Beilstein J. Org. Chem. 2011, 7, 127–134.
3,4,5-Trimethylisoxazole (7e) [26]. A mixture of 3 (20.55 g, quartz NMR tube, degassed with argon and irradiated in a photo
0.18 mol), hydroxylamine hydrochloride (12.50 g, 0.18 mol) reactor (LZ-C4, 300 nm) for 6 h. The mixture was examined
and water (80 mL) was stirred at rt for 24 h. The solution was before and after irradiation by 1H NMR spectroscopy.
then extracted three times with chloroform and the combined
organic layers were dried over magnesium sulfate and filtered. Photocycloaddition reactions of 7e with aryl substituted
After removal of the solvent, the residue was distilled (165 °C) aldehydes: General procedure. A solution of 7e (2.22 g,
to yield 13.0 g (65%) of 7e as a colorless liquid. 13C NMR 20 mmol) and the corresponding aldehyde (20 mmol) in aceto-
(75.5 MHz, CDCl3): 163.4 (1C, Cq), 159.0 (1C, Cq), 108.2 (1C, nitrile (200 mL) was transferred to a quartz vessel, mixed with
Cq), 9.9 (1C, CH3), 9.2 (1C, CH3), 5.8 (1C, CH3). 1H NMR potassium carbonate (0.28 g, 10 mol %) and degassed with
(300 MHz, CDCl3): 2.01 (s, 3H, CH3), 1.91 (s, 3H, CH3), 1.61 nitrogen. The mixture was stirred and irradiated in a Rayonet
(s, 3H, CH3).
photochemical reactor (RPR 208, lamps centered 300 nm) at
−10 °C for 48–72 h. The resulting yellow solution was concen-
3,5-Diphenylisoxazole (7f) [25]. A solution of 4 (4.49 g, trated (35 °C) and extracted three times with ether. The
20 mmol) and hydroxylamine hydrochloride (1.74 g, 20 mmol) combined organic layers were dried over magnesium sulfate
in water (30 mL) and ethanol (20 mL) was refluxed for 90 min. and the solvent was evaporated. The solid residue was mixed
The mixture was cooled to rt, poured onto ice (50 mL) and with a little ethanol, stirred for 30 min, filtered and recrystal-
extracted two times with dichloromethane. The combined lized at −10 °C from ethanol.
organic phases were dried over magnesium sulfate, filtered and
evaporated. The resulting solid was recrystallized from ether to exo-1, 4, 5-trimethyl-6-phenyl-2, 7-dioxa-3-aza-
yield 4.09 g (92%) of 7f as colorless crystals. 13C NMR bicyclo[3.2.0]hept-3-en (9a). Yield: 50%. 13C NMR
(75.5 MHz, CDCl3): 170.3 (1C, Cq), 162.9 (1C, Cq), 130.1 (1C, (500 MHz, CD3CN): 163.6 (1C, CN), 138.9 (1C, Ar-C), 129.3
CH), 129.9 (1C, CH), 129.0 (1C, Cq), 128.9 (2C, CH), 128.8 (2C, Ar-C), 128.9 (1C, Ar-C), 126.2 (2C, Ar-C), 115.6 (1C,
(2C, CH), 127.4 (1C, Cq), 126.7 (2C, CH), 125.7 (2C, CH), OCO), 86.4 (1C, CO), 63.7 (1C, Cq), 19.1 (1C, CH3), 11.1 (1C,
97.4 (1C, CH). 1H NMR (300 MHz, CDCl3): 7.90–7.84 (m, 4H, CH3), 10.2 (1C, CH3). 1H NMR (500 MHz, CD3CN):
CH), 7.52 (m, 6H, CH), 6.83 (s, 1H, CH).
7.45–7.35 (m, 5H, 3J = 7.6 Hz, 3J = 7.0 Hz, Ar-CH), 5.59 (s,
1H, CH), 2.05 (s, 3H, CH3CN), 1.60 (s, 3H, CH3), 0.78 (s, 3H,
3-Phenyl-5-Methoxy-isoxazole (7g) [22]. A solution of 6 CH3). Anal. calcd. for C13H15NO2: C 71.87, H 6.96, N 6.45.
(2.88 g, 16 mmol) and sodium methoxide (7.78 g, 144 mmol) in Found: C 71.67, H 6.98, N 6.40. Colorless needles, unit cell
methanol (40 mL) and THF (40 mL) was heated under reflux parameters: a = 9.3804(6), b = 14.8345(10), c = 9.080(6), β =
for 63 h and then concentrated. The residue was poured in iced 116.646(3), space group P21/c.
water (100 mL), extracted with ether and washed with aqueous
sodium carbonate to neutralize hydrochloric acid. The organic exo-1,4,5-trimethyl-6-(4'-methylphenyl)-2,7-dioxa-3-aza-bi-
layer was dried over magnesium sulfate followed by removal of cyclo[3.2.0]hept-3-en (9b). Yield: 35%. 13C NMR (75.5 MHz,
the solvent. The remaining solid was recrystallized from warm CD3CN): 163.7 (1C, CN), 138.8 (1C, Ar-C), 136.1 (1C, Ar-C),
cyclohexane (50 °C) to yield 1.57 g (56%) of 7g as yellow crys- 130.0 (2C, Ar-C), 126.4 (2C, Ar-C), 115.6 (1C, OCO), 86.6
tals. 13C NMR (75.5 MHz, CDCl3): 174.4 (1C, Cq), 163.9 (1C, (1C, CO), 63.8 (1C, Cq), 21.1 (1C, CH3), 19.2 (1C, CH3), 11.1
Cq), 129.8 (1C, CH), 129.3 (1C, Cq), 128.6 (2C, CH), 126.2 (1C, CH3), 10.3 (1C, CH3). 1H NMR (300 MHz, CD3CN):
(2C, CH), 75.1 (1C, CH), 58.7 (1C, CH3). 1H NMR: 7.75–7.72 7.26–7.24 (m, 4H, Ar-CH), 5.55 (s, 1H, CH), 2.35 (s, 3H,
(m, 2H, CH), 7.42–7.39 (m, 3H, CH), 5.51 (s, 1H, CH), 3.96 (s, Ar-CH3), 2.03 (s, 3H, CH3CN), 1.60 (s, 3H, CH3), 0.79 (s, 3H,
3H, CH3).
CH3). Anal. calcd. for C14H17NO2: C 72.70, H 7.41, N 6.06.
Found: C 72.71, H 7.45, N 6.08. Colorless needles, unit cell
3-Phenyl-5-(trimethylsilyloxy)isoxazole (7h) [27]. To 2.72 g parameters: a = 17.196(2), b = 5.9199(4), c = 12.807(2), β =
(25 mmol, 3.2 mL) of chlorotrimethylsilane, was added a mix- 109.944(4), space group P21/c.
ture of 5 (3.22 g, 20 mmol) and hexamethyldisilazane (7.26 g,
45 mmol, 9.5 mL). The solution was heated at 120 °C for 30 exo-1,4,5-trimethyl-6-(3'-methylphenyl)-2,7-dioxa-3-aza-bi-
min and then concentrated. The resulting product was unstable cyclo[3.2.0]hept-3-en (9c). Yield: 65%. 13C NMR (75.5 MHz,
and therefore used immediately.
CD3CN): 163.7 (1C, CN), 139.3 (1C, Ar-C), 139.0 (1C, Ar-C),
129.7 (1C, Ar-C), 129.3 (1C, Ar-C), 126.9 (1C, Ar-C), 123.4
NMR-photolyses of isoxazoles 7a–h with aldehydes: General (1C, Ar-C), 115.7 (1C, OCO), 86.6 (1C, CO), 63.8 (1C, Cq),
procedure. A solution of isoxazole (0.05 mmol) and aldehyde 21.4 (1C, CH3), 19.2 (1C, CH3), 11.1 (1C, CH3), 10.3 (1C,
(0.1 M) in deuterated acetonitrile (0.5 mL) was transferred to a CH3). 1H NMR (300 MHz, CD3CN): 7.35–7.13 (m, 4H,
133