Hydrochloric Acid → 3-Isoxazol-5-yl-3-oxo-propionic acid ethyl ester

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To a stirred solution of diethyl malonate (2.6 ml, 2.74 g) in THF (40 ml) under an atmosphere of nitrogen at −60° C. was added nbutyl lithium (2.5 M, 18.9 ml) dropwise over 15 mins, keeping the temperature constant. The reaction became a cloudy white colour from the formation of the di-lithium salt then became yellowish. After 10 mins, the reaction was cooled to −78° C., when a solution of Isoxazole-5-carbonyl chloride (1 g) in THF (10 ml) was added dropwise over 15 mins. The reaction then warmed to −45° C. and stirred for 1 h, the solution had gone pale brown. The reaction was then poured into 1 M HCl solution (50 ml) with stirring and extracted with CH2Cl2 (2×150 ml). The combined organic layers were washed with sat. NaCO3 soln (1×30 ml), dried over MgSO4, filtered and concentrated in vacuo to give the product as a pale oil which was used as such in the next step. Yield 1.1 g, 35.1%.

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