Synthesis and Biological activity of 4-(4,6-Disubstituted-pyrimidin-2-yloxy)phenoxy Acetates

Ten novel 4-(4,6-dimethoxypyrimidin-2-yloxy)phenoxy acetates and 4-(4,6-dimethylpyrimidin-2-yloxy)phenoxy acetates were synthesized with hydroquinone, 2-methylsulfonyl-4,6-disubstituted-pyrimidine and chloroacetic ester as starting materials. The products were characterized by IR, 1H-NMR, MS spectra and elemental analyses. Preliminary bioassay indicates that the target compounds possess high herbicidal activity against monocotyledonous plants such as Digitaria sanguinalis L. at concentrations of 100 mg/L and 50 mg/L.


Introduction
Aryloxy-phenoxy propionates are an important class of herbicides due to their high efficiency, broad spectrum, low toxicity and good selectivity. They act by blocking the biosynthesis of fatty acids by inhibiting acetyl-coenzyme A carboxylase [1][2][3]. Since the first herbicide of this series, diclofopmethyl, was synthesized in 1972, more than twenty aryloxy-phenoxy propionate herbicides such as fluazifop-butyl, heloxyfop-methyl, quizalofop-ethyl and cyhalofop-butyl have been developed [4], and are widely used to control gramineous weeds. In addition, some aryloxy-phenoxy acetates exhibit good herbicidal activity. For example, two substituted pyrazolo [3,4-d] pyrimidin-4-yloxy phenoxy acetates OPEN ACCESS display considerable activities [5], with 100% inhibition against the root growth of Brassica napus L. at a concentration of 100 mg/L, and 98.1% and 100% against the root growth of Echinochloa crusgalli L. at the same concentration, respectively.

Synthesis
There are two pathways (route A and route B, Scheme 1) in the synthesis of aryloxy-phenoxy propionate herbicides [12]. In route A, hydroquinone reacts with 2-chloropropionates to yield 4hydroxyphenoxy propionates which react with aryl chlorides to give the target compounds. In route B, hydroquinone firstly reacts with an aryl chloride to give a 4-hydroxyphenyl aryl ether which reacts with 2-chloropropionates to give the target compound. Considering the lower yield of 4-hydroxyphenoxy propionate in the reaction of hydroquinone with 2-chloropropionate in route A, we choose route B as the synthetic strategy in our work. Moreover, we used 2-methylsulfonyl-4,6-disubstitutedpyrimidines instead of 2-chloro-4,6-disubstitutedpyrimidines to produce 2-(4-hydroxyphenoxy)-4,6-disubstitutedpyrimidines 2a, 2b owing to their higher activity [13]. The reaction of hydroquinone with 2-methylsulfonyl-4,6-dimethoxylpyrimidine in tetrahydrofuran or N,N-dimethylformamide at 70-80 ºC [14] gives 2a in very low yield and low purity. Hence, depending on the preparation method of 2-(4-hydroxyphenoxy)-6-chlorobenzoxazole [15], we used sodium hydroxide as a base, and benzyltriethylammonium chloride as a phase transfer catalyst, thus, refluxing the hydroquinone with 2-methylsulfonyl-4,6-dimethoxylpyrimidine for 3.5 h with stirring in a mixed solvent of toluene and water affords compound 2a with a satisfactory yield. The reaction of 2a (or 2b) with the appropriate chloroacetates in acetonitrile in the presence of potassium carbonate gives compounds 3a-3j (Scheme 2). Scheme 2. Synthetic route of target compounds 3a-3j. 3 R Both spectral and elemental analyses data of the prepared compounds 3a-3j were all in agreement with the suggested molecular structures (see Experimental). Their IR spectra exhibited absorption bands assignable to C=O stretching vibrations and characteristic bands near υ 1208-1193 cm −1 and 1084-1062 cm −1 , corresponding to the C-O-C linkages. In addition, the 1 H-NMR spectra of 3a-3j showed characteristic singlet signals at δ 4.59-4.64, 3.82-3.83, and 2.39 ppm, assignable to the methylene, methoxy and methyl protons, respectively, and singlet signals at δ 5.76 ppm or at δ 6.74-6.75 ppm corresponding to the dimethoxy-or dimethylpyrimidine ring protons (5-H), in addition to the expected aromatic protons appear as double doublet signals at 6.91-6.93 and 7.12-7.14 ppm, respectively.

Biological Activity
The herbicidal activities are summarized in Table 1. In all tested compounds, the rates of inhibition of Brassica napus L. root growth are 15.9%-58.4%, 13.1%-36.4% and 9.2%-22.9% at concentrations of 100 mg/L, 50 mg/L and 10 mg/L, respectively, which means that the synthesized compounds display low herbicidal activities against this plant. However, the rates of inhibition of Digitaria sanguinalis L. root growth are all 100% at a concentration of 100 mg/L, 95.2%-98.5% at 50 mg/L, and 32.5%-44.7% at 10 mg/L. The results demonstrate that the target compounds exhibit excellent herbicidal activities against this kind of weed at 100 mg/L and 50 mg/L, but their activities are not satisfactory at a lower concentration (10 mg/L).

General
Melting points were measured on an X-5 microscopic melting-point apparatus and uncorrected. IR spectra were recorded in KBr pellets on a Shimadzu IR-440 infrared spectrophotometer. 1 H-NMR spectra were registered on an Inova-600 spectrometer (in CDCl 3 solvent, TMS as internal standard). Mass spectra were recorded on an Agilent 1100 LC-MS spectrometer (APCI source). Elemental analyses were performed with a Vario EL III Elemental Analyzer.

Biological activity
The herbicidal activities of the target compounds were determined using Brassica napus L. and Digitaria sanguinalis L. as samples of dicotyledonous and monocotyledonous plants, respectively [16,17]. Emulsions of the tested compounds were prepared by dissolving them in N,N-dimethylformamide (100 μL) with the addition of Tween 20 (2 μL), and then diluting with distilled water. The germinated seeds were placed on two filter papers in a 9-cm Petri plate, to which 5 mL of tested solution was added in advance. Usually, 15 seeds were used on each plate. The plates were placed in a dark room and allowed to germinate for 65 h at 28 (±1) ºC. The lengths of 10 seed roots selected from each plate were measured and the means were calculated. Moreover, quizalofop-Pethyl, a commercial aryloxy-phenoxy propionate herbicide and the emulsion which does not contain tested compounds were used as control and blank respectively. For all of the bioassay tests, each treatment was repeated three times. The inhibitory rate was calculated relative to the blank. The bioassay results are shown in Table 1.