Design, synthesis and effect of the introduction of a propargyloxy group on the fungicidal activities of 1-substituted phenoxypropan-2-amino valinamide carbamate derivatives

Abstract

The cell walls of oomycetes are composed of cellulose, making cellulose synthase enzymes good targets for carboxylic acid amide fungicides. Valinamide carbamates are amino acid fungicides that represent excellent alternatives to conventional synthetic pesticides in terms of their ability to reduce the negative impacts of these compounds on human health and the environment. In a continuation of our research towards the development of new cellulose synthase inhibitors, we have developed a series of “stretched” analogues of iprovalicarb by the introduction of an additional OCH₂ linker. The bioassay results indicated that compounds containing a small group at the para-position of phenyl gave excellent fungicidal activities with EC₅₀ values ranging from 0.59 to 2.06 μmol L⁻¹. Most notably, the introduction of a propargyloxy group led to a pronounced increase in the fungicidal activity. Furthermore, compound 7o bearing a propargyloxy group was identified as the most promising candidate because of its excellent fungicidal potency against oomycete diseases and good fungicidal activity against non-oomycete diseases.

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Introduction

Oomycetes can lead to the occurrence of several destructive diseases in a range of important crop plants, such as late blight on potatoes, blue mold on tobacco, and grape downy mildew.¹ Fungicides are therefore vital for increasing the yield of food production processes. However, the overuse of traditional pesticides has led to several environmental issues during the last decade, which have raised public concerns regarding their use. The use of environmental friendly and biodegradable green pesticides could address some of these environmental problems, while maintaining crop yields. Amino acids are important biochemical molecules that are critical to life. The valinamide carbamates, which are amino acid fungicides, represent an excellent alternative to conventional synthetic pesticides in terms of their impact on human health and the environment. The valinamide carbamates, including benthiavalicarb,² iprovalicarb,³ and valifenalate,⁴ belong to one of three sub-classes of carboxylic acid amide (CAA) fungicides (FRAC code: 40),⁵ which were officially announced by the Fungicide Resistance Action Committee (FRAC) in 2005.¹ In terms of their mode of action, these compounds target the cellulose synthase enzymes found in oomycete plant pathogens.^6,7

Mandipropamid,^8–10 which was developed by Syngenta, is the only mandelic acid amide fungicide to have been commercially marketed throughout the world to date. Lamberth et al.^10,11 reported that mandelamide fungicides bearing an OCH₂ or CH₂OCH₂ linker between the 4-chlorophenyl ring and the 2-propargyloxyacetamide function of mandipropamid exhibited improved levels of fungicidal activity. For example, these “stretched” mandelamide systems showed improved activity against P. infestans with an increase in activity from 0.1 to 0.02 mg L⁻¹ versus the parent system (Fig. 1).

Fig. 1 Structures of commercial valinamide carbamates fungicides.

In our previous study,^12–18 we reported the synthesis and evaluation of the fungicidal activities of a series of mandelic acid amide and valinamide carbamate derivatives. We also found that “stretched” valinamide carbamate derivatives bearing an OCH₂ linker between both of their phenyl rings exhibited much higher fungicidal activities against Phytophthora capsici and Pseudoperonospora cubensis than the corresponding lead compounds with two phenyl fragments. Although it has been reported¹⁹ that individual valinamide carbamate compounds bearing an OCH₂ linker between their phenyl ring (p-Cl and p-CN) and amide group possess good fungicidal activities, we describe herein the synthesis and fungicidal evaluation of a series of “stretched” iprovalicarb derivatives bearing an OCH₂ or CH₂OCH₂ linker between the phenyl ring and the valinamide function of iprovalicarb. We have also shown for the first time that the introduction of a propargyloxy group had a positive effect on the antioomycetic activity (Fig. 2).

Fig. 2 Design of the target compounds.

Results and discussion

Chemistry

A series of “stretched” compounds bearing an OCH₂ linker 7a–o was synthesized as shown in Scheme 1. The precursor compounds 3a–m, 4a–m, and 5a–m were synthesized according to a previously reported procedure. All of the aryloxy acetophenones 3 were prepared by the alkylation of the corresponding substituted phenols with chloroacetone in DMF in the presence of potassium carbonate and sodium iodide. The oximes 4 were subsequently prepared by the addition of NH₂OH·HCl and NaOH to the aryloxy acetophenones 3 at room temperature. The result of a previous study showed that the hydrogenation of oximes over palladium on carbon provided facile access to the corresponding amines in high yield. However, the hydrogenation of the oximes 4 under these conditions failed to afford the corresponding amines 5. To overcome this issue, the oximes 4 were instead reduced with LiAlH₄ in dry THF to give the corresponding amines 5 in good yields. Amine 5m was subsequently debenzylated using Pd/C and H₂ to give amine 5n. L-Valine was treated with isopropyl chloroformate under basic conditions in THF to give the mixed anhydride 6, which was used without purification or isolation. The subsequent treatment of 6 with triethylamine and amines 5 in tetrahydrofuran gave the title compounds 7a–n in 80–86% yields (Scheme 1). The propargylation of compound 7n (R¹ = OH) with propargyl bromide in the presence of K₂CO₃ under reflux conditions for 10 h gave the corresponding ether 7o (Scheme 1).

Scheme 1 Synthetic route to compounds 7a–n and 7o.

A series of “stretched” compounds bearing a CH₂OCH₂ linker 11a–g was synthesized as shown in Scheme 2. Aminopropanol 9 was prepared by the reduction of L-alanine with LiAlH₄ according to a previously reported procedure. Compound 10 was successfully prepared using the method described above for the synthesis of 7. The subsequent alkylation of compound 10 with various benzyl bromides gave the desired products 11a–g.

Scheme 2 Synthetic route to compounds 11a–g.

All of the synthesized compounds were characterized by ¹H and ¹³C NMR and HR-MS. All of the spectral and analytical data were consistent with the assigned structures.

Fungicidal activity

The in vitro fungicidal activities of compounds 7a–o and 11a–g towards Phytophthora capsici are shown in Table 1 (reported as EC₅₀ values in μmol L⁻¹). The EC₅₀ values were obtained from Petri dish trials and represent the concentrations at which the test compounds inhibited the growth of the fungus by 50%. Most of the valinamide carbamates 7 bearing an OCH₂ bridge showed excellent fungicidal activity against Phytophthora capsici, especially for the para-substituted compounds. Once again, the experimental data confirmed that the para-substituted compounds exhibited stronger activity than their meta- or ortho-substituted counterparts. In contrast to previously reported valinamide fungicides, the methyl- and chloro-substituted compounds prepared in the current study were not the most active examples from this series. The EC₅₀ values of compounds 7f (R = 4-CH₃) and 7i (R = 4-CF₃) were 22.5 and 11.7 μmol L⁻¹, respectively. Compounds containing a sterically bulky alkyl group, e.g. 7h (EC₅₀ = 56.4 μmol L⁻¹), showed much lower levels of fungicidal activity. In contrast, compounds containing a halogen group showed much higher levels of fungicidal activity. The EC₅₀ values of compounds 7b (R = 4-F), 7d (R = 4-Cl), and 7e (R = 4-Br) were 2.06, 1.89 and 2.38 μmol L⁻¹, respectively. Compounds bearing small alkyloxy groups achieved the highest activity of all of the compounds in the valinamide family. The EC₅₀ values of compounds 7j (R = 4-OCH₃) and 7l (R = 4-OCF₃) were 1.53 and 1.36 μmol L⁻¹, respectively. Interestingly, the introduction of a propargyloxy group at the para-position of the phenyl ring led to a pronounced increase in fungicidal activity. The most active compound 7o had an EC₅₀ value of 0.59 μmol L⁻¹ (three-fold greater than that of the known compound 7d (R = 4-Cl) and two-fold greater than that of compound 7l (R = 4-OCF₃)) against Phytophthora capsici. The propargyloxy group has been reported to play an important role in the fungicidal activity of mandipropamid.¹⁰ However, there have been no reports to date suggesting that the introduction of a propargyloxy group can be used as a strategy for increasing the fungicidal activity of any other CAA fungicides (i.e., valinamide carbamate and cinnamic acid fungicides).^17,18 This result therefore provides clear evidence that the inclusion of a propargyl group can have a pronounced impact on the fungicidal activity of valinamide carbamate derivatives, although the extent of this effect is dependent on the other substituents. Steric congestion can lead to a sharp reduction in the fungicidal activity, as demonstrated in our previous study, where steric hindrance made it difficult for a propargyl group to interact with the binding pocket of cellulose synthase.

Table 1 Fungicidal activities of compounds 7a–o and 11a–g against Phytophthora capsici (in vitro) along with their predicted activities

No.	Substituents		y = a + bx	r^2	EC50
	R^1	0			μmol L^−1	95% CI^a
a CI: confidence interval.
7a	H	0	y = 2.4517 + 3.6457x	0.9650	14.8	13.5–16.4
7b	p-F	0	y = 5.1629 + 1.2016x	0.9755	2.06	1.33–3.19
7c	o-F	0	y = 3.2447 + 1.8472x	0.9973	25.2	21.3–29.7
7d	p-Cl	0	y = 5.813 + 5.3012x	0.9857	1.89	1.11–3.26
7e	p-Br	0	y = 5.3748 + 5.509x	0.9937	2.38	1.04–5.47
7f	p-CH3	0	y = 2.6767 + 2.6063x	0.9863	22.5	19.4–25.5
7g	m-CH3	0	y = 2.9982 + 1.7720x	0.9883	38.5	32.8–45.1
7h	p-t-Bu	0	y = 3.1965 + 1.3407x	0.9850	56.4	44.5–71.6
7i	p-F3C	0	y = 4.2399 + 1.8009x	0.9991	11.7	8.93–14.9
7j	p-CH3O	0	y = 2.0601x + 5.3874	0.9733	1.53	1.01–2.32
7k	o-CH3O	0	y = 3.9365 + 1.4030x	0.9835	15.6	12.1–20.2
7l	p-CF3O	0	y = 5.5047 + 2.0758x	0.9776	1.36	1.17–1.57
7m	p-BnO	0	y = 4.9551 + 1.7729x	0.9883	2.40	2.10–2.92
7n	p-OH	0	y = 3.7098 + 1.0486x	0.9932	48.2	37.1–62.9
7o	p-HCCCH2O	0	y = 9.2377 + 6.6440x	0.9917	0.59	0.41–0.87
11a	H	1	y = −3.7477 + 5.9951x	0.9866	99.7	56.5–173
11b	p-F	1	y = −4.0438 + 5.6919x	1.0360	96.3	58.4–163
11c	m-F	1	y = 1.0083 + 2.7566x	0.9975	90.9	71.5–115
11d	o-F	1	y = −4.0638 + 6.2018x	0.9917	108	67.6–158
11e	p-Cl	1	y = −3.8533 + 4.0541x	0.9856	106	62.4–174
11f	p-CH3	1	y = −4.1271 + 6.1156x	0.9565	90.4	57.5–150
11g	p-CF3O	1	y = 2.1463 + 3.7225x	0.9906	15.0	11.7–19.2
	Dimethomorph		y = 9.1342 + 6.3811x	0.9349	0.59	0.49–0.80
	Iprovalicarb		y = 6.2088 + 2.1245x	0.9966	0.84	0.69–1.03

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In contrast to compounds 7a–o, the inclusion of three linker atoms to give compounds 11a–g appeared to be unsuitable for good fungicidal activity. Most of the compounds belonging to the latter of these two series showed poor fungicidal activity. This result could be attributed to the adverse steric effect of the CH₂OCH₂ bridge.

A further in vivo assay was conducted in a greenhouse to estimate the fungicidal activities of the most active compounds against P. capsici and P. cubensis. The results of our previous study^17,18 revealed that the high clog P values of the valinamide carbamate derivatives led to a significant decrease in their in vivo fungicidal potency against P. capsici, because of their poor absorption through the root. As shown in Table 2, the clog P values of most of the compounds prepared in this study were in the range of 2.6–3.3, except for compounds 7l and 7m bearing trifluoromethoxy and benzyloxy moieties, respectively. Most of these compounds exhibited very good in vivo fungicidal activity, with a strong association between their EC₅₀ value and in vivo fungicidal potency against P. capsici and P. cubensis. Compound 7o displayed high levels of control of 79 and 46% against P. capsici and P. cubensis at 6.25 μg mL⁻¹, respectively, and showed higher levels of antioomycetic activity than dimethomorph (85 and 27%) and iprovalicarb (70%) at the same concentration. However, high clog P compounds, e.g. 7l and 7m, showed much lower fungicidal activity for root irrigation treatments (P. capsici) than leaf spray (P. cubensis).

Table 2 Fungicidal activities of selected compounds against Phytophthora capsici and Pseudoperonospora cubensis (in vivo)

No.	P. capsici (% control at given concentration mg L^−1)					P. cubensis (% control at given concentration mg L^−1)					clog P
	100	50	25	12.5	6.25	100	50	25	12.5	6.25
a No test.
7b	100	100	82	80	49	81	72	61	44	40	2.89
7d	100	100	100	66	62	84	70	66	42	35	3.29
7j	100	100	88	79	21	81	70	62	54	30	2.60
7l	100	100	74	30	30	84	72	66	47	42	4.26
7m	87	35	30	30	26	78	69	58	41	37	4.34
7o	100	100	100	92	79	86	72	70	54	46	2.82
Iprovalicarb	100	100	100	100	70	—^a	—	—	—	—	3.20
Dimethomorph	100	100	100	100	85	82	74	54	37	27	2.73

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Interestingly, these compounds showed good broad-spectrum fungicidal activities against non-oomycete diseases at 50 μg mL⁻¹, with almost all of them inhibiting the growth of S. sclerotiorum by more than 90%, which was similar to that of chlorothalonil. Compound 7j showed the highest level of activity towards P. piricola at 100%, which was greater than that of chlorothalonil (Table 3).

Table 3 Fungicidal spectrum test of selected compounds at 50 μg mL⁻¹

No.	FV^a	CA^b	PP^c	AS^d	GZ^e	SS^f	BC^g	TC^h
a FV: Fusarium vasinfectum.b CA: Cercospora arachidicola.c PP: Physalospora piricola.d AS: Alternaria solani.e GZ: Gibberella zeae.f SS: Sclerotinia sclerotiorum.g BC: Botrytis cinerea.h TC: Thanatephorus cucumber.
7a	48.1	57.1	65.0	72.7	73.1	75.0	61.0	67.2
7b	46.2	52.9	90.9	63.2	66.7	90.9	81.4	86.3
7c	42.3	35.3	100	68.4	66.7	86.4	81.4	88.2
7d	50.0	47.1	63.6	68.4	66.7	90.9	83.7	92.2
7e	47.4	36.8	68.4	63.2	53.3	90.2	76.1	88.2
7f	38.5	47.1	79.5	63.2	60.0	84.1	79.1	86.3
7g	42.3	47.1	100	63.2	66.7	88.6	83.7	92.2
7h	42.3	5.9	100	63.2	66.7	95.5	79.1	86.3
7i	46.2	41.2	100	63.2	73.3	93.2	79.1	88.2
7j	50.0	35.3	100	68.4	53.3	90.9	76.7	84.3
7k	46.2	35.3	90.9	68.4	66.7	90.9	79.1	84.3
7l	33.3	31.6	35.7	76.2	47.1	91.0	90.2	79.1
7m	33.3	36.8	44.6	71.4	38.2	90.2	76.1	68.7
7n	38.1	47.4	46.4	76.2	52.9	90.2	84.8	90.3
7o	52.4	63.2	25.0	71.4	41.2	90.2	76.1	73.1
11a	—	23.8	58.1	45.5	61.9	93.4	69.0	62.2
11b	—	23.8	58.1	50.0	59.5	90.8	66.7	58.1
11d	—	28.6	48.4	45.5	38.1	88.2	69.0	62.2
11f	—	28.6	64.5	72.7	71.4	93.4	73.8	64.9
11g	—	23.8	58.1	54.5	61.9	93.4	69.0	67.6
Chlorothalonil	83	75	92	74	73	96	96	96

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We subsequently determined the EC₅₀ values of a selection of the synthesized compounds against a variety of different fungi, and the results are listed in Table 4. These compounds 7 and 11 showed fungicidal activities against S. sclerotiorum similar to or higher than those of the control compound chlorothalonil (21.8 μmol L⁻¹). Compounds 7b, 7h, 7i, 7j, 7o, 11a, 11d, 11e and 11g gave EC₅₀ values of 7.28, 7.62, 11.5, 12.9, 14.7, 15.8, 16.4, 17.5 and 10.4 μmol L⁻¹ against S. sclerotiorum, respectively. Compound 7j also provided good levels of activity against P. piricola with an EC₅₀ value of 9.6 μg mL⁻¹.

Table 4 EC₅₀ values of fifteen selected compounds against four different fungi (μmol L⁻¹)

No.	SS^a	PP^b	BC^c	TC^d
a SS: Sclerotinia sclerotiorum.b PP: Physalospora piricola.c BC: Botrytis cinerea.d TC: Thanatephorus cucumber.
7b	7.28	36.1	—
7d	22.1	—	—	16.3
7g	—	34.5	—	15.9
7h	7.62	29.4	—	—
7i	11.5	—	—	—
7j	12.9	26.1	—	—
7l	—	—	15.8	—
7n	36.7	—	—	27.5
7o	14.7	—	—	—
11a	15.8	—	—	—
11b	25.8	—	—	—
11d	16.4	—	—	—
11e	17.5	—	—	—
11f	25.3	—	—	—
11g	10.4	—	—	—
Chlorothalonil	21.8	27.5	4.14	6.39

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Experimental

Materials and methods

¹H and ¹³C NMR spectra were measured on a Bruker AC-P500 instrument (Bruker, Fallanden, Switzerland) using TMS as internal standard and CDCl₃ as a solvent. Melting points were determined on an X-4 binocular microscope melting point apparatus (Beijing Tech Instruments, Beijing, China) and were uncorrected. HRMS were recorded on an Ionspec 7.0-T Fourier-transform ion-cyclotron resonance (FTICR) mass spectrometer (Bruker, Billerica, USA). All of the reagents were purchased as the analytical grade.

General procedure for the synthesis of the 1-substituted phenoxypropan-2-ones 3a–m

The intermediate phenoxypropan-2-ones were prepared according to a previously reported method by the reaction of the corresponding substituted phenols with α-chloroacetone.²⁰ Analytical data for the characterization of compounds 3a–m can be found in the ESI.†

General synthetic procedure for the synthesis of 1-substituted phenoxypropan-2-one oximes 4a–m

The intermediate oximes were prepared according to a previously reported method by the reaction of the corresponding ketone 3 with hydroxylamine hydrochloride.²¹ Analytical data for the characterization of compounds 4a–m can be found in the ESI.†

General procedure for the synthesis of the 1-substituted phenoxypropan-2-amines 5a–m

Lithium aluminium hydride (45.3 mmol) was suspended in 50 mL of ether at 0 °C. Oxime 4 (18.1 mmol) was then added to the mixture in small portions, and the resulting mixture was heated at reflux for 3 h. The mixture was then cooled to room temperature and slowly treated with a 2 N NaOH solution. The mixture was subsequently filtered and evaporated to dryness to give a yellow residue, which was purified by flash column chromatography. Analytical data for the characterization of compounds 5a–m can be found in the ESI.†

Procedure for the synthesis of the 4-(2-aminopropoxy)phenol (5n)

Amine 5m (2.0 g, 7.78 mmol) was dissolved in 50 mL of ethanol, and the resulting solution was flushed with N₂. Palladium on carbon (0.2 grams, 10% wt) was added to the solution, and the resulting mixture was hydrogenated under 15 atm of H₂ pressure for 12 h. The mixture was then filtered to remove the catalyst, and the filtrate was concentrated under vacuum to afford a yellow residue, which was purified by flash column chromatography to give the desired product (0.95 g, 73.1%. Mp: 72–74 °C).

General procedure for the synthesis of the isopropyl((2S)-3-methyl-1-oxo-1-((1-substituted phenoxypropan-2-yl)amino)butan-2-yl)carbamates 7a–n

4-Methylmorpholine (6 mmol) was added to a solution of (S)-2-((isopropoxycarbonyl)amino)-3-methylbutanoic acid (5 mmol) in anhydrous tetrahydrofuran (20 mL), followed by ethyl chloroformate (5 mmol), and the resulting mixture was stirred at 0 °C for 1 h. A solution of amine 5 (6 mmol) in anhydrous tetrahydrofuran (10 mL) was then added to the reaction in a drop-wise manner, and the resulting mixture was stirred at room temperature for 10 h. The reaction mixture was then filtered and the filtrate concentrated under vacuum to give a residue, which was extracted with ethyl acetate (3 × 20 mL). The combined organics were washed with brine (2 × 15 mL), dried over anhydrous sodium sulfate, and concentrated in vacuo to give the crude product, which was purified by flash column chromatography to give the desired product 7a as a white solid (78.5%). Mp: 89–91 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.36–7.23 (m, 2H, Ar–H), 7.04–6.85 (m, 3H, Ar–H), 6.25 (t, J = 8.6 Hz, 1H, CHCONH), 5.24 (dd, J = 25.3, 8.1 Hz, 1H, OCONH), 4.89 (dt, J = 12.4, 6.1 Hz, 1H, OCH(CH₃)₂), 4.41 (d, J = 3.3 Hz, 1H, CH₂CHCH₃), 3.95 (qd, J = 9.4, 4.4 Hz, 3H, OCONHCH + OCH₂), 2.26–2.00 (m, 1H, CHCH(CH₃)₂), 1.33 (dd, J = 6.8, 3.4 Hz, 3H, CHCH₃), 1.23 (dd, J = 8.1, 6.4 Hz, 6H, OCH(CH₃)₂), 1.03–0.84 (m, 6H, CHCH(CH₃)₂); ¹³C NMR (101 MHz, CDCl₃) δ 170.66, 158.50, 156.28, 129.46, 121.13, 114.73, 70.30, 68.60, 60.37, 44.71, 31.07, 22.07, 19.24, 17.59; HRMS (MALDI) m/z calcd. for C₁₈H₂₈N₂O₄Na⁺ [M + Na]⁺ = 359.1941, found 359.1945.

Analytical data for the characterization of compounds 7b–n can be found in the ESI.†

Procedure for the synthesis both diastereoisomers of isopropyl((2S)-3-methyl-1-oxo-1-((1-(4-(prop-2-yn-1-yloxy)phenoxy)propan-2-yl)amino)butan-2-yl)carbamate (7o)

To a solution of compound 7n (2.83 mmol) in dry acetone (30 mL) was added anhydrous K₂CO₃ (4.26 mmol), and the resulting mixture was stirred for 1 h. Propargyl bromide (4.3 mmol) was then added to the mixture in a drop-wise manner over 30 min, and the resulting mixture was heated at reflux for 10 h. The reaction mixture was then cooled to ambient temperature, filtered, and evaporated to dryness to give a brown oily residue. The residue was dissolved in methylene chloride and washed with water (2 × 50 mL). The organic layer was then dried over anhydrous Na₂SO₄ and evaporated under vacuum to give the crude product, which was purified by flash column chromatography to give diastereoisomer 7o as a white solid (80.9%). Mp: 81–83 °C; ¹H NMR (300 MHz, CDCl₃) δ 6.90–6.69 (m, 4H, Ar–H), 6.03 (s, 1H, CHCONH), 5.08 (s, 1H, OCONH), 4.90–4.73 (m, 1H, OCH(CH₃)₂), 4.57 (d, J = 1.9 Hz, 2H, HCCCH₂), 4.29 (s, 1H, CH₂CHCH₃), 3.82 (dd, J = 13.8, 6.6 Hz, 3H, OCONHCH + OCH₂), 2.44 (s, 1H, OCH₂CCH), 2.06 (s, 1H, CHCH(CH₃)₂), 1.24 (d, J = 6.8 Hz, 3H, CHCH₃), 1.15 (t, J = 6.2 Hz, 6H, OCH(CH₃)₂), 0.92–0.77 (m, 6H, CHCH(CH₃)₂); ¹³C NMR (101 MHz, CDCl₃) δ 170.95, 156.30, 153.37, 152.06, 116.13, 115.39, 78.82, 75.41, 71.02, 68.58, 60.26, 56.54, 44.68, 31.37, 21.96, 19.18, 17.56; HRMS (MALDI) m/z calcd. for C₂₁H₃₀N₂O₅Na⁺ [M + Na]⁺ = 413.2047, found 413.2049.

Procedure for the synthesis of (S)-2-aminopropan-1-ol (9)

The intermediate amino alcohol 9 was prepared according to a previously reported method by the reduction reaction of L-alanine with LiAlH₄.²²

Procedure for the synthesis of isopropyl((S)-1-(((S)-1-hydroxypropan-2-yl)amino)-3-methyl-1-oxobutan-2-yl)carbamate (10)

Intermediate 10 was prepared according to the procedure reported in Chapter 2.2.6 to give a white solid (82.1%). Mp: 179–181 °C; ¹H NMR (400 MHz, CDCl₃) δ 6.37 (d, J = 6.8 Hz, 1H, CHCONH), 5.31 (d, J = 7.4 Hz, 1H, OCONH), 5.02–4.77 (m, 1H, OCH(CH₃)₂), 4.09 (dt, J = 10.9, 6.8 Hz, 1H, CH₂CHCH₃), 3.93 (dt, J = 9.8, 5.0 Hz, 1H, OCONHCH), 3.69 (dd, J = 11.1, 3.6 Hz, 1H, OCH₂CH), 3.56 (dd, J = 11.1, 5.8 Hz, 1H, OCH₂CH), 2.59 (s, 1H, OH), 2.12 (d, J = 6.6 Hz, 1H, CHCH(CH₃)₂), 1.26 (dd, J = 6.2, 3.6 Hz, 6H, OCH(CH₃)₂), 1.21 (d, J = 6.9 Hz, 3H, CHCH₃), 1.04–0.93 (m, 6H, CHCH(CH₃)₂); ¹³C NMR (101 MHz, CDCl₃) δ 171.97, 156.51, 68.84, 66.65, 60.61, 47.83, 30.88, 22.06, 19.26, 17.95, 16.86; HRMS (ESI) m/z calcd. for C₁₂H₂₅N₂O₄⁺ [M + H]⁺ = 261.1809, found 261.1810.

Procedure for the synthesis of isopropyl((S)-1-(((S)-1-(substituted benzyloxy)propan-2-yl)amino)-3-methyl-1-oxobutan-2-yl)carbamate (11)

To a stirred suspension of NaH (0.23 g, 0.57 mmol, 60%) in DMF (10 mL) was added intermediate 10 (1.0 g, 3.8 mmol) in a portion-wise manner at −10 °C, and the resulting mixture was stirred for 20 min at −10 °C. A solution of benzyl bromide (5.7 mmol) in DMF (10 mL) was then added to the reaction in a drop-wise manner over 30 min at −10 °C, and the resulting mixture was stirred for 6 h at the same temperature. The mixture was then poured into 200 mL of ice water to give a white solid, which was collected by filtration and purified by recrystallization from ethanol to give compound 11a as a white solid (68.9%). Mp: 145–147 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.49–7.19 (m, 5H, Ar–H), 6.12 (s, 1H, CHCONH), 5.23 (s, 1H, OCONH), 4.90 (s, 1H, OCH(CH₃)₂), 4.53 (s, 2H, Ar–CH₂), 4.22 (s, 1H, CH₂CHCH₃), 3.91 (s, 1H, OCONHCH), 3.43 (s, 2H, OCH₂CH), 2.08 (s, 1H, CHCH(CH₃)₂), 1.23 (t, J = 7.2 Hz, 9H, CHCH₃ + OCH(CH₃)₂), 1.01–0.87 (m, 6H, CHCH(CH₃)₂); ¹³C NMR (101 MHz, CDCl₃) δ 170.65, 156.27, 137.99, 128.43, 127.75, 127.66, 73.17, 72.84, 68.50, 60.30, 44.96, 31.24, 22.09, 19.18, 17.70; HRMS (ESI) m/z calcd. for C₁₉H₃₁N₂O₄⁺ [M + H]⁺ = 351.2278, found 351.2277.

Analytical data for the characterization of compounds 7b–g can be found in the ESI.†

Fungicidal activities

In vitro fungicidal activity. The in vitro fungicidal activities of the synthesized compounds were evaluated against Fusarium vasinfectum (FV), Cercospora arachidicola (CA), Physalospora piricola (PP), Alternaria solani (AS), Gibberella zeae (GZ), Sclerotinia sclerotiorum (SS), Botrytis cinerea (BC), and Thanatephorus cucumber (TC), as previously described.^17,23–25 The results of these analyses are summarized in Tables 3 and 4.

Fungicidal activity against Phytophthora capsici (in vivo). The in vivo fungicidal activities of the synthesized compounds against P. capsici were determined as previously described.^17,26 The results are summarized in Table 1.

Fungicidal activity against Pseudoperonospora cubensi (in vivo). The in vivo fungicidal activities of the synthesized compounds against P. capsici were determined as previously described.^17,27 The results are summarized in Table 2.

Conclusions

In summary, we have designed and synthesized a series of 1-substituted phenoxypropan-2-amino valinamide carbamates as potential cellulose synthase inhibitors. The subsequent evaluation of the fungicidal activities of these compounds revealed that the introduction of an additional OCH₂ linker into iprovalicarb resulted in very good levels of fungicidal activity against Phytophthora capsici and Pseudoperonospora cubensis. Most interestingly, the introduction of a propargyloxy group led to a considerable increase in the fungicidal activity of the “stretched” iprovalicarb. Compound 7o was identified as the most promising candidate based on its excellent fungicidal potency against oomycete diseases and good fungicidal activity against non-oomycete diseases. Further studies are currently underway in our laboratory involving field trials.

Acknowledgements

We are grateful for financial support for this work from the National Natural Science Foundation of China (21172124).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17908h

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