Synthesis of Highly Functionalized Isoxazoles via Base-Promoted Cyclocondensation of Stable Nitrile Oxides with Active Methylene Compounds

Hiroshi Takikawa; Katsuyoshi Hikita; Keisuke Suzuki

doi:10.1055/s-2007-985561

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Synlett 2007(14): 2252-2256
DOI: 10.1055/s-2007-985561

LETTER

Synthesis of Highly Functionalized Isoxazoles via Base-Promoted Cyclocondensation of Stable Nitrile Oxides with Active Methylene Compounds

Hiroshi Takikawa, Katsuyoshi Hikita, Keisuke Suzuki*
Department of Chemistry, Tokyo Institute of Technology and SORST-JST Agency, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8551, Japan
Fax: +81(3)57342788; e-Mail: ksuzuki@chem.titech.ac.jp;

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Publikationsverlauf

Received 8 June 2007
Publikationsdatum:
24. Juli 2007 (online)

Abstract
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Abstract

Base-promoted cyclocondensation of ortho-disubstituted benzonitrile oxides with active methylene compounds provides a concise route to highly functionalized isoxazole derivatives.

Key words

benzonitrile oxide - isoxazole - active methylene compounds - cyclocondensation - molecular sieves

References and Notes

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Although the pKa of methanesulfonylacetone (6) has not been documented so far, we estimated it to be higher than that of ethyl acetoacetate (3) by analogy from the pKa of dimethyl sulfone (31.1 in DMSO) and EtOAc (29.5 in DMSO):
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7

Structure of 21 was confirmed by X-ray crystallographic analysis (CCDC 647875).

9

Typical Experimental Procedure for the Synthesis of Isoxazole 14 Using 4 Å MS: To a solution of 2-bromo-6-methoxybenzonitrile oxide (12; 214 mg, 0.937 mmol) and benzoylacetonitrile (4; 161 mg, 1.11 mmol) in i-PrOH (3.1 mL) was added 4 Å MS (938 mg) at r.t. The red suspension was heated at 50 °C for 2 h. After cooling, the mixture was filtered through a Celite^® pad, and the filtrate was concentrated in vacuo. The residue was purified by silica gel column chromatography (hexane-EtOAc, 85:15) to give 14 (253 mg, 76%) as a yellow solid. Recrystallization of 14 from acetone-hexane gave a colorless prism; mp 177.2-177.6 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.85 (s, 3 H), 7.00 (dd, J = 2.0, 7.6 Hz, 1 H), 7.33 (dd, J = 2.0, 7.6 Hz, 1 H), 7.37 (dd, J = 7.6, 7.6 Hz, 1 H), 7.55-7.64 (m, 3 H), 8.12-8.19 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 56.1, 90.0, 110.2, 111.8, 116.3, 124.1, 124.9, 125.1, 126.7, 129.3, 132.5, 132.8, 158.8, 160.4, 173.8. Anal. Calcd for C₁₇H₁₁BrN₂O₂: C, 57.49; H, 3.12; N, 7.89. Found: C, 57.25; H, 3.32; N, 7.72. IR (ATR): 3014, 2941, 2836, 2239, 1590, 1573, 1456, 1395, 1268, 1032, 784 cm^-1.
Typical Experimental Procedure for the Synthesis of Isoxazole 16 Using i -PrONa: To a mixture of 2-bromo-6-methoxybenzonitrile oxide (12; 199 mg, 0.872 mmol) and methanesulfonylacetone (6; 142 mg, 1.05 mmol) in i-PrOH (8.7 mL) at r.t. was added i-PrONa (0.3 M in i-PrOH, 0.43 mL, 0.13 mmol). After stirring at 40 °Cfor 1 h, the reaction was stopped at r.t. by adding H₂O. After the products were extracted with EtOAc (3 ×), the combined organic layer was dried over Na₂SO₄, and concentrated in vacuo. The residue was purified by silica gel column chromatography (hexane-EtOAc, 4:1) to give 16 (252 mg, 83%) as a colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 2.80 (s, 3 H), 2.96 (s, 3 H), 3.79 (s, 3 H), 6.96 (dd, J = 1.2, 8.1 Hz, 1 H), 7.28 (dd, J = 1.2, 8.1 Hz, 1 H), 7.35 (dd, J = 8.1, 8.1 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 12.8, 44.8, 56.1, 110.1, 117.5, 117.6, 124.3, 124.7, 132.5, 157.5, 159.1, 174.4. Anal. Calcd for C₁₂H₁₂BrNO₄S: C, 41.63; H, 3.49; N, 4.05; S, 9.26. Found: C, 41.42; H, 3.60; N, 3.84; S, 9.03. IR (neat): 3009, 2929, 1571, 1431, 1378, 1313, 1259, 1134, 1026, 952, 773 cm^-1.
Data of Other Isoxazole Products:
8: colorless prism; mp 159.5-160.0 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.34 (d, J = 13.5 Hz, 1 H), 2.09-2.17 (m, 1 H), 3.46 (s, 3 H), 3.80 (ddd, J = 2.4, 10.6, 12.3 Hz, 2 H), 4.13 (dd, J = 5.1, 10.6 Hz, 2 H), 5.20 (s, 2 H), 5.60 (s, 1 H), 7.33 (dd, J = 1.9, 7.1 Hz, 1 H), 7.48-7.52 (m, 2 H), 7.59-7.62 (m, 3 H), 8.17-8.20 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 25.5, 56.4, 67.2, 91.1, 95.1, 98.9, 112.3, 113.9, 115.0, 119.9, 125.2, 126.8, 129.4, 132.1, 132.5, 139.6, 155.5, 160.4, 173.3. Anal. Calcd for C₂₂H₂₀N₂O₅: C, 67.34; H, 5.14; N, 7.14. Found: C, 67.36; H, 5.43; N, 6.98. IR (KBr): 2960, 2856, 2238, 1604, 1569, 1475, 1382, 1261, 1159, 1108, 981, 896, 796 cm^-1.
9: colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 0.93 (t, J = 6.9 Hz, 3 H), 1.31 (d, J = 13.4 Hz, 1 H), 2.05-2.20 (m, 1 H), 2.77 (s, 3 H), 3.35 (s, 3 H), 3.62 (br, 1 H), 3.80 (br, 1 H), 4.00 (br, 1 H), 4.02 (q, J = 6.9 Hz, 2 H), 4.16 (br, 1 H), 5.07 (s, 2 H), 5.29 (s, 1 H), 7.15-7.23 (m, 1 H), 7.40-7.43 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 13.0, 13.5, 25.5, 55.9, 60.1, 67.1, 94.7, 99.3, 111.0, 114.6, 118.0, 119.2, 130.4, 138.8, 155.2, 158.3, 162.0, 174.2. Anal. Calcd for C₁₉H₂₃NO₇: C, 60.47; H, 6.14; N, 3.71. Found: C, 60.73; H, 6.25; N, 3.62. IR (neat): 2991, 2933, 2869, 2844, 1712, 1608, 1475, 1309, 1157, 1108, 1039, 987 cm^-1.
10: colorless prism; mp 106.0-107.0 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.35 (d, J = 13.5 Hz, 1 H), 2.06-2.16 (m, 1 H), 2.80 (s, 3 H), 2.84 (s, 3 H), 3.42 (s, 3 H), 3.64 (ddd, J = 2.4, 11.2, 12.2 Hz, 1 H), 3.84 (ddd, J = 2.4, 11.2, 12.2 Hz, 1 H), 3.97 (dd, J = 5.0, 11.2 Hz, 1 H), 4.21 (dd, J = 5.0, 11.2 Hz, 1 H), 5.04 (d, J = 7.0 Hz, 1 H), 5.17 (d, J = 7.0 Hz, 1 H), 5.46 (s, 1 H), 7.28 (d, J = 7.97 Hz, 1 H), 7.40 (d, J = 7.97 Hz, 1 H), 7.48 (d, J = 7.97 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 12.9, 25.4, 44.4, 56.3, 66.9, 67.2, 95.6, 98.9, 114.7, 115.1, 118.5, 119.2, 131.7, 138.9, 156.4, 156.8, 173.4. Anal. Calcd for C₁₇H₂₁NO₇S: C, 53.25; H, 5.52; N, 3.65; S, 8.36. Found: C, 53.27; H, 5.69; N, 3.64; S, 8.60. IR (KBr): 2967, 2927, 2852, 1579, 1473, 1402, 1317, 1257, 1155, 1105, 1045, 1008, 987, 954, 775 cm^-1.
15: colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 1.03 (t, J = 7.1 Hz, 3 H), 2.77 (s, 3 H), 3.75 (s, 3 H), 4.09 (q, J = 7.1 Hz, 2 H), 6.91 (dd, J = 2.2, 6.8 Hz, 1 H), 7.22-7.30 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 13.2, 13.6, 56.0, 60.1, 109.5, 109.6, 120.0, 124.2, 124.4, 131.2, 158.7, 159.0, 161.4, 175.0. Anal. Calcd for C₁₄H₁₄BrNO₄: C, 49.43; H, 4.15; N, 4.12. Found: C, 49.13; H, 4.36; N, 4.00. IR (neat): 2942, 1718, 1610, 1460, 1429, 1308, 1263, 1105, 1031, 776 cm^-1.
17: colorless prism; mp 113.8-114.3 °C. ¹H NMR (300 MHz, CDCl₃): δ = 1.19 (d, J = 13.4 Hz, 1 H), 1.67-1.80 (m, 1 H), 3.75 (s, 3 H), 3.78-3.82 (m, 2 H), 3.85 (s, 3 H), 3.92-4.14 (m, 2 H), 6.01 (s, 1 H), 6.97 (d, J = 9.3 Hz, 1 H), 7.04 (d, J = 9.3 Hz, 1 H), 7.57-7.59 (m, 3 H), 8.14-8.19 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 25.6, 56.5, 56.6, 67.1, 91.7, 96.4, 112.6, 112.7, 114.5, 115.4, 125.5, 126.7, 128.0, 129.3, 132.0, 151.0, 152.3, 161.0, 172.0. Anal. Calcd for C₂₂H₂₀N₂O₅: C, 67.34; H, 5.14; N, 7.14. Found: C, 67.13; H, 5.35; N, 6.93. IR (ATR): 3065, 2957, 2915, 2837, 2233, 1607, 1595, 1519, 1479, 1446, 1394, 1259, 1082, 995, 809 cm^-1.
18: colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 0.94 (t, J = 7.1 Hz, 3 H), 1.18 (d, J = 13.2 Hz, 1 H), 1.63-1.79 (m, 1 H), 2.73 (s, 3 H), 3.69 (s, 3 H), 3.66-3.79 (m, 2 H), 3.83 (s, 3 H), 3.87-3.98 (m, 2 H), 4.01 (q, J = 7.1 Hz, 2 H), 5.89 (s, 1 H), 6.91 (d, J = 9.0 Hz, 1 H), 6.96 (d, J = 9.0 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 12.6, 13.4, 25.3, 56.6, 56.7, 59.7, 66.8, 67.0, 96.7, 111.6, 112.4, 113.1, 119.3, 127.0, 150.1, 152.3, 158.8, 162.2, 172.1. Anal. Calcd for C₁₉H₂₃NO₇: C, 60.47; H, 6.14; N, 3.71. Found: C, 60.23; H, 5.96; N, 3.50. IR (neat): 3434, 2962, 2842, 2730, 1712, 1619, 1509, 1482, 1465, 1396, 1307, 1259, 1176, 1106, 1083, 998 cm^-1.
19: colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 1.24 (d, J = 13.4 Hz, 1 H), 1.70-1.86 (m, 1 H), 2.76 (s, 3 H), 2.90 (s, 3 H), 3.71 (s, 3 H), 3.75 (br d, J = 2.5 Hz, 1 H), 3.78 (br d, J = 2.5 Hz, 1 H), 3.83 (s, 3 H), 4.13 (br d, J = 4.7 Hz, 1 H), 4.17 (br d, J = 4.7 Hz, 1 H), 5.96 (s, 1 H), 6.93 (d, J = 9.3 Hz, 1 H), 7.01 (d, J = 9.3 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 12.6, 25.5, 44.4, 56.3, 56.6, 66.8, 67.2, 96.6, 112.3, 114.1, 116.1, 118.3, 127.2, 150.8, 152.8, 157.3, 172.2. Anal. Calcd for C₁₇H₂₁NO₇S: C, 53.25; H, 5.52; N, 3.65; S, 8.36. Found: C, 53.49; H, 5.35; N, 3.68; S, 8.47. IR (neat): 3002, 2962, 2931, 2892, 2840, 1586, 1481, 1461, 1380, 1313, 1257, 1241, 1156, 1101, 1080, 996, 956 cm^-1.
Isoxazolone 21: yellow prism; mp 120.0-120.8 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.38 (d, J = 13.5 Hz, 1 H), 1.79 (s, 3 H), 2.05-2.17 (m, 1 H), 2.54 (s, 3 H), 3.39 (s, 3 H), 3.76 (ddd, J = 2.4, 11.4, 12.3 Hz, 1 H), 3.92 (ddd, J = 2.4, 11.4, 12.3 Hz, 1 H), 3.99 (dd, J = 5.1, 11.4 Hz, 1 H), 4.24 (dd, J = 5.1, 11.4 Hz, 1 H), 5.13 (d, J = 6.8 Hz, 1 H), 5.16 (d, J = 6.8 Hz, 1 H), 5.56 (s, 1 H), 7.18 (d, J = 7.2 Hz, 1 H), 7.41 (d, J = 7.2 Hz, 1 H), 7.64 (t, J = 7.2 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 22.8, 24.0, 25.5, 56.3, 67.2, 67.2, 94.6, 98.8, 114.7, 117.9, 118.7, 119.6, 131.4, 139.0, 155.0, 158.7, 169.4, 172.3. Anal. Calcd for C₁₈H₂₁NO₆: C, 62.24; H, 6.09; N, 4.03. Found: C, 62.32; H, 5.98; N, 4.00. IR (KBr): 2962, 2857, 1754, 1635, 1467, 1369, 1251, 1157, 1093, 1041, 987, 865, 800 cm^-1.