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Synlett 2006(20): 3427-3430  
DOI: 10.1055/s-2006-956475
LETTER
© Georg Thieme Verlag Stuttgart · New York

A Convenient Procedure for Transformation of Tertiary Cyclopropanols into 5-Substituted Isoxazoles

Dzmitry H. Churykau, Oleg G. Kulinkovich*
Department of Organic Chemistry, Belarusian State University, Nezavisimosty Av. 4, 220030 Minsk, Belarus
Fax: +375(17)2265609; e-Mail: kulinkovich@bsu.by;
Further Information

Publication History

Received 28 September 2006
Publication Date:
08 December 2006 (online)

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Abstract

Tertiary cyclopropanols, when treated with an excess of amyl nitrite at room temperature, are smoothly converted into dimeric β-nitrosoketones. Heating the methanolic solutions of the latter under reflux gives 5-substituted isoxazoles in good yields.

Key words

cyclopropanols - nitrosation - β-nitrosoketones - isoxazoles - isoxazolines

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Cyclopropanols 1a-h were synthesized by the reductive cyclopropanation of the corresponding esters with ethylmagnesium bromide (compounds 1a-g) or propylmagnesium bromide (1h) in the presence of titanium(IV) isopropoxide (see ref. 1).

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Utilization of amyl nitrite that was stored in a refrigerator for more than two weeks led to a significant reduction of the reaction rate and to a decrease in the yields of products.

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After crystallization from MeOH pure E isomer was obtained.

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Preparation of β-Nitrosoketones 2; Typical Procedure: Freshly prepared amyl nitrite (17 mL, 124 mmol) was added at 5 °C under Ar atmosphere to a solution of 1a (4.9 g, 31 mmol) in anhyd benzene (5 mL) in one portion. The mixture was stirred for 3 h and was kept at r.t. until the reaction was completed as monitored by TLC (2-3 d, see ref. 20). The mixture was concentrated in vacuo and was used for the preparation of isoxazoles without further purification. In order to obtain solid samples of 2a (as a mixture of Z and E isomers), the residue was diluted with petroleum ether, cooled and the crystals were filtered off. Single E isomer of 2a (3.45 g, 20.2 mmol, 65%) was obtained by the crystallization from hot MeOH as a yellowish solid (mp 86-87 °C).

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Analytical data of selected nitrosoketones 2.
2a: 1H NMR (400 MHz, CDCl3): δ = 0.83 (t, J = 6.8 Hz, 3 H), 1.18-1.30 (m, 6 H), 1.49-1.59 (m, 2 H), 2.44 (t, J = 7.4 Hz, 2 H), 2.90 (t, J = 6.2 Hz, 2 H), 4.40 (t, J = 6.2 Hz, 2 H). 13C NMR (100 MHz, CDCl3): δ = 13.84, 22.31, 23.41, 28.64, 31.39, 36.40, 42.74, 53.48, 206.63. IR (CCl4): 1722, 1371, 1250 cm-1. Anal. Calcd for C9H17NO2 (171.24): C, 63.13; H, 10.01. Found: C, 63.28; H, 9.75.
2c: 1H NMR (400 MHz, CDCl3): δ = 2.04-2.13 (m, 2 H), 2.72 (t, J = 7.0 Hz, 2 H), 2.97 (t, J = 6.1 Hz, 2 H), 3.58 (t, J = 6.2 Hz, 2 H), 4.47 (t, J = 6.1 Hz, 2 H). 13C NMR (100 MHz, CDCl3): δ = 26.10, 36.69, 39.44, 44.21, 53.60, 205.58. IR (CCl4): 1720, 1370, 1247 cm-1. Anal. Calcd for C6H10ClNO2 (163.61): C, 44.05; H, 6.16. Found: C, 43.90; H, 5.89.

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Preparation of Isoxazoles; Typical Procedure: Crude β-nitrosoketone 2a, prepared from 1e (4.9 g, 31 mmol) and amyl nitrite (17.0 mL, 124 mmol) as described above (see ref. 14) was diluted with anhyd MeOH (45 mL). The solution was heated under reflux until TLC indicated that no β-nitrosoketone 2a and intermediate isoxazoline 4a remained (2-3 days, see ref. 20). After removal of the solvent under reduced pressure, the isoxazole 3a was isolated by column chromatography (SiO2, PE-EtOAc as eluent) as a yellowish oil (4.4 g, 91%).

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Analytical data of selected isoxazoles 3.
3c: 1H NMR (400 MHz, CDCl3): δ = 2.13-2.22 (m, 2 H), 2.97 (t, J = 7.4 Hz, 2 H), 3.57 (t, J = 6.3 Hz, 2 H), 6.03-6.05 (m, 1 H), 8.14-8.17 (m, 1 H). 13C NMR (100 MHz, CDCl3): δ = 23.70, 30.14, 43.54, 100.58, 150.22, 170.93. IR (CCl4): 1606 cm-1. Anal. Calcd for C6H8ClNO (145.59): C, 49.50; H, 5.54. Found: C, 49.33; H, 5.75.
3e: 1H NMR (400 MHz, CDCl3): δ = 1.22-1.40 (m, 10 H), 1.65-1.73 (m, 2 H), 1.99-2.06 (m, 2 H), 2.76 (t, J = 7.7 Hz, 2 H), 4.92 (ddt, J 1 = 10.2 Hz, J 2 = 2.2 Hz, J 3 = 1.1 Hz, 1 H), 4.98 (ddt, J 1 = 16.9 Hz, J 2 = 2.2 Hz, J 3 = 1.5 Hz, 1 H), 5.80 (ddt, J 1 = 16.9 Hz, J 2 = 10.2 Hz, J 3 = 6.7 Hz, 1 H), 5.95-5.97 (m, 1 H), 8.12-8.14 (m, 1 H). 13C NMR (100 MHz, CDCl3): δ = 26.49, 27.49, 28.83, 28.98, 28.99, 29.11, 29.24, 33.73, 99.76, 114.13, 139.10, 150.12, 173.02. IR (CCl4): 3079, 1640, 1593 cm-1. Anal. Calcd for C13H21NO (207.32): C, 75.32; H, 10.21. Found: C, 75.59; H, 10.02.
3g: 1H NMR (400 MHz, CDCl3): δ = 1.16 (t, J = 7.0 Hz, 6 H), 3.09 (d, J = 5.7 Hz, 2 H), 3.29 (dq, J 1 = 9.4 Hz, J 2 = 7.0 Hz, 2 H), 3.66 (dq, J 1 = 9.4 Hz, J 2 = 7.0 Hz, 2 H), 4.78 (t, J = 5.7 Hz, 1 H), 6.09 (d, J = 1.6 Hz, 1 H), 8.14 (d, J = 1.6 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ = 15.06, 31.92, 61.91, 100.18, 101.56, 150.19, 168.18. IR (CCl4): 2874, 1734, 1597, 1125, 1064 cm-1. Anal. Calcd for C9H15NO3 (185.22): C, 58.36; H, 8.16. Found: C, 58.60; H, 8.01.

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Analytical data of 4: 1H NMR (400 MHz, CDCl3): δ = 0.88 (t, J = 6.9 Hz, 3 H), 1.22-1.53 (m, 8 H), 1.82-1.97 (m, 2 H), 2.89 (dd, J 1 = 18.4 Hz, J 2 = 1.3 Hz, 1 H), 2.92 (dd, J 1 = 18.4 Hz, J 2 = 1.6 Hz, 1 H), 3.10 (br s, 1 H), 7.20-7.23 (m, 1 H). 13C NMR (100 MHz, CDCl3): δ = 13.97, 22.44, 24.60, 29.13, 31.58, 37.95, 44.86, 106.61, 147.16. IR (CCl4): 3598, 3395, 1722, 1601 cm-1. Anal. Calcd for C9H17NO2 (171.24): C, 63.13; H, 10.01. Found: C, 63.28; H, 9.75.

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The reaction proceeded less smoothly under elevated temperatures or in the presence of acidic or basic catalysts.