Step 2 - Synthesis of 2-(5-methylisoxazol-3-yl)but-3-yn-2-ol
A stirred solution of ethynylmagnesium bromide (0.5 M in THF, 50 mL, 25 mmol) was cooled below 0 °C under nitrogen protection. Subsequently, a tetrahydrofuran (20 mL) solution of 1-(5-methyl-3-isoxazolyl)ethanone (2.5 g, 19.98 mmol) was slowly added dropwise over 5 min. After dropwise addition, the reaction mixture was gradually warmed to room temperature and stirring was continued for 3 hours. Upon completion of the reaction, the reaction was quenched by the addition of saturated ammonium chloride solution (100 mL) and then extracted with ethyl acetate (2 x 100 mL). The organic phases were combined, washed with brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography with the eluent ethyl acetate/petroleum ether (0:1 to 1:4) to afford the target compound 2-(5-methylisoxazol-3-yl)but-3-yn-2-ol (2.3 g, 75%) as a colorless oil. Its 1H NMR (300 MHz, CDCl3) data were as follows: δ 6.12 (s, 1H), 3.47 (s, 1H), 2.63 (s, 1H), 2.42 (s, 3H), 1.86 (s, 3H).
