Chinese Chemical Letters  2017, Vol. 28 Issue (8): 1780-1783   PDF    
Synthesis of trifluoromethyl-/cyclopropyl-substituted 2-isoxazolines by DBU-promoted domino reaction
Xiao-Dong Liua, Hai-Yan Maa, Chun-Hui Xingb, Long Lub    
a School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China;
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
Abstract: NTrifluoromethyl and cyclopropyl substituted 2-isoxazolines were synthesized via a DBU-promoted domino reaction of β-trifluoromethyl-/β-cyclopropyl-substituted enones with hydroxylamine. The domino reaction consists of a Michael addition and the followed cyclization. A wide range of 3-substituted 5-cyclopropyl-5-trifluoromethyl-2-isoxazolines were obtained in good to excellent yields under mild reaction conditions. The method could also apply to other trifluoromethyl-substituted enones.
Key words: Isoxazoline     Enone     Michael addition     Cyclization     Domino reaction    
1. Introduction

2-Isoxazolines are important heterocycle structures in agricultural and medicinal chemistry [1]. Their derivatives exhibit multiple bioactivities such as anti-inflammation, anti-thrombus, and antibacterial activity [2]. Therefore, the synthesis of 2-isoxazolines has attracted more attentions [3-5]. Construction of these compounds generally involves 1, 3-dipolar cycloaddition of nitrile oxides with alkenes or intramolecular cyclization of oxime derivatives [4, 5]. Recently, 5-trifluoromethyl-2-isoxazolines were emerged as a class of antiparasitic compounds and drew certain interests [6]. For preparation of these fluorinated 2-isoxazolines, a more efficient method was developed through the domino reaction of Michael addition/cyclization of trifluoromethyl-substituted enone and hydroxylamine [7].

As is well-known, organofluorine chemicals are vital to pharmaceutical, agrochemical and material sciences. The employment of fluorine or fluorine-containing substituents especially the trifluoromethyl group into organic molecules, might lead to the enhancement of their bioactivities [8, 9]. On the other hand, cyclopropanes are also crucial synthons in organic synthesis and significant structural units in many natural products [10]. We are recently focusing on exploring the applications of cyclopropyl trifluoromethyl ketone (1) in constructing complex compounds. As a novel fluorinated building blocks, 1 combines trifluoromethyl group with cyclopropyl unit. Both important functional groups would be installed into various molecules through the transformation of the synthon 1.

We have already investigated bisindolylation reaction of 1 catalyzed by trifluoromethanesulfonic acid [11]. As the continuing study, we prepared trifluoromethyl-/cyclopropyl-substituted enone 2 from 1, and explored the domino reaction of Michael addition/cyclization of 2 with hydroxylamine (Scheme 1). It provides an efficient method for the syntheses of 5-cyclopropyl-5-trifluoromethyl-2-isoxazoline derivatives 3 with broad substrate generality. Herein, we report the results.

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Scheme1. General route for the syntheses of 5-cyclopropyl-5-trifluoromethyl-2-isoxazolines

2. Results and discussion

The initial study began with the examination of the reaction of trifluoromethyl and cyclopropyl substituted enone 2a with hydroxylamine. No reaction took place in the absence of base at room temperature. Carbonates were tested at first and could not trigger the reaction (Table 1, entries 1-3). Neither could other weaker inorganic bases (Table 1, entries 9, 10). With the aid of TBAB (tetrabutylammonium bromide), NaOH, KOH and CsOH may promote 2a to react with hydroxylamine, and afford the desired 2-isoxazoline product 3a in 27%, 51% and 87% yields, respectively, after being stirred at room temperature for 3 h (Table 1, entries 5-7). A few organic bases were examined as well. The reaction did not happen with Et3N as base (Table 1, entry 12). Low yield was detected in the existence of TMG (tetramethylguanidine) (Table 1, entry 11). To our pleased, DBU displayed excellent activation ability, and 99% yield of 2-isoxazoline was obtained (Table 1, entry 13).

Table 1
Screening of bases for the reaction of 2a with NH2OHa

Then, a brief screening of solvents was performed with DBU as base. The results listed in Table 2 showed that the reaction proceeded smoothly in all kinds of solvents. 1, 2-Dichloroethane was chosen as the optimal solvent since the highest yield was obtained in it (Table 2, entry 2). Actually, phase transfer catalyst TBAB had few effects on this DBU-promoted reaction. The reaction still worked well and gave the comparable yield without it (Table 2, entry 3).

Table 2
Screening of solvents for the reaction of 2a with NH2OHa

With the optimal conditions established, the scope of substrates enones 2 was then explored. As listed in Table 3, a wide range of substituents including electron-donating and electron-withdrawing groups on the aromatic ring of trifluoromethyl substituted enones 2 are tolerated, and good to excellent yields of the corresponding tirfluoromethyl and cyclopropyl substituted 2-isoxazoline 3 were obtained (Table 3, 3a-3l). In addition, tirfluoromethyl-substituted heteroaryl enones 2m was applied to this reaction, nearly quantitative yield of 3-(thiophen-2-yl)-5-(trifluoromethyl)-4, 5-dihydroisoxazole was isolated (Table 3, 3m). Besides, alkyl-substituted enones also reacted with hydroxylamine smoothly and afforded the corresponding products in satisfied yields (Table 3, 3n & 3o).

Table 3
Scope of β-cyclopropyl-/β-trifluoromethyl-substituted enones 2 for the domino reaction with NH2OHa

Furthermore, under the same conditions, the method was applied to other enones derived from aryl or heteroaryl trifluoromethyl ketones and the corresponding products were isolated in moderate to excellent yields (Fig. 1, 3p-3v).

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Fig. 1. The products and yields of reactions between enones derivatives under the same conditions

3. Conclusion

In summary, we have demonstrated a DBU-mediated Michael addition/cyclization domino reaction of trifluoromethyl and cyclopropyl substituted enones with hydroxylamine. The mild conditions and convenient manipulation made this protocol a practical method to synthesis various 5-cyclopropyl-5-trifluoromethyl-2-isoxazoline derivatives efficiently. This method may also apply to other enones derived from aryl or heteroaryl trifluoromethyl ketones.

4. Experimental

1H NMR and 19F NMR spectra were obtained with an Agilent AM-400 instrument with TMS as the internal standard and CFCl3 as the external standard, respectively. 13C NMR spectra were recorded on an Agilent AM-400 or AM-500 instrument with TMS as the internal standard. Chemical shifts (δ) are reported in ppm, and coupling constants (J) are in Hertz (Hz). FT-IR spectra were obtained with a Nicolet AV-360 spectrophotometer. Mass spectra were obtained on an Agilent 5973 Network or a Waters Micromass GCT Premier instrument. All melting points were determined on a Büchi B-545 melting point apparatus and are uncorrected. All commercially available reagents were purchased from commercial sources and used directly. All reactions were monitored by TLC, 19F NMR or 1H NMR. Flash column chromatograph was carried out using 300-400 mesh silica gel at medium pressure.

General procedure for the syntheses of cyclopropyl-/trifluoromethyl-substituted 2-isoxazolines: Enone 2 (1.0 mmol, 1.0 equiv.), NH2OH (50 wt% aq., 1.5 mmol, 1.5 equiv.), DBU (2.0 mmol, 2.0 equiv.) and DCE (10 mL) were added to a Schlenk tube equipped with a stir bar. The reaction mixture was stirred at room temperature for appropriate time. The completion of the reaction was monitored by 19F NMR. When the reaction was completed, DCE was evaporated and the residue was purified by column chromatography on silica gel to afford 2-isoxazoline 3 as the desired product.

Spectral data of representative products were listed below. Others and general method for the preparation of enones 2 are provided in Supporting information.

5-Cyclopropyl-3-phenyl-5-(trifluoromethyl)-4, 5-dihydroisoxazole (3a): White solid. Mp: 72-73 ℃. 1H NMR (400 MHz, CDCl3): δ 7.67-7.60 (m, 2H), 7.48-7.38 (m, 3H), 3.67 (d, 1H, J = 17.2 Hz), 3.35 (d, 1H, J = 17.3 Hz), 1.36-1.29 (m, 1H), 0.88-0.81 (m, 1H), 0.68-0.60 (m, 1H), 0.59-0.51 (m, 1H), 0.47-0.41 (m, 1H). 19F NMR (376 MHz, CDCl3): δ -80.61 (s, 3F). 13C NMR (101 MHz, CDCl3): δ 156.32, 130.70, 128.84, 128.33, 126.77, 125.04 (q, J = 284.3 Hz), 85.81 (q, J = 29.1 Hz), 41.86, 13.27 (d, J = 0.9 Hz), 1.74, -0.48. IR (KBr, cm-1): 3095, 3017, 2961, 2942, 1604, 1572, 1497, 1447, 1438, 1366, 1328, 1221, 1189, 1163, 1154, 1140, 1032, 983, 920, 779. MS (EI): m/z (%) 255 (M+, 54.50), 69 (100), 77, 186, 41, 119, 51, 91. HRMS (EI): Calculated for C13H12NOF3: 255.0871; Found: 255.0867.

5-Cyclopropyl-3-(4-methoxyphenyl)-5-(trifluoromethyl)-4, 5-dihydroisoxazole (3b): White solid. Mp: 73-75 ℃. 1H NMR (400 MHz, CDCl3): δ 7.57 (d, 2H, J = 8.7 Hz), 6.93 (d, 2H, J = 8.8 Hz), 3.85 (s, 3H), 3.64 (d, 1H, J = 17.2 Hz), 3.32 (d, 1H, J = 17.4 Hz), 1.39-1.26 (m, 1H), 0.90-0.79 (m, 1H), 0.69-0.59 (m, 1H), 0.59-0.50 (m, 1H), 0.49-0.38 (m, 1H). 19F NMR (376 MHz, CDCl3): δ -80.59 (s, 3F). 13C NMR (101 MHz, CDCl3): δ 161.45, 155.81, 128.32, 125.06 (q, J = 284.2 Hz), 120.75, 114.20, 85.42 (q, J = 29.1 Hz), 55.38, 42.10, 13.24 (d, J = 1.1 Hz), 1.74, -0.50. IR (KBr, cm-1): 3097, 3017, 2962, 2937, 2909, 2840, 1610, 1520, 1438, 1421, 1365, 1327, 1256, 1186, 1162, 1141, 1127, 1034, 985, 919, 832. MS (EI): m/z (%) 285 (M+, 82.08), 69 (100), 149, 147, 135, 133, 77, 41. HRMS (EI): Calculated for C14H14NO2F3: 285.0977; Found: 285.0975.

5-Cyclopropyl-3-(o-tolyl)-5-(trifluoromethyl)-4, 5-dihydroisoxazole (3c): White solid. Mp: 51-52 ℃. 1H NMR (400 MHz, CDCl3): δ 7.36-7.22 (m, 4H), 3.71 (d, 1H, J = 17.3 Hz), 3.42 (d, 1H, J = 17.3 Hz), 2.54 (s, 3H), 1.38-1.29 (m, 1H), 0.91-0.83 (m, 1H), 0.69-0.62 (m, 1H), 0.60-0.53 (m, 1H), 0.50-0.43 (m, 1H). 19F NMR (376 MHz, CDCl3): δ -80.57 (s, 3F). 13C NMR (101 MHz, CDCl3): δ 157.22, 138.20, 131.71, 129.90, 128.78, 127.33, 125.88, 125.08 (q, J = 284.4 Hz), 84.56 (q, J = 29.1 Hz), 44.39, 22.89, 13.21, 1.80, -0.49. IR (KBr, cm-1): 3102, 3075, 3023, 2971, 2936, 1603, 1593, 1560, 1495, 1460, 1441, 1386, 1348, 1326, 1287, 1189, 1166, 1125, 1038, 899, 866, 828, 759. MS (EI): m/z (%) 269 (M+, 32.85), 200 (100), 69, 130, 91, 65, 41, 132. HRMS (EI): [39_TD$DIFF]Calculated for C14H14NOF3: 269.1027; Found: 269.1032.

5-Cyclopropyl-3-(4-fluorophenyl)-5-(trifluoromethyl)-4, 5-dihydroisoxazole (3f): Yellow liquid. 1H NMR (400 MHz, CDCl3): δ 7.73-7.52 (m, 2H), 7.11 (t, 2H, J = 8.7 Hz), 3.64 (d, 1H, J = 17.3 Hz), 3.34 (d, 1H, J = 17.3 Hz), 1.37-1.29 (m, 1H), 0.88-0.80 (m, 1H), 0.68-0.61 (m, 1H), 0.59-0.52 (m, 1H), 0.47-0.40 (m, 1H). 19F NMR (376 MHz, CDCl3): δ -80.66 (s, 3F), -108.73-108.83 (m, 1F). 13C NMR (101 MHz, CDCl3): δ 164.02 (d, J = 251.8 Hz), 155.39 (d, J = 0.8 Hz), 128.76 (d, J = 8.6 Hz), 124.95 (q, J = 284.3 Hz), 124.51 (d, J = 3.4 Hz), 115.99 (d, J = 22.1 Hz), 85.90 (q, J = 29.2 Hz), 41.85 (d, J = 0.6 Hz), 13.17 (d, J = 1.3 Hz), 1.72, -0.52. IR (KBr, cm-1): 3095, 3020, 1604, 1516, 1437, 1414, 1362, 1325, 1204, 1237, 1204, 1165, 1127, 1040, 917, 836. MS (EI): m/z (%) 273 (M+, 0.83), 69 (100), 41, 300, 231, 76, 67, 89, 162. HRMS (EI): Calculated for C13H11NOF4: 273.0777; Found: 273.0781.

5-Cyclopropyl-3-(thiophen-2-yl)-5-(trifluoromethyl)-4, 5-dihydroisoxazole (3m): White solid. Mp: 62-64 ℃. 1H NMR (400 MHz, CDCl3): δ 7.43 (dd, 1H, J = 5.1, 1.1 Hz), 7.22 (dd, 1H, J = 3.6, 1.0 Hz), 7.08 (dd, 1H, J = 5.1, 3.7 Hz), 3.67 (d, 1H, J = 17.1 Hz), 3.36 (d, 1H, J = 17.1 Hz), 1.36-1.28 (m, 1H), 0.89-0.79 (m, 1H), 0.70-0.60 (m, 1H), 0.60-0.51 (m, 1H), 0.49-0.40 (m, 1H). 19F NMR (376 MHz, CDCl3): δ -80.55 (s, 3F). 13C NMR (101 MHz, CDCl3): δ 152.04, 130.50, 129.03, 129.01, 127.40, 124.90 (q, J = 284.2 Hz), 85.98 (q, J = 29.3 Hz), 42.57, 13.20, 1.78 (s), -0.44. IR (KBr, cm-1): 3108, 3027, 2922, 2852, 1598, 1438, 1366, 1325, 1161, 1179, 1145, 1120, 1039, 897, 727. MS (EI): m/z (%) 261 (M+, 56.77), 69 (100), 125, 123, 109, 70, 192, 97. HRMS (EI): Calculated for C11H10NOF3S: 261.0435; Found: 261.0439.

3-Butyl-5-cyclopropyl-5-(trifluoromethyl)-4, 5-dihydroisoxazole (3n): Colorless liquid. 1H NMR (400 MHz, CDCl3): δ 3.22 (d, 1H, J = 17.6 Hz), 2.91 (dd, 1H, J = 17.6, 0.7 Hz), 2.32 (t, 2H, J = 7.6 Hz), 1.58-1.47 (m, 2H), 1.42-1.29 (m, 2H), 1.26-1.14 (m, 1H), 0.93 (t, 3H, J = 7.3 Hz), 0.81-0.71 (m, 1H), 0.62-0.54 (m, 1H), 0.54-0.45 (m, 1H), 0.40-0.33 (m, 1H). 19F NMR (376 MHz, CDCl3): δ -80.83 (s, 3F). 13C NMR (101 MHz, CDCl3): δ 158.83, 125.12 (q, J = 284.3 Hz), 84.37 (q, J = 29.1 Hz), 43.88, 28.29, 26.86, 22.12, 13.63, 13.12, 1.66, -0.68. IR (KBr, cm-1): 3095, 3019, 2961, 2935, 2875, 1635, 1469, 1435, 1327, 1163, 1124, 1035, 1009. MS (EI): m/z (%) 235 (M+, 1.06), 193 (100), 69, 57, 166, 67, 98, 110, 206. HRMS (EI): Calculated for C11H16NOF3S: 235.1184; Found: 235.1191.

3-Phenyl-5-(pyridin-2-yl)-5-(trifluoromethyl)-4, 5-dihydroisoxazole (3v): White solid. Mp: 97-99 ℃. 1H NMR (400 MHz, CDCl3): δ 8.65 (dd, 1H, J = 4.8, 0.6 Hz), 7.88 (d, 1H, J = 7.9 Hz), 7.81 (td, 1H, J = 7.8, 1.7 Hz), 7.74-7.66 (m, 2H), 7.49-7.39 (m, 3H), 7.35 (ddd, 1H, J = 7.4, 4.8, 1.1 Hz), 4.47 (d, 1H, J = 17.8 Hz), 4.00 (d, J = 17.8 Hz, 1H). 19F NMR (376 MHz, CDCl3): δ -78.78 (s, 3F). 13C NMR (101 MHz, CDCl3): δ 156.77, 154.11, 148.95, 137.02, 130.82, 128.84, 128.09, 126.96, 124.32, 124.01 (q, J = 284.4 Hz), 122.96, 88.82 (q, J = 29.7 Hz), 41.87. IR (KBr, cm-1): 3070, 2978, 1589, 1574, 1473, 1438, 1361, 1308, 1289, 1170, 1091, 1005, 986, 889, 759. MS (EI): m/z (%) 292 (M+, 7.03), 223 (100), 261, 262, 78, 106, 193, 79, 51. HRMS (EI): Calculated for C15H11N2OF3: 292.0823; Found: 292.0818.

Acknowledgment

Supports from the National Natural Science Foundation of China (Nos. 21272257, 21274041 and 21474028) are gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.03.031.

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