Approach to thiazole-containing tetrahydropyridines via Aza–Rauhut–Currier reaction and their potent fungicidal and insecticidal activity

Abstract

A convenient and efficient synthesis of multi-substituted thiazole-containing tetrahydropyridine moieties was reported using the phosphine-catalyzed Aza–Rauhut–Currier reaction with excellent yields and diastereoselectivity. Thiazole-containing tetrahydropyridines were further transformed into the corresponding piperidine derivatives. The biological activity of the title compounds was explored; they exhibited moderate insecticidal activity against Aphis laburni Kaltenbach at 100 μg mL. A 3D QSAR model accurately predicted the insecticidal activity of the structurally diverse set of test compounds. Thiazole-containing tetrahydropyridines were active against normal fungi and also had good activity against resistant fungi mutations without cross resistance; thus, these compounds will be valuable for resistance management. The predicted potential fungicidal target of the title compounds is fumarate reductase.

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Introduction

Heterocycles are important key pharmacophores in drug discovery.¹ Specifically, nitrogen-containing heterocycles constitute a major class of natural products and bioactive ingredients.² Piperidine is one N-heterocycle that is widely found in pharmaceutically active and natural compounds, therefore it has attracted the interest of chemists, biologists and pharmacists alike for its significant, diverse and promising biological activity.³ Numerous investigations have shown that sulfur-containing heterocycles can induce apoptosis in multiple cell lines and experimental animals.⁴ Sulfur- and nitrogen-containing heterocyclic compounds are recognized as important leads.⁵ In particular, thiazole presents an important opportunity to access a wide variety of natural and synthetic products with a broad spectrum of biological activities.^6,7 A combination of these two active substructures into one target molecule is a simple and effective method for designing novel pesticides.⁸ Certain compounds that combine thiazole and piperidine substructures have been identified for their medicinal activity. The fungicide oxathiapiprolin, which has excellent efficacy against downy mildew, was discovered by DuPont via high throughput screening.⁹ Our group has concentrated on the agricultural activities of nitrogen- and sulfur-containing heterocycles for novel pesticide discovery.⁴ Thiazole-containing piperidine structures without chiral carbon atoms were constructed with certain types of difficulties; however, inclusion of a chiral carbon center generally resulted in unexpected biological activities.¹⁰ In our continued exploration of this field, the phosphine-catalyzed Aza–Rauhut–Currier reaction was a useful tool to combine nitrogen- and sulfur-heterocycles with chiral carbon atoms in a single molecule.¹¹ Inspired by this reaction, herein we aimed to synthesize thiazole-containing piperidine derivatives for novel pesticide discovery via the method shown in Scheme 1. Using thiazole aldehydes as raw materials, an efficient and convenient Aza–Rauhut–Currier reaction was employed to obtain thiazole-containing tetrahydropyridine derivatives, which could then be easily transformed into the corresponding thiazole-containing piperidines. To our delight, thiazole N-sulfonyl-1-aza-1,3-dienes reacted smoothly with vinyl ketones with high diastereoselectivity to produce targets with moderate biological activities. Several target compounds were found to be valuable for fungicide resistance management and novel pesticide activities.

Scheme 1 Synthesis route for the target compounds.

Results and discussion

Chemistry

The results of the optimization study for the synthesis of benzenesulfonyl-substituted thiazole-containing tetrahydropyridine are summarized in Table 1. Observations at different temperatures were made in CH₂Cl₂ with 20% PPh₃ and 20% 4-methoxyphenol for a 24 h reaction (Table 1, entries 1–3). The reaction did not proceed at room temperature, but was effectively advanced at 30 °C. The quantity of PPh₃ and 4-methoxyphenol affected the yield of 5ea (Table 1, entries 4–8); when the reaction was conducted in CH₂Cl₂ with 40% PPh₃ and 40% 4-methoxyphenol at 30 °C for 12 h, the yield of 5ea reached 90% (Table 1, entry 9). Further increases the reaction time and PPh₃ and 4-methyoxyphenol loading did not improve the yield (Table 1, entries 7–8). However, under the other reaction conditions (Table 1, entries 1–6), reaction times were longer (24 h) and the yields were lower. Solvent screening (Table 1, entries 9–12) indicated that CH₂Cl₂ was the most effective solvent. Thus, the optimum reaction conditions were those illustrated in Table 1, entry 9.

Table 1 Optimization conditions for the synthesis of benzenesulfonyl substituted thiazole-containing tetrahydropyridine

Entry	PPh3	Methoxyphenol	Solvent	Time, h	T, °C	Yield^a, %
a Isolated yield.
1	20%	30%	CH2Cl2	24	25	None
2	20%	30%	CH2Cl2	24	30	33
3	20%	30%	CH2Cl2	24	35	32
4	10%	30%	CH2Cl2	24	30	None
5	30%	30%	CH2Cl2	24	30	51
6	40%	30%	CH2Cl2	24	30	60
7	40%	40%	CH2Cl2	24	30	90
8	50%	50%	CH2Cl2	24	30	90
9	40%	40%	CH2Cl2	12	30	90
10	40%	40%	CHCl3	12	30	74
11	40%	40%	Toluene	12	30	41
12	40%	40%	CH3CN	12	30	57

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The scope of the reaction was assessed using various substrates as 3 and 4. The group R¹ was held constant as N-tert-butyl piperidine, whereas R² and R³ groups were varied. Regardless of the identity of R³, when R² was an electron-donating group such as 4-MeOC₆H₄, the reaction gave moderate yields with an excellent diastereoselectivity (Table 2, entries 1–6).

Table 2 Scope of synthesis for benzenesulfonyl substituted tert-butyl piperidinyl thiazole-containing tetrahydropyridine from piperidinylthiazoles 3 and ketene 4 by phosphine-catalyzed Aza–Rauhut–Currier reaction

Entry	3	4	R^2	R^3	5	d.r.,^a yield^b%
a d.r. = trans/cis, determined by crude ^1H NMR.b Yield of isolated trans isomer.
1	3a	4a	4-MeOC6H4	Et	5aa	>20 : 1, 74
2	3a	4b	4-MeOC6H4	Phenyl	5ab	>20 : 1, 83
3	3a	4c	4-MeOC6H4	4-MeC6H4	5ac	>20 : 1, 70
4	3a	4d	4-MeOC6H4	2-Thienyl	5ad	>20 : 1, 79
5	3a	4e	4-MeOC6H4	2-Furyl	5ae	>20 : 1, 71
6	3a	4f	4-MeOC6H4	Cyclohexyl	5af	>20 : 1, 68
7	3b	4a	Ph	Et	5ba	>20 : 1, 88
8	3b	4b	Ph	Phenyl	5bb	>20 : 1, 68
9	3b	4c	Ph	4-MeC6H4	5bc	>20 : 1, 70
10	3b	4d	Ph	2-Thienyl	5bd	=12 : 1, 64
11	3b	4e	Ph	2-Furyl	5be	=12 : 1, 74
12	3b	4f	Ph	Cyclohexyl	5bf	>20 : 1, 76
13	3b	4g	Ph	4-CF3C6H4	5bg	>20 : 1, 72
14	3c	4h	4-NO2C6H4	Et	5bh	>20 : 1, 80

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The reaction was also very tolerant to the substrates when the R² group was changed into phenyl (Table 2, entries 7–13). Irrespective of the electronic characteristics of the substituent at the R³ position, good yields and excellent diastereomeric ratios (d.r.) were obtained (Table 2, entries 8–9, and entry 13). Even when the R³ group was ethyl and cyclohexyl, the protocol afforded good yields and excellent d.r. values (Table 2, entries 7 and 12). Furthermore, it was applicable to heteroaryl substituents such as 2-thiophenyl and 2-furyl with moderate yields and d.r. values (Table 2, entries 10–11).

When R² remained unchanged, and R³ was an alkyl substituent such as ethyl and cyclohexyl, the reaction afforded excellent yields with low d.r. values (Table 3, entries 1 and 6). When R³ was changed to substituted phenyls with both electron-donating and electron-withdrawing groups, the reaction gave lower yields and d.r. values as compared with the equivalent reagent with a unsubstituted phenyl (Table 3, entries 2–3, 7). Substrate 4 with heteroaryl substituents such as 2-thiophenyl and 2-fury gave excellent yields and d.r. values (Table 3, entries 4–5).

Table 3 Synthesis of benzenesulfonyl substituted 4-methylthiazole-containing tetrahydropyridines from different substrate ketene 4 by phosphine-catalyzed Aza–Rauhut–Currier reaction

Entry	4	R^3	5	d.r.,^a yield^b%
a d.r. = trans/cis, determined by crude ^1H NMR.b Yield of isolated trans isomer.
1	4a	Et	5da	=15 : 1, 89
2	4b	Phenyl	5db	>20 : 1, 82
3	4c	4-MeC6H4	5dc	>17 : 1, 90
4	4d	2-Thienyl	5dd	>20 : 1, 96
5	4f	2-Furyl	5de	>20 : 1, 92
6	4g	Cyclohexyl	5df	=10 : 1, 97
7	4h	4-CF3C6H4	5dg	=17 : 1, 45

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The scope of the reaction was further explored using unsubstituted thiazole for substrate 3. As compared to the reactions of phenyl-substituted 3 and alkyl-substituted 4, ethyl-substituted 4 afforded excellent yield and d.r. value, whereas cyclohexyl-substituted 4 gave both lower yield and d.r. value (Table 4, entries 1 and 6). The reaction in which unsubstituted phenyl R³ gave lower yields than electron-donating substituent 4-methylphenyl, the d.r. values contrastingly were higher (Table 4, entries 2–3). Both substrates with 2-thiophenyl and 2-furyl at the R³ position gave excellent d.r. values (Table 4, entries 4–5), but only the 2-furyl substituted 4 provided excellent yields. If R² (cyclopropyl) and R³ (ethyl) were both alkyl groups, lower d.r. value and moderate yield were obtained (Table 4, entry 7).

Table 4 Synthesis of benzenesulfonyl-substituted thiazole-containing tetrahydropyridines from unsubstituted thiazoles 3 using phosphine-catalyzed Aza–Rauhut–Currier reaction

Entry	3	4	R^2	R^3	5	d.r.,^a yield^b%
a d.r. = trans/cis, determined by crude ^1H NMR.b Yield of isolated trans isomer.
1	3e	4a	Ph	Et	5ea	>20 : 1, 90
2	3e	4b	Ph	Phenyl	5eb	>20 : 1, 66
3	3e	4c	Ph	4-MeC6H4	5ec	=17 : 1, 93
4	3e	4d	Ph	2-Thienyl	5ed	>20 : 1, 65
5	3e	4e	Ph	2-Furyl	5ee	>20 : 1, 96
6	3e	4f	Ph	Cyclohexyl	5ef	=8 : 1, 60
7	3f	4i	Cyclopropyl	Et	5ia	=8 : 1, 66

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Furthermore, the benzenesulfonyl substituted thiazole-containing tetrahydropyridine underwent smooth desulfonation under mild reaction conditions with TFA and thioanisole to give imine intermediate 6; the configuration of which was assigned by X-ray crystallography. This intermediate was easily reduced with sodium cyanoborohydride to obtain thiazole-containing piperidine 7 in moderate yield (Scheme 2).

Scheme 2 Deprotection and reduction of the compound 5aa.

On the basis of the above experimental explorations and existing literature descriptions,¹² a reaction mechanism was proposed as shown in Scheme 3. Triphenylphosphate played the role of catalyst and 4-methoxyphenol acted as an additive.

Scheme 3 Proposed mechanisms for the formation of compounds 5.

Biological activity

After obtaining the benzenesulfonyl substituted thiazole-containing tetrahydropyridines, biological screening was carried out. The insecticidal activity of all the title compounds against Aphis laburni Kaltenbach was evaluated. The results shown in Table 5 indicated that, to some extent, almost all of the compounds possessed activity against aphids at the tested concentration. Compounds 5bc, 5bd, 5df, 5ec, 5ee and 5ef presented good insecticidal activity at 100 μg mL⁻¹ with over 50% mortality. Compounds 5bc, 5df, 5ee and 5ef were particularly active and presented insecticidal activities at 100 μg mL⁻¹ with over 60% mortality.

Table 5 Insecticidal activity against Aphis Laburni Kaltenbach and the experimental (Exp), predicted (Pred) and their residuals (Res) for the training set molecules using topomer CoMFA between structures of benzenesulfonyl substituted thiazole-containing tetrahydropyridines

No.	Compound	Mortality% (A)	lg A
			Exp	Pred	Res
1	5aa	22.84	−3.34	−3.31	0.03
2	5ab	45.78	−2.92	−2.91	0.01
3	5ad	9.41	−3.83	−3.83	0.00
4	5af	46.98	−2.90	−2.90	0.00
5	5ba	14.83	−3.55	−3.56	−0.01
6	5bc	60.37	−2.65	−2.63	0.02
7	5bd	55.20	−2.74	−2.79	−0.05
8	5bg	33.18	−3.17	−3.17	−0.00
9	5bh	35.86	−3.08	−3.09	−0.01
10	5da	34.81	−2.93	−2.92	0.01
11	5dc	28.70	−3.11	−3.17	−0.06
12	5de	27.73	−3.11	−3.04	0.07
13	5dg	24.63	−3.24	−3.26	−0.02
14	5eb	14.83	−3.45	−3.43	0.02
15	5ec	58.92	−2.54	−2.51	0.03
16	5ed	26.92	−3.13	−3.15	−0.02
17	5ef	70.28	−2.32	−2.34	−0.02
18	5ia	40.56	−2.77	−2.79	−0.02
19	5ac	48.78	−2.87	−2.95	−0.08
20	5ae	21.28	−3.41	−3.32	0.09
21	5bb	45.37	−2.91	−2.76	0.15
22	5be	27.27	−3.25	−3.30	−0.05
23	5bf	21.28	−3.40	−3.22	0.18
24	5db	17.93	−3.36	−3.06	0.30
25	5dd	48.70	−2.73	−2.84	−0.09
26	5df	68.70	−2.36	−2.90	−0.54
27	5ea	39.24	−2.83	−2.63	0.20
28	5ee	64.73	−2.41	−2.76	−0.35

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The fungicidal screening results against Alternaria solani (AS), Botrytis cinerea (BC), Cercospora arachidicola (CA), Gibberella zeae (GZ), Phytophthora infestans (Mont) de Bary (PI), Physalospora piricola (PP), Pellicularia sasakii (Shirai) (PS), Sclerotinia sclerotiorum (SS), and Rhizoctonia cerealis (RC) are shown in Table 6. The majority of the target compounds showed good fungicidal activity against CA, SS, and BC. Compound 5bh exhibited over 50% activity against six of the fungi tested, whereas compound 5ia exhibited over 50% activity against seven of the tested fungi. Compound 5ba stood out as being particularly active, with growth inhibition of 93.85% against SS. Compound 5da showed 89.29% inhibition against BC. Compound 5df was also active against BC, with 91.07% inhibition.

Table 6 Fungicidal activity of benzenesulfonyl substituted thiazole-containing tetrahydropyridines at 50 μg mL⁻¹ (%)^a

Compd	Growth inhibition (%)
	AS	CA	GZ	PP	SS	BC	PI	RC	PS
a PC: positive control, azoxystrobin; AS: A. solani, BC: B. cinerea, CA: C. arachidicola, GZ: G. zeae, PI: P. infestans, PP: P. piricola, PS: P. sasakii (Shirai), SS: S. sclerotiorum, RC: R. cerealis.
5aa	25.00	29.17	28.00	29.63	20.00	41.07	15.38	30.00	14.29
5ab	40.00	58.33	38.67	29.63	56.92	39.29	34.62	27.50	19.05
5ac	30.00	41.67	36.00	46.30	53.85	58.93	26.92	35.00	4.76
5ad	35.00	37.50	20.00	59.26	50.77	30.36	15.38	37.50	19.05
5ae	25.00	45.83	28.00	14.81	26.15	44.64	15.38	42.50	19.05
5af	30.00	41.67	20.00	45.29	26.15	87.50	23.08	30.00	4.76
5ba	25.00	37.50	52.00	51.85	93.85	62.50	19.23	7.50	4.76
5bb	15.00	38.10	22.67	16.67	26.25	23.21	15.38	36.25	4.76
5bc	30.00	37.50	28.00	48.15	26.15	41.07	15.38	37.50	4.76
5bd	29.63	38.10	46.51	58.70	38.46	62.50	46.15	68.63	23.08
5be	25.00	37.50	25.33	33.33	26.15	46.43	15.38	47.50	0
5bf	30.00	37.50	36.00	42.59	35.38	53.57	23.08	50.00	19.05
5bg	30.00	8.33	25.33	33.33	35.38	39.29	11.54	5.00	9.52
5bh	67.23	47.88	68.35	73.89	56.45	80.30	60.23	69.66	58.87
5da	26.53	28.57	50.00	39.53	31.82	89.29	22.58	46.43	50.00
5db	20.00	45.83	33.33	38.89	35.38	35.71	23.08	42.50	19.05
5dc	20.41	21.43	50.00	55.81	45.45	67.86	8.06	28.57	27.27
5dd	30.00	37.50	36.00	42.59	35.38	53.57	23.08	50.00	19.05
5de	45.00	54.71	49.33	51.89	50.77	57.14	34.62	67.50	23.81
5df	40.00	50.00	44.00	35.19	44.62	91.07	34.62	60.00	33.33
5dg	30.00	41.67	22.67	42.59	35.38	14.29	19.23	40.00	9.52
5ea	8.16	21.43	42.86	20.93	68.18	78.57	22.58	25.00	22.73
5eb	35.00	45.83	33.33	46.30	44.62	58.93	19.23	37.50	19.05
5ec	29.63	47.62	46.15	32.61	38.46	56.25	19.23	54.90	0
5ed	35.00	33.33	25.33	31.48	53.85	37.50	19.23	25.00	19.05
5ee	44.44	23.81	19.23	47.83	7.69	50.00	19.23	68.63	19.23
5ef	22.22	23.81	0	43.48	19.23	12.50	15.38	68.63	34.62
5ia	60.47	67.56	50.83	78.69	60.12	68.99	64.32	60.78	68.91
PC	75.00	55.56	100	100	91.18	100	100	80.77	100

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The detailed potency of precision toxicity was detected and the results were shown in Table 7. Compound 5ba exhibited excellent activity against SS with EC₅₀ of 13.19 μg mL⁻¹.

Table 7 The precision toxicity of three active compounds

Compound	Fungi	Regression equation	R^2	EC50/μg mL^−1
5ba	S. sclerotiorum	y = 1.3860x + 3.4470	0.9610	13.19
5da	B. cinerea	y = 0.9390x + 2.4960	0.9310	463.35
5df	B. cinerea	y = 0.5820x + 3.7070	0.9560	166.62

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BC resistance is a worldwide problem; in order to provide alternative measures for BC resistance management, four types of BC mutations were chosen for further screening using the compounds that possessed good activity against normal BC, and the results were shown in Table 8. Compound 5ba exhibited excellent activity, even higher than that of the positive control trifloxystrobin against all the tested mutations of BC, with almost 100% activity at 50 μg mL⁻¹. This is the first report on diastereoselective thiazole piperdine compounds with biological activity against resistant fungi mutations.

Table 8 Fungicidal activity of resistant Botrytis cinerea mutations (%, 50 mg mL⁻¹)^a

Compound	T3–10	T1–12	FJ1–10	BC2
a FJ1–10 of B. cinerea strains were mutations of 143^th amino acid of azoxystrobin target point CYTB protein by GGT into GCT. T1–12 and T3–10 of B. cinerea strains were multiresistant to azoxystrobin and Boscalid. BC2 of B. cinerea strains were multiresistant to carbendazim, diethofencarb, procynidone, pyrimethanil and kresoxim-methyl.
Trifloxystrobin	66.00	87.30	59.26	80.33
5ba	100	92.06	100	93.44
5da	60.00	79.38	48.15	68.03
5df	41.00	85.71	77.78	79.51

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This provided the opportunity to find more potential novel leads with anti-resistant fungicide activity for resistance management.

Possible drug target prediction

Representative compound 5df was used to find putative targets based on the high fit score using Pharm Mapper Server. A total of 300 top potential target candidates were reserved. Among these target candidates, Escherichia coli fumarate reductase had a high fit score value of 3.75. The lowest fit score value was 3.48 for aldose reductase. Further sequence analysis studies on the fungicidal targets of this study were carried out to identify that the fumarate reductase in E. coli and B. cinerea possessed a low degree of sequence homology (about 18.6%). This suggested that benzenesulfonyl substituted thiazole-containing tetrahydropyridines might be inhibitors for fumarate reductase, a possibility deserving further experimental validation.

3D-QSAR model analysis for insecticidal activity against Aphis laburni Kaltenbach

To obtain the relationship between structure and insecticidal activities against Aphis laburni Kaltenbach, 18 compounds, including 5aa, 5ab, 5ad, 5af, 5ba, 5bc, 5bd, 5bg, 5bh, 5da, 5dc, 5de, 5dg, 5eb, 5ec, 5ed, 5ef and 5ia, were chosen to perform 3D-QSAR analysis (Table 5, entries 1–18). The result showed an excellent statistical correlation between experimental (Exp) insecticidal activities and predicted (Pred) insecticidal activities (Fig. 1). The non-cross-validated correlation coefficient r² was 0.993, which served as a measure of the quality of the model. The F-statistic and cross-validation q² values were 472.65 and 0.593, respectively. The optimal number of components obtained from the cross-validated analysis was 4. In addition, the proportions of steric and electrostatic contributions were 83.1% and 16.9%, respectively. These results suggested that prediction using the 3D-QSAR model was credible for insecticidal activity of benzenesulfonyl substituted thiazole-containing tetrahydropyridine compounds. The 3D-QSAR analysis provided more information about the relationship between structure and insecticidal activities against Aphis laburni Kaltenbach.

Fig. 1 Experimental and predicted (lg A) for CoMFA model.

The CoMFA steric and electrostatic fields were plotted as 3D colored contour maps, which were helpful to identify important regions of change in the steric and electrostatic fields.

The compound 5ef, which exhibited the highest rate of insecticidal activity against Aphis laburni Kaltenbach at 100 mg mL⁻¹, was used as a reference structure for the 3D-QSAR model. The steric and electrostatic contour maps from CoMFA analysis were presented in Fig. 2. The yellow regions represented areas where increasing the steric bulk would decrease the activity and the green regions illustrated areas where increasing the steric bulk might enhance the activity. Similarly, the red regions indicated areas where increasing electronegative substitution could enhance the activity, whereas the blue regions showed areas where increasing electropositive substitution might enhance the activity.

Fig. 2 (A) The contour maps for CoMFA model in steric fields. (B) The contour maps for CoMFA model in electrostatic fields. Green areas presented the sterically favoured areas; yellow areas presented the sterically unfavored areas. Blue areas presented positive charge favored areas; red areas presented negative charge favoured areas.

For comparison, the weakest compound 5ad and the most potent compound 5ef were overlaid on the map shown in Fig. 3. A small green contour around the cyclohexyl substituent of 5ef indicated that a bulky group was favored in this region. The least active compound 5ad tended to locate its piperidine moiety in a sterically unfavorable yellow contour. In general, extending the backbone into yellow areas would decrease the activity in steric fields, and green areas indicated that a large group at the R³ position would lead to increased activity. In addition, the influence of the electrostatic field was exerted mainly on the R² position and the carbonyl oxygen of the R³ position.

Fig. 3 (A) The contour maps of compound 5ad for CoMFA model in steric fields. (B) The contour maps of compound 5ef for CoMFA model in steric fields.

To test the utility of the model as a predictive tool, it was applied to predict the activity of a set of compounds not used in the model generation. The results were shown in entries 19–28 in Table 5. All of the 10 compounds in the test set were accurately predicted with relatively small residuals values except compound 5df. The predictive r², which was calculated based on Patel's work¹³ from the CoMFA model, was found to be 0.707. The results showed that the 3D QSAR model could accurately predict the activity of the structurally diverse test set compounds and yield reliable information for further optimization of benzenesulfonyl substituted thiazole-containing tetrahydropyridine derivatives.

Experimental

Instruments

¹H NMR and ¹³C NMR spectra were recorded at 400 MHz and 100 MHz in CDCl₃, respectively. Chemical shifts were reported in ppm relative to TMS as an internal reference. FT-IR spectra were obtained on KBr plates and major peaks were reported in inverse centimeters. HRMS spectra were recorded on a spectrometer with an ESI source. X-ray single-crystal diffraction was performed on a CCD area detector.

General procedure for synthesis of compounds 2 (2a as example)

Compound 2 was synthesized according to a revised reference procedure.¹⁴ A solution of 1a (200 mg, 0.67 mmol) and the corresponding vinyl ketone compound (100 mg, 0.81 mmol) in methanol (30 mL) was slowly dropped into 2.5% potassium hydroxide solution (110 mg, 2.01 mmol) in an ice bath. The obtained mixture was stirred for 30 min before removing the ice bath for another 2 h of stirring at room temperature. Methanol was evaporated under vacuum and the residue was extracted with ethyl acetate (3 × 10 mL). After condensation of the ethyl acetate phase, the crude product was purified by silica gel column chromatography with petroleum ether/ethyl acetate (6 : 1 to 2 : 1) as the eluent to afford the pure 2a as a yellow solid.

General procedure for synthesis of compounds 3 (3a as example)

Compound 3 was synthesized according to a revision of reference procedures.^11,15 To a solution of benzenesulfonamide (1140 mg, 7.22 mmol) and α,β-unsaturated carbonyl compound 2 (2400 mg, 6.0 mmol) in anhydrous DCM (40 mL) cooled to 0 °C, Et₃N (1830 mg, 18.1 mmol) and TiCl₄ (0.36 mL, 3.0 mmol), which was added slowly, were successively added under nitrogen atmosphere. The reaction mixture was then heated for 5 h at 50 °C. The solution was cooled to room temperature, and quenched with water (100 mL); the mixture was then extracted with DCM (2 × 30 mL). After condensation of the DCM phase, the residue was purified by column chromatography with petroleum ether/ethyl acetate (6 : 1 to 2 : 1) as the eluent to afford pure 3a as a yellow solid.

General procedure for the syntheses of N-sulfonyl-1-aza-1,3-diene compounds 5 (5aa as example)

Compound 5 was synthesized according to a revision of reference procedures.^11,15 To a solution of compound 3a (100 mg, 0.18 mmol), PPh₃ (8.9 mg, 0.07 mmol) and 4-methoxyphenol (9 mg, 0.07 mmol) in CH₂Cl₂, the corresponding vinyl ketone compound 4a was slowly added. The reaction was maintained for 12 hours at a constant temperature of 30 °C and the reaction mixture was directly condensed and purified by silica gel column chromatography with petroleum ether/ethyl acetate (10 : 1 to 6 : 1) as the eluent to afford pure 5aa as a yellow solid.

Synthesis procedure of 1-((3S,4R)-6-(4-methoxyphenyl)-4-(2-(piperidin-4-yl)thiazol-4-yl)-2.3,4,5-tetrahydropyridin-3-yl)propan-1-one (6)

To a solution of 5aa (760 mg, 1.68 mmol) in thioanisole (5 mL), TFA was slowly added in an ice bath, and stirred for another 3 h at room temperature. Finally, the pH was adjusted to 8–9 with a 1 mol L⁻¹ sodium hydroxide solution, and the mixture was extracted with DCM (3 × 10 mL). After condensation of the DCM phase, the residue was purified by column chromatography with DCM/MeOH (20 : 1) as the eluent to afford pure 6 as a pale yellow solid.

Synthesis procedure of (1R)-1-((3S,4R)-6-(4-methoxyphenyl)-4-(2-(piperidin-4-yl) thiazol-4-yl)piperidin-3-yl)propan-1-ol (7)

Sodium cyanoborohydride (45.2 mg, 0.72 mmol) was added to a solution of compound 6 (100 mg, 0.24 mmol) in glacial acetic acid (5 mL) in an ice bath, followed by stirring for another 2 h at room temperature. The mixture was then adjusted to pH 9–10 with 1 mol L⁻¹ sodium hydroxide solution, and extracted with EA (3 × 10 mL). After condensation of the EA phase, the residue was purified by column chromatography with DCM/MeOH (30 : 1) as the eluent to afford pure 7 as a pale yellow solid.

Insecticidal screening method for Aphis laburni Kaltenbach

The target compounds were evaluated for insecticidal activity against Aphis laburni.¹⁶ About 50 aphids were transferred to the shoot with 3 to 5 fresh leaves of horsebean. The shoot with aphids was cut and dipped into a solution of 100 μg mL⁻¹ of the test compound for 2 seconds. After removing the extra solution on the leaf, the aphids were raised in the shoot at 27 °C ± 1 °C and 85% of relative humidity for 16 h by cutting it into the foam saturated with water, with a glass cylinder covering to avoid their escape. Each experiment for one compound was repeated for three times. The corrected death rate was calculated by Abbott's formula.

Fungicide screening

Preliminary screening was conducted by fungal growth inhibition method.¹⁰ Fungi used in this study included Alternaria solani (AS), Botrytis cinerea (BC), Cercospora arachidicola (CA), Gibberella zeae (GZ), Phytophthora infestans (Mont) de Bary (PI), Physalospora piricola (PP), Pellicularia sasakii (Shirai) (PS), Sclerotinia sclerotiorum (SS), and Rhizoctonia cerealis (RC). Precision toxicity studies were conducted according to the above methods by determination of growth inhibition of specific fungi using the same compound in five to seven different of concentrations; the median effective concentration (EC₅₀) was calculated by linear regression of the logarithm of the concentration with probability of the corresponding growth inhibition using Excel. The active compounds were evaluated against Botrytis cinerea resistant mutation fungi strains FJ1–10, SQ15, SJ4–8, T1–12 and T3–10 and multi-resistant mutation fungi strains Botrytis cinerea BC1 and BC2.

Potential target prediction

PharmMapper server (http://59.78.96.61/pharmmapper/) was used to predict potential drug targets for benzenesulfonyl substituted thiazole-containing tetrahydropyridine compounds. The SDF format of the compound was submitted to PharmMapper server to retrieve targets with fit scores. Other settings were the default parameters.

Molecular modelling and alignment for insecticidal activity

The 3D-QSAR analysis was carried out using comparative molecular field analysis (CoMFA) in SYBYL 8.0 (Tripos, St. Louis, MO, USA) molecular modeling software.¹⁷ Two-thirds of each packet was randomly selected to construct a training set, and the rest of the total compounds were used for the test set. Therefore, 18 compounds in entries 1–18 of Table 5 were chosen for CoMFA analysis, which showed insecticidal activities of the target compounds against aphids at 100 mg mL⁻¹ and their activities were within 100%. The activity data was transformed and expressed in terms of A by the formula A = lg{a/[100 − a]M_w}, where a was the percentage of inhibition and M_w was the molecular weight of the tested compounds. Energy minimizations were carried out using the Tripos force field¹⁸ and Gasteiger-Hückel charges.¹⁹ The potent compounds in lines 1–18 were chosen as templates for molecular alignment. In this study, model validation was achieved using a standard Leave-One Out (LOO) cross-validation, and the optimal number of components was obtained. Genetic algorithm (GA) and setup stimulated annealing methods were used to modify the conformation with biggest residual values for optimizing the model.

Conclusions

In summary, we reported a simple synthetic procedure to obtain substituted thiazole-containing tetrahydropyridines with high diastereoselectivity by a phosphine-catalyzed [4 + 2] annulation Aza–Rauhut–Currier reaction. Biological activity studies indicated that all the title compounds possessed moderate insecticidal activity against Aphis laburni Kaltenbach. The QSAR studies clearly elucidated the relationship between activity and structure characteristics. Some target compounds were effective against multi-resistant fungi mutations. Benzenesulfonyl substituted thiazole-containing tetrahydropyridines were predicted to be potential fumarate reductase inhibitors.

Acknowledgements

Z.-J. Fan is grateful to the financial support from the National Natural Science Foundation of China (Grant No. 31571991, 21372132) and the International Science & Technology Cooperation Program of China (Grant No. 2014DFR41030), and the Tianjin Natural Science Foundation (No. 14JCYBJC20400). V. A. Bakulev thanks the Ministry of Education and Science of the Russian Federation (State task 4.1626.2014/K). Resistant mutation fungi strains FJ1–10, T1–12 and T3–10 were kindly provided by Professor Liu Xili at China Agricultural University, and resistant mutation fungi strains BC2 were kindly provided by Professor Zhou Mingguo and Duan Yabing at Nanjing Agricutural University with multi-resistance characteristics.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1491770. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra24342h

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