Water → Ethyl 4-oxo-3,4-dihydro-2H-pyran-2-carboxylate

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A solution of ZnCl (0.63 g, 4.6 mmol) in anhydrous THF (15 mL) was added to a solution of 1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene (7.94 g, 46.0 mmol) and ethyl glyoxalate (7.05 g, 69.0 mmol) in toluene (30 mL) at room temperature. After stirring for 30 minutes, water (30 mL) and TFA (2 mL) were added, and the mixture was stirred vigorously for 20 minutes. After concentration, the residue was partitioned between EtOAc (200 mL) and water (100 mL). The separated organic layer was washed with brine, dried over sodium sulfate, and concentrated to give 4-oxo-3,4-dihydro-2H-pyran-2-carboxylic acid ethyl ester (8.0 g, 100% yield) as an oil, which was carried to the next step without further purification. 1H NMR (500 MHz, CDCl3) δ 7.40 (d, J=1.0 Hz, 1H), 5.48 (d, J=6.5 Hz, 1H), 5.02 (t, J=8.0 Hz, 1H), 4.26 (q, J=7.0 Hz, 2H), 2.85 (d, J=8.0 Hz, 2H), 1.32 (t, J=7.5 Hz 3H); LCMS (ESI) m/z: 171 [M+H]+.

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