An
efficient method for the synthesis of 5-vinyl-2-isoxazolines
by Pd-catalyzed intramolecular O-allylation of ketoximes
has been developed. The reaction involves Pd(0)-catalyzed π-allyl
formation via leaving group ionization or Pd(II)-catalyzed allylic
C–H activation followed by intramolecular nucleophilic oxime
oxygen attack. This methodology has been elaborated to various value-added
products by epoxidation, Wacker oxidation, cross-metathesis, hydroboration–oxidation,
dihydroxylation, and catalytic hydrogenation.
Flasks were oven or flame dried and cooled in a desiccator. Dry reactions were carried out under an atmosphere of Ar or N2. Solvents and reagents were purified by standard methods.Thin-layer chromatography was performed on EM 250 Kieselgel 60 F254 silica gel plates. The spots were visualized by either staining with KMnO4 or under UV lamp. 1 H NMR and 13 C NMR were recorded on Bruker, AVANCE III 400 spectrometer and the chemical shifts are based on TMS peak at = 0.00 ppm for 1 H NMR and CDCl3 peak at = 77.00 ppm (t) in 13 C NMR. IR spectra were obtained on Perkin Elmer FT-IR spectrometer. HRMS was recorded using Micromass: Q-Tof micro (YA-105) spectrometer using positive electrospray ionization.
Experimental Procedure for the synthesis of (E)-Ethyl 4,4-dimethyl-5-oxopent-2-enoate (12)To a solution of DMSO (2.73 mL, 38.4 mmol, 8.0 equiv) in CH2Cl2 (40 mL) at -78 ºC was added oxalyl chloride (1.65 mL, 19.2 mmol, 4.0 equiv). The resulting mixture was stirred for 30 min, then a solution of diol 11 (0.5 g, 4.8 mmol, 1.0 equiv) in CH2Cl2 (20 mL) was added and stirring continued for 1 h. It was then quenched with Et3N (6.65 mL, 48.0 mmol, 10.0 equiv) and the reaction mixture was kept at -78 ºC for 10 min and then slowly warmed to room temperature. After stirring at room temperature for 2 h, LiCl (0.61 g, 14.4 mmol, 3.0 equiv) was added followed by the addition of triethyl phosphonoacetate (2.2 g, 9.6 mmol, 2.0 equiv). The reaction mixture was stirred for 16 h. It was then quenched with sat. aq. NH4Cl (15 mL) and extracted with CH2Cl2 (3 30 mL). The combined organic layers were washed with water, brine, dried (Na2SO4) and concentrated. The crude residue was purified by silica gel column chromatography using petroleum ether/EtOAc (7:1) as eluent to give 12 (0.588 g, 72%)as pale yellow oil. 1 H NMR (CDCl3, 400 MHz): δ 9.44 (s, 1H) 6.95 (d, J = 16.0 Hz, 1H) 5.87
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