temperature	273(2) K
Wavelength	0.71073 A
Crystal system, space group	Triclinic, P-1
Unit cell dimensions	a = 7.7063(3) A b = 14.9927(7) A c = 15.2846(4) A alpha = 86.157(3)° beta = 84.261(3)° gamma = 82.603(4)°
Volume	1739.86(12) Å3
Z, calculated density	4, 1.345 g/cm3
Absorption coefficient	0.205 mm–1
F(000)	736
Crystal size	0.11 mm × 0.06 mm × 0.05 mm
Theta range for data collection	3.495–26.371°
Limiting indices	–9 ⩽ h ⩽ 9; –18 ⩽ k ⩽ 18; –19 ⩽ l ⩽ 19
Reflections collected/unique	7104/5701 (R(int) = 0.0213)
Data completeness	99.8%
Absorption correction	Spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm
Data/restraints/parameters	7104/0/457
Goodness of fit on F2	1.032
Final R indices (I > 2sigma(I))	R1 = 0.0458, wR2 = 0.1133

Herbicidal activity

The target compounds 4a–4k were screened for herbicidal activities against radish (Raphanus sativus), grain sorghum (Sorghum bicolor), cucumber (Cucumis sativus), purple medic (Medicago sativa), rape (Brassica napus), and barnyard grass (Echinochloa crusgalli) at concentrations of 100 mg L⁻¹ and 10 mg L⁻¹ using a Petri dish method. ²² As shown in Table S2 (in Supporting Information), compound 4i shows a >90% inhibition rate to the stalk and root of barnyard grass at 100 mg L⁻¹. Compound 4g displays 100% inhibition rate to the stalk and root of grain sorghum at 100 mg L⁻¹. Compounds 4e–4h exhibit a >90% inhibition rate to the root of rape. However, compounds 4d and 4h have a positively stimulating action to the stalk and root of cucumber at 10 mg L⁻¹.

Conclusion

In summary, we designed and synthesized 11 pyrido[4,3-d][1,3]oxazine derivatives via tandem aza-Wittig reaction. The preliminary bioassay of these title compounds was studied. Based on the herbicidal testing, we found that some target compounds possessed excellent herbicidal activity against the root and stalk of radish, grain sorghum, cucumber, purple medic, rape, and barnyard grass. The study in this work demonstrated an effective method to synthesize fused heterocycles containing pyrido[4,3-d][1,3]oxazines with good herbicidal activity.

Experimental analysis

General experimental methods

All commercial reagents were purchased from J&K Chemical Co. Et₃N and dichloromethane (DCM) were dried according to standard methods and distilled before use. Melting points were determined on an electrothermal melting-point apparatus and are uncorrected. A Nicolet 6700 spectrometer was used to record IR spectra (KBr pellets). ¹H NMR, ¹³C NMR, and ¹⁹F NMR spectra were recorded on a Bruker 400 spectrometer with CDCl₃ or dimethyl sulfoxide-d₆ (DMSO-d₆) as the solvent. The HRMS analysis was acquired on a micro-TOF II instrument. The X-ray single-crystal diffraction was performed with a SuperNova E G8910B Instrument.

Synthesis of {N-[3-Acetyl-5-cyano-2-methyl-6-(methylthio)pyridin-4-yl]imino}triphenylphosphorane 2

1 (2.21 g, 10 mmol), Ph₃P (5.24 g, 20 mmol), and C₂Cl₆ (4.74 g, 20 mmol) were dissolved in dry DCM (40 mL), and Et₃N (4.04 g, 40 mmol) was added dropwise to the solution. After the addition was complete, the reaction mixture was stirred at room temperature for 6 h. Then, the solution was concentrated under vacuum, and the solid residue was washed with ethanol to provide compound 2 in 72% yield; m.p. 145–146 °C; ¹H NMR (CDCl₃, 400 MHz) δ 7.64 (m, 6H), 7.54 (m, 3H), 7.49–7.42 (m, 6H), 2.48 (s, 3H), 2.38 (s, 3H), 2.31 (s, 3H); ¹³C NMR (CDCl₃, 100 MHz) δ 206.2, 163.3, 157.2, 154.2, 132.6, 132.5, 132.4, 132.3, 130.9, 130.5, 130.4, 129.9, 128.8, 128.7, 117.6, 98.2, 31.9, 22.5, 13.3; HRMS (ESI) calcd for C₂₈H₂₄N₃OPS [M + H]⁺: 482.1456; found: 482.1452.

General procedure for compound 4

Intermediate 2 (0.962 g, 2 mmol) was dissolved in dry DCM (10 mL), and aryl isocyanate (2 mmol) was quickly added to the solution under nitrogen at room temperature. After the reaction mixture was left unstirred for 6–8 h, the precipitate was isolated by filtration under washing with DCM (50 mL) and recrystallized from DMSO/petroleum ether to give pure 2-(arylamino)-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile derivatives (4a–4k).

5-methyl-4-methylene-7-(methylthio)-2-(phenylamino)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4a). Yellow solid; yield: 71%; m.p. 156–158 °C; ¹H NMR (400 MHz, DMSO) δ 10.55 (s, 1H), 7.80 (s, 2H), 7.34 (t, J = 7.9 Hz, 2H), 7.11 (t, J = 7.3 Hz, 1H), 5.05 (d, J = 3.3 Hz, 1H), 4.96 (d, J = 3.2 Hz, 1H), 2.58 (s, 3H), 2.55 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.2, 157.3, 154.8, 153.7, 148.4, 137.8, 129.2, 124.4, 120.6, 115.2, 110.3, 97.3, 97.0, 26.4, 12.9; IR (KBr): υ=3313, 2926, 2218, 1639, 1093 cm^–1; HRMS (ESI) calcd for C₁₇H₁₄N₄OS [M + H]⁺: 323.0967; found: 323.0962.

5-methyl-4-methylene-7-methylthio-2-(p-tolylamino)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4b). Yellow solid; yield: 73%; m.p. 228–230 °C; ¹H NMR (400 MHz, DMSO) δ 10.45 (s, 1H), 7.71 (s, 2H), 7.13 (d, J = 8.2 Hz, 2H), 5.02 (d, J = 3.3 Hz, 1H), 4.92 (d, J = 3.1 Hz, 1H), 2.55 (s, 3H), 2.54 (s, 3H), 2.28 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.1, 157.2, 154.8, 153.3, 148.5, 135.2, 133.4, 129.6, 120.4, 115.2, 110.1, 97.3, 96.8, 26.4, 20.9, 12.8; IR (KBr): υ=3305, 2926, 2216, 1641, 1084 cm^–1; HRMS (ESI) calcd for C₁₈H₁₆N₄OS [M + H]⁺: 337.1123; found: 337.1118.

2-[(3,4-dichlorophenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4c). White solid; yield: 68%; m.p. 253–255 °C; ¹H NMR (400 MHz, DMSO) δ 10.79 (s, 1H), 8.31 (s, 1H), 7.59–7.50 (m, 2H), 5.05 (d, J = 3.3 Hz, 1H), 4.98 (d, J = 3.2 Hz, 1H), 2.58 (s, 3H), 2.56 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.2, 157.5, 154.2, 153.4, 148.3, 138.1, 131.7, 130.8, 125.8, 121.7, 120.3, 115.0, 110.3, 97.6, 97.3, 26.4, 12.9; IR (KBr): υ=3279, 2925, 2221, 1645, 1090 cm^–1; HRMS (ESI) calcd for C₁₇H₁₂Cl₂N₄OS [M + H]⁺: 391.0187; found: 391.0182.

2-[(4-chlorophenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4d). Yellow solid; yield: 77%; m.p. 251–253 °C; ¹H NMR (400 MHz, DMSO) δ 10.66 (s, 1H), 7.79 (s, 2H), 7.36 (d, J = 8.7 Hz, 2H), 5.05 (d, J = 3.1 Hz, 1H), 4.96 (d, J = 3.2 Hz, 1H), 2.57 (s, 3H), 2.55 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.2, 157.4, 154.6, 153.6, 148.4, 136.9, 129.0, 128.1, 122.2, 115.2, 110.3, 97.3, 97.1, 26.4, 12.9; IR (KBr): υ=3306, 2928, 2219, 1639, 1084 cm^–1; HRMS (ESI) calcd for C₁₇H₁₃ClN₄OS [M + H]⁺: 357.0577; found: 357.0581.

5-methyl-4-methylene-7-(methylthio)-2-{[2-(trifluoromethyl)phenyl]amino}-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4e). Yellow solid; yield: 70%; m.p. 243–245 °C; ¹H NMR (400 MHz, DMSO) δ 10.97 (s, 1H), 7.97–7.85 (m, 2H), 7.75–7.70 (m, 1H), 7.67–7.60 (d, J = 7.8 Hz, 1H), 4.88 (d, J = 2.5 Hz, 1H), 4.16 (d, J = 2.6 Hz, 1H), 2.64 (s, 3H), 2.61 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.4, 158.3, 149.4, 147.0, 139.6, 135.6, 134.5, 133.9, 132.0, 131.9, 130.1, 129.3, 129.2, 128.2, 128.2, 127.3, 127.0, 125.0, 122.3, 113.4, 110.5, 100.5, 89.5, 26.3, 13.2; ¹⁹F NMR (376 MHz, DMSO) δ –60.85; IR (KBr): υ=3200, 2931, 2216, 1691, 1056 cm^–1; HRMS (ESI) calcd for C₁₈H₁₃F₃N₄OS [M + H]⁺: 391.0840; found: 391.0835.

2-[(3-chlorophenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4f). White solid; yield: 79%; m.p. 160–162 °C; ¹H NMR (400 MHz, DMSO) δ 10.71 (s, 1H), 8.07 (s, 1H), 7.61 (d, J = 7.3 Hz, 1H), 7.35 (t, J = 8.1 Hz, 1H), 7.15 (d, J = 7.5 Hz, 1H), 5.07 (d, J = 3.3 Hz, 1H), 5.00 (d, J = 3.2 Hz, 1H), 2.61 (s, 3H), 2.58 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.3, 157.5, 154.4, 153.6, 148.3, 139.4, 133.8, 130.8, 124.0, 120.1, 118.9, 115.1, 110.4, 97.5, 97.3, 26.4, 12.9; IR (KBr): υ=3274, 2925, 2217, 1646, 1099 cm^–1; HRMS (ESI) calcd for C₁₇H₁₃ClN₄OS [M + H]⁺: 357.0577; found: 357.0571.

2-[(4-methoxyphenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4H-pyrido [4,3-d][1,3]oxazine-8-carbonitrile (4g). Yellow solid; yield: 74%; m.p. 214–216 °C; ¹H NMR (400 MHz, DMSO) δ 10.42 (s, 1H), 7.74 (s, 2H), 6.90 (d, J = 8.4 Hz, 2H), 5.02 (d, J = 2.8 Hz, 1H), 4.94 (d, J = 2.8 Hz, 1H), 3.75 (s, 3H), 2.57 (s, 3H), 2.55 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.1, 157.2, 156.2, 154.9, 153.3, 148.5, 121.8, 120.4, 115.3, 114.4, 110.2, 97.2, 96.9, 55.7, 26.3, 12.8; IR (KBr): υ=3271, 2928, 2224, 1643, 1081 cm^–1; HRMS (ESI) calcd for C₁₈H₁₆N₄O₂S [M + H]⁺: 353.1072; found: 353.1067.

2-(benzylamino)-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3d][1,3] oxazine-8-carbonitrile (4h); White solid; yield: 66%; m.p. 216–218 °C; ¹H NMR (400 MHz, DMSO) δ 8.97 (s, 1H), 7.42–7.28 (m, 5H), 4.98 (d, J = 3.1 Hz, 1H), 4.88 (d, J = 2.9 Hz, 1H), 4.48 (s, 2H), 2.55 (s, 3H), 2.54 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.1, 157.0, 156.6, 155.3, 148.6, 138.6, 128.8, 128.5, 127.7, 115.2, 109.9, 97.1, 96.7, 44.9, 26.3, 12.8; IR (KBr): υ=3357, 2927, 2219, 1634, 1082 cm^–1; HRMS (ESI) calcd for C₁₈H₁₆N₄OS [M + H]⁺: 337.1123; found: 337.1118.

2-[(3,5-dimethylphenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4i); White solid; yield: 79%; m.p. 218–220 °C; ¹H NMR (400 MHz, DMSO) δ 10.48 (s, 1H), 7.57 (d, J = 40.0 Hz, 2H), 6.74 (s, 1H), 5.06 (d, J = 3.4 Hz, 1H), 4.98 (d, J = 3.2 Hz, 1H), 2.60 (s, 3H), 2.58 (s, 3H), 2.25 (s, 6H); ¹³C NMR (100 MHz, DMSO) δ 162.1, 157.4, 155.0, 148.5, 138.3, 137.6, 125.7, 118.2, 117.1, 115.3, 110.4, 97.1, 96.0, 26.3, 21.5, 12.9; IR (KBr): υ=3353, 2919, 2219, 1638, 1098 cm^–1; HRMS (ESI) calcd for C₁₉H₁₈N₄OS [M + H]⁺: 351.1280; found: 351.1276.

5-methyl-4-methylene-7-(methylthio)-2-{[4-(trifluoromethyl)phenyl]amino}-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4j); White solid; yield: 72%; m.p. 256–258 °C; ¹H NMR (400 MHz, DMSO) δ 10.92 (s, 1H), 7.96 (s, 2H), 7.66 (d, J = 8.7 Hz, 2H), 5.09 (d, J = 3.4 Hz, 1H), 5.01 (d, J = 3.3 Hz, 1H), 2.60 (s, 3H), 2.57 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.2, 157.5, 154.5, 153.7, 148.3, 142.0, 126.4, 126.3, 126.3, 126.3, 120.7, 115.2, 110.4, 97.4, 97.3, 26.4, 12.9; ¹⁹F NMR (376 MHz, DMSO) δ –60.12; IR (KBr): υ=3299, 2931, 2222, 1649, 1116 cm^–1; HRMS (ESI) calcd for C₁₈H₁₃F₃N₄OS [M + H]⁺: 391.0840; found: 391.0836.

2-[(4-fluorophenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile (4k); Yellow solid; yield: 70%; m.p. 235–237 °C; ¹H NMR (400 MHz, DMSO) δ 10.60 (s, 1H), 7.82 (s, 2H), 7.17 (t, J = 8.7 Hz, 2H), 5.05 (d, J = 3.4 Hz, 1H), 4.96 (d, J = 3.2 Hz, 1H), 2.58 (s, 3H), 2.56 (s, 3H); ¹³C NMR (100 MHz, DMSO) δ 162.2, 159.0 (d, J = 237.3 Hz), 157.3, 154.8, 153.6, 148.4, 134.2, 122.4, 115.8 (d, J = 22.5 Hz), 115.2, 110.3, 97.3, 97.1, 26.4, 12.9; ¹⁹F NMR (376 MHz, DMSO) δ –118.82; IR (KBr): υ=3311, 2932, 2217, 1639, 1084 cm^–1; HRMS (ESI) calcd for C₁₇H₁₃FN₄OS [M + H]⁺: 341.0872; found: 341.0873.

Supplemental Material

Supporting_Information – Supplemental material for Design, synthesis, and herbicidal activity of novel 2-(arylamino)-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile derivatives

Supplemental material, Supporting_Information for Design, synthesis, and herbicidal activity of novel 2-(arylamino)-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile derivatives by Yi Sun, Wenfei Huang, Zhiqiang Li, Tao Wang and Jin Luo in Journal of Chemical Research