2013 Volume 38 Issue 3 Pages 96-104
Recently, pesticides have had to meet strict requirements for registration such as those regarding ecotoxicology and safety to mammals. These strict regulations have inevitably narrowed the range of choices for pesticide types. Consequently, the risk of chemical-resistant pathogens emerging has increased. Therefore, different types of pesticides are needed for effective crop protection. Several approaches have been taken to find a new potential lead for pesticides. One of these strategies is in the analects of Sir James Black: “The most fruitful basis for the discovery of a new drug is to start with an old drug.”1,2) In other words, the structural modification of known active molecules is a useful approach to discover superior products or new leads with different biological features of the original.
Hymexazol (1), 3-hydroxy-5-methylisoxazol, is a fungicide that controls soil-borne diseases caused by Fusarium spp., Aphanomyces spp., Pythium spp., Sclerotium spp., and some isolates of Rhizoctonia solani on rice or vegetables and has been contributing to crop protection for three decades.3–6) A number of structural modifications of hymexazol have been carried out to spread the fungicidal spectrum or provide different biological activity features, partly owing to the preparative availability and structural versatility of hymexazol derivatives.7–11) In a previous study, we reported that the antifungal activities of 2N-acyl hymexazol derivatives against some pathogens such as Rhizoctonia solani were higher than those of hymexazol.12) In this study, we examined the antifungal activities or disease control activities of the 3O-esters of 3-hydroxy-5-methylisoxazol (Fig. 1) against several plant pathogens.
IR spectra were measured with a Perkin Elmer FTIR 1600 spectrometer. NMR spectra were obtained with a Jeol JNM-ECA600 (600 MHz). Chemical shifts were recorded in δ (ppm) and the coupling constants J in Hz. Mass spectra were recorded with a Jeol JMS-700. The compounds tested are given in Figs. 2-1 and 2-2. Hymexazol (1) was isolated from a commercial 30% solution (Tachigaren®, Mitsui-Chemical Agro, Tokyo, Japan). Compounds 2,13) 3,13) 6,14) 11,9) 17,9) 18,9) 20,9) 41,15) 43,16) and 4417) were prepared according to the reported procedures and their structures were confirmed spectroscopically. Compounds 45, 46, and 47, listed in Fig. 2-1, were prepared as described in the previous study.12) The new 3O-acyl and alkyl compounds listed in Fig. 2-2 were prepared according to the scheme in Fig. 3.
5-Methyl-3-isoxazolyl-1-adamantylcarboxylate (31) A solution of 1-adamantanecarbonyl chloride (199 mg, 1.0 mmol) in acetonitrile (5 mL) was slowly added to a solution of 5-methyl-3(2H)-isoxazolone (1) (99 mg, 1.0 mmol) and triethylamine (110 mg, 1.1 mmol) in acetonitrile (10 mL) in an ice bath, and the reaction solution was then stirred at room temperature for 5 hr. The solution was concentrated in vacuo, and water was then added to the residue. The crude product was solidified, and the solid was filtered and washed with water. The solid was subjected to column chromatography (silica gel, ethyl acetate/hexane 1 : 10–1 : 5), giving the title compound as colorless crystals.
Yield: 83%. Mp: 66–68°C. IR νmax (KBr) cm−1: 1764, 1610, 1455, 1419, 1207, 1055. 1H NMR δ (CDCl3): 1.7–1.8 (6H, m), 2.0–2.1 (9H, m), 2.41 (3H, s), 6.10 (1H, s). 13C NMR δ (CDCl3): 13.2, 27.8, 36.3, 38.5, 41.4, 96.2, 166.4, 171.2, 173.6. EIMS m/z (%): 261 (M+, 3), 135 (100), 113 (10), 98 (4).
Compounds 12, 14, 15, 16, 19, 22, 23, 24, 25, 29, 35, 36, and 38 were prepared as described for compound 31.
5-Methyl-3-isoxazolyl 1-cyclohexylacetate (12) Yield: 49%. Liquid. IR νmax (KBr) cm−1: 1777, 1615, 1449, 1428, 1105. 1H NMR δ (CDCl3): 0.9–1.4 (5H, m), 1.6–1.8 (6H, m), 2.41 (3H, s), 2.45 (2H, d, J=7.3 Hz), 6.12 (1H, s). 13C NMR δ (CDCl3): 13.2, 26.0, 33.0, 34.8, 41.8, 96.1, 166.0, 169.0, 171.3. EIMS m/z (%): 223 (M+, 0.3), 181 (100), 152 (43), 126 (16), 99 (3).
5-Methyl-3-isoxazolyl 2-methyl-2-phenylpropionate (14) Yield: 75%. Liquid. IR νmax (KBr) cm−1: 1770, 1614, 1447, 1427, 1130, 1117, 1090. 1H NMR δ (CDCl3): 1.71 (6H, s), 2.37 (3H, s), 6.04 (1H, s), 7.27 (1H, m), 7.35 (2H, m), 7.41 (2H, m). 13C NMR δ (CDCl3): 13.2, 26.3, 47.2, 95.9, 125.7, 127.4, 128.8, 143.0, 166.2, 171.3, 172.9. EIMS m/z (%): 245 (M+, 2), 126 (65), 119 (100), 99 (15).
5-Methyl-3-isoxazolyl 2-methyl-2-(p-chlorophenoxy)propionate (15) Yield: 76%. Liquid. IR νmax (KBr) cm−1: 1776, 1614, 1489, 1427, 1256, 1236, 1130, 1094. 1H NMR δ (CDCl3): 1.70 (6H, s), 2.42 (3H, s), 6.07 (1H, s), 6.87 (2H, d, J=8.6 Hz), 7.23 (2H, d, J=8.6 Hz). 13C NMR δ (CDCl3): 13.2, 25.3, 79.9, 95.7, 121.6, 128.4, 129.4, 153.6, 165.8, 170.2, 171.8. EIMS m/z (%): 295 (M+, 0.9), 169 (13), 127 (94), 99 (17), 57 (100).
5-Methyl-3-isoxazolyl stearate (16) Yield: 50%. Mp: 73–75°C. IR νmax (KBr) cm−1: 2916, 2848, 1729, 1710, 1633, 1320, 1182. 1H NMR δ (CDCl3): 0.88 (3H, t, J=7.4 Hz), 1.20–1.45 (28 H, m), 1.73 (2H, m), 2.42 (3H, s), 2.58 (3H, t, J=7.4 Hz), 6.12 (1H, s). 13C NMR δ (CDCl3): 13.6, 14.2, 22.8, 24.1, 29.1, 29.2, 29.4 29.5, 29.7, 29.7, 29.8, 32.0, 35.2, 98.5, 163.5, 166.1, 172.2. EIMS m/z (%): 365 (M+, 2), 267 (75), 127 (10), 99 (21), 71 (100).
5-Methyl-3-isoxazolyl p-iodobenzoate (19) Yield: 82%. Mp: 118–120°C. IR νmax (KBr) cm−1: 1747, 1584, 1421, 1393, 1274, 1092, 1004. 1H NMR δ (CDCl3): 2.47 (3H, s), 6.26 (1H, s), 7.89 (4H, s). 13C NMR δ (CDCl3): 13.2, 96.2, 102.9, 127.4, 131.9, 138.3, 162.3, 166.1, 171.7. EIMS m/z (%): 329 (M+, 46), 231 (100), 203 (99), 127 (7), 99 (1).
5-Methyl-3-isoxazolyl p-phenylbenzoate (22) Yield: 87%. Mp: 127–129°C. IR νmax (KBr) cm−1: 1748, 1606, 1483, 1451, 1426, 1406, 1255, 1200, 1072. 1H NMR δ (CDCl3): 2.47 (3H, s), 6.29 (1H, s), 7.42 (1H, m), 7.55 (2H, m), 7.64 (2H, d, J=7.4 Hz), 7.73 (2H, d, J=7.4 Hz), 8.25 (2H, d, J=8.1 Hz). 13C NMR δ (CDCl3): 13.2, 96.2, 126.5, 127.3, 127.4, 128.5, 129.0, 131.1, 139.5, 147.1, 162.5, 166.2, 171.4. EIMS m/z (%): 279 (M+, 33), 198 (14), 182 (99), 152 (100), 127 (37), 99 (4).
5-Methyl-3-isoxazolyl pentafluorobenzoate (23) Yield: 60%. Mp: Liquid. IR νmax (KBr) cm−1: 1769, 1614, 1526, 1503, 1413, 1330, 1202, 1004. 1H NMR δ (CDCl3): 2.48 (3H, s), 6.26 (1H, s). 13C NMR δ (CDCl3): 13.2, 95.8, 106.0 (d, JC–F=13.0 Hz), 137.1 (d, JC–F=13.1 Hz), 138.8 (d, JC–F=13.1 Hz), 143.6 (d, JC–F =13.0 Hz), 155.0, 165.1, 172.2. 19F NMR δ (CDCl3): −159.3, −144.7, −135.3. EIMS m/z (%): 293 (M+, 2), 212 (13), 195 (100), 167 (34), 117 (26), 99 (6).
5-Methyl-3-isoxazolyl 1-naphthoate (24) Yield: 38%. Mp: 42–44°C. IR νmax (KBr) cm−1: 1745, 1610, 1428, 1263, 1233, 1189, 1112. 1H NMR δ (CDCl3): 2.49 (3H, s), 6.33 (1H, s), 7.56 (2H, m), 7.68 (1H, m), 7.93 (1H, d, J=8.2 Hz), 8.13 (1H, d, J=8.3 Hz), 8.52 (1H, d, J=6.9 Hz), 9.02 (1H, d, J=8.30 Hz). 13C NMR δ (CDCl3): 13.3, 96.4, 123.8, 124.6, 125.5, 126.7, 128.7, 128.9, 131.9, 132.4, 134.0, 135.5, 162.8, 166.4, 171.6. EIMS m/z (%): 253 (M+, 2), 155 (100), 127 (51), 99 (1).
5-Methyl-3-isoxazolyl 2-naphthoate (25) Yield: 34%. Mp: 79–81°C. IR νmax (KBr) cm−1: 1750, 1615, 1427, 1268, 1223, 1191, 1129, 1084. 1H NMR δ (CDCl3): 2.48 (3H, s), 6.33 (1H, s), 7.55–7.60 (2H, m), 7.90–8.0 (3H, m), 8.15 (1H, dd, J=8.7 Hz, J=1.8 Hz), 8.80 (1H, d, J=1.8 Hz). 13C NMR δ (CDCl3): 13.2, 96.2, 124.9, 125.3, 127.1, 127.8, 128.6, 129.1, 129.6, 132.3, 132.8, 136.1, 162.7, 166.3, 171.4. EIMS m/z (%): 253 (M+, 5), 155 (100), 127 (80), 99 (1).
5-Methyl-3-isoxazolyl 1-phenyl-2,2-dichlorocyclopropanecarboxylate (29) Yield: 51%. Liquid. IR νmax (KBr) cm−1: 1771, 1613, 1448, 1427, 1264, 1166, 698. 1H NMR δ (CDCl3): 2.21 (1H, d, J=7.8 Hz), 2.39 (3H, s), 2.79 (1H, d, J=7.8 Hz), 6.09 (1H, s), 7.39–7.42 (3H, m), 7.55–7.58 (2H, m). 13C NMR δ (CDCl3): 13.1, 31.0, 44.8, 61.4, 95.7, 128.6, 129.1, 130.9, 163.8, 165.7, 171.6. EIMS m/z (%): 312 (M+, 2), 213 (24), 149 (100), 115 (52), 99 (2).
5-Methyl-3-isoxazolyl nicotinate (35) Yield: 12%. Mp: 49–50°C. IR νmax (KBr) cm−1: 1764, 1614, 1505, 1435, 1276, 1260. 1H NMR δ (CDCl3): 2.49 (3H, s), 6.29 (1H, s), 7.50 (1H, m), 8.47 (1H, m), 8.90 (1H, m), 9.40 (1H, m). 13C NMR δ (CDCl3): 13.3, 96.1, 123.8, 124.3, 138.1, 151.7, 154.7, 161.4, 165.8, 171.9. EIMS m/z (%): 204 (M+, 10), 105 (56), 99 (10), 78 (51), 57 (100).
5-Methyl-3-isoxazolyl isonicotinate (36) Yield: 44%. Mp: 83–84°C. IR νmax (KBr) cm−1: 1766, 1613, 1455, 1440, 1412, 1216. 1H NMR δ (CDCl3): 2.47 (3H, s), 6.27 (1H, s), 7.98 (2H, d, J=6.2 Hz), 8.88 (2H, d, J=6.2 Hz). 13C NMR δ (CDCl3): 13.3, 96.0, 123.4, 135.2, 151.1, 161.4, 165.8, 172.0. EIMS m/z (%): 204 (M+, 4), 106 (100), 99 (4), 78 (53).
5-Methyl-3-isoxazolyl 2-quinolinate (38) Yield: 81%. Mp: 110–112°C. IR νmax (KBr) cm−1: 1766, 1609, 1449, 1421, 1262, 1232, 1207, 1136, 1096. 1H NMR δ (CDCl3): 2.49 (3H, s), 6.34 (1H, s), 7.71 (1H, m), 7.84 (1H, m), 7.93 (1H, d, J=8.6 Hz), 8.30 (1H, d, J=8.6 Hz), 8.34 (1H, d, J=8.6 Hz), 8.38 (1H, d, J=8.6 Hz). 13C NMR δ (CDCl3): 13.3, 96.3, 121.5, 127.7, 129.5, 129.8, 130.8, 131.1, 137.7, 145.8, 148.0, 161.4, 166.3, 171.7. EIMS m/z (%): 254 (M+, 4), 210 (98), 156 (90), 129 (100), 101 (85).
3-t-Butylcarbamoyloxy-5-methyl-isoxazole (42) A mixture of 5-methyl-3(2H)-isoxazolone (1) (198 mg, 2.0 mmol) and t-butylisocyanate (2 mL) with cesium carbonate (ca. 10 mg) in a sealed glass tube was heated at 140°C (bath temperature) for 9 hr. The tube was opened and the reaction mixture was subjected to column chromatography on silica gel with ethyl acetate and hexane (1 : 2, v/v). Compound 42 was separated as a colorless liquid. Yield: 40%. IR νmax (KBr) cm−1: 3340, 1750, 1721, 1554, 1365, 1225, 1086. 1H NMR δ (CDCl3): 1.40 (9H, s), 2.10 (3H, s), 6.83 (1H, s), 7.86 (1H, bs). 13C NMR δ (CDCl3): 11.4, 28.8, 51.6, 107.2, 137.4, 147.1, 153.5. EIMS m/z (%): 198 (M+, 2), 184 (19), 140 (13), 126 (100), 99 (23).
5-Methyl-3-isoxazolyl picolinate (34) A suspension of 5-methyl-3(2H)-isoxazolone (1) (99 mg, 1.0 mmol), pyridine-2-carboxylic acid (133 mg, 1.0 mmol), N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (191 mg, 1.0 mmol), and 4,4-dimethylaminopyridine (133 mg, 1.1 mmol) in dichloromethane (20 mL) was stirred at room temperature for 12 hr. The reaction was quenched by the addition of 10 mL 1% aq. HCl. After the addition of 20 mL of dichloromethane, the organic layer was washed with water, 1% aq. HCl, 1% aq. NaOH, and water before drying over magnesium sulfate. The dichloromethane was distilled off. Purification by column chromatography with silica gel (ethyl acetate/hexane 1:10–1:5) provided the crude title compound. The product was recrystallized from a mixture of ether and hexane. Yield: 85%. Mp: 60–62°C. IR νmax (KBr) cm−1: 1758, 1613, 1448, 1425, 1288, 1241. 1H NMR δ (CDCl3): 2.48 (3H, s), 6.28 (1H, s), 7.58 (1H, m), 7.93 (1H, m), 8.29 (1H, d, J=7.7 Hz), 8.86 (1H, m). 13C NMR δ (CDCl3): 13.3, 96.2, 126.4, 128.1, 137.5, 141.6, 150.5, 161.1, 166.1, 171.8. EIMS m/z (%): 204 (M+, 0.2), 160 (59), 106 (100), 99 (1).
Compounds 13, 18, 20, 21, 26, 27, 28, 30, 32, 33, 37, and 39 were prepared as described for compound 34.
5-Methyl-3-isoxazolyl 2-phenylpropionate (13) Yield: 55%. Liquid. IR νmax (KBr) cm−1: 1777, 1715, 1614, 1453, 1427, 1257, 1133. 1H NMR δ (CDCl3): 1.59 (3H, d, J=7.5 Hz), 2.34 (3H, s), 3.97 (1H, q, J=7.5 Hz), 6.05 (1H, s), 7.25–7.29 (1H, m), 7.33–7.34 (4H, m). 13C NMR δ (CDCl3): 12.9, 18.2, 45.3, 95.7, 127.4, 127.5, 128.8, 138.8, 165.8, 170.3, 171.2. EIMS m/z (%): 231 (M+, 5), 133 (80), 105 (100), 99 (5).
5-Methyl-3-isoxazolyl p-chlorobenzoate (18) Yield: 61%. Mp: 103–105°C. IR νmax (KBr) cm−1: 1747, 1617, 1591, 1430, 1400, 1269, 1257, 1090. 1H NMR δ (CDCl3): 2.47 (3H, s), 6.27 (1H, s), 7.50 (2H, d, J=8.3 Hz), 8.13 (2H, d, J=8.3 Hz). 13C NMR δ (CDCl3): 13.2, 96.0, 126.3, 129.2, 131.9, 141.1, 161.7, 166.0, 171.5. EIMS m/z (%): 237 (M+, 1), 156 (23), 139 (52), 126 (100), 111 (15), 99 (12).
5-Methyl-3-isoxazolyl o-toluate (20) Yield: 67%. Mp: 69–71°C. IR νmax (KBr) cm−1: 1758, 1609, 1479, 1428, 1260, 1248, 1231, 1064. 1H NMR δ (CDCl3): 2.46 (3H, s), 2.67 (3H, s), 6.25 (1H, s), 7.26–7.34 (2H, m), 7.49–7.52 (1H, m), 8.18 (1H, m). 13C NMR δ (CDCl3): 13.1, 22.0, 96.0, 126.1, 126.6, 131.7, 132.1, 133.6, 142.2, 162.8, 166.2, 171.3. EIMS m/z (%): 218 (M+, 1), 219 (43), 163 (100), 126 (58), 120 (100), 99 (5).
5-Methyl-3-isoxazolyl o-acetoxybenzoate (21) Yield: 78%. Liquid. IR νmax (KBr) cm−1: 1762, 1607, 1450, 1428, 1285, 1241, 1193. 1H NMR δ (CDCl3): 2.34 (3H, s), 2.39 (3H, s), 6.21 (1H, s), 7.16 (1H, m), 7.37 (1H, m), 7.63 (1H, m), 8.18 (1H, m). 13C NMR δ (CDCl3): 12.7, 20.6, 95.8, 120.7, 124.0, 126.0, 132.1, 135.2, 151.2, 159.9, 165.5, 169.2, 171.3. EIMS m/z (%): 261 (M+, 2), 219 (43), 163 (100), 126 (58), 120 (100).
5-Methyl-3-isoxazolyl 1-phenylcyclopropanecarboxylate (26) Yield: 64%. Mp: 84–87°C. IR νmax (KBr) cm−1: 1755, 1619, 1443, 1280, 1252, 1142, 1130, 1085. 1H NMR δ (CDCl3): 1.40 (2H, m), 1.83 (2H, m), 2.38 (3H, s), 6.06 (1H, s), 7.28–7.42 (5H, m). 13C NMR δ (CDCl3): 13.2, 17.9, 29.2, 96.2, 127.9, 128.5, 130.7, 138.0, 166.2, 170.9, 171.2. EIMS m/z (%): 243 (M+, 1), 145 (86), 117 (100), 115 (94).
5-Methyl-3-isoxazolyl 2,2-dichloro-1-ethyl-3-methylcyclopropanecarboxylate (27) Yield: 82%. Mp: 75–78°C. IR νmax (KBr) cm−1: 1772, 1717, 1615, 1428, 1233, 1147. 1H NMR δ (CDCl3): 1.06 (3H, t, J=6.6 Hz), 1.27 (3H, d, J=6.9 Hz), 2.40 (1H, q, J=6.9 Hz), 2.43 (3H, s), 2.45 (2H, q, J=6.6 Hz), 6.12 (1H, s). 13C NMR δ (CDCl3): 9.0, 10.9, 13.8, 20.4, 30.6, 44.3, 66.2, 98.3, 162.2, 162.8, 173.4. EIMS m/z (%): 278 (M+, 4), 242 (36), 179 (100), 151 (90), 115 (80).
5-Methyl-3-isoxazolyl 2,2-dichloro-1,3,3-trimethylcyclopropanecarboxylate (28) Yield: 32%. Mp: 68–70°C. IR νmax (KBr) cm−1: 1730, 1638, 1389, 1277, 1254, 1227, 986. 1H NMR δ (CDCl3): 1.32 (3H, s), 1.48 (3H, s), 1.62 (3H, s), 2.42 (3H, s), 6.10 (1H, s). 13C NMR δ (CDCl3): 13.2, 15.4, 18.8, 21.4, 33.1, 39.2, 71.7, 95.9, 165.0, 165.7, 171.6. EIMS m/z (%): 278 (M+, 21), 242 (19), 179 (50), 151 (49), 143 (44), 115 (75), 79 (80), 43 (100).
5-Methyl-3-isoxazolyl 1-phenylcyclopentanecarboxylate (30) Yield: 45%. Mp: 54–55°C. IR νmax (KBr) cm−1: 1757, 1611, 1455, 1418, 1264, 1144, 1133. 1H NMR δ (CDCl3): 1.80 (4H, m), 2.06 (2H, m), 2.35 (3H, s), 2.77 (2H, m), 5.98 (1H, s), 7.26 (1H, m), 7.35 (2H, m), 7.44 (2H, m). 13C NMR δ (CDCl3): 13.1, 23.6, 36.2, 59.5, 95.9, 127.0, 127.4, 128.7, 141.7, 166.3, 171.2, 172.2. EIMS m/z (%): 271 (M+, 0.3), 190 (2), 173 (3), 145 (100).
5-Methyl-3-isoxazolyl 2-furancarboxylate (32) Yield: 58%. Mp: 78–80°C. IR νmax (KBr) cm−1: 1753, 1614, 1472, 1392, 1293, 1095. 1H NMR δ (CDCl3): 2.46 (3H, s), 6.27 (1H, s), 6.62 (1H, dd, J=4.0 Hz, J=1.7 Hz), 7.46 (1H, m), 7.72 (1H, d, J=1.7 Hz). 13C NMR δ (CDCl3): 13.3, 96.1, 112.6, 121.3, 142.5, 148.4, 154.1, 165.6, 171.6. EIMS m/z (%): 193 (M+, 0.5), 165 (6), 95 (100).
5-Methyl-3-isoxazolyl 3-thiophenecarboxylate (33) Yield: 62%. Mp: 58–60°C. IR νmax (KBr) cm−1: 1754, 1613, 1390, 1255, 1182, 1092. 1H NMR δ (CDCl3): 2.46 (3H, s), 6.26 (1H, s), 7.40 (1H, m), 7.65 (1H, d, J=5.0 Hz), 8.38 (1H, s). 13C NMR δ (CDCl3): 13.3, 96.2, 126.9, 128.3, 131.1, 135.7, 158.3, 166.1, 171.5. EIMS m/z (%): 209 (M+, 3), 111 (100), 83 (20).
5-Methyl-3-isoxazolyl 2-benzofurancarboxylate (37) Yield: 78%. Mp: 117–119°C. IR νmax (KBr) cm−1: 1758, 1607, 1561, 1423, 1300, 1258, 1171, 1142, 1090, 748. 1H NMR δ (CDCl3): 2.48 (3H, s), 6.33 (1H, s), 7.36 (1H, m), 7.53 (1H, m), 7.63 (1H, m), 7.75 (1H, m), 7.81 (1H, s). 13C NMR δ (CDCl3): 13.2, 95.9, 112.5, 117.1, 123.3, 124.3, 126.6, 128.7, 143.0, 155.0, 156.4, 165.5, 171.6. EIMS m/z (%): 243 (M+, 26), 243 (26), 215 (44), 145 (100), 126 (18).
5-Methyl-3-isoxazolyl 3-quinolinate (39) Yield: 38%. Mp: 124–126°C. IR νmax (KBr) cm−1: 1748, 1615, 1457, 1419, 1287, 1213, 1195. 1H NMR δ (CDCl3): 2.32 (3H, s), 6.17 (1H, s), 7.52 (1H, m), 7.74 (1H, m), 7.83 (1H, m), 8.02 (1H, m), 8.87 (1H, d, J=2.0 Hz), 9.35 (1H, d, J=2.0 Hz). 13C NMR δ (CDCl3): 13.2, 96.1, 120.8, 126.6, 128.0, 129.4, 129.5, 132.9, 140.3, 149.7, 150.3, 161.4, 165.8, 171.8. EIMS m/z (%): 254 (M+, 14), 173 (24), 156 (95), 128 (100).
5-Methyl-3-isoxazolyl benzoate (17)9) A solution of 5-methyl-3(2H)-isoxazolone (1) (198 mg, 2.0 mmol) and benzoyl chloride (281 mg, 2.0 mmol) in toluene (20 mL) was stirred at reflux for 3 hr. The solution was concentrated in vacuo, and then water was added to the residue. The crude product was solidified, and the solid was then filtered and washed with water and cold ether. Yield: 81%. Mp: 42–43°C. IR νmax (KBr) cm−1: 1758, 1615, 1450, 1429, 1248, 1062. 1H NMR δ (CDCl3): 2.46 (3H, s), 6.28 (1H, s), 7.52 (1H, t, J=7.8 Hz), 7.65 (2H, m), 8.19 (2H, dd, J=7.8 Hz, J=1.3 Hz). 13C NMR δ (CDCl3): 13.3, 96.3, 128.0, 128.9, 130.6, 134.5, 162.7, 166.3, 171.5. EIMS m/z (%): 203 (M+, 2), 105 (100), 99 (3).
Compound 11 was prepared as described for compound 17.
5-Methyl-3-isoxazolyl 2,2-dimethylpropionate (11)9) Yield: 22%. Liquid. Added proof for the structure. 1H NMR δ (CDCl3): 1.11 (9H, s), 2.46 (3H, s), 6.12 (1H, s). 13C NMR δ (CDCl3): 13.2, 29.6, 47.5, 96.1, 165.9, 168.2, 171.3.
2-(5-Methyl-3-isoxazolyloxycarbonyl)-5-methyl-3(2H)-isoxazolone (40) The published procedure18) was slightly modified.
A solution of 5-methyl-3(2H)-isoxazolone (1) (99 mg, 1.0 mmol) and bis(trichloromethyl) carbonate (50 mg, 0.17 mmol) in toluene (10 mL) was stirred at reflux for 40 hr. The reaction mixture was left at room temperature overnight. The needle crystals were filtered and washed with a small amount of ether. Yield: 40%. Mp: 135–136°C (136–139°C18)). IR νmax (KBr) cm−1: 1703, 1635, 1607, 1455, 1259, 1120, 956. 1H NMR δ (CDCl3): 2.37 (3H, s), 2.47 (3H, s), 5.62 (1H, s), 6.25 (1H, s). 13C NMR δ (CDCl3): 13.3, 13.8, 95.7, 98.1, 142.4, 162.5, 164.9, 172.3, 173.5. EIMS m/z (%): 224 (M+, 0.2), 180 (10), 165 (13), 126 (100), 99 (8).
2-t-Butyl-dimethylsilyl-5-methyl-3(2H)-isoxazolone (4) A mixture of 5-methyl-3(2H)-isoxazolone (1) (99 mg, 1.0 mmol), t-butyldimethylsilyl chloride (180 mg, 1.2 mmol), and 4-dimethyaminopyridine (158 mg, 1.3 mmol) in 6 mL of dichloromethane was stirred at room temperature for 20 hr. The dichloromethane phase was separated and the aqueous phase was extracted with dichloromethane (2×10 mL). The combined dichloromethane solution was washed successively with saturated aq. NaHCO3 and brine before drying over CaCl2. Column chromatography on silica gel with dichloromethane afforded the desired product as a colorless liquid. Yield: 42%. IR νmax (KBr) cm−1: 3032, 2957, 2671, 1635, 1530, 1341, 1250, 790. 1H NMR δ (CDCl3): 0.30 (6H, s), 0.98 (9H, s), 2.33 (3H, s), 5.57 (1H, s). 13C NMR δ (CDCl3): −4.7, 18.1, 25.5, 95.1, 169.7, 170.2. EIMS m/z (%): 213 (M+, 1), 198 (1), 185 (1), 156 (100), 126 (12), 99 (92).
3-Isopropyloxy-5-methyl-isoxazole (7) A mixture of 5-methyl-3(2H)-isoxazolone (1) (99 mg, 1.0 mmol) and potassium carbonate (276 mg, 2 mmol) in 5 mL of dry benzene was stirred at room temperature for 20 min before adding isopropyl bromide (184 mg, 1.5 mmol). The resulting mixture was stirred at 75°C for 5 hr. The cooled reaction mixture was poured onto 50 mL of brine and extracted with ether (20 mL×3). The ether phase was washed with water and dried over magnesium sulfate. The ether was distilled off and the residue was subjected to column chromatography on silica gel using ethyl acetate as the eluent, giving the product as a viscous liquid. Yield: 72%. IR νmax (KBr) cm−1: 1652, 1635, 1558, 1541, 1507, 1456. 1H NMR δ (CDCl3): 1.37 (6H, d, J=5.7 Hz), 2.31 (3H, s), 4.86 (1H, sep, J=5.7 Hz), 5.56 (1H, s). 13C NMR δ (CDCl3): 12.7, 21.9 72.8, 93.4, 169.8, 171.2. EIMS m/z (%): 141 (M+, 42), 127 (50), 113 (66), 99 (75), 57 (100).
Compounds 9, 10, and 8 were prepared as described for compound 7.
3-Benzyloxy-5-methyl-isoxazole (9) Yield: 34%. IR νmax (KBr) cm−1: 1619, 1504, 1459, 1364, 1259, 1144, 1034, 991, 970, 908, 781, 745. 1H NMR δ (CDCl3): 2.33 (3H, s), 5.24 (2H, s), 5.64 (1H, s), 7.33–7.45 (5H, m). 13C NMR δ (CDCl3): 12.8, 71.2, 93.0, 128.1, 128.4, 128.5, 135.9, 170.4, 171.8. EIMS m/z (%): 189 (M+, 10), 105 (66), 91 (38), 85 (55), 69 (79), 57 (100).
3-Propargyloxy-5-methyl-isoxazole (10) Yield: 51%. IR νmax (KBr) cm−1: 3193, 2122, 1618, 1505, 1468, 1357, 1260, 1147, 1044, 1004, 788. 1H NMR δ (CDCl3): 2.34 (3H, s), 2.58 (1H, t, J=2.6 Hz), 4.85 (2H, d, J=2.6 Hz), 5.61 (1H, s). 13C NMR δ (CDCl3): 12.9, 57.0, 75.8, 77.5, 92.9, 170.8, 170.9. EIMS m/z (%): 137 (M+, 33), 127 (19), 121 (20), 113 (22), 107 (29), 99 (21), 69 (100).
3-Propenyloxy-5-methyl-isoxazole (8) The above procedure gave a ca. 2:1 mixture of 2- and 3-propenyl isomers, and 3-propenyl isomer was separated by column chromatography. Yield: 45%. IR νmax (KBr) cm−1: 2927, 1621, 1507, 1462, 1348, 1262, 1144. 1H NMR δ (CDCl3): 2.32 (3H, s), 4.72 (2H, dd, J=7.6 Hz, J=1.4 Hz), 5.30 (1H, dd, J=10.3 Hz, J=1.4 Hz), 5.42 (1H, m), 5.63 (1H, s), 6.04(1H, s). 13C NMR δ (CDCl3): 12.9, 70.1, 93.0, 118.6, 132.2, 170.3, 171.7. EIMS m/z (%): 139 (M+, 7), 127 (36), 113 (42), 99 (45), 91 (100).
2-Formyl-5-methyl-3(2H)-isoxazolone (5) A mixture of 5-methyl-3(2H)-isoxazolone (1) (99 mg, 1.0 mmol) and N,N-diformylacetamide (115 mg, 1 mmol) in 5 mL of dry benzene was heated under reflux for 9 hr. Distilling off the benzene, the solid residue was rinsed with a small amount of cold ethyl acetate. Yield: 60%. Mp: 101–103°C. IR νmax (KBr) cm−1: 3134, 1723, 1711, 1691, 1629, 1304, 1255, 956, 842. 1H NMR δ (CDCl3): 2.37 (3H, s), 5.58 (1H, s), 8.92 (1H, s). 13C NMR δ (CDCl3): 13.8, 97.9, 150.2, 164.5, 174.7. EIMS m/z (%): 127 (M+, 22), 113 (25), 99 (24), 97 (26), 57 (85), 18 (100).
2. Evaluation of antifungal activity with a Petri dishThe mycelial growth of each fungus listed in Table 1 was measured on the potato dextrose agar (PDA) medium containing the synthesized compounds, and the inhibition rate was calculated on the basis of growth without the compound. Each test compound was dissolved in dimethyl sulfoxide (DMSO) to prepare a 2,500 mg/L solution. This solution was added to the PDA (Difco Laboratories, Sparks, MD, USA) medium after cooling down to about 60°C following autoclave sterilization for preparation of the medium containing the compound (25 mg/L) and DMSO (1%). The PDA medium containing the compound was mixed thoroughly in a plastic tube, poured into Petri dishes, and kept at room temperature to be hardened. Mycelial agar discs (4 mm in diameter) of the pathogens cultured on a PDA medium were cut out from the medium and transferred to the PDA plate containing each test compound. After inoculation, the PDA cultures were incubated at the temperatures and for the periods shown in Table 1. The diameter of the mycelial colony of each fungus was measured at two points, with three replications.
| Fungi | Origin | Incubation temperature (°C) | Incubation days | |
|---|---|---|---|---|
| P. g | Pyrenophora graminea IFO-7507 | NBRC a) | 25 | 3 |
| F. g | Fusarium graminearum H3 | KONARC b) | 25 | 3 |
| A. m | Alternaria alternata APS-38 | NIFTS c) | 25 | 3 |
| C. b | Cercospora beticola Be-2-6 | KONARC d) | 25 | 7 |
| R. sec | Rhynchosporium secalis IFO-5290 | NBRC a) | 25 | 14 |
| S. t | Septoria tritici S3 | Germany e) | 25 | 14 |
| M. n | Microdochium nivale IFO-7432 | NBRC a) | 25 | 3 |
| R. s | Rhizoctonia solani ET0007 | Fukushima Pref. f) | 25 | 1 |
| G. g | Gaeumannomyces graminis MAFF305168 | NIAS Genebank g) | 20 | 3 |
a) NITE Biological Resource Center (Chiba, Japan).b) National Agricultural Research Center for Kyushu-Okinawa Region (Kumamoto, Japan).c) National Institute of Fruit Tree Science (Iwate, Japan).d) National Agricultural Research Center for Hokkaido Region (Hokkaido, Japan).e) Wheat field in Germany (stored at Kureha Corporation, Fukushima, Japan).f) Paddy field in Fukushima Prefecture (stored at Kureha Corporation, Fukushima, Japan).g) National Institute of Agrobiological Science Genebank (Ibaraki, Japan).
The compound was dissolved in acetone with Gramin S (Sankyo, Tokyo, Japan) and diluted with water to reach final compound concentrations of 1,000 and 500 mg/L. Gramin S was used to facilitate the adherence of the solution to the surface of a plant to maximize the efficacy of the compound. The final concentrations of acetone and Gramin S were adjusted to 6% and 60 mg/L, respectively. The solution was then sprayed (spray volume: 1,000 L/ha) onto the seedlings of wheat (Norin-#61), and the solution was then left to dry. The plants were inoculated with the urediospores of Puccinia recondita or the conidia of Blumeria graminis as described below.
P. recondita urediospores were collected from wheat plants on which leaf rusts fully appeared and suspended with deionized water to adjust to about 1×106 spores /mL. The spore suspension was sprayed onto wheat seedlings previously treated with the test chemicals and the plants were kept in a humid chamber at 25°C for 2 days. The plants were transferred to a greenhouse and were kept at about 25°C for 12 days, and the percentage of lesion areas on the first and second leaves was then assessed.
The spores of B. graminis were dusted directly from the diseased plant onto the wheat seedlings previously treated with the test chemicals. The plants were kept in a greenhouse at about 20°C for 10 days and then the percentage of lesion areas on the first and second leaves was assessed.
The 3O-acyl or alkyl derivatives were prepared by (i) mixing the acid chlorides or the alkyl halides with hymexazol (1) in the presence of a base, (ii) condensation of hymexazol (1) with the corresponding acid in the presence of a dehydrating agent, or (iii) heating the acid chlorides with hymexazol (1) in an inert solvent like toluene (Fig. 3). In some cases, 2N-acyl or alkyl isomers were formed as side products. Since the 2N-substituted derivatives had a longer retention time than the fully conjugated isoxazole molecules on silica gel chromatography, the 3O-isomers were easily separable. The desired esters were characterized on the IR spectra with carbonyl stretching of the –OCOR appearing at around 1750 cm−1, higher than that of –NCOR. The most distinguishable feature was the chemical shift for the 4th position on 1H NMR, which was around 6 ppm, and around 5.5 ppm for the corresponding –NCOR molecule. 13C NMR and mass spectra also supported these structures. The absence of a 13C peak around 170 ppm due to the CO was a distinct sign of 3O-alkyl structures. Formylation by a conventional method such as refluxing with alkylformates or the treatment of DMF/POCl3 yielded various products. Ziegler’s method with N,N-diformylacetamide19) gave the products. However, the product was not the desired formate but 2N-formylisoxazolone (5).
The antifungal activities of the compounds were shown in Table 2. At first, the antifungal activities of N-methyl-5-methyl-3(2H)-isoxazolone (2), N-allyl-5-methyl-3(2H)-isoxazolone (3), and 2N-formylisoxazolone (5) derivatives were examined. N-alkyl derivatives (2, 3) were inactive against all pathogens, while the simplest acyl derivative (5) showed activity against Pyrenophora graminea, Alternaria alternata, Cercospora beticola, Rhynchosporium secalis, and Septoria tritici (Table 1). The acetyl derivative (45) also showed activity against P. graminea, A. alternate, C. beticola, R. secalis, S. tritici, Microdochium nivale, and Gaeumannomyces graminis.12) These results coincided well with the previous result that the CO-bond linkage bound to the 2N-position was crucial for antifungal activity against P. graminea, A. alternata, C. beticola, R. secalis, and S. tritici.12) However, interestingly, the non-acyl compound (4) showed significant activity. Considering that the activity pattern of the compound was similar to hymexazol, hymexazol may be produced from this silylamine derivative (4) due to the fragile N–Si bond.
| Compound | Mycelial growth inhibition (%) a) | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| P. g | F. g | A. m | C. b | R. sec | S. t | M. n | R. s | G. g | |
| 2 | 0 | 3 | 8 | 0 | 2 | 0 | 9 | 6 | 0 |
| 3 | 50 | 10 | 7 | 0 | 0 | 0 | 11 | 0 | 0 |
| 4 | 80 | 22 | 64 | 87 | 25 | 78 | 34 | 45 | 0 |
| 5 | 81 | 36 | 71 | 85 | 98 | 88 | 44 | 16 | 32 |
| 6 | 45 | 7 | 13 | 0 | 0 | 0 | 7 | 0 | 3 |
| 7 | 24 | 10 | 4 | 0 | 0 | 0 | 10 | 0 | 0 |
| 8 | 49 | 17 | 6 | 0 | 0 | 0 | 13 | 0 | 3 |
| 9 | 85 | 19 | 8 | 0 | 28 | 19 | 29 | 36 | 3 |
| 10 | 13 | 4 | 8 | 0 | 0 | 0 | 13 | 6 | 1 |
| 11 | 80 | 35 | 72 | 92 | 99 | 79 | 78 | 53 | 53 |
| 12 | 85 | 30 | 72 | 89 | 100 | 79 | 48 | 66 | 3 |
| 13 | 66 | 23 | 58 | 90 | 100 | 69 | 31 | 49 | 3 |
| 14 | 96 | 64 | 72 | 91 | 100 | 70 | 68 | 79 | 49 |
| 15 | 80 | 23 | 54 | 88 | 99 | 68 | 41 | 48 | 11 |
| 16 | 53 | 3 | 7 | 58 | 43 | 22 | 16 | 27 | 5 |
| 17 | 79 | 28 | 62 | 89 | 100 | 79 | 42 | 60 | 11 |
| 18 | 77 | 31 | 55 | 78 | 100 | 70 | 54 | 61 | 24 |
| 19 | 81 | 21 | 51 | 78 | 100 | 68 | 52 | 71 | 21 |
| 20 | 95 | 37 | 70 | 88 | 99 | 77 | 48 | 100 | 33 |
| 21 | 64 | 22 | 60 | 89 | 97 | 74 | 34 | 62 | 3 |
| 22 | 89 | 53 | 52 | 82 | 100 | 85 | 65 | 71 | 15 |
| 23 | 74 | 20 | 55 | 88 | 99 | 70 | 35 | 58 | 5 |
| 24 | 93 | 49 | 63 | 89 | 100 | 74 | 61 | 100 | 24 |
| 25 | 92 | 29 | 67 | 91 | 100 | 88 | 83 | 66 | 43 |
| 26 | 80 | 30 | 71 | 90 | 99 | 81 | 41 | 69 | 13 |
| 27 | 97 | 78 | 48 | 45 | 100 | 62 | 76 | 71 | 42 |
| 28 | 99 | 43 | 69 | 77 | 100 | 83 | 100 | 88 | 68 |
| 29 | 91 | 20 | 61 | 90 | 99 | 73 | 40 | 64 | 0 |
| 30 | 95 | 39 | 70 | 91 | 100 | 85 | 56 | 85 | 31 |
| 31 | 97 | 46 | 80 | 93 | 100 | 74 | 65 | 86 | 0 |
| 32 | 85 | 27 | 69 | 90 | 100 | 80 | 54 | 66 | 30 |
| 33 | 96 | 27 | 63 | 87 | 100 | 80 | 44 | 57 | 28 |
| 34 | 82 | 29 | 64 | 89 | 100 | 73 | 37 | 51 | 22 |
| 35 | 86 | 24 | 65 | 88 | 100 | 72 | 44 | 55 | 28 |
| 36 | 84 | 27 | 69 | 89 | 100 | 70 | 44 | 54 | 26 |
| 37 | 90 | 25 | 63 | 87 | 99 | 83 | 44 | 57 | 0 |
| 38 | 88 | 31 | 62 | 88 | 99 | 75 | 46 | 54 | 13 |
| 39 | 88 | 17 | 64 | 87 | 99 | 79 | 33 | 45 | 16 |
| 40 | 71 | 41 | 75 | 89 | 99 | 91 | 60 | 54 | 44 |
| 41 | 2 | 24 | 2 | 11 | 11 | 12 | 10 | 10 | 0 |
| 42 | 0 | 13 | 3 | 8 | 10 | 3 | 19 | 0 | 0 |
| 43 | 0 | 13 | 6 | 9 | 6 | 0 | 13 | 0 | 0 |
| 44 | 0 | 16 | 11 | 8 | 7 | 10 | 20 | 10 | 6 |
| Hymexazol (1) | 82 | 46 | 73 | 87 | 99 | 85 | 62 | 28 | 44 |
a) The inhibition rate of mycelial growth was calculated with the formula 100×(dc−dt)/dc, where dc is the diameter of mycelial colony on the untreated PDA medium and dt is that on a PDA medium containing the compound. Data are the mean of three replications.
We then investigated whether the OCO-moiety at the 3O-position also plays a crucial role in antifungal activities. All the 3-alkoxy derivatives (6–10) had low activities against the tested pathogens except for benzyl ether (9) against P. graminea. Carbamates (41, 42) and amides (43, 44) were also inactive. We synthesized many types of esters composed with 3-OH as the alcohol part, and their antifungal activities were examined. Most of the ester derivatives showed high activities against P. graminea, C. beticola, R. secalis, and S. tritici. Ester derivatives were much more active than 3-alkoxy, carbamate, and amide derivatives, especially against R. secalis, an important pathogen of barley that needs to be controlled. This suggests that the OCO-moiety at the 3O-position on the ring is crucial for antifungal activity, as is the case with the 2N-position.
Unlike hymexazol, esters such as 20, 24, 28, 30, and 31 inhibited the mycelial growth of Rhizoctonia solani. Complete inhibition of the mycelial growth of o-tolyl (20) and α-naphtyl esters (24) was observed among these derivatives. The wide range of activities of cumyl (14) and dichlorotrimethylcyclopropyl (28) derivatives was also noteworthy. Compound (28), in particular, completely inhibited the mycelial growth of M. nivale. On the other hand, the activity of stearic ester (16) was lower than that of other esters, suggesting that a long alkyl chain is disadvantageous to antifungal activity.
We picked up several 2N-acyl and 3O-acyl derivatives and examined their activities against pathogens that cannot be evaluated on/in a medium. The selected compounds showed activities on wheat rust and wheat powdery mildew at 500–1,000 g/ha and, except for the formyl derivative (5), had higher activities against wheat rust than that of hymexazol. In a series of 2N-acyl derivatives, the activities of compounds (46, 47) composed of the bulky-acyl moiety were higher than those of compounds (5, 45) composed of simple-acyl moiety.
We found that the 3O-acyl and 2N-acyl derivatives showed high antifungal activities or disease control activities against S. tritici, Puccinia spp., B. glaminis, and R. secalis. These fungi have been shown to be the most economically important pathogens of wheat and barley.20,21) The diseases caused by these pathogens need to be controlled for the world’s food supply, and azole and succinate dehydrogenase inhibitor (SDHI) fungicides are frequently used to control these diseases. Using same type of pesticide repetitively in a season increases the risk of the emergence of chemical-resistant pathogens; therefore the development of new chemicals having a novel mode of action is desired. In this situation, identifying the antifungal activities and disease control activities of 3O-acyl and 2N-acyl derivatives may assist in the discovery of novel fungicides.
In this study, we introduced various functional groups to the 3O-position of hymexazol, and antifungal activities or disease control activities were discussed. Combined with the findings of a previous study regarding 2N-modification, the long-standing fungicide hymexazol can be a lead for new fungicides that have activity against a wide range of economically important pathogens. One of the prospective modification guidelines is the introduction of the bulky lipophilic-acyl moiety to the 2N or 3O atom of hymexazol.
| Compound a) | Concentration (mg/L) | Wheat rust b) | Wheat powdery mildew b) | ||||
|---|---|---|---|---|---|---|---|
| Disease severity | S.D. | Protective value | Disease severity | S.D. | Protective value | ||
| 5 | 1000 | 21.8 | ±7.0 | 51.3 | 15.7 | ±7.4 | 68.8 |
| 45 | 1000 | 27.2 | ±10.2 | 39.4 | 8.5 | ±5.5 | 83.1 |
| 500 | 18.7 | ±3.4 | 58.4 | 6.4 | ±4.5 | 87.3 | |
| 46 | 1000 | 3.1 | ±0.9 | 93.0 | 4.5 | ±2.4 | 91.1 |
| 500 | 8.1 | ±2.9 | 81.8 | 5.4 | ±2.9 | 89.2 | |
| 47 | 1000 | 7.9 | ±2.8 | 82.3 | 1.5 | ±1.6 | 96.9 |
| 500 | 9.2 | ±6.1 | 79.6 | 4.1 | ±2.4 | 91.8 | |
| 13 | 1000 | 9.8 | ±3.0 | 78.2 | 11.3 | ±7.2 | 77.6 |
| 500 | 17.1 | ±5.8 | 61.9 | 10.6 | ±5.6 | 78.9 | |
| 21 | 1000 | 12.0 | ±4.1 | 73.2 | 15.5 | ±5.4 | 69.2 |
| 500 | 17.9 | ±8.0 | 60.2 | 8.7 | ±4.5 | 82.7 | |
| 23 | 1000 | 4.5 | ±2.6 | 90.0 | 6.5 | ±3.3 | 87.2 |
| 500 | 17.3 | ±5.6 | 61.3 | 7.3 | ±3.7 | 85.6 | |
| 24 | 1000 | 11.8 | ±3.8 | 73.6 | 7.6 | ±3 | 84.9 |
| 500 | 21.3 | ±8.1 | 52.4 | 13.2 | ±4.8 | 73.8 | |
| 27 | 1000 | 13.7 | ±4.9 | 69.5 | 8.7 | ±3.6 | 82.7 |
| 500 | 22.3 | ±7.3 | 50.2 | 7.0 | ±2.8 | 86.2 | |
| 29 | 1000 | 7.5 | ±3.4 | 83.3 | 3.4 | ±2.7 | 93.2 |
| 500 | 5.9 | ±1.8 | 86.8 | 7.9 | ±5.2 | 84.3 | |
| 30 | 1000 | 5.8 | ±1.9 | 87.0 | 5.3 | ±3.3 | 89.5 |
| 500 | 10.7 | ±5.1 | 76.1 | 8.5 | ±4.8 | 83.2 | |
| Hymexazol (1) | 1000 | 29.3 | ±7.4 | 34.8 | 10.1 | ±3.8 | 80.0 |
| 500 | 19.0 | ±3.5 | 57.7 | 13.2 | ±3.5 | 73.8 | |
| Untreated | 44.8 | ±9.8 | 0.0 | 50.4 | ±14.2 | 0.0 | |
a) Compound 5 was 2N-formyl and compounds 45, 46, and 47 were 2N-acyl derivatives. Compounds 13, 21, 23, 24, 27, 29, and 30 were 3O-acyl derivatives.b) A protective index was calculated with the formula 100×(dc−dt)/dc, where dc is the percentage of lesion area on an untreated plot and dt is that of the plot treated with the test compound. The fourteen to sixteen seedlings were assessed by lesion area.
