English

• 1.   Introduction

  The β-triketone skeleton is a basic structural feature of many natural products (Figure 1).^[1-11] Over the last few decades, compounds with a β-triketone skeleton have played an important role in the pharmaceutical and agrochemical fields because of the antioxidative, ^[12] antiplasmodial, ^[13] anti-inflammatory, ^[14] antibacterial, ^{[15, 16]} antiviral, ^[17] herbicidal, ^[18] antifungal, ^[19-23] and insecticidal activities.^[24] Such characteristics of β-triketones have made them important, and numerous novel compounds with a β-triketone skeleton, mainly served as herbicides, have been synthesized in recent years. For example, Zhu et al.^[25] have reported that β-triketones containing a 2H-benzo[b]^{[1, 4]}oxazin-3(4H)-one moiety exhibited good herbicidal activity at a dosage of 375 g•ha^－1. Yang et al.^[26-29] have reported that β-triketones containing a quinazoline-2, 4-dione, quinoline, or aryloxyacetyl moiety exhibited excellent herbicidal activity at a lower dosage.

  Figure 1

  

  Figure 1.  Examples of natural products containing a β-triketone skeleton

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  Barbituric acid is a key intermediate in the synthesis of biologically active compounds, and the derivatives (Figure 2) with substituents in the 5-position have demonstrated anticancer, ^[30-31] antibacterial, ^[32-33] antifungal, ^[34] antitubercular, ^[35-36] anti-inflammatory, ^[37-38] sedative-hypnotic and anticonvulsant properties.^[39-40] Moreover, a number of compounds with a barbituric acid moiety are radio-

  Figure 2

  

  Figure 2.  Design strategy of target compounds

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  sensitizing agents, ^[41] reactive oxygen scavengers, ^[42] methionine aminopeptidase-1 inhibitors, ^[43] and human immuno-deficiency virus-1 (HIV-1) and human immuno-deficiency virus-2 (HIV-2) (HIV-2) protease inhibitors.^[44] Although barbituric acid derivatives exhibit fascinating biological activities, relatively little is known about their application as herbicides.^[45] Recently, we^[46-47] constructed β-triketone derivatives by introducing an acyl group in the 3-positions of 4-hydroxy-6-methyl-2H-pyran- 2-one and 2, 1-benzothiazine, and found that some compounds possessed good herbicidal activity. Inspired by our recent work, we anticipated that introducing an acyl group in the 5-position of barbituric acid can help construct β-triketone compounds containing a barbituric acid moiety (Figure 2), which should possess good herbicidal activity. Therefore, as a part of our continuous efforts to develop new β-triketone herbicides, ^{[46, 48]} 31 5-acylbarbituric acid derivatives were synthesized and tested for herbicidal activity. Here, the synthesis, herbicidal activity, and structure-activity relationship of 5-acyl- barbituric acid derivatives, as well as the molecular mode of action study results were reported.

  2.   Results and discussion

  2.1   Chemistry

  As depicted in Scheme 1 (Route A), the target compounds BBA-1~BBA-6 were prepared via a two-step synthetic route. First, 1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (3) was obtained in moderate yield by reacting 1, 3-dimethylurea (1) with diethyl malonate (2) in absolute ethanol under basic conditions. Then, target compounds BBA-1~BBA-6 were synthesized in yield of 47.2%~69.5% by condensation of 3 with a commercial acetyl chloride derivative 4 in the presence of trimethylamine. To obtain the target compounds BBA-7~BBA-31, the intermediate aryloxyacetyl chloride derivative 9 was first synthesized according to Scheme 1 (Route B). In the presence of potassium carbonate as a base, phenol 5 was reacted with methyl chloroacetate to yield the corresponding methyl aryloxyacetate 7. Subsequently, hydrolysis of methyl aryloxyacetate 7 with sodium hydroxide in a methanol/water system resulted in the corresponding aryloxyacetyl acid 8, which was reacted with oxalyl chloride in the presence of a catalytic amount of N, N-dimethylform- amide (DMF) to produce the corresponding aryloxyacetyl chloride derivative 9. Finally, the condensation of compound 3 with aryloxyacetyl chloride 9 in the presence of trimethylamine afforded the target compounds BBA-7~ BBA-31 in 34.7%~63.3% yield.

  Scheme 1

  

  Scheme 1.  Synthetic route of target compounds BBA-1~BBA-31

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  The structures of all the target compounds were determined using ¹H NMR and ¹³C NMR spectroscopy and HRMS. In addition, the structures of compounds BBA-27 and BBA-29 were confirmed by X-ray diffraction analysis (CCDC 2002865 and 2002866, Figure 3).

  Figure 3

  

  Figure 3.  X-ray crystal structures of compounds BBA-27 (top) and BBA-29 (bottom)

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  2.2   In vitro Herbicidal activity and evaluation of the molecular mode of action

  For initial evaluation of the target compounds, the B. campestris root test and E. crusgalli cup test were performed. Mesotrione and 2, 4-D were selected as positive controls. As shown in Figure 4, most compounds displayed good herbicidal activity against B. campestris at a dosage of 100 μg•mL⁻¹. Compounds BBA-10, BBA-18, BBA-22, BBA-23 and BBA-27 displayed > 90% growth control of B. campestris even at a dosage of 10 μg•mL⁻¹, which is comparable to that of a commercial herbicide 2, 4-D and superior to that of mesotrione. However, most of the target compounds showed worse herbicidal activity against E. crusgalli. Only few compounds (BBA-2, BBA-3, BBA-22, and BBA-27) exhibited > 70% growth control of E. crusgalli at a dosage of 100 μg•mL⁻¹. These findings indicate that most of the target compounds exhibit better herbicide activity against dicotyledonous plants than that of monocotyledonous plants.

  Figure 4

  

  Figure 4.  Inhibition of mustard root and barnyard grass seedling growth by BBA-1~BBA-31 at concentrations of (A) 100 µg• mL^－1 and (B) 10 µg•mL^－1 (error bars represent standard deviation from three biological replicates)

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  Furthermore, it was found that the herbicidal activity of the target compounds was significantly influenced by the R group. When R is a simple alkyl group (BBA-1~BBA-3), the herbicidal activity against the tested weeds at a dosage of 100 μg•mL⁻¹ is higher than those of the compounds containing benzene rings (BBA-4~BBA-6). When the phenylethylcarbonyl β-carbon atom ofBBA-6 was replaced by an oxygen atom, the herbicidal activity of BBA-7 at a dosage of 100 μg•mL⁻¹ decreased sharply. These results indicated that the type of acyl group in the 5-postion of barbituric acid played an important role in influencing herbicidal activity.

  Upon analyzing the herbicidal activity of compoundsBBA-7~BBA-16 against B. campestris at a dosage of 100 μg•mL⁻¹, it was found that the herbicidal activity could be influenced by the benzene ring substituents of compound BBA-7. Among the compounds with a single substituent, those with substituent in the 4-position exhibited better herbicidal activity than compounds with a substituent in the 2- or 3-position, compounds with a chlorine atom on the benzene ring (BBA-8~BBA-10) displayed better herbicidal activity than those with a methyl or trifluoromethyl substituent (BBA-11~BBA-16). Herbicidal activity of the compounds with two chlorine atoms on the benzene ring (BBA-17~BBA-21) can be summarized as follows: BBA- 18 (2-Cl-4-Cl) > BBA-19 (2-Cl-5-Cl) > BBA-17 (2-Cl-3- Cl) > BBA-20 (3-Cl-5-Cl) > BBA-21 (2-Cl-6-Cl). The pattern demonstrates that the 2, 4-disubstituted molecule was the most active and the 2, 6-disubstituted one was the least. Based on the above analysis, it was concluded that introducing a halogen atom at the 2, 4-position of the benzene ring of compound BBA-7 is optimal for improving the herbicidal activity of the target compounds against B. campestris. Our conclusion was verified by the bioassay results of compounds BBA-22~BBA-27. It was found that double-substituted compounds BBA-22~BBA-27 at the 2, 4-position of the benzene ring displayed good herbicidal activity against B. campestris even at a dosage of 10 μg•mL⁻¹. Noteworthy, replacing the benzene ring of compound BBA-7 with naphthalene sharply decreased the herbicidal activity of the resulting compound BBA-29. Even though two halogen atoms were introduced into naphthalene of compound BBA-29, the herbicidal activity did not improve significantly (BBA-30 and BBA-31).

  To understand the herbicidal mechanism of the target compounds, the phenotypes of A. thaliana, treated with compound BBA-22, mesotrione, and 2, 4-D, were investigated. It was found that A. thaliana treated with 10 µmol/L mesotrione exhibited bleaching symptoms, whereas A. thaliana treated with compound BBA-22 did not (Figure 5A), indicating that the herbicidal mode of action of compound BBA-22 is different from that of mesotrione. Interestingly, compound BBA-22 at a lower concentration (0.01 µmol/L) induced lateral root hairs formation in A. thaliana, which was consistent with the plant exposure to 2, 4-D (Figure 5B). The phenotype indicates that compound BBA-22 may be an auxin-type compound. To verify the hypothesis, the effects of compound BBA-22 on the expression of the auxin-induced gene (IAA5), auxin-regulated gene (GH3.3), and auxin transport gene (AUX1) were investigated (Figure 5C).^[49-52] It was found that the expression levels of IAA5, GH3.3, and AUX1 were downregulated after the treatment of A. thaliana with mesotrione. In contrast, upon prolonging the treatment time with compound BBA-22, the expression levels of IAA5, GH3.3, and AUX1 were upregulated, and the trend was consistent with that observed for 2, 4-D. The above finding confirmed that compound BBA-22 behaves as an auxin-type compound.

  Figure 5

  

  Figure 5.  Photographs illustrating seedling (A) and root phenotype (B) of A. thaliana and effect of BBA-22, mesotrione, and 2, 4-D on the expression levels of IAA5, GH3.3, and AUX1 (C) (error bars represent standard deviation from three biological replicates)

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  Furthermore, the degradation of compound BBA-22 in A. thaliana was investigated to explain why BBA-22 has a similar herbicidal mechanism to that of 2, 4-D, but not mesotrione. As shown in Figure 6A, it was found that with the degradation of compound BBA-22 in A. thaliana, two new peaks were observed at 13.5 and 17.2 min. The peak at 13.5 min was also observed in the spectrum of 2-(2- chloro-4-fluorophenoxy) acetic acid (Figure 6B), which demonstrates that compound BBA-22 was degraded to 2-(2-chloro-4-fluorophenoxy) acetic acid in A. thaliana. The observation supports the hypothesis that the herbicidal mechanism of compound BBA-22 is similar to that of auxin herbicides. Notably, 2-(2-chloro-4-fluorophenoxy) acetic acid was completely degraded after 2.5 h, while compound BBA-22 undergone continuous degradation to release 2-(2-chloro-4-fluorophenoxy) acetic acid. Therefore, compound BBA-22 can exert sustained herbicidal activity due to the prolonged lifetime of phenoxycarboxylic acid.

  Figure 6

  

  Figure 6.  Degradation analysis of compound BBA-22 (A) and 2-(2-chloro-4-fluorophenoxy) acetic acid (B) in A. thaliana

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  2.3   Herbicidal activity in greenhouse tests

  Based on the in vitro bioassay results, compounds BBA-22 and BBA-27 were chosen for further study, and the herbicidal activity against four species representative of monocotyledonous and dicotyledonous greenhouse plants was tested. 2, 4-D was selected as the positive control. As the results shown in Table 1, it was found that compounds BBA-22 and BBA-27 showed good herbicidal activity against all tested weeds at a dosage of 1500 g•ha^－1. When the dosage reduced from 1500~187.5 g•ha^－1, the herbicidal activity of the tested compounds and 2, 4-D reduced progressively. However, when the dosage was reduced to 187.5 g•ha^－1, compound BBA-22 still displayed good pre-emergent herbicidal activity against B. campestris, A. retroflexus, and D. sanguinalis (better than that of 2, 4-D). The promising result indicates that compound BBA-22 possesses good pre-emergent herbicidal activity that can potentially be further optimized.

  Table 1

  

  Table 1.  Further herbicidal testing of compounds BBA-22 and BBA-27 in a greenhouse (percent inhibition)^a

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  Compd.	Rate/(g•ha－1)	B. campestris			A. retroflexus			E. crusgalli			D. sanguinalis
  		Pre	Post		Pre	Post		Pre	Post		Pre	Post
  BBA-22	1500	100	100		100	100		100	100		100	100
  	750	100	100		100	100		85.5±2.4	50.5±2.0		97.3±1.2	55.1±2.0
  	375	92.7±3.2	53.3±3.1		100	84.0±2.3		36.6±2.7	0		72.6±2.7	17.7±1.7
  	187.5	55.0±2.5	17.4±2.2		93.4±2.6	58.0±2.1		10.0±2.0	0		47.4±3.0	0
  BBA-27	1500	100	100		100	100		100	100		100	100
  	750	94.6±1.3	52.1±3.3		100	96.3±2.7		81.7±1.6	44.2±2.4		81.3±2.1	51.3±3.3
  	375	82.6±2.3	39.2±1.7		90.0±3.3	66.3±2.0		37.2±3.3	0		66.3±1.5	13.4±2.0
  	187.5	43.3±1.6	12.7±2.0		77.2±2.4	43.3±3.1		0	0		44.2±2.3	0
  2, 4-D	1500	100	100		100	100		100	100		100	100
  	750	100	100		100	100		94.6±1.9	46.6±1.7		100	45.0±1.0
  	375	90.1±2.1	100		100	100		73.3±1.6	21.1±2.3		55.0±3.1	10.5±1.3
  	187.5	38.8±3.6	100		91.3±1.5	100		47.6±3.3	10.0±2.0		22.4±1.6	0
  a Value represents the mean±SD of three experiments.

  3.   Conclusions

  In summary, by combining the structural features of β-triketones and barbituric acid, a series of 5-acylbarbituric acid derivatives have been designed and synthesized. The greenhouse bioassay results showed that the target compounds BBA-22 and BBA-27 possessed good preemergent herbicidal activity. In particular, compound BBA-22 displayed good pre-emergent herbicidal activity against B. campestris, A. retroflexus, and D. sanguinalis even at a dosage of 187.5 g•ha^－1. By investigating the phenotypes of A. thaliana, detecting the effect on the auxin response genes, and studying the degradation of compound BBA-22, we found that BBA-22 is metabolized to form a corresponding aryloxy acetic acid in plants and, thus, has a herbicidal mechanism similar to that of auxin-type herbicides. It is worth mentioning that in our current work, the compounds generated by molecular hybridization prolong the lifetime of herbicides in plants, which may exert a sustained herbicidal effect. This is a step forward as less herbicide need be used, which would have a very important meaning in practical applications. Further studies on the structural optimization of compound BBA-22 are ongoing in our laboratory.

  4.   Experimental section

  4.1   Reagents and instruments

  Usually, chemicals and solvents (purchased from Energy Chemical and Tokyo Chemical Industry) were used as received unless otherwise stated. Melting points for target compounds were determined on a X-4 binocular microscope (Gongyi Tech. Instrument Co., Henan, China). ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) were recorded with CDCl₃ or DMSO-d₆ as solvent. The chemical shifts (δ) are reported relative to tetramethylsilane (TMS). High-resolution mass spectrometry was recorded using an Ionspec 7.0 T spectrometer (Varian, Palo Alto, CA), equipped with an ESI source. The crystal structure was determined on a Saturn 724 CCD area-detector diffractometer (Rigaku, Tokyo, Japan). Real-time PCRs was performed with a Bio-Rad real-time thermal cycling system (Bio-Rad CFX96, USA). High-performance liquid chromatography (HPLC) data were obtained on a Thermo UltiMate3000 (USA).

  4.2   Chemical synthesis procedures

  4.2.1   Synthesis of 1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (3)

  1, 3-Dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (3)was synthesized by a previously reported method with slight modification.^[53] Metal sodium (0.5 mol) was added gradually to 400 mL of absolute ethanol at room temperature, and the solution was stirred for 1 h. Subsequently, 1, 3-dimethylurea 1 (0.5 mol) and diethyl malonate 2 (0.55 mol) were slowly added, and the reaction mixture was heated to 80 ℃ for 12 h. After completion of the reaction, the solvent was evaporated to one-third of the original volume under reduced pressure, 500 mL of water was added, and the mixture was acidified with aqueous hydrochloric acid solution (1 mol/L) to pH＝2. The mixture was cooled to 0 ℃ and the product was crystallized. The residue was collected by filtration and dried under vacuum to produce a pale yellow solid 3 in 41.7% yield. m.p. 123~125 ℃; ¹H NMR (500 MHz, CDCl₃) δ: 3.61 (s, 2H), 3.23 (s, 6H); ¹³C NMR (125 MHz, CDCl₃) δ: 164.8, 151.9, 39.4, 28.4; HRMS calcd for C₆H₉N₂O₃ [M+H]⁺ 157.0608, found 157.0611.

  4.2.2   General procedure for the synthesis of aryloxyacetyl chloride derivative 9

  Aryloxyacetyl chloride derivative 9 was synthesized by a previously reported method.^[46] Phenol 5 (20 mmol) was dissolved in 100 mL of DMF, potassium carbonate (30 mmol) was added and the reaction mixture was heated to 50 ℃. Subsequently, methyl chloroacetate 6 (22 mmol) was added dropwise over 10 min, and the suspension was stirred at 50 ℃ for 12 h. After completion of the reaction, the solution was partitioned between ethyl acetate and water, and the aqueous layer was extracted three times with ethyl acetate. The combined organic layers were washed with brine, dried over anhydrous sodium sulfate, and concentrated to obtain the crude product 7. Next, the crude product 7 was dissolved in a methanol/water (50 mL/5 mL) system, and sodium hydroxide (40 mmol) was added. The solution was heated to 70 ℃ and stirred for 2 h. After completion of the reaction, the solution volume was halved. The reaction mixture was cooled to 0 ℃ and acidified with aqueous hydrochloric acid solution (1 mol/L) to pH＝1. The resulting solid was collected by filtration, washed with water, and dried under vacuum to afford aryloxyacetic acid 8. Aryloxyacetic acid 8 (10 mmol) was dissolved in dichloromethane (20 mL), followed by the addition of oxalyl chloride (20 mmol) and DMF (one drop). The mixture was stirred at room temperature for 12 h until no more gas evolved. Then, the solvent was removed by rotary evaporation to yield the aryloxyacetyl chloride derivative 9, which was used without purification.

  4.2.3   General preparation of 5-acylbarbituric acid derivatives BBA-1~BBA-31

  The target compounds were prepared by modifying previously reported method.^[54] 1, 3-Dimethylpyrimidine- 2, 4, 6(1H, 3H, 5H)-trione (3) (5 mmol) was dissolved in dichloromethane (20 mL), followed by the addition of acetyl chloride derivative 4 or 9 (10 mmol) and triethylamine (20 mmol) at 0 ℃. The solution was stirred at 25 ℃ for 12 h. After completion of the reaction, 20 mL of aqueous hydrochloric acid solution (1 mol/L) was added. The organic layer was separated, washed with water and brine, dried over anhydrous sodium sulfate, and concentrated by rotary evaporation. The residue was scratched in ethyl acetate and petroleum ether (1/5 by volume) to yield the target compounds BBA-1~BBA-31.

  5-(1-Hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-1): White solid, yield 67.3%. m.p. 97~98 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 3.37 (s, 3H), 3.33 (s, 3H), 2.72 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 196.2, 169.4, 161.2, 150.5, 95.9, 28.0, 27.8, 24.8; HRMS calcd for C₈H₁₁N₂O₄ [M+H]⁺ 199.0713, found 199.0715.

  5-(1-Hydroxypropylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-2): White solid, yield 61.6%. m.p. 124~125 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 3.38 (s, 3H), 3.34 (s, 3H), 3.19 (q, J＝7.4 Hz, 2H), 1.25 (t, J＝7.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 200.7, 169.8, 160.9, 150.4, 95.0, 30.5, 28.0, 27.9, 9.5; HRMS calcd for C₉H₁₃N₂O₄ [M+H]⁺ 213.0870, found 213.0872.

  5-(1-Hydroxy-2-methylpropylidene)-1, 3-dimethyl-pyri-midine-2, 4, 6(1H, 3H, 5H)-trione (BBA-3): White solid, yield 47.2%. m.p. 93~94 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 4.36~4.18 (m, 1H), 3.38 (s, 3H), 3.34 (s, 3H), 1.24 (s, 3H), 1.22 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 204.3, 170.1, 160.7, 150.4, 94.2, 33.5, 28.1, 27.9, 19.0; HRMS calcd for C₁₀H₁₅N₂O₄ [M+H]⁺ 227.1026, found 227.1027.

  5-(Hydroxy(phenyl)methylene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-4): White solid, yield 53.0%. m.p. 102~104 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.60~7.52 (m, 3H), 7.48~7.44 (m, 2H), 3.44 (s, 3H), 3.28 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 191.7, 170.3, 160.0, 150.5, 134.7, 132.0, 128.4, 127.8, 95.2, 28.3, 28.1; HRMS calcd for C₁₃H₁₃N₂O₄ [M+H]⁺ 261.0870, found 261.0872.

  5-(1-Hydroxy-2-phenylethylidene)-1, 3-dimethylpyrimi-dine-2, 4, 6(1H, 3H, 5H)-trione (BBA-5): White solid, yield 61.0%. m.p. 92~94 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.54~7.19 (m, 5H), 4.51 (s, 2H), 3.35 (s, 6H); ¹³C NMR (100 MHz, CDCl₃) δ: 196.4, 169.9, 160.9, 150.3, 134.4, 129.7, 128.7, 127.4, 95.3, 41.9, 28.2, 28.0; HRMS calcd for C₁₄H₁₅N₂O₄ [M+H]⁺ 275.1026, found 275.1028.

  5-(1-Hydroxy-3-phenylpropylidene)-1, 3-dimethylpyri-midine-2, 4, 6(1H, 3H, 5H)-trione (BBA-6): White solid, yield 69.5%. m.p. 93~94 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.33~7.27 (m, 4H), 7.25~7.17 (m, 1H), 3.47 (t, J＝8.0 Hz, 2H), 3.37 (s, 3H), 3.34 (s, 3H), 3.00 (t, J＝8.0 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 198.4, 169.8, 160.8, 150.4, 140.3, 128.5, 128.5, 126.4, 95.4, 38.7, 31.5, 28.1, 27.9; HRMS calcd for C₁₅H₁₇N₂O₄ [M+H]⁺289.1183, found 289.1185.

  5-(1-Hydroxy-2-phenoxyethylidene)-1, 3-dimethyl-pyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-7): White solid, yield 63.3%. m.p. 203~204 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.35~7.28 (m, 2H), 7.01 (t, J＝7.4 Hz, 1H), 6.98~6.93 (m, 2H), 5.51 (s, 2H), 3.39 (s, 6H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.6, 169.6, 160.8, 157.9, 150.2, 129.6, 121.8, 114.6, 94.0, 68.5, 28.0; HRMS calcd for C₁₄H₁₅N₂O₅ [M+H]⁺ 291.0975, found 291.0976.

  5-(2-(2-Chlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-8): White solid, yield 47.1%. m.p. 210~211 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.43 (d, J＝7.4 Hz, 1H), 7.21 (t, J＝6.9 Hz, 1H), 6.98 (t, J＝6.0 Hz, 1H), 6.90 (d, J＝7.5 Hz, 1H), 5.60 (s, 2H), 3.43 (s, 3H), 3.38 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.8, 169.5, 160.7, 153.5, 150.1, 130.7, 127.7, 123.1, 122.5, 113.7, 93.9, 69.3, 28.1, 28.0; HRMS calcd for C₁₄H₁₄ClN₂O₅ [M+H]⁺ 325.0586, found 325.0588.

  5-(2-(3-Chlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-9): White solid, yield 48.0%. m.p. 198~199 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.24 (t, J＝8.0 Hz, 1H), 7.01 (d, J＝7.9 Hz, 1H), 6.97 (s, 1H), 6.87 (d, J＝8.2 Hz, 1H), 5.51 (s, 2H), 3.43 (s, 3H), 3.39 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.0, 169.6, 160.7, 158.6, 150.1, 135.1, 130.4, 122.0, 115.2, 113.1, 94.0, 68.6, 28.1, 28.0; HRMS calcd for C₁₄H₁₄ClN₂O₅ [M+H]⁺ 325.0586, found 325.0588.

  5-(2-(4-Chlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-10): White solid, yield 53.6%. m.p. 172~174 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.26 (d, J＝7.7 Hz, 2H), 6.89 (d, J＝7.7 Hz, 2H), 5.48 (s, 2H), 3.42 (s, 3H), 3.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.1, 169.6, 160.7, 156.5, 150.1, 129.5, 126.7, 116.0, 94.0, 68.8, 28.1, 28.0; HRMS calcd for C₁₄H₁₄ClN₂O₅ [M+H]⁺ 325.0586, found 325.0588.

  5-(1-Hydroxy-2-(o-tolyloxy)ethylidene)-1, 3-dimethyl-pyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-11): White solid, yield 51.0%. m.p. 213~214 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.18 (d, J＝7.3 Hz, 1H), 7.12 (t, J＝7.6 Hz, 1H), 6.91 (t, J＝7.3 Hz, 1H), 6.74 (d, J＝8.1 Hz, 1H), 5.53 (s, 2H), 3.40 (s, 3H), 3.36 (s, 3H), 2.33 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.9, 169.6, 160.8, 156.1, 150.2, 131.1, 127.2, 126.8, 121.4, 111.2, 94.0, 68.7, 28.1, 28.0, 16.2; HRMS calcd for C₁₅H₁₇N₂O₅ [M+H]⁺ 305.1132, found 305.1133.

  5-(1-Hydroxy-2-(m-tolyloxy)ethylidene)-1, 3-dimethyl-pyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-12): White solid, yield 55.3%. m.p. 185~187 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.18 (t, J＝7.8 Hz, 1H), 6.82 (d, J＝7.5 Hz, 1H), 6.78 (s, 1H), 6.75 (d, J＝8.2 Hz, 1H), 5.49 (s, 2H), 3.41 (s, 3H), 3.36 (s, 3H), 2.33 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.8, 169.6, 160.8, 157.8, 150.2, 139.8 129.4, 122.6, 115.4, 111.4, 93.9, 68.4, 28.1, 28.0, 21.6; HRMS calcd for C₁₅H₁₇N₂O₅ [M+H]⁺ 305.1132, found 305.1134.

  5-(1-Hydroxy-2-(p-tolyloxy)ethylidene)-1, 3-dimethyl-pyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-13): White solid, yield 50.6%. m.p. 183~185 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.10 (d, J＝8.2 Hz, 2H), 6.85 (d, J＝8.0 Hz, 2H), 5.48 (s, 2H), 3.40 (s, 3H), 3.36 (s, 3H), 2.29 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.9, 169.6, 160.8, 155.7, 150.2, 131.1, 130.1, 114.4, 94.0, 68.7, 28.1, 28.0, 20.5; HRMS calcd for C₁₅H₁₇N₂O₅ [M+H]⁺ 305.1132, found 305.1134.

  5-(1-Hydroxy-2-(2-(trifluoromethyl)phenoxy)ethyl-idene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-14): White solid, yield 45.3%. m.p. 211~212 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.63 (dd, J＝7.7, 0.8 Hz, 1H), 7.51~7.41 (m, 1H), 7.07 (t, J＝7.6 Hz, 1H), 6.91 (d, J＝8.4 Hz, 1H), 5.60 (s, 2H), 3.41 (s, 3H), 3.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.5, 169.5, 160.8, 155.8, 150.1, 133.2, 127.44 (q, J＝5.1 Hz), 123.5 (q, J＝272.5 Hz), 121.1, 119.17 (q, J＝31.2 Hz), 112.8, 94.0, 68.8, 28.1, 28.0; HRMS calcd for C₁₅H₁₄F₃N₂O₅ [M+H]⁺ 359.0849, found 359.0852.

  5-(1-Hydroxy-2-(3-(trifluoromethyl)phenoxy)ethyl-idene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione(BBA-15): White solid, yield 47.5%. m.p. 165~166 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.42 (t, J＝8.0 Hz, 1H), 7.30~7.24 (m, 1H), 7.19 (s, 1H), 7.12 (dd, J＝8.3, 2.4 Hz, 1H), 5.54 (s, 2H), 3.42 (s, 3H), 3.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.8, 169.6, 160.8, 158.0, 150.1, 132.0 (q, J＝32.6 Hz), 130.2, 123.8 (q, J＝272.5 Hz), 118.5 (q, J＝3.7 Hz), 117.9, 111.8 (q, J＝3.8 Hz), 94.0, 68.7, 28.1, 28.0; HRMS calcd for C₁₅H₁₄F₃N₂O₅ [M+H]⁺359.0849, found 359.0852.

  5-(1-Hydroxy-2-(4-(trifluoromethyl)phenoxy)ethyl-idene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-16): White solid, yield 40.1%. m.p. 175~176 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.57 (d, J＝8.6 Hz, 2H), 7.02 (d, J＝8.6 Hz, 2H), 5.55 (s, 2H), 3.42 (s, 3H), 3.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.7, 169.6, 160.8, 160.3, 150.1, 127.1 (q, J＝3.8 Hz), 124.2 (q, J＝272.7 Hz), 123.9 (q, J＝32.8 Hz), 114.6, 94.0, 68.5, 28.2, 28.0; HRMS calcd for C₁₅H₁₄F₃N₂O₅ [M+H]⁺ 359.0849, found 359.0852.

  5-(2-(2, 3-Dichlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-17): White solid, yield 55.0%. m.p. 201~203 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.19~7.14 (m, 2H), 6.79~6.77 (m, 1H), 5.60 (s, 2H), 3.43 (s, 3H), 3.38 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.3, 169.6, 160.7, 154.9, 150.1, 134.3, 127.3, 123.8, 123.4, 111.5, 93.9, 69.5, 28.1, 28.0; HRMS calcd for C₁₄H₁₃Cl₂N₂O₅ [M+H]⁺ 359.0196, found 359.0199.

  5-(2-(2, 4-Dichlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trionee (BBA-18): White solid, yield 59.0%. m.p. 193~194 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.42 (s, 1H), 7.16 (d, J＝8.7 Hz, 1H), 6.81 (d, J＝8.8 Hz, 1H), 5.55 (s, 2H), 3.41 (s, 3H), 3.36 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.4, 169.9, 160.7, 152.5, 150.1, 130.4, 127.6 127.0, 124.1, 114.5, 93.9, 69.6, 28.1, 28.0; HRMS calcd for C₁₄H₁₃Cl₂N₂O₅ [M+H]⁺359.0196, found 359.0199.

  5-(2-(2, 5-Dichlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-19): White solid, yield 48.2%. m.p. 185~187 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.34 (dd, J＝8.4, 0.9 Hz, 1H), 6.96 (d, J＝8.5 Hz, 1H), 6.87 (s, 1H), 5.57 (s, 2H), 3.43 (s, 3H), 3.38 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.1, 169.6, 160.7, 154, 0, 150.0, 133.1, 131.2, 122.5, 121.6, 114.1, 94.0, 69.4, 28.1, 28.0; HRMS calcd for C₁₄H₁₃Cl₂N₂O₅ [M+H]⁺ 359.0196, found 359.0198.

  5-(2-(3, 5-Dichlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-20): White solid, yield 51.7%. m.p. 196~197 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.01 (s, 1H), 6.85 (s, 1H), 6.84 (s, 1H), 5.47 (s, 2H), 3.42 (s, 3H), 3.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.4, 169.6, 160.7, 158.9, 150.0, 135.6, 122.2, 113.8, 94.0, 68.8, 28.2, 28.0; HRMS calcd for C₁₄H₁₃Cl₂N₂O₅ [M+H]⁺ 359.0196, found 359.0199.

  5-(2-(2, 6-Dichlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-21): White solid, yield 44.6%. m.p. 196~198 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.32 (d, J＝8.1 Hz, 2H), 7.10~7.01 (t, J＝8.0 Hz, 1H), 5.47 (s, 2H), 3.43 (s, 3H), 3.31 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.3, 169.6, 160.6, 150.5, 150.2, 129.4, 129.1, 126.0, 93.6, 72.6, 28.1, 27.9; HRMS calcd for C₁₄H₁₃Cl₂N₂O₅ [M+H]⁺ 359.0196, found 359.0198.

  5-(2-(2-Chloro-4-fluorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA- 22): White solid, yield 63.2%. m.p. 198~199 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.18 (dd, J＝8.0, 2.9 Hz, 1H), 6.95~6.83 (m, 2H), 5.54 (s, 2H), 3.42 (s, 3H), 3.36 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.7, 169.6, 160.7, 157.2 (d, J＝243.8 Hz), 150.2 (d, J＝2.9 Hz), 150.1, 124.0 (d, J＝10.6 Hz), 118.0 (d, J＝26.1 Hz), 114.9 (d, J＝8.9 Hz), 114.1 (d, J＝22.8 Hz), 93.9, 70.1, 28.1, 28.0; HRMS calcd. for C₁₄H₁₃ClFN₂O₅ [M+H]⁺ 343.0492, found 343.0494.

  5-(2-(4-Chloro-2-methylphenoxy)-1-hydroxyethyl-idene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-23): White solid, yield 50.6%. m.p. 219~220 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.16 (s, 1H), 7.07 (d, J＝8.3 Hz, 1H), 6.66 (d, J＝8.6 Hz, 1H), 5.50 (s, 2H), 3.41 (s, 3H), 3.36 (s, 3H), 2.30 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.5, 169.6, 160.8, 154.7, 150.2, 130.9, 129.1, 126.3, 126.2, 112.3, 94.0, 68.9, 28.1, 28.0, 16.2; HRMS calcd for C₁₅H₁₆ClN₂O₅ [M+H]⁺ 339.0742, found 339.0744.

  5-(2-(2, 4-Dimethylphenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-24): White solid, yield 48.3%. m.p. 222~224 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 6.99 (s, 1H), 6.91 (d, J＝8.2 Hz, 1H), 6.64 (d, J＝8.2 Hz, 1H), 5.50 (s, 2H), 3.40 (s, 3H), 3.36 (s, 3H), 2.29 (s, 3H), 2.26 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 195.2, 169.6, 160.8, 154.0, 150.3, 131.9, 130.7, 126.9, 126.9, 111.2, 94.0 68.9, 28.1, 28.0, 20.5, 16.2; HRMS calcd for C₁₆H₁₉ClN₂O₅ [M+H]⁺ 319.1288, found 319.1291.

  5-(2-(4-Fluoro-2-methylphenoxy)-1-hydroxyethyl-idene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-25): White solid, yield 44.2%. m.p. 220~221 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 6.90 (dd, J＝8.9, 3.0 Hz, 1H), 6.80 (td, J＝8.5, 3.1 Hz, 1H), 6.69 (dd, J＝8.9, 4.5 Hz, 1H), 5.48 (s, 2H), 3.41 (s, 3H), 3.36 (s, 3H), 2.31 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.8, 169.6, 169.6, 160.8, 157.4 (d, J＝239.1 Hz), 152.2 (d, J＝2.2 Hz), 150.2, 129.3 (d, J＝7.7 Hz), 117.8 (d, J＝22.9 Hz), 112.5 (d, J＝14.1 Hz), 112.4, 94.0, 69.4, 28.1, 28.0, 16.4; HRMS calcd for C₁₅H₁₆FN₂O₅ [M+H]⁺ 323.1038, found 323.1040.

  5-(2-(2-Bromo-4-chlorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA- 26): White solid, yield 50.5%. m.p. 199~201 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.58 (d, J＝2.5 Hz, 1H), 7.21 (dd, J＝8.8, 2.5 Hz, 1H), 6.77 (d, J＝8.8 Hz, 1H), 5.55 (s, 2H), 3.42 (s, 3H), 3.36 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.4, 169.6, 160.7, 153.3, 150.1, 133.3, 128.3, 127.3, 114.1, 112.8, 93.9, 69.6, 28.2, 28.0; HRMS calcd for C₁₄H₁₃BrClN₂O₅ [M+H]⁺ 402.9691, found 402.9694.

  5-(2-(2-Bromo-4-fluorophenoxy)-1-hydroxyethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA- 27): White solid, yield 57.1%. m.p. 212~213 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.34 (dd, J＝7.7, 3.0 Hz, 1H), 6.97 (ddd, J＝9.0, 7.7, 3.0 Hz, 1H), 6.83 (dd, J＝9.1, 4.6 Hz, 1H), 5.53 (s, 2H), 3.42 (s, 3H), 3.36 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.7, 169.6, 160.7, 157.3 (d, J＝244.5 Hz), 151.1 (d, J＝2.8 Hz), 150.1, 120.8 (d, J＝25.8 Hz), 114.8 (d, J＝22.8 Hz), 114.5 (d, J＝8.5 Hz), 112.6 (d, J＝10.0 Hz), 93.9, 70.2, 28.2, 28.0; HRMS calcd for C₁₄H₁₃BrFN₂O₅ [M+H]⁺ 386.9986, found 386.9988.

  5-(1-Hydroxy-2-(4-phenoxyphenoxy)ethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-28): White solid, yield 55.2%. m.p. 125~127 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.34~7.27 (m, 2H), 7.06 (t, J＝7.4 Hz, 1H), 7.00~6.92 (m, 6H), 5.49 (s, 2H), 3.41 (s, 3H), 3.36 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.7, 169.6, 160.7, 158.1, 154.1, 151.1, 150.2, 129.7, 122.7, 120.7, 117.9, 115.9, 94.0, 69.2, 28.1, 28.0; HRMS calcd for C₂₀H₁₉N₂O₆ [M+H]⁺ 383.1238, found 383.1240.

  5-(1-Hydroxy-2-(naphthalen-2-yloxy)ethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-29): White solid, yield 49.6%. m.p. 219~220 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.79~7.76 (m, 2H), 7.72 (d, J＝8.2 Hz, 1H), 7.44 (t, J＝7.4 Hz, 1H), 7.35 (t, J＝7.4 Hz, 1H), 7.27 (dd, J＝8.7, 2.7 Hz, 1H), 7.14 (d, J＝2.3 Hz, 1H), 5.58 (s, 2H), 3.39 (s, 3H), 3.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.3, 169.6, 160.8, 155.7, 150.1, 134.3, 129.8, 129.4, 127.7, 126.9, 126.6, 124.1, 118.5, 107.1, 94.0, 68.5, 53.5, 28.1, 28.0; HRMS calcd for C₁₈H₁₇N₂O₅ [M+H]⁺ 341.1132, found 341.1133.

  5-(2-((1-Bromonaphthalen-2-yl)oxy)-1-hydroxyethyl-idene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-30): White solid, yield 37.1%. m.p. 235~237 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.26 (d, J＝8.7 Hz, 1H), 7.79 (d, J＝8.7 Hz, 2H), 7.60 (t, J＝7.2 Hz, 1H), 7.44 (t, J＝7.1 Hz, 1H), 7.19 (d, J＝9.0 Hz, 1H), 5.71 (s, 2H), 3.42 (s, 3H), 3.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 194.0, 169.5, 160.7, 152.3, 150.1, 133.2, 130.3, 129.0, 128.1, 127.9, 126.4, 124.9, 114.7, 93.9, 70.2, 28.1, 28.0; HRMS calcd for C₁₈H₁₆BrN₂O₅ [M+H]⁺ 419.0237, found 419.0239.

  5-(2-((1, 6-Dibromonaphthalen-2-yl)oxy)-1-hydroxy-ethylidene)-1, 3-dimethylpyrimidine-2, 4, 6(1H, 3H, 5H)-trione (BBA-31): White solid, yield 34.7%. m.p. 221~223 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (d, J＝9.0 Hz, 1H), 7.95 (s, 1H), 7.75~7.60 (m, 2H), 7.18 (d, J＝9.0 Hz, 1H), 5.70 (s, 2H), 3.41 (s, 3H), 3.37 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 193.8, 169.6, 160.8, 152.6, 150.1, 131.8, 131.2, 131.2, 129.9, 128.4, 128.0, 118.9, 115.6, 109.9, 93.9, 70.1; HRMS calcd for C₁₈H₁₅Br₂N₂O₅ [M+H]⁺ 496.9342, found 496.9344.

  4.3   X-ray diffraction of compounds BBA-27 and BBA-29

  BBA-27 andBBA-29 were recrystallized from a mixture of dichloromethane and ethanol (1/1 by volume) to afford a suitable single crystal for each compound. Crystallographic data for compounds BBA-27 and BBA-29 were deposited with the Cambridge Crystallographic Data Centre as supplementary publications with deposition numbers 2002865 and 2002866, respectively. The detailed data can be acquired free of charge via http://www.ccdc.cam.ac.uk/.

  4.4   Herbicidal activity

  The herbicidal activity of the target compounds was evaluated by a previously reported method.^[46] The preliminary herbicidal activity was assessed using the Brassica campestris root test and Echinochloa crusgalli cup test. Commercial herbicides 2, 4-dichlorophenoxyacetic acid (2, 4-D) and mesotrione were selected as positive controls. Further herbicidal activity of the target compounds BBA-22 and BBA-27 against monocotyledonous weeds (Echinochloa crusgalli, Digitaria sanguinalis) and dicotyledonous plants (Brassica campestris, Amaranthus retroflexus) was tested in a greenhouse. Details of the experimental procedure are available in the Supporting Information.

  4.5   Molecular mode of action

  To evaluate the molecular mode of action of the target compounds, Arabidopsis thaliana Columbia (Col-0) background was cultivated. The phenotypic, quantitative real- time PCR (qRT-PCR), and compound degradation studies were performed according to a previously described procedure.^[46]

  Supporting Information ¹H NMR and ¹³C NMR spectra of the target compounds. The procedure of herbicidal activity test and evaluation of molecular mode of action. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

  
  
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  Figure 1  Examples of natural products containing a β-triketone skeleton

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  Figure 2  Design strategy of target compounds

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  Scheme 1  Synthetic route of target compounds BBA-1~BBA-31

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  Figure 3  X-ray crystal structures of compounds BBA-27 (top) and BBA-29 (bottom)

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  Figure 4  Inhibition of mustard root and barnyard grass seedling growth by BBA-1~BBA-31 at concentrations of (A) 100 µg• mL^－1 and (B) 10 µg•mL^－1 (error bars represent standard deviation from three biological replicates)

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  Figure 5  Photographs illustrating seedling (A) and root phenotype (B) of A. thaliana and effect of BBA-22, mesotrione, and 2, 4-D on the expression levels of IAA5, GH3.3, and AUX1 (C) (error bars represent standard deviation from three biological replicates)

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  Figure 6  Degradation analysis of compound BBA-22 (A) and 2-(2-chloro-4-fluorophenoxy) acetic acid (B) in A. thaliana

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  Table 1.  Further herbicidal testing of compounds BBA-22 and BBA-27 in a greenhouse (percent inhibition)^a

  Compd.	Rate/(g•ha－1)	B. campestris			A. retroflexus			E. crusgalli			D. sanguinalis
  		Pre	Post		Pre	Post		Pre	Post		Pre	Post
  BBA-22	1500	100	100		100	100		100	100		100	100
  	750	100	100		100	100		85.5±2.4	50.5±2.0		97.3±1.2	55.1±2.0
  	375	92.7±3.2	53.3±3.1		100	84.0±2.3		36.6±2.7	0		72.6±2.7	17.7±1.7
  	187.5	55.0±2.5	17.4±2.2		93.4±2.6	58.0±2.1		10.0±2.0	0		47.4±3.0	0
  BBA-27	1500	100	100		100	100		100	100		100	100
  	750	94.6±1.3	52.1±3.3		100	96.3±2.7		81.7±1.6	44.2±2.4		81.3±2.1	51.3±3.3
  	375	82.6±2.3	39.2±1.7		90.0±3.3	66.3±2.0		37.2±3.3	0		66.3±1.5	13.4±2.0
  	187.5	43.3±1.6	12.7±2.0		77.2±2.4	43.3±3.1		0	0		44.2±2.3	0
  2, 4-D	1500	100	100		100	100		100	100		100	100
  	750	100	100		100	100		94.6±1.9	46.6±1.7		100	45.0±1.0
  	375	90.1±2.1	100		100	100		73.3±1.6	21.1±2.3		55.0±3.1	10.5±1.3
  	187.5	38.8±3.6	100		91.3±1.5	100		47.6±3.3	10.0±2.0		22.4±1.6	0
  a Value represents the mean±SD of three experiments.

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